Acta Metallurgica Sinica, 2016, 52(10): 1267-1278
doi: 10.11900/0412.1961.2016.00369
Mg的{101̅2}形变孪晶机制*
THE MECHANISM OF {101̅2} DEFORMATION TWINNING IN MAGNESIUM
单智伟, 刘博宇

摘要:

Mg在室温下的强度和塑性较差, 其根源之一在于Mg的{101̅2}形变孪晶在极低的应力下即可形核和扩展, 而且研究表明目前应用于镁合金的时效强化法通常无法显著抑制{101̅2}形变孪晶. 尽管对Mg及其合金的力学性能至关重要, 迄今为止, 对{101̅2}形变孪晶的形核和扩展的机制仍存在很大争议. 本文首先回顾了有关形变孪晶的定义以及{101̅2}孪晶机制的研究历史, 然后着重介绍了最新的基于原位TEM的研究结果: 即Mg的{101̅2}形变孪晶迥异于孪晶的经典定义, 它事实上是一种新的室温变形机制, 即塑性的产生可以通过局部的晶胞重构来完成, 而不需要孪晶位错的参与; 由晶胞重构机制所产生的界面为{0002}/{101̅0}界面(BP界面), 而且该界面在三维空间呈现梯田状的不规则形貌. 晶胞重构机制迥异于基于位错的孪晶变形机制, 因此基于对该机制进行抑制的设计思路可能是开发未来高强韧镁合金的关键.

关键词: Mg ; 形变孪晶 ; 基面-柱面界面 ; 强度 ; 合金设计

Abstract:

The {101̅2} deformation twinning with extremely low activation stress is considered to be one of main reasons for the low strength of magnesium and its alloys at room temperature. In addition, it was found that those generally adopted age-strengthening methods are less effective for magnesium alloys in which postmortem investigation found that {101̅2} deformation twinning is still profuse. The formation and propagation mechanism of {101̅2} deformation twinning, which are of great importance for designing high strength magnesium alloy, remains elusive or under fervent debate. This paper reviewed the classical definition of deformation twinning, the existing twinning mechanisms, and the recent achievements through in-situ TEM studies on {101̅2} deformation twinning. It was found that the {101̅2} deformation twinning observed in magnesium are distinct from the classical definition on twinning. It is indeed a brand new room temperature deformation mechanism that can be carried out through unit-cell-reconstruction, without involving twinning dislocations. In addition, the boundaries generated through unit-cell-reconstruction are composed of {0002}/{101̅0} interfaces (BP interfaces) and exhibit a terrace-like morphology in 3D space. The unit-cell-reconstruction is essentially different from the traditional dislocation-based twinning mechanism. As a consequence, to develop an effective strengthening strategy based on the nature of this new deformation mechanism would be the key for designing high strength magnesium alloy.

Key words: Mg ; deformation twinning ; basal/prismatic interface ; strength ; alloy design

Mg在室温塑性变形过程中容易形成大量的{ 10 1 ̅ 2 }形变孪晶[1-3], 导致了Mg的屈服强度低[1,4], 各向异性加剧[5-7], 还会导致局部的应力集中[8-11]. 因此, 对{ 10 1 ̅ 2 }形变孪晶的调控是镁合金强韧化设计中需要重点考虑的问题[12]. 根据传统的形变孪晶理论, 形变孪晶是由孪晶位错沿着孪晶方向在孪晶面上逐层滑移形成的[13,14]. 据此推测, { 10 1 ̅ 2 }形变孪晶的本质仍属于位错滑移, 与面心立方晶体中的{111}孪晶的情况类似. 基于这样的认识, 若要抑制{ 10 1 ̅ 2 }形变孪晶, 一个最直接的方法就是在Mg基体中生成沉淀相, 使孪晶位错像普通位错一样被析出相阻碍[15-19], 从而使得合金的强度提高, 即沉淀相强化. 人们仿照铝合金中的时效强化方法对镁合金采取了类似的措施, 众多成分各异的镁合金被研发出来, 诸如Mg-Al基、Mg-Zn基、Mg-Zn-Al基、Mg-Ca基、Mg-Sn基、Mg-Nd基、Mg-Ce基、Mg-Gd基和Mg-Y基等, 这些镁合金中都含有大量的形状、尺寸和分布情况各异的析出相[20]. 然而, { 10 1 ̅ 2 }形变孪晶均没有因为析出相的存在而得到有效的控制, 相应的镁合金的强度也没有得到期望中的大幅提升, 例如AZ31镁合金[21-28]、AZ61镁合金[29]、AZ91镁合金[30]和Z5合金[30,31]等. 这从一个侧面反映出{ 10 1 ̅ 2 }形变孪晶在本质上很可能与传统上所理解的位错滑移有所不同. 因此, 充分理解{ 10 1 ̅ 2 }形变孪晶的成核和扩展机理就成为设计高强韧镁合金的前提和基础.

本文首先回顾了Mg及其合金中{ 10 1 ̅ 2 }形变孪晶的一系列反常现象, 然后从晶体学分析入手, 结合有关{ 10 1 ̅ 2 }形变孪晶机制的发展历史及最新的研究结果, 探讨{ 10 1 ̅ 2 }形变孪晶与经典的形变孪晶的异同及其对镁合金力学行为的影响.

1 { }形变孪晶的异常行为

早在1964年, Partridge 和Roberts[32]就发现在孪晶区域进行压入实验后, 观察到原本平直的2条孪晶界分别向外鼓出. 这种孪晶长大方式的奇特之处在于所形成的孪晶界显著偏离期望中{ 10 1 ̅ 2 }孪晶面的共格孪晶界. 文章作者估算[32], 若用孪晶位错来解释这样大的偏离现象, 则在偏离现象最严重的孪晶界上, 孪晶位错的间距仅为0.4 nm, 而这在物理上是不现实的. 遗憾的是, 上述现象在当时并未引起人们的广泛关注. 1965年, Clark[33]报道了对镁锌合金中时效强化的研究结果, 发现尽管析出相能够阻碍位错的滑移, 但是它们对{ 10 1 ̅ 2 }形变孪晶的抑制作用非常有限,{ 10 1 ̅ 2 }形变孪晶仍然能够在充满析出相的地方形成. 1968年, Clark[34]又报道了对镁铝合金的时效强化研究结果, 发现由于析出相的存在, { 10 1 ̅ 2 }形变孪晶的数量有所减少, 一些{ 10 1 ̅ 2 }形变孪晶的前端终止在析出相附近. 在之后近30年的时间内, 鲜有关于镁合金中孪晶与析出相相互作用的研究报道.

直到1998年, Gharghouri等[35]报道了镁铝合金中的{ 10 1 ̅ 2 }形变孪晶在遇到Mg17Al12析出相时的行为. 在对实验结果综合分析的基础上, 文章作者认为析出相与孪晶的相互作用方式取决于两者尺寸差: 当{ 10 1 ̅ 2 }形变孪晶的宽度远大于Mg17Al12析出相时, 孪晶会将析出相的整体吞没; 当{ 10 1 ̅ 2 }形变孪晶的长度远大于Mg17Al12析出相时, 析出相会将孪晶阻挡住; 而当{ 10 1 ̅ 2 }形变孪晶的宽度与Mg17Al12析出相的宽度相当时, { 10 1 ̅ 2 }形变孪晶会像河流绕过山丘一样绕过阻碍它扩展的析出相. 文章作者进一步指出: “我们相信这是第一次观察到形变孪晶绕过障碍物, 这些孪晶的表观惯习面偏离理论孪晶面达15°之多”.

2009年和2011年, Barnett和其合作者[30,31]对镁铝合金和镁锌合金进行了研究, 结果表明尽管在时效之后的镁合金中产生了大量形状各异的析出相, 但{ 10 1 ̅ 2 }形变孪晶仍然大量地形成. 文章作者发现: “析出相与孪晶的作用十分微弱. 没有任何一个孪晶界的扩展会被析出相钉扎住, 被孪晶扫过的析出相的形状或方向也没有变化”.

上述实验现象表明, 镁合金中{ 10 1 ̅ 2 }形变孪晶的行为与经典的形变孪晶十分不同, 因此在成核和扩展机理上也应与基于位错的孪晶有所不同. 下文将综述有关{ 10 1 ̅ 2 }形变孪晶机制的研究进展及其对高强韧镁合金设计的启发意义.

2 经典的孪晶定义

根据经典文献[14,36-38], 形变孪晶可以按照如下方式定义. 形变孪晶是晶体材料的一种塑性变形方式, 在应力的作用下, 晶体中的部分区域发生剪切变形并伴随着晶向的改变, 变形后的晶体结构与其基体完全一样, 并以一个低指数晶面为界面, 变形的部分与其基体关于这个界面形成镜面对称关系或其它对称关系, 这样的塑性变形过程就是形变孪晶, 发生晶向改变的区域叫做孪晶, 孪晶与其基体互为孪晶关系, 它们的界面称为孪晶面. 在这一定义的基础上, 人们常用4个孪生要素来描述形变孪晶[14]: (1)第一不畸变面K1, 也就是孪晶面和共格孪晶界所对应的晶面; (2)第二不畸变面K2, 也叫共轭不畸变面; (3)剪切方向η1; (4)共轭剪切方向η2.

以fcc结构晶体中的{111}< 11 2 ̅ >孪晶为例, 如图1[38]所示, 观察方向为 [ 1 ̅ 10 , (111)面的迹线用斜向下的虚线表示. 左侧为基体(白色), 右侧为孪晶(灰色), 孪晶界平行于(111)面. (111)面为孪晶与基体共享, 即第一不畸变面K1. 孪晶与基体关于K1面呈镜面对称关系. 孪晶的剪切方向, 也就是Shockley分位错的滑移方向为 [ 11 2 ̅ ] , 即η1. 此外, 基体中的 11 2 ] 方向在经过切应变之后在孪晶中仍是 11 2 ] 方向, 没有发生畸变, 这就是共轭剪切方向η2. 沿η2且垂直于纸面的面为( 11 1 ̅ ), 它在经过了切应变之后在孪晶中的仍是( 11 1 ̅ )方向, 这就是第二不畸变面K2. 根据晶体学关系可以算出η2η2'的夹角为38.9°, η1η2'的夹角为70.5°, 该过程产生的切应变量为0.707. 在fcc的{111}< 11 2 ̅ >孪晶中, 经过孪晶位错Burgers矢量(bT)的整数倍的移动量之后, 所有的位点都可以到达正确的孪晶位点上, 这说明{111}< 11 2 ̅ >孪晶变形等价于一个应变为0.707的剪切变形. 而剪切变形可以由位错滑移产生, 这是孪晶位错理论的晶体学基础.

图1 fcc晶体中的111孪晶的孪生要素[38]

Fig.1 Twinning elements of 111 twin in fcc structure[38] (K1 is the first undistorted (invariant) plane, K2 is the second undistorted (conjugate) plane, η1 is the shear direction, η2 and η2 are the conjugate shear directions in matrix and twin, respectively. and represent alternative (11̅0) planes. A, B and C represent the stacking sequence of (111) planes. a refers to the lattice constant. Dashed lines towards lower right are traces of (111) planes)

3 { }形变孪晶机制的研究历史

在{ 10 1 ̅ 2 }孪晶中, 切应变仅能使部分原子被转移到正确的孪晶位点上, 其它原子则还需经历原子重组(atomic shuffling)才能到达孪晶位置[14,36], 有关{ 10 1 ̅ 2 }形变孪生的晶体学分析的详细论述参见文献[39]. 此外, 在孪生过程中, { 10 1 ̅ 2 }面上的原子位置也发生了变化, 不再满足“不畸变面”的定义[40]. 因此, 不能简单地照搬适用于fcc晶体中的孪生位错理论来解释hcp结构中的{ 10 1 ̅ 2 }孪晶. 关于{ 10 1 ̅ 2 }形变孪晶的形核机制, Thompson和Millard[41]在1952年提出了极位错机制. 该机制从晶体学的角度出发, 认为在基体中的一根Burgers矢量为c的螺位错经过交滑移后形成了一段不可动的位错, 并在此过程中分解出2根孪晶位错, 进而由这些孪晶位错在孪晶面上的滑移完成孪晶的形核过程. 但后来的一些理论计算认为这种孪晶位错的产生十分困难, 相关的模拟工作也预测了这个极位错分解过程和新形成的孪晶不能稳定存在[42]. 更为重要的是, 这个极位错理论一直缺乏直接实验证据支持. 1986~1999年, 由Serra, Bacon和Pond[43-48]共同提出并发展了一套{ 10 1 ̅ 2 }形变孪晶长大的理论. 该理论是晶界理论中的disconnection (即由于2个晶粒间存在晶格错配而在晶界上形成的台阶, 我们称之为“错阶”)概念的拓展, 认为由于{ 10 1 ̅ 2 }孪晶与其基体在晶体学上的差异, 其界面上也会存在disconnection. 这个disconnection可以被认为是一个孪晶位错, 并由其沿着{ 10 1 ̅ 2 }孪晶面的滑移完成孪晶的长大过程. 1997年, Braisaz等[49]用TEM对金属Zn中的{ 10 1 ̅ 2 }孪晶界进行了表征, 发现{ 10 1 ̅ 2 }孪晶界确实存在扭折现象, 并将其归结为disconnection机制的直接证据. 然而, disconnection机制是从纯几何的角度出发, 其可动性尚未得到有效的论证, 且至今尚没有令人信服的实验证据证明 { 10 1 ̅ 2 }形变孪晶的长大就是通过上述TEM照片中看到的disconnection的滑移完成的. 2009年, Wang等[50,51]通过分子动力学模拟和密度泛函理论计算, 提出{ 10 1 ̅ 2 }形变孪晶的形核过程是由“分位错+孪晶位错”的共同作用完成的. 然而, 受限于当时的技术条件, 上述孪晶机制均未得到强有力的实验证据支持.

2009年, Li和Ma[52]通过分子动力学模拟提出{ 10 1 ̅ 2 }形变孪晶可以单纯依靠原子重组过程来完成, 而无需借助位错滑移或disconnection滑移. 由于不同研究者对原子重组(atomic shuffling)的理解或定义不同, 该理论一度引起了相当大争议[40,53-57]. 经典的孪晶理论从晶体学的角度出发, 认为hcp金属中的原子从基体位点转移到{ 10 1 ̅ 2 }孪晶位点上时必须同时经历剪切和原子重组, 这里并没有强调具体的变形机制和路径. 而Li和Ma[52]所使用的原子重组指的是原子在局部范围内的运动, 是一个具体的过程或路径, 在这个过程中, 基体中的原子就像被“洗牌”一样重新定位到{ 10 1 ̅ 2 }孪晶位点上. 该机制不涉及位错滑移, 与经典的形变孪晶理论十分不同. 尽管该机制是基于分子动力学模拟的结果, 在实验上缺乏有力的证据, 但其从一个全新的视角审视了{ 10 1 ̅ 2 }形变孪晶的机制问题, 一些由传统的位错机制所不能解释的实验现象也因此有了解惑的新思路. 随后, 这种基于原子重组的孪晶变形机理得到了广泛关注, 研究人员使用计算机模拟和理论计算对{ 10 1 ̅ 2 }孪晶形核和长大的原子重组机制及非共格孪晶界的组成单元进行了探索[40,57-65].

4 { }形变孪晶机制的最新研究进展
4.1 { }孪晶界异常行为的原位电镜研究

从前文可以看出, 有关{ 10 1 ̅ 2 }孪晶的成核和扩展机制仍存在相当大的争议: 多数关于孪晶变形机制的描述是基于位错滑移的概念, 尽管Li和Ma[52]提出了{ 10 1 ̅ 2 }形变孪晶的非位错机制——原子重排, 但相关的理论研究大都建立在计算机模拟的基础上, 缺乏有力的实验依据. 而{ 10 1 ̅ 2 }孪晶对镁合金的力学性能起着至关重要的作用, 对{ 10 1 ̅ 2 }孪晶机制认知上的不足无疑对设计高强和高韧镁合金十分不利. 因此, 从实验上直观地揭示{ 10 1 ̅ 2 }孪晶的成核和扩展机理意义重大.

以前人们对镁合金孪晶行为的研究方法大多是传统的“先测试再观察”, 即先对块体材料进行加载, 再将变形后的样品进行抛光或减薄处理, 然后用光学显微镜或电子显微镜进行观察与表征, 常用方法包括EBSD分析、电子衍射分析或衍衬分析. 这种“非原位”方法不能在微观尺度上直观地展现和揭示塑性流变的机理. 近年来, 随着基于电子显微镜的力学测试技术的快速发展, 研究人员把材料变形过程中的微观结构演化过程(如孪晶界的迁移、位错滑移等)与样品的力学数据(如应力应变曲线)一一对应起来, 从而可以在小尺度上定量地测量和揭示材料塑性变形的物理机制[66].

2010年, 西安交通大学单智伟研究团队开始将最新的原位透射电镜力学测试技术用于研究金属Mg的塑性变形机理[67-70]. Liu等[69]使用聚焦离子束制备出微纳尺度的纯Mg压缩和拉伸样品, 如图2[69]所示. 原位压缩实验的加载方向为[ 1 1 ̅ 00 ], 即平行于基面但垂直于柱面; 原位拉伸实验的加载方向为[0001], 即平行于c轴但垂直于基面. 根据以往的研究结果, 在这2种加载条件下, 样品均应发生{ 10 1 ̅ 2 }形变孪晶[2,3]. 理论计算表明, 在上述加载条件下, { 10 1 ̅ 2 }孪晶界与加载方向的夹角应为43.15°(压缩)或46.85°(拉伸). 然而统计分析表明, 所形成的{ 10 1 ̅ 2 }孪晶界均偏移了理论上的{ 10 1 ̅ 2 }孪晶面, { 10 1 ̅ 2 }孪晶界与加载方向的夹角大部分落在40°~60°, 如图3[69]所示.

图 2 微纳尺度纯镁力学测试样品的SEM像[69]

Fig.2 SEM images of micro-pillar (a) and ‘dog-bone’ sample (b) of pure magnesium[69]

图3 101̅2孪晶界与加载方向的夹角[69]

Fig.3 Measured angle between the {1012̅} twin boundary and the loading direction[69] (a and b point to the twin boundary that is approximately parallel to and perpendicular to the loading direction, respectively. c points to the twin boundary generally following the twinning plane)

值得注意的是2种极限情况: 第一种, { 10 1 ̅ 2 }孪晶界几乎与加载方向平行, 孪晶像匕首一样“插入”样品根部, 其右侧的界面几乎与样品标距区域的右边缘平行, 这相当于一个偏离了{ 10 1 ̅ 2 }孪晶面达45°之多的{ 10 1 ̅ 2 }孪晶界, 如图4a[68,69]所示. 按照经典的孪晶位错理论, 孪晶位错在孪晶面上逐层滑移, 使得孪晶向下长大. 但是, 当孪晶生长到样品根部时, 所有的位错停止在标距区域向下的延长线位置(图中白色虚线), 就像遇到了一堵平行于样品右边界的“墙”. 这种行为显然难以用传统的基于孪晶位错的理论来解释; 第二种,{ 10 1 ̅ 2 }孪晶界几乎与加载方向垂直, 如图4b[68,69]所示. 该孪晶界呈拱形, 并几乎与加载方向成90°的夹角, 这也相当于一个偏离了{ 10 1 ̅ 2 }孪晶面达45°之多的孪晶界. 与此同时, 在孪晶界运动过程中, 晶体的两侧同时变“胖”, 并没有观察到该孪晶应有的约为12.9%的切应变. 按照经典的孪晶位错理论, 孪晶位错在孪晶面上逐层滑移, 使得孪晶向上生长. 但是, 所有的位错都同时停止在相同的高度, 就像遇到了一堵垂直于样品轴向的拱形墙似的. 这一现象同样很难用传统的孪晶位错理论解释.

图4 {101̅2}孪晶界与加载方向近似平行或垂直[68,69]

Fig.4 {101̅2} twin boundary is almost parallel (a) or perpendicular (b) to the loading direction[68,69]

有趣的是, 当观察方向为[ 11 2 ̅ 0 ]时, 一些{ 10 1 ̅ 2 }孪晶界并没有像期望中那样显示为一条迹线, 而是呈现出一定的“宽度”. 图5给出了一个典型的案例, 尽管孪晶界与加载方向的夹角约为44°, 非常接近理论值(43.15°), 但它并不显示为一条明锐的迹线, 而是呈现为一个由2条清晰界线界定的条带(如图5a[69]中的虚白线所示). 这种具有一定“宽度”的{ 10 1 ̅ 2 }孪晶界应该是实际孪晶界沿[ 11 2 ̅ 0 ]方向的投影. 这说明孪晶界并不全在期望的{ 10 1 ̅ 2 }面上, 而是在空间上呈三维分布. 图5b是根据实验观察构造出来的一种可能的晶界构型, OMNP为“孪晶界”, 注意其与电子束观察方向[ 11 2 ̅ 0 ]不平行. 该界面在样品背面的投影为 O M P . 可以根据实测的晶界条带宽度来估算 ∠OM M 的值. 如图5a所示, 实测的投影宽度w约为120 nm, 样品的厚度t约为424 nm, OP与样品轴线的夹角约为44°, 因此 O M 120 nm cos 44 ° 167 nm . 假定 O M 垂直于 M M , 则 ∠OM M arctan 167 424 21 ° . 为了进一步揭示孪晶界的实际形貌, Liu等[69]以[0001]为观察方向对样品进行了测试, 结果发现所生成的孪晶界呈连续的弧形, 如图6[69]所示. 因此, 当这个界面沿[ 11 2 ̅ 0 ]方向产生投影时, 就一定会导致图5中所观察到的具有一定“宽度”的带状晶界. 值得注意的是, 在该观察条件下, 孪晶界的形貌一直在变化, 并且其弓出的程度与应力有关, 当外加应力去除之后, 孪晶界趋向于平直.

图5 具有一定“宽度”的{101̅2}孪晶界投影[69]及成因示意图

Fig.5 The projection of an inclined {101̅2} twin boundary [69] (w—width of projection, t—thickness of sample)
(a) dark field TEM image showing a band-like twin boundary
(b) schematic illustrates that the band-like feature comes from the projection of a inclined twin boundary along the e-beam direction

图6 沿[0001]方向观察到的{101̅2} 孪晶界迁移过程的原位录像截图[69]

Fig.6 Snapshots from an in-situ video showing the {101̅2} twin boundary migration viewed along [0001] [69]
(a) the twin (dark contrast) just formed (b) the twin is expanding with an arch shaped boundary (c) the pillar was under the largest strain (d) the diamond punch was completely retracted

综上所述, 在微纳米尺度下, { 10 1 ̅ 2 }孪晶界并非只是简单地偏离{ 10 1 ̅ 2 }孪晶面. 实际上, 它在三维空间中可呈现出一定的形状, 而不是平行于任何一个晶体学面. 为了揭示其在原子尺度上的构成, 必须借助原子分辨率的电子显微镜对其进行系统的剖析.

4.2 BP界面的发现

为了揭示{ 10 1 ̅ 2 }孪晶界的三维形貌及其在原子尺度上的构成, Liu等[69]逐层减薄了一个具有{ 10 1 ̅ 2 }孪晶界的样品, 发现当样品的厚度逐渐变薄时, 其投影区域的宽度也在逐渐变窄. 尽管在低倍下, 这些晶界看起来很直, 但在高分辨观察模式下, 这些晶界具有显著的弯曲和带状特征, 且不同位置的晶界具有不同的形状[70]. 图7给出了6种典型的形貌. 各图的左上部分为基体, 右下部分为孪晶, 基体和孪晶中的基面及{ 10 1 ̅ 2 }面用白色实线标出, 基体和孪晶中的基面呈近似垂直的关系,孪晶界的位置及其轮廓用白色虚线标出. 图7a中的孪晶界近似平行于{ 10 1 ̅ 2 }面, 但是在该晶界的右上方有一个台阶(箭头1), 这个台阶近似平行于孪晶中的基面. 图7b中的孪晶界明显偏离{ 10 1 ̅ 2 }面, 其大致方向与孪晶中的基面方向较为接近. 在该晶界的右侧也有一个台阶(箭头1), 但这个台阶近似平行于基体中的基面. 此外, 在基体中还可观察到一个类似层错的特征(箭头2). 图7c中的孪晶界可分为4段, 第1段和第3段晶界较为狭窄, 且近似平行于孪晶中的基面方向. 第2段晶界近似平行于{ 10 1 ̅ 2 }面. 第4段是由2个界线包夹的带状区域, 该区域的高分辨像较为模糊. 图7d和e中, 部分带状区域的宽度可达5~10 nm,其形貌呈现一定程度的扭折. 图7f中的这种带状区域更为明显,其宽度可达15~20 nm. 有趣的是, 这种带状区域的界线或近似平行于孪晶中的基面方向, 或近似平行于基体中的基面方向.

图7 {101̅2}孪晶界的高分辨像[70]

Fig.7 HRTEM images of {101̅2} twin boundaries[70] (white dashed lines outline the profile of twin boundary)
(a) twin boundary is approximately parallel to the {101̅2} plane (1 points to a step parallel to the basal plane in twin)
(b) twin boundary is approximately perpendicular to the basal plane of the matrix (1 points to a step parallel to the basal plane in matrix. 2 points to a stacking fault in matrix)
(c) twin boundary with zig-zag shape (1 and 3 point to segments of the twin boundary parallel to basal plane in twin. 2 point to a segment approximately along the {101̅2} plane. 4 points to a band-like boundary area with its width of about 2 nm)
(d, e) serrated twin boundaries exhibit considerable width of about 5~10 nm
(f) a band-like twin boundary with its width of about 15 nm

为了更清晰地表征{ 10 1 ̅ 2 }孪晶界的精细结构, Liu等[68,69]运用消球差电镜技术对这些界面进行了系统的表征. 结果表明, 实验中得到的{ 10 1 ̅ 2 }孪晶界的基本组成单元有2类: { 10 1 ̅ 2 }共格孪晶界面(CTB)和基面-柱面界面(BP或PB). CTB平行于{ 10 1 ̅ 2 }孪晶面, 也就是传统上认为的{ 10 1 ̅ 2 }孪晶界. 这里BP界面是指孪晶中的(0002)基面(basal plane)和基体中的( 1 1 ̅ 00 )柱面(prismatic plane)的界面; PB界面是指孪晶中的( 1 1 ̅ 00 )面和基体中的(0002)面的界面. BP与PB相互垂直, 与CTB的夹角约为45°. 图8[39]给出了3个典型案例. 为了表示晶向关系, 将基体中的基面、孪晶中的基面和{ 10 1 ̅ 2 }面用白色实线标出. 可以看出, 2个晶粒中的基面互相垂直, 且与{ 10 1 ̅ 2 }面的夹角约为45°. 为了清晰地显示各个原子的位置, 进而勾勒出界面的精细结构, 文章作者为每张HAADF-STEM照片制作了相应的原子位置示意图(黑色空心圆圈标出基体中原子的位置, 黑色实心圆标出孪晶中原子的位置). 图8a和b是一个CTB与BP共存的典型例子, 图中CTB的长度约为3 nm, BP的长度约为3 nm, BP界面直接连接了孪晶中的基面和基体中的柱面. 图8c和d是一个BP与PB共存的例子, PB界面直接连接了孪晶中的柱面和基体中的基面, 且BP与PB相互垂直. 若考虑这个样品的加载方向, 则BP界面与压缩方向垂直, PB界面与压缩方向平行. 这几个BP界面和PB界面的长度也在2~4 nm. 图8e和f是一个较长的BP界面, 在该界面上有一些台阶. 这些台阶可以看做是最小的PB界面或CTB界面.

图 8 BP界面的原子尺度成像及界面示意图[39]

Fig.8 Atomic scale images of BP interfaces (a, c, e) and the corresponding schematics (b, d, f) [39]
(a, b) the coexistence of CTB and BP interfaces
(c, d) the coexistence of BP and PB interfaces
(e, f) a long BP inteface with several steps

由上述结果可以推测, BP界面、PB界面和CTB界面可以组成形状各异的{ 10 1 ̅ 2 }孪晶界. 而且, 由于BP界面和PB界面相互垂直, 因此即使没有CTB的参与, 通过这2种界面的组合也能形成任意形状的{ 10 1 ̅ 2 }孪晶界, 即图 4中所示的案例. 值得注意的是, 最近的研究[58,65,71,72]表明, 其它变形后的hcp金属中也发现了BP和PB界面的存在, 这说明该现象对hcp金属而言很可能是一种普遍的现象. 鉴于PB和BP界面的类似性, 后文中将二者统称为BP界面.

4.3 晶胞重构

上述实验结果表明, 在Mg中所发现的基于BP界面运动的塑性变形方式与传统的位错滑移和经典的形变孪晶有本质区别. 一方面, { 10 1 ̅ 2 }孪晶界上存在大量的BP界面, 使得其整体偏离{ 10 1 ̅ 2 }孪晶面, 并在三维空间呈现出不规则形状, 且孪晶与基体的晶体学取向关系不符合{ 10 1 ̅ 2 }孪晶的晶体学取向关系. 另一方面, BP界面的迁移倾向于产生正应变, 而非理论上的沿< 10 1 ̅ 1 ̅ >的切应变. 再次审视孪晶与基体的相对关系: 通过剪切变形不能使基体位点全部转移到孪晶位点上, 在此基础上还需要进行额外的原子重组, 且原子重组过程中涉及到的局部位移量也与切变位移量相当. 基于上述认识, 本文作者提出了金属Mg在室温下塑性变形的新机制——晶胞重构. 在由该机制主导的变形过程中, 基体中的位点直接通过局部重组到达新晶粒中的位点, 新形成的晶粒与基体关于 a 轴< 2 1 ̅ 1 ̅ 0 >的“旋转”角度为90°, 新晶胞相对于基体晶胞沿基体的[0001]方向产生了正应变, 而无需引入切应变. 新晶胞中的(0002)面与基体晶胞中的( 1 1 ̅ 00 )面相互平行, 新晶胞中的( 1 1 ̅ 00 )面与基体晶胞中的(0002)面相互平行, 而2个晶胞中的(0002)面成垂直关系, 与BP界面恰好吻合. 图 9[68,70]给出了一种可能的重构路径, 该图将晶胞右下角的2个原子位置作为参考点, 在三维空间中显示了一个hcp晶胞中各个原子与其在晶胞重构后的相对位置关系.

图 9 一种可能的晶胞重构路径[68,70]

Fig.9 One possible route for the unit-cell-reconstruction[68,70] (The matrix hcp cell and its atoms are outlined by dashed lines and circles (light gray) respectively. The new hcp cell and the atoms are outlined by solid lines and circles (dark gray) respectively)

尽管晶胞重构产生的晶向改变与形变孪晶十分相似, 它们都使晶体的晶向发生改变且保持晶体结构不变, 但这2种变形机制有着本质上的区别. 形变孪晶需要一个晶体学面充当剪切过程中的不变面(孪晶面), 而晶胞重构使得晶体沿一个晶向发生正应变. 形变孪晶中产生的孪晶界通常平行于孪晶面, 具有特定的方向性, 而晶胞重构产生的界面是两组相互垂直的BP界面. 这些BP界面以不同的比例进行组合可以在三维空间中形成任意形貌的晶界.

目前的研究表明, 以BP界面为特征的晶胞重构机制不仅发生于微纳米尺度的样品中, 也存在于高应变速率变形后的块体材料中[72,73]. 从能量的角度来看, 这似乎是不合理的, 因为{ 10 1 ̅ 2 }共格孪晶界具有较低的界面能, 约为125 mJ/m2, BP界面的界面能约为170 mJ/m2, 而其它非共格界面的界面能一般要高于250 mJ/m2 [65]. 众所周知, 金属材料中存在着“越小越强”的趋势, 即随着被测试样品尺寸的降低,特别是降低到微米、纳米级别时, 其强度将远高于在宏观块体样品上测得的强度[74]. 而高应变速率通常也意味着材料要承受更高的应力水平. 因此, 高应力可能是晶胞重构机制替代基于位错机制的{ 10 1 ̅ 2 }孪晶的主要诱因. 以小尺度试样为例, 随着样品尺寸的减小, 激活孪晶位错所需的应力水平可能急剧上升, 并直接导致可用于进行形变孪晶的位错数量大大降低, 从而使得样品所能承受的应力提高[75]. 而应力的提高使得晶胞重构机制得以克服能量势垒替代基于位错的孪晶成为主导的变形机制. 此外, 微纳米尺度的样品的比表面积更大, 利于释放BP界面上错配位错产生的晶格畸变也可能对晶胞重构机制起到了一定的促进作用.

5 结语与展望

本文简述了Mg的{ 10 1 ̅ 2 }形变孪晶的反常现象, 如孪晶界偏离孪晶面, 不具备明显的尺寸效应, 不易被析出相阻碍等. 总结了{ 10 1 ̅ 2 }孪晶的位错机制和非位错机制的研究历史, 并详细介绍了基于原位电镜定量研究Mg的{ 10 1 ̅ 2 }孪晶的最新进展, 提出了不同于传统的位错滑移和形变孪晶的新型塑性变形机制——晶胞重构.

Mg的{ 10 1 ̅ 2 }形变孪晶是Mg和镁合金的最重要的塑性变形方式之一, 其行为将显著地影响Mg和镁合金的力学性能. 一方面, 由于{ 10 1 ̅ 2 }孪晶的形核和扩展应力很低, 提升镁合金强度的方法之一是抑制该类孪晶或提升其成核和扩展应力. 另一方面, 晶胞重构机制与传统的位错滑移及经典的形变孪晶在本质上有所不同, 这可能导致析出相、颗粒增强相等“障碍物”对其阻碍作用较弱. 在以位错滑移为主要塑性变形方式的金属材料中, 如铝合金, 析出相对位错的强烈的钉扎作用产生了明显的时效强化效果. 然而晶胞重构是由原子的局部重组完成的, 不需要依赖位错滑移. 根据这一特点可以推测, 当迁移中的{ 10 1 ̅ 2 }孪晶界遇到障碍物时, 被阻碍的部分可以通过晶胞重构的方式绕过析出相, 该过程可以用流沙没过石头进行类比. 因此, 只要镁合金的镁基体是连续的, 不论析出相是何种形状、尺寸和密度, { 10 1 ̅ 2 }孪晶都可以形核并长大, 使得由孪晶导致的一系列缺点在镁合金中被一定程度地遗留了下来, 如较低的屈服和流变应力, 变形的各向异性和基面织构等.若要使镁合金达到更高的强度, 需要针对晶胞重构这种特殊的变形机制进行强韧化设计. 此外, 晶胞重构还可能与{ 10 1 ̅ 2 }孪晶中大量存在的基面层错有内在联系[39,68], 而基面层错对镁合金的强化具有一定的积极作用[71,76-78], 因此晶胞重构产生基面层错的成因, 以及对其的控制方法值得进一步研究.

The authors have declared that no competing interests exist.

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This paper seeks to expose the different modes of production of subjectivities engendered by clinical psychological practices and modes of translation and coordination between them. Such research is based on the conceptual Political Epistemology of Stengers and Despret and the Actor-Network Theory of Latour and Law. For these authors, scientific knowledge is produced not as a representation of reality through well-formed sentences, but as modes of articulation between researchers and authorities investigated. In general, these articulation modes can generate a recalcitrant or docility effect by the entities investigated. More specifically, we will accompany therapeutic techniques coming from different orientations on the way they are performed by the Division of Applied Psychology at the Federal University of Rio de Janeiro. For this, besides the description of the artifacts, the people, interns, the triage team and counselors were interviewed at the beginning and end of the therapy.Este trabalho visa trazer à cena os diferentes modos de produ0400o de subjetividades engendrados pelas práticas psicológicas clínicas e os modos de tradu0400o e articula0400o entre elas. Tal investiga0400o tem como base conceitual a Epistemologia Política de Stengers e Despret e a Teoria Ator-Rede de Latour e Law. Para estes autores, o conhecimento científico se produz n00o como representa0400o da realidade através de senten04as bem formadas, mas como modos de articula0400o entre pesquisadores e entes pesquisados. De modo geral, estes modos de articula0400o podem engendrar um efeito de recalcitr09ncia ou docilidade por parte dos entes investigados. De modo mais específico acompanharemos técnicas terapêuticas vindas de orienta0401es distintas na maneira como s00o performadas na Divis00o de Psicologia Aplicada da Universidade Federal do Rio de Janeiro. Para tal, além da descri0400o dos artefatos presentes, foram entrevistadas pessoas em início e em término de terapia, estagiários, a equipe de triagem e orientadores.Este trabajo busca traer a colación los diferentes modos de producción de subjetividades engendrados por las prácticas psicológicas clínicas y los modos de traducción y articulación que hay entre ellas. Esta investigación tiene como base conceptual la Epistemología política de Stengers y Despret y la Teoría Actor-Red de Latour y Law. Para estos autores, el conocimiento científico se produce, no como representación de la realidad a través de sentencias bien formadas, sino como modos de articulación entre investigadores y entes investigados. De modo general, estos modos de articulación pueden generar un efecto recalcitrante o de docilidad por parte de los entes investigados. De modo más específico, acompa09aremos las técnicas terapéuticas provenientes de orientaciones distintas tal como son desempe09adas por la División de Psicología Aplicada de la Universidad Federal de Río de Janeiro. Para esto, además de la descripción de los artefactos presentes, se entrevistaron al inicio y al final de la terapia, a las personas, los pasantes, el equipo de triage y los orientadores.
DOI:10.11144/3835      URL     [本文引用:]
[11] Yu Q, Jiang Y, Wang J.Scr Mater, 2015; 96: 41
The 0 relaxation typically plays an important role in the plastic deformation of glassy materials. Compared with amorphous polymers, most of the metallic glasses do not show evident beta-relaxation based on mechanical spectroscopy. However, La60Ni15Al25 bulk metallic glass (BMG) exhibits a prominent beta relaxation process, which could be an ideal model alloy to investigate the correlation between the beta relaxation and mechanical behavior of metallic glasses. In this work, compressive properties and stress relaxation at high temperature (below glass transition temperature T-g) were studied. Stress relaxation of La60Ni15Al25 BMG was measured by uniaxial compressive tests and mechanical spectroscopy around both a and beta relaxation temperature domain. At higher temperatures and sufficiently low strain rate, the flow behavior of the La50Ni15Al25 BMG could be simulated by a master curve, showing that the behavior is independent of temperature, especially on the proximity of the beta relaxation process. Because the existence of the beta relaxation, a high value of the activation volume for the plastic deformation could be ascribed to the existence of a specific atomic arrangement in the La(60)Nt(15)Al(25) BMG. It is found that compressive stress relaxation kinetics parameter remains temperature independent below T-g. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
DOI:10.1016/j.actamat.2015.07.020      URL     [本文引用:1]
[12] Nie J F, Zhu Y M, Liu J Z, Fang X Y.Science, 2013; 340: 957
The formability and mechanical properties of many engineering alloys are intimately related to the formation and growth of twins. Understanding the structure and chemistry of twin boundaries at the atomic scale is crucial if we are to properly tailor twins to achieve a new range of desired properties. We report an unusual phenomenon in magnesium alloys that until now was thought unlikely: the equilibrium segregation of solute atoms into patterns within fully coherent terraces of deformation twin boundaries. This ordered segregation provides a pinning effect for twin boundaries, leading to a concomitant but unusual situation in which annealing strengthens rather than weakens these alloys. The findings point to a platform for engineering nano-twinned structures through solute atoms. This may lead to new alloy compositions and thermomechanical processes.
DOI:10.1126/science.1229369      PMID:23704567      URL     [本文引用:1]
[13] Mahajan S, Chin G Y.Acta Metall, 1973; 21: 1353
It is proposed that the dislocation reaction may govern the nucleation of three-layer twins in f.c.c. crystals. Conceivably a macroscopic twin may evolve when these three-layer embryonic twins, distributed at different levels within a localized slipped region, grow into each other.Using transmission electron microscopy, the circumstantial evidence in support of the model has been developed by correlating the crystallographic features of twins, faults and associated slip in cobalt-9.5 wt. % iron alloy deformed at 77°K. The observed correlations are totally consistent with the suggested reaction. They cannot, however, be explained in terms of the dislocation reaction considered by Venables and Cohen and Weertman.Our “glide” sources for twins are compared with Venables' “glide” and “prismatic” sources. On the basis of both models, it is possible to satisfactorily account for the orientation dependence of the twinning behavior observed by Blewitt, Coltman and Redman, and Suzuki and Barren. However, the crystallography of twins observed by Chin, Hosford and Mendorf during constrained deformation of the [11000] (110) oriented cobalt-8 wt. % iron crystal cannot be rationalized on the basis of Venables' and Cohen-Weertman's approach, although it can be explained in a satisfactory manner in terms of our model.Les auteurs proposent un modèle suivant lequel la réaction de dislocations peut régir la formation de macles à trois couches atomiques dans les cristaux c.f.c. On peut imaginer qu'une macle macroscopique puisse se former quand ces maeles embryonnaires à trois couches, distribuées à différents niveaux dans une région de glissement localisé, grossissent en s'interpénétrant.Ce modèle a pu être confirmé à partir d'observations au microscope électronique en transmission effectuées sur l'alliage Co-9,5%Fe en poids déformé à 77K, en reliant les caractéristiques cristallographiques des maeles, des fautes et du glissement associé. Les relations observées sont tout à fait compatibles avec la réaction proposée. On ne peut pas par contre les expliquer à l'aide de la réaction de dislocations utilisée par Venables, et par Cohen et Weertman.Les auteurs comparent leurs “sources de glissement” pour les maeles aux sources prismatiques et de glissement de Venables. Les deux modèles permettent d'expliquer de fa04on satisfaisante l'influence de l'orientation sur le maclage, observée par Blewitt, Coltman et Redman ainsi que par Suzuki et Barrett. Cependant, la cristallographie des maeles observées par Chin, Hosford et Mendorf au cours de la déformation sous contrainte du cristal [11000] (110) de Co-8%Fe en poids ne peut pas être interprétée à l'aide du modèle de Venables et de Cohen et Weertman, alors que le modèle proposé ici fournit une interprétation satisfaisante.Es wird vorgeschlagen, daβ die Keimbildung von Dreischichtzwillingen in f.c.c. Kristallen durch die Versetzungsreaktion bestimmt ist. Es ist denkbar, daβ sich aus diesen Dreischicht-Zwillingskeimen makroskopische Zwillinge entwickeln, wenn die innerhalb eies lokalisierten Abgleitbereichs über mehrere Ebenen verteilten Keime ineinander wachsen.Mit Hilfe der Durchstrahlungs-Elektronenmikroskopie wurden experimentelle Hinweise gefunden, die dieses Modell unterstützen. Zu diesem Zweck wurden die kristallographischen Eigenschaften von Zwillingen, Stapelfehlern und der in den bei 77°K verformten Kobalt-9,5 Gew. % Eisen-Legierungen damit verbundenen Gleitung untersucht. Die gefundenen Zusammenh01nge sind vollst01ndig konsistent mit der vorgeschlagenen Reaktion. Sie k02nnen jedoch nicht mit der von Venables und Cohen und Weertman betrachteten Versetzungsreaktion erkl01rt werden.Unsere “Gleit”-quellen f02r Zwillinge werden mit Venables “Gleit”- und prismatischen Quellen verglichen. Beide Modelle erlauben eine befriedigende Erkl01rung der Orientierungsabh01ngigkeit des von Blewitt, Coltman und Redman und von Suzuki und Barrett beobachteten Zwillingsverhaltens. Die Kristallographie der von Chin, Hosford und Mendorf w01hrend der Druckverformung von [11000](110)-orientierten Kobalt-8 Gew. % Eisen-Kristallen kann jedoch nicht anhand der Modelle von Venables und Cohen und Weertman verstanden werden; dagegen erlaubt unser Modell eine befriedgende Erkl01rung.
DOI:10.1016/0001-6160(73)90085-0      URL     [本文引用:1]
[14] Christian J W, Mahajan S.Prog Mater Sci, 1995; 39: 1
[本文引用:4]
[15] Raeisinia B, Agnew S R, Akhtar A.Metall Mater Trans, 2011; 42A: 1418
Discrepancies between the strength of slip systems determined directly from Mg single-crystal studies and those estimated from polycrystal simulations of Mg alloys are known to exist. These discrepanc
DOI:10.1007/s11661-010-0527-5      URL     [本文引用:1]
[16] Akhtar A, Teghtsoonian E.Acta Metall, 1969; 17: 1339
Die Legierungsverfestigung in Mg-Zn-Einkristallen mit einem Zinkgehalt bis zu 0,45 At. % wurde im Temperaturbereich zwischen 78°K und 423°K untersucht. Die kritische Schubspannung für Basisgleitung nahm mit der Wurzel aus der Zn-Konzentration in zwei linearen Stufen mit einem 05bergang bei 0,025 At. % Zn zu. Bei Fremdstoffgehalten von weniger als 0,025 At. % wurde oberhalb 330°K keine Legierungsverfestigung beobachtet. Die elektronenmikroskopische Untersuchung dünner Folien zeigte, daβ die Versetzungsdichte in der Basisebene mit der Wurzel aus der Fremdstoffkonzentratiori zunahm. Diese Zunahme konnte jedoch nicht die bei Zulegierung beobachtete 02nderung der athermischen Komponente der kritischen Schubspannung erkl01ren. Die Zunahme des weitreichenden Spannungsfeldes kann am besten an Hand des Fleischer-Mechanismus erkl01rt werden. Die Hindernisse kurzer Reichweite wurden mit Hilfe der “'rate theorz” identifiziert; die Legiorungsverfestigung bei tiefen Temperaturen wird diskutiert.
DOI:10.1016/0001-6160(69)90151-5      URL     [本文引用:]
[17] Akhtar A, Teghtsoonian E.Acta Metall, 1969; 17: 1351
Magnesiumeinkristalle und eine Reihe Zn und Al enthaltender Legierungen wurden im Zugversuch bei verschiedenen Temperaturen und in einer Orientierung verformt, so daβ {10l000}〈112000〉-prismatisches Gleiten erster Ordnung induziert und sowohl Basisgleitung als auch {101002}-Zwillingsbildung unterdrückt wurde. Bei tiefen Temperaturen nimmt die kritische Schubspannung für prismatisches Gleiten mit der Zugabe der Legierungskomponente kontinuierlich ab. Bei h02heren Temperaturen jedoch nimmt sie bei kleinen Konzentrationen der Legierungskomponente zu und bei gr02βeren Konzentrationen ab. Die Ergebnisse k02nnen am besten mit einem Anwachsen der athermischen Komponente und einer Abnahme der thermisch aktivierten Komponente der Flieβspannung beim Zulegieren erkl01rt werden. Die Beobachtungen deuten darauf hin, daβ die als geschwindigkeitsbestimmender Schritt bei der prismatischen Gleitung in Magnesium und seinen Legierungen identifizierte Abnahme der H02he des Peierls-Potentials weder notwendigerweise mit einer Abnahme des Achsenverh01ltnisses ( ja:math ) noch mit der Einwertigkeit der Legierungskomponente verbunden ist.
DOI:10.1016/0001-6160(69)90152-7      URL     [本文引用:]
[18] Nie J F.Scr Mater, 2003; 48: 1009
This paper reports results on the development of the Orowan equation appropriate for magnesium alloys strengthened by rationally-oriented, shear-resistant precipitate plates/rods. Comparisons of identical volume fractions and number densities of precipitates per unit volume indicate that plate-shaped precipitates that form on prismatic planes of the magnesium matrix phase are most effective for dispersion strengthening.
DOI:10.1016/S1359-6462(02)00497-9      URL     [本文引用:]
[19] Liao M, Li B, Horstemeyer M F.Comput Mater Sci, 2013; 79: 534
Zirconia based ceramics are strong, hard, inert, and smooth, with low thermal conductivity and good biocompatibility. Such properties made zirconia ceramics an ideal material for different applications form thermal barrier coatings (TBCs) to biomedicine applications like femoral implants and dental bridges. However, this unusual versatility of excellent properties would be mediated by the metastable tetragonal (or cubic) transformation to the stable monoclinic phase after a certain exposure at service temperatures. This transformation from tetragonal to monoclinic, known as LTD (low temperature degradation) in biomedical application, proceeds by propagation of martensite, which corresponds to transformation twinning. As such, tetragonal to monoclinic transformation is highly sensitive to mechanical and chemomechanical stresses. It is known in fact that this transformation is the source of the fracture toughening in stabilized zirconia as it occurs at the stress concentration regions ahead of the crack tip.
DOI:10.1016/j.actamat.2013.05.015      URL     [本文引用:1]
[20] Nie J F.Metall Mater Trans, 2012; 43A: 3891
Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted efforts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identified and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys via precipitation hardening.
DOI:10.1007/s11661-012-1217-2      URL     [本文引用:1]
[21] Hong S G, Park S H, Lee C S.J Mater Res, 2010; 25: 784
Low-cycle fatigue (LCF) tests were carried out to investigate the effect of anisotropy on the fatigue resistance of rolled AZ31 magnesium (Mg) alloy. The alloy showed a strong basal texture indicating that most of the basal planes of hexagonal close-packed lattice were located parallel to the rolling direction. Specimens loaded parallel to the rolling direction (RD) were easy to yield in compression, while those loaded normal to the rolled plane (ND) were easily yielded in tension by twinning. This anisotropy caused superior fatigue resistance of ND specimen by introducing the beneficial compressive mean stress. An energy-based model considering the effect of the mean stress was used for the prediction of fatigue life and the predicted results showed a good agreement with experimental data.
DOI:10.1016/j.msea.2009.08.044      URL     [本文引用:1]
[22] Lou X Y, Li M, Boger R K, Agnew S R, Wagoner R H.Int J Plast, 2007; 23: 44
The monotonic and cyclic mechanical behavior of O-temper AZ31B Mg sheet was measured in large-strain tension/compression and simple shear. Metallography, acoustic emission (AE), and texture measurements revealed twinning during in-plane compression and untwinning upon subsequent tension, producing asymmetric yield and hardening evolution. A working model of deformation mechanisms consistent with the results and with the literature was constructed on the basis of predominantly basal slip for initial tension, twinning for initial compression, and untwinning for tension following compression. The activation stress for twinning is larger than that for untwinning, presumably because of the need for nucleation. Increased accumulated hardening increases the twin nucleation stress, but has little effect on the untwinning stress. Multiple-cycle deformation tends to saturate, with larger strain cycles saturating more slowly. A novel analysis based on saturated cycling was used to estimate the relative magnitude of hardening effects related to twinning. For a 4% strain range, the obstacle strength of twins to slip is 3 MPa, approximately 1/3 the magnitude of textural hardening caused by twin formation (10 MPa). The difference in activation stress of twinning versus untwinning (11 MPa) is of the same magnitude as textural hardening.
DOI:10.1016/j.ijplas.2006.03.005      URL     [本文引用:]
[23] Xiong Y, Yu Q, Jiang Y.Mater Sci Eng, 2012; A546: 119
ABSTRACT Multiaxial fatigue behavior of extruded AZ31B magnesium alloy was experimentally studied in ambient air under strain-controlled axial-torsion loading using tubular specimens. Four fully reversed loading paths were employed: tension&ndash;compression, pure torsion, proportional axial-torsion, and 90 out-of-phase nonproportional axial-torsion. Tension&ndash;compression asymmetry in cyclic plastic deformation was observed for tension&ndash;compression and combined axial-torsion loading. For the same equivalent strain amplitude, the 90 out-of-phase nonproportional loading path resulted in the shortest fatigue life and the proportional loading path produced the longest fatigue life. Two critical plane multiaxial fatigue models, namely, the modified Smith&ndash;Watson&ndash;Topper (SWT) model and the Jiang multiaxial fatigue criterion, were evaluated based on the experimentally obtained fatigue results. Both models were found to be able to correlate the fatigue experiments reasonably well.
DOI:10.1016/j.msea.2012.03.039      URL     [本文引用:]
[24] Wan G, Wu B L, Zhang Y D, Sha G Y, Esling C.Mater Sci Eng, 2010; A527: 2915
Textures always exert an intensive influence on the deformation and fracture of magnesium alloys. A hot extruded AZ31 alloy with intensive texture was selected for impact tests with the impact direction parallel to the respective normal, extrusion and transverse directions (ND, ED and TD) of the initial extrusion plate. The results showed that the stress–strain response of the alloy is highly anisotropic and sensitive to the strain rate. The maximum flow stress can reach nearly as high as 600MPa for TD impact. Microstructure responses show that when impacted along normal direction, {106111}–{106112} double twins is common in large grains, however, small grains show {106112} tension twins. After impacted along transverse direction, grains of the alloy are evidently refined and the texture changes from {0001}〈116120〉 to that with {0001} pole in TD direction. The stress–strain, microstructure and orientation responses could well be interpreted with the Schmid factor distributions.
DOI:10.1016/j.msea.2010.01.023      URL     [本文引用:]
[25] Proust G, Tome C N, Jain A, Agnew S R.Int J Plast, 2009; 25: 861
Hexagonal materials deform plastically by activating diverse slip and twinning modes. The activation of such modes depends on their relative critical stresses, and the orientation of the crystals with respect to the loading direction. To be reliable, a constitutive description of these materials has to account for texture evolution associated with reorientations due to both dislocation slip and twinning, and for the effect of the twin boundaries as barriers to dislocation propagation. We extend a previously introduced twin model, which accounts explicitly for the composite character of the grain formed by a matrix with embedded twin lamellae, to describe the influence of twinning on the mechanical behavior of the material. The role of the twins as barriers to dislocations is explicitly incorporated into the hardening description of slip deformation via a directional Hall etch mechanism. We introduce here an improved hardening law for twinning, which discriminates for specific twin/dislocation interactions, and a detwinning mechanism. We apply this model to the interpretation of compression and tension experiments done in rolled magnesium alloy AZ31B at room temperature. Particularly challenging cases involve strain-path changes that force strong interactions between twinning, detwinning, and slip mechanisms.
DOI:10.1016/j.ijplas.2008.05.005      URL     [本文引用:]
[26] Chino Y, Kimura K, Mabuchi M.Mater Sci Eng, 2008; A486: 481
Complicated deformation mechanisms such as twinning, non-basal slips and grain boundary sliding besides the basal slip occur at room temperature for Mg and its alloys. The roles of these mechanisms should be understood from the viewpoint of the homogeneous deformation of a polycrystalline and the relaxation of stress concentrations at the grain boundaries. In the present paper, twinning behavior of AZ31 Mg alloy extrusions with different texture intensity and grain sizes was investigated. Twinning played a vital role in a coarse-grained Mg, not in a fine-grained Mg. Thus, the deformation behavior was drastically changed by grain refinement. Based on the results, the deformation mechanisms are discussed from the viewpoint of the deformation and relaxation mechanisms.
DOI:10.1016/j.msea.2007.09.058      URL     [本文引用:]
[27] Wang Y N, Huang J C.Acta Mater, 2007; 55: 897
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">This paper examines the effect of compressive pre-deformation on subsequent tensile deformation behavior in a hot-extruded AZ31 Mg alloy bar with a ring fiber texture, and with the basal planes parallel to the extrusion direction. Such an orientation favors extensive <span id="mmlsi6" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645406006677&amp;_mathId=si6.gif&amp;_pii=S1359645406006677&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=5e7b14b355521a88d9edf2285fc1695d')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="19" border="0" style="vertical-align:bottom" width="54" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645406006677-si6.gif"></span> twinning under compressive loading, resulting in a comparably low compressive yield stress. In contrast, the basal slip and <span id="mmlsi7" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645406006677&amp;_mathId=si7.gif&amp;_pii=S1359645406006677&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=2c578b11a6f2ccdb4c4f692eaac1b7f9')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="19" border="0" style="vertical-align:bottom" width="54" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645406006677-si7.gif"></span> twinning are difficult to operate under tensile testing, resulting in a high tensile yield strength. Compressive pre-deformation causes a significant drop in tensile yield strength, from &sim;265 to &sim;160&#xA0;MPa, irrespective of the amount of pre-deformation strain. The latter value of &sim;160&#xA0;MPa nearly coincides with the compressive yield strength. The lattice reorientation of 86.3&deg; caused by twinning during compressive loading favors untwinning in the twinned areas during subsequent tensile reloading, leading to a significant drop in tensile yield strength.</p>
DOI:10.1016/j.actamat.2006.09.010      Magsci     URL     [本文引用:]
[28] Knezevic M, Levinson A, Harris R, Mishra R K, Doherty R D, Kalidindi S R.Acta Mater, 2010; 58: 6230
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="sp005">This paper describes the main results from an experimental investigation into the consequences of deformation twinning in AZ31 on various aspects of plastic deformation, including the anisotropic strain-hardening rates, the tension/compression yield asymmetry, and the evolution of crystallographic texture. It was seen that AZ31 exhibited unusually high normalized strain-hardening rates compared to α-Ti that occurred beyond the strain levels where extension twins have completely altered the underlying texture. This observation challenges the validity of the generally accepted notion in the current literature that the high strain-hardening rates in AZ31 are directly caused by extension twins. It is postulated here that the thin contraction twins are very effective in strain hardening of the alloy by restricting the slip length associated with pyramidal 〈<em>c&#xA0;</em>+&#xA0;<em>a</em>〉 slip. This new hypothesis is able to explain the major experimental observations made in this study and in the prior literature. We have also presented a new hypothesis for the physical origin of the observed differences in the thicknesses of the extension and contraction twins. The stress fields in selected matrix&ndash;twin configurations were modeled using crystal plasticity finite element models. The contraction twin <span id="mmlsi1" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645410004854&amp;_mathId=si1.gif&amp;_pii=S1359645410004854&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=9fb883d5fe85ef9d2e63da44b3e62811')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="19" border="0" style="vertical-align:bottom" width="50" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645410004854-si1.gif"></span><span id="mmlsi2" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645410004854&amp;_mathId=si2.gif&amp;_pii=S1359645410004854&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=f40d67747f49f8a2c6005b3900317c09')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="18" border="0" style="vertical-align:bottom" width="45" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645410004854-si2.gif"></span> was predicted to form an internal extension twin <span id="mmlsi3" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645410004854&amp;_mathId=si3.gif&amp;_pii=S1359645410004854&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=834f5d53c08cadbf70547530e8fcb876')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="19" border="0" style="vertical-align:bottom" width="50" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645410004854-si3.gif"></span><span id="mmlsi4" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645410004854&amp;_mathId=si4.gif&amp;_pii=S1359645410004854&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=f867a5671bd1c680ffe6826402382024')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="18" border="0" style="vertical-align:bottom" width="45" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645410004854-si4.gif"></span>, resulting in the commonly observed &ldquo;double twin&rdquo; sequence. The extension twin is suggested to inhibit thickening of this double twin by loss of twin&ndash;matrix coherency. Extension twins were predicted to retain their coherency and thus thicken.</p>
DOI:10.1016/j.actamat.2010.07.041      Magsci     URL     [本文引用:1]
[29] Kleiner S, Uggowitzer P J.Mater Sci Eng, 2004; A379: 258
Deformation anisotropy of extruded Mg-6% Al-1% Zn alloy has been investigated on specimens with different tilt angles relative to the extrusion direction. Calculations of the orientation factors for basal slip and of the strains caused by {1 0 1 2} twinning were done for a slightly idealised texture. This quantification of the two dominating deformation modes was used to explain the marked mechanical anisotropy of the extruded magnesium alloy. Basal slip as well as {1 0 1 2} twinning is inhibited in extrusion direction under tensile loads, which results in high yield strength. Any other testing direction and/or compressive loads are capable of activating slip and/or twinning and yield stress is significantly lower under such conditions. The lattice reorientation of 86.3 caused by twinning has a large influence on the deformation behaviour of a pre-deformed specimen, since the twinned areas are capable of untwinning during reloading in the opposite direction.
DOI:10.1016/j.msea.2004.02.020      URL     [本文引用:1]
[30] Robson J D, Stanford N, Barnett M R.Acta Mater, 2011; 59: 1945
The predicted strengthening effect of precipitates of different shape and habit on the basal, prismatic and {1 0 (1) over bar 2} twinning deformation systems in magnesium has been calculated. In parent material, rod precipitates parallel to the c-axis are predicted to be more effective than plates parallel to the basal plane in hardening the basal and prismatic slip systems. However, in twinned material, non-sheared basal plates are highly effective in inhibiting the basal slip necessary to relieve incompatibility stresses. The predictions suggest basal plates will reduce asymmetry in strongly textured extrusions by preferentially hardening against twin growth compared to prismatic slip, whereas c-axis rods can have the opposite effect. The predictions have been compared with the measured asymmetry for two magnesium alloys that form either c-axis rods (Z5) or basal plates (AZ91). In agreement with the model, it is shown that precipitation in Z5 leads to an increase in asymmetry, whereas in AZ91 precipitation reduces asymmetry. These results suggest that designed precipitation may provide a useful tool for reducing asymmetry in wrought magnesium alloys. (c) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
DOI:10.1016/j.actamat.2010.11.060      Magsci     URL     [本文引用:3]
[31] Stanford N, Barnett M R.Mater Sci Eng, 2009; A516: 226
The deformation behaviour of the age hardenable alloy Mg–5%Zn after different precipitation treatments has been examined. It has been found that during compressive deformation, fine particles increase the number of twins that form, but reduce the size and total volume fraction of twins. Visco-plastic self-consistent modelling has been used to show that the presence of precipitates hardens the twin and prismatic slip systems more than the basal slip system. It is proposed that because the { 1 65 0 65 1 04 65 2 } mathContainer Loading Mathjax twin requires basal slip to accommodate the twinning shear, this twin type will always be hardened equal to, or more than, the basal slip system in response to precipitation.
DOI:10.1016/j.msea.2009.04.001      URL     [本文引用:2]
[32] Partridge P G, Roberts E.Acta Metall, 1964; 12: 1205
Die Bewegung von {101002}〈100011〉-Zwillingsgrenzen in Zink und Magnesium wurde bei kleinen Spannungen untersucht; diese wurden mit einem Mikro-H01rte-Prüfger01t erzeugt. Die lokale Bewegung von Zwillingsgrenzen unter diesen Spannungen rief extrem inkoh01rente Zwillingsgrenzen hervor, in denen der Abstand der Zwillingsgrenzen 4 03 betrug. Traf jedoch die Zwillingsgrenze auf eine groβe Zahl von Gleitversetzungen, wurde die Bewegung erschwert und der Grad der Inkoh01renz verringert. Es fanden sich direkte Gründe für die Ansicht, daβ die Bewegung vorhandener Zwillingsgrenzen weniger Spannung erfordert als die Keimbildung eines neuen Zwillings.
DOI:10.1016/0001-6160(64)90103-8      URL     [本文引用:2]
[33] Clark J B.Acta Metall, 1965; 13: 1281
Im Temperaturbereich 149–260°C verfestigt sich eine Mg-5 Gew.-% Zn-Legierung beim Auslagern durch Ausscheidung eines koh01renten 05bergangsgitters MgZn ja:math in Form von langen dünnen St01bchen senkrecht zur Basisebene. Unterhalb 149°C wird die Legierung nach sehr langem Auslagern durch Bildung von G.P. Zonen geh01rtet. Nach Verlust der Koh01renz wandeln sich die MgZn ja:math St01bchen um in Idiomorphe der Gleichgewichtsphase MgZn. Basisgleitung und 1012-Zwillingsbildung sind die wichtigsten Verformungsmechanismen zwischen 25 und 61 196°C. Es wurde eine schwache Temperaturabh01ngigkeit der kritischen Schubspannung beobachtet. 1012-Zwillingsbildung schert die MgZn01t01bchen. Gleitversetzungen w02lben sich jedoch zwischen den weit verteilten (1000–2000 03) MgZn ja:math Ausscheidungen aus. Folglich führt die Ausscheidung von MgZn ja:math nur zu einer schwachen Zunahme der Festigkeit.
DOI:10.1016/0001-6160(65)90039-8      URL     [本文引用:1]
[34] Clark J B.Acta Metall, 1968; 16: 141
Wenn man die W01rmebehandlung vom gew02hnlichen Abschrecken und Anlassen ab01ndert in eine isotherme Umwandlung, so wird die Wachstumsgeschwindigkeit der Zellen und der Anteil der gebildeten Zellstruktur um fast eine Gr02ssenordnung reduziert und der Abstand zwischen Lamellen nimmt um 50–100% zu. Diese Effekte h01ngen wahrscheinlich mit einer weiteren Abnahme der treibenden Kraft zusammenhangen. Diese Abnahme wird der verst01rkten Einführung von Fehlpassungsversetzungen in die breiten Fl01chen der γ′-Pl01ttchen zugeschrieben, d.h. kleineren Werten des koh01renten Solvus.
DOI:10.1016/0001-6160(68)90104-1      URL     [本文引用:1]
[35] Gharghouri M A, Weatherly G C, Embury J D.Philos Mag, 1998; 78A: 1137
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[36] Bilby B A, Crocker A G.Proc R Soc Lon, 1965; 288A: 240
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Publication &raquo; Twinned crystals.
DOI:10.1080/00018735400101223      URL     [本文引用:]
[38] Yu Y N.The Principle of Physical Metallurgy. 2nd Ed., Beijing: Metallurgica Industry Press, 2013: 763
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(余永宁. 金属学原理. 第2版, 北京: 冶金工业出版社, 2013: 763)
[39] Liu B Y.PhD Dissertation, Xi'an Jiaotong University, 2015
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(刘博宇, 西安交通大学博士学位论文, 2015)
[40] Li B, Zhang X Y.Scr Mater, 2016; 125: 73
Magnetoelectric (ME) coupling by acoustic waveguide was developed. A very strong axial ME response was observed. The dependences of the sample size and the frequency of the ac field on the ME coupling were investigated. Several resonant points were observed in the frequency range applied (<50 kHz). Analysis shows that the standing waves transmitted in the waveguide were responsible for those resonances. And the resonant frequencies were closely influenced by the geometrical size of the waveguide. A resonant condition related to the size of the sample was obtained. The axial (or longitudinal) and transversal ME coefficients were observed to be up to 62 and 6 (V cm611 Oe611) at resonant points, respectively, indicating that the axial ME effect in this device was much higher than its transversal ones. A series of double-peak curves of axial ME coefficient versus magnetic field were observed. The significance of the double-peak curves was discussed.
DOI:10.1063/1.4945676      URL     [本文引用:3]
[41] Thompson N, Millard D J.Philos Mag, 1952; 43: 422
Summary Experiments on the formation of twins in single crystal wires of cadmium under tension demonstrate the essentially discontinuous nature of the process of twin growth, and produce evidence in favour of a critical resolved shear stress law for twinning. Twinning can contribute to creep at room temperature but not at 90 a. A dislocation mechanism for the growth of a twin is discussed in some detail.
DOI:10.1080/14786440408520175      URL     [本文引用:1]
[42] Capolungo L, Beyerlein I J.Phys Rev, 2008; 78B: 2
We propose a three-dimensional model for twin nucleation in hcp materials based on the nonplanar dissociation of the leading dislocation in a pile-up of 70 a 71 slip dislocations. Continuum linear elastic dislocation theory is used to calculate the change in free energy with extension of the dissociated configuration, consisting of a stair rod and glissile twinning dislocation loops. The model is applied to Mg, which deforms primarily by basal slip, and to Zr, which deforms primarily by prismatic slip. It is found that dissociations from an isolated 70 a 71 slip dislocation are energetically unable to produce a stable twin fault loop, at least larger than 2 r 0 , the core width of the initial 70 a 71 slip dislocation. For some reactions, dissociations of the lead dislocation in a basal or prismatic dislocation pile-up can, however, lead to a stable and sizable twin loop. In these, the loop size is found to increase with decreasing twin boundary energy and increasing number of dislocations in the pile-up.
DOI:10.1103/PhysRevB.78.024117      URL     [本文引用:1]
[43] Serra A, Bacon D J, Pond R C.Acta Mater, 1999; 47: 1425
Atomic-scale computer simulation techniques have been used to investigate the interaction of crystal dislocations with two interfaces in hexagonal-close-packed (h.c.p.) metals, namely the ja:math twin boundary and a ja:math tilt boundary which is incommensurate in the direction perpendicular to the tilt axis. Crystal dislocations are always found to be absorbed in the tilt boundary with concomitant reconstruction of their cores. In the twin boundary, a broader range of interactions is observed, including defect transmission from matrix to twin and decomposition in the interface into discrete defects. The easy generation of mobile twinning dislocations facilitates the latter behaviour. The simulations demonstrate that the core structures of localized interfacial defects exhibit preferred riser configurations. For the twin, the favoured structure is the asal-on-prism configuration, whereas risers in the tilt boundary resemble ja:math twin forms. By comparing interaction processes in two interfaces, this investigation elucidates the role of crystallographic considerations and interfacial structure. It also illustrates that the core structure of interfacial defects can be complex and contributes significantly to total defect energy.
DOI:10.1016/S1359-6454(99)00016-6      URL     [本文引用:1]
[44] Pond R C, Serra A, Bacon D J.Acta Mater, 1999; 47: 1441
Atomic-scale computer simulation techniques have been used to investigate the interaction of crystal dislocations with two interfaces in hexagonal-close-packed (h.c.p.) metals, namely the ja:math twin boundary and a ja:math tilt boundary which is incommensurate in the direction perpendicular to the tilt axis. Crystal dislocations are always found to be absorbed in the tilt boundary with concomitant reconstruction of their cores. In the twin boundary, a broader range of interactions is observed, including defect transmission from matrix to twin and decomposition in the interface into discrete defects. The easy generation of mobile twinning dislocations facilitates the latter behaviour. The simulations demonstrate that the core structures of localized interfacial defects exhibit preferred riser configurations. For the twin, the favoured structure is the asal-on-prism configuration, whereas risers in the tilt boundary resemble ja:math twin forms. By comparing interaction processes in two interfaces, this investigation elucidates the role of crystallographic considerations and interfacial structure. It also illustrates that the core structure of interfacial defects can be complex and contributes significantly to total defect energy.
DOI:10.1016/S1359-6454(99)00016-6      URL     [本文引用:]
[45] Serra A, Bacon D J.Philos Mag, 1996; 73A: 333
0} Burgers vector is inclined at 60 to the interface. It is found that slip is not transferred from one crystal to the other with a residual dislocation left at the interface. Instead, the matrix dislocation decomposes into interfacial defects. We show that as a result of this decomposition the matrix dislocation becomes a new source of twinning dislocations that produce twin growth when the appropriate stress is applied to the crystal. The mechanism described does not require twinning dislocations to multiply by a pole process.
DOI:10.1080/01418619608244386      URL     [本文引用:]
[46] Pond R C, Bacon D J, Serra A, Sutton A P.Metall Trans, 1991; 22A: 1185
The crystallographic analysis of line defects in interfaces is discussed and applied to the particular case of twinning dislocations in hexagonal-close-packed (hop) metals, which have been studied here by atomistic simulation. Two crystallographic approaches are used; first, the concept of bicrystal structure maps is developed for the case of interfaces between crystals having multiple-atom bases, and second, the topological theory of line defects based on symmetry theory is used. On the basis of the atomistic calculations, some general conclusions concerning the relative contribution to the total energy of dislocations made by their elastic fields and core structures are presented.
DOI:10.1007/BF02660650      URL     [本文引用:]
[47] Serra A, Bacon D J, Pond R C.Acta Metall, 1988; 36: 3183
Versetzungen in (101002)-, (112002)- und (112001)-Zwillingsgrenzfl01chen in hexagonal dichtest gepackten Metallen werden mit Computersimulation untersucht. Die geometrischen Aspekte dieser Defekte, d.h. die Burgersvektoren b und die Stufenh02hen h , ergeben sich mit einer neulich entwickelten Behandlung von Grenzfl01chendefekten; diese berücksichtigt die gesamte Symmetrie des Kristalles. Es wird gezeigt, daβ zus01tzlich zu den früher auf der Grundlage der Symmetrie von Kristallgittern gefundenen Zwillingsversetzungen zwei weitere Versetzungstypen bei geeigneter Symmetrie der betrachteten Grenzfl01chenstruktur auftreten k02nnen. Verschiedene Versetzungen mit unterschiedlichen Burgersverktoren und Stufenh02hen werden für drei Grenzfl01chen im Modell beschrieben, um deren m02gliche Bedeutung beim Verformungsprozeβ zu ermitteln. Es ergibt sich, daβ die klassischen Zwillingsversetzungen, die Kombinationen von relativ kleinen Burgersvektoren und Stufenh02hen aufweisen, in (101002)- und ( 112001) Grenzfl01chen eine kleinere Linienenergie haben als in der (112002)-Grenzfl01che. Allerdings sind die Kernstrukturen, insbesondere die Kernweite, sehr empfindlich hinsichtlich des benutzten interatomaren Potentaiales und k02nnen sehr weit sein, besonders bei den (1012)-und (112001)-Grenzfl01chen. Im Falle der (l 12001)-Grenzfl01che ergibt sich, daβ die klassischen Zwillingsdefekte mit Burgersvektor b und Stufenh02he h in andere Defekte zerfallen, nach Aussage der kristallografischen Theorie mit den Parametern ja:math b und ja:math , zerfallen. Diese Defekte k02nnen zur Zwillingsbildung in einer Weise führen, daβ alle Atome direkt in die richtigen Positionen geschert werden (ohne “shuffling”). Ein weiterer Typ des Zerfalls von Versetzungen wird für den Fall der ( 112002)-Grenzflache angeführt; diese Versetzungen k02nnen aber nicht zur Zwillingsbildung führen.
DOI:10.1016/0001-6160(88)90054-5      URL     [本文引用:]
[48] Serra A, Bacon D J.Philos Mag, 1986; 54A: 793
Not Available
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[49] Braisaz T, Ruterana P, Nouet G, Pond R C.Philos Mag, 1997; 75A: 1075
Not Available
DOI:10.1080/01418619708214012      URL     [本文引用:1]
[50] Wang J, Hoagland R G, Hirth J P, Capolungo L, Beyerlein I J, Tome C N.Scr Mater, 2009; 61: 903
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[51] Wang J, Hirth J P, Tome C N.Acta Mater, 2009; 57: 5521
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">Mechanisms for <span id="mmlsi2" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645409004832&amp;_mathId=si2.gif&amp;_pii=S1359645409004832&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=967f36dbed712487fb149a960aaf802f')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="19" border="0" style="vertical-align:bottom" width="50" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645409004832-si2.gif"></span> twinning in hexagonal-close-packed crystals at an atomic scale were studied using topological analysis and atomistic simulations. Two twinning mechanisms were found: a normal-twinning mechanism in which a stable twin nucleus is created by simultaneous nucleation of multiple twinning dislocations; and a zonal-twinning mechanism in which a stable twin nucleus is created by simultaneous nucleation of a partial dislocation and multiple twinning dislocations. The twinning direction, dependent on the ratio of lattice parameters <em>c</em>/<em>a</em>, is along <span id="mmlsi3" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645409004832&amp;_mathId=si3.gif&amp;_pii=S1359645409004832&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=b1563ada847412dbe5d3bca076ade9fd')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="18" border="0" style="vertical-align:bottom" width="45" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645409004832-si3.gif"></span> when <span id="mmlsi4" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645409004832&amp;_mathId=si4.gif&amp;_pii=S1359645409004832&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=5fa4abedd70712b84f99fba3636a458b')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="19" border="0" style="vertical-align:bottom" width="68" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645409004832-si4.gif"></span>, but along the opposite direction when <span id="mmlsi5" onclick="submitCitation('/science?_ob=MathURL&amp;_method=retrieve&amp;_eid=1-s2.0-S1359645409004832&amp;_mathId=si5.gif&amp;_pii=S1359645409004832&amp;_issn=13596454&amp;_acct=C000228598&amp;_version=1&amp;_userid=10&amp;md5=b499cd1c3f364553c763dfe9473ba66a')" style="cursor:pointer;" alt="Click to view the MathML source" title="Click to view the MathML source"><img height="19" border="0" style="vertical-align:bottom" width="68" alt="View the MathML source" title="View the MathML source" src="http://ars.els-cdn.com/content/image/1-s2.0-S1359645409004832-si5.gif"></span>. Atomistic simulations, using density function theory for Mg, Zr and Zn and an empirical potential for Mg, were performed to study the kinetics and energetics associated with the two twinning mechanisms. The results show that the zonal-twinning mechanism is energetically favorable relative to the normal-twinning mechanism, because the zonal dislocation has a smaller Burgers vector.</p>
DOI:10.1016/j.actamat.2009.07.047      Magsci     URL     [本文引用:1]
[52] Li B, Ma E.Phys Rev Lett, 2009; 103: 035503
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[53] Serra A, Bacon D J, Pond R C.Phys Rev Lett, 2010; 104: 029603
A Comment on the Letter by B. Li and E. Ma, Phys. Rev. Lett.PRLTAO0031-9007 103, 035503 (2009)10.1103/PhysRevLett.103.035503. The authors of the Letter offer a Reply.
DOI:10.1103/PhysRevLett.104.029603      PMID:20366631      URL     [本文引用:1]
[54] Li B, Ma E.Phys Rev Lett, 2010; 104: 029604
[本文引用:]
[55] Pond R C, Hirth J P, Serra A, Bacon D J.Mater Res Lett, 2016; 4: 185
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[56] Hirth J P, Wang J, Tomé C N.Prog Mater Sci, 2016; 83: 417
The general topological model for interfacial defects is reviewed and expanded, and the role of these defects in the coupled shear - migration of interfaces is explored. We focus on twinning in hexagonal metals for many defect examples. The definition of shuffles within the topological model is presented. The concept of partitioning of the rotational component of elastic distortions at a grain boundary or interphase interface has recently been elucidated. This work shows that rotational coherency has an important role in twinning. The role of disconnections in type II twins is presented.
DOI:10.1016/j.pmatsci.2016.07.003      URL     [本文引用:]
[57] Ishii A, Li J, Ogata S.Int J Plast, 2016; 82: 32
The atomistic pathways of deformation twinning can be computed ab initio 02 , and quantified by two variables: strain which describes shape change of a periodic supercell, and shuffling which describes non-affine displacements of the internal degrees of freedom. The minimum energy path involves juxta-position of both. But if one can obtain the same saddle point by continuously increasing the strain and relaxing the internal degrees of freedom by steepest descent, we call the path strain-controlled, and vice versa. Surprisingly, we find the { 10 1 04 2 } 〈 10 1 04 1 04 〉 mathContainer Loading Mathjax twinning of Mg is shuffling-controlled at the smallest lengthscale of the irreducible lattice correspondence pattern, that is, the reaction coordinate at the level of 4 atoms is dominated by non-affine displacements, instead of strain. Shuffling-controlled deformation twinning is expected to have different temperature and strain-rate sensitivities from strain-controlled deformation twinning due to relatively weaker strength of long-range elastic interactions, in particular at the twin nucleation stage. As the twin grows large enough, however, elastic interactions and displacive character of the transformation should always turn dominant.
DOI:10.1016/j.ijplas.2016.01.019      URL     [本文引用:2]
[58] Zong H, Ding X, Lookman T, Li J, Sun J.Acta Mater, 2015; 82: 295
Stress-driven grain boundary (GB) migration has been evident as a dominant mechanism accounting for plastic deformation in crystalline solids. Using molecular dynamics (MD) simulations on a Ti bicrystal model, we show that a uniaxial stress-driven coupling is associated with the recently observed 90 GB reorientation in shock simulations and nanopillar compression measurements. This is not consistent with the theory of shear-induced coupled GB migration. In situ atomic configuration analysis reveals that this GB motion is accompanied by the glide of two sets of parallel dislocation arrays, and the uniaxial stress-driven coupling is explained through a composite action of symmetrically distributed dislocations and deformation twins. In addition, the coupling factor is calculated from MD simulations over a wide range of temperatures. We find that the coupled motion can be thermally damped (i.e., not thermally activated), probably due to the absence of the collective action of interface dislocations. This uniaxial coupled mechanism is believed to apply to other hexagonal close-packed metals.
DOI:10.1016/j.actamat.2014.09.010      URL     [本文引用:1]
[59] Yuasa M, Hayashi M, Mabuchi M, Chino Y.J Phys: Condens Matter, 2014; 26: 015003
(101042) and (101041) twinning and (101042) detwinning for Mg were investigated from the viewpoint of mobility of twinning dislocations and atomic shuffling. First-principles calculations suggested that the twinning dislocations glide more readily for the (101041) twinning than for the (101042) twinning. However, this conflicts with the experimental fact of easier (101042) twin . On the other hand, molecular dynamics simulations showed that the atomic shuffling was more activated for the (101042) twinning than for the(101041) twinning, which corresponds to the experimental fact. Therefore, it is suggested that the rate-controlling process for the twin is the atomic shuffling. Moreover, the calculations and simulations showed that the twinning dislocations glide more readily for the (101042) detwinning than for the (101042) twinning, whereas the atomic shuffling is less activated for the detwinning, suggesting that the detwinning occurs easily but is unstable, resulting in easy repetition of twinning-detwinning.
DOI:10.1088/0953-8984/26/1/015003      PMID:24263246      URL     [本文引用:]
[60] Li B, Zhang X Y.Scr Mater, 2014; 71: 45
[本文引用:]
[61] Li B, McClelland Z, Horstemeyer S J, Aslam I, Wang P T, Horstemeyer M F.Mater Des, 2014; 66(Part B): 575
Three-point bending and tensile tests were performed on rare earth containing ZEK100 at room temperature and various displacement rates (1.0, 5.0, 10.0, 20.0 and 50.0mm/min). The bend was either parallel to the rolling direction (RD specimens) or the transverse direction (TD specimens). The TD specimens presented higher bendability than the RD specimens. For the TD specimens, fracture did not occur at low displacement rates of 1.0, 5.0 and 10.0mm/min, but occurred at 20.0 and 50.0mm/min. Fracture occurred in the RD specimens at all the displacement rates. Cracks were observed in both the compression zone and the tension zone. Tensile tests in the RD and the TD show that the ductility in the TD is about three times as much as in the RD, leading to the better bendability in the RD specimens. The initial texture was examined by X-ray diffraction (XRD) and the results show that the (0001) basal texture was weakened and spread along the TD due to the addition of the rare earth elements. In-situ electron backscatter diffraction (EBSD) analysis was performed when the specimen was being bent, and the results show that {10 1 04 mathContainer Loading Mathjax 2}〈10 1 04 mathContainer Loading Mathjax 1 04 mathContainer Loading Mathjax 〉 twinning was activated in both the compression zone and the tension zone, different from highly textured AZ31 sheets. Our results indicate that the ZEK100 Mg alloy still presents anisotropy in the tensile properties and in the bending behavior, despite the weakened basal texture.
DOI:10.1016/j.msea.2013.10.030      URL     [本文引用:]
[62] Barrett C D, El Kadiri H.Acta Mater, 2014; 63: 1
Consider a magnesium bicrystal created from two single crystal lattices referred to as black and white. The bicrystal abuts a basal plane of the black crystal with a first-order prismatic plane of the white crystal. At zero stress, the boundary relaxes by developing coherent terraces, while two types of disconnection dipoles and a misfit dislocation above each dipole nucleate, in combination, to remove the attendant long-range elastic strains and maximize local coherency. When the bicrystal is stretched normal to its semicoherent interface, stress motivates the misfit dislocation at the far side of the boundary to glide and annihilate at the triple point. The removal of this bounding misfit dislocation allows the positive step of the disconnection dipole to conservatively move away. The negative step of this dipole, however, remains sessile at the grain boundary as it cannot glide conservatively. This sessile step facilitates nucleation of a new disconnection dipole. The first process of disconnection dipole widening then repeats, and the new residual negative step coalesces with the first one to form a novel disconnection of double step height. The whole process cycles once more, and two of these double-height disconnections pile-up at the boundary. The pile-up quickly relaxes into a coherent boundary along the {10 (1) over bar2} plane; a {10 (1) over bar2} twin facet thus nucleates, and a disclination dipole arises to bound the twin facet at the two junctions. Soon thereafter, the lower disclination, at the opposite side from where the misfit was removed, begins emitting twinning disconnections toward the upper disclination. These twinning disconnections caused their source junction to recede in favor of an overwhelming lengthening of the twin facet. All twinning disconnections end up moving through the upper disclination, thereby undergoing a dislocation transformation event. The new transformed disconnection glides freely along the remaining basal-prismatic boundary segment, and enables this boundary to be dragged with the propagating twin facet. This steady stage proceeds until the entire bicrystal transforms in to a single twinned crystal. This paper examines the fundamentals of these twin nucleation mechanisms at the grain boundary on the basis of the interfacial defect theory advanced by Pond and co-workers [1,2]. It introduces an important twin nucleation mechanism with dynamics of twin faceting in relation to an important asymmetric low-energy boundary in hexagonal close-packed materials. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
DOI:10.1016/j.actamat.2013.09.012      Magsci     URL     [本文引用:]
[63] Xu B, Capolungo L, Rodney D.Scr Mater, 2013; 68: 901
The growth process of of (-1012) twins is studied in Magnesium using atomistic simulations. Two twin seeds are considered and both cases, a specific interface, which places face-to-face prismatic and basal planes, plays an important role. This interface has a low energy corresponding to a cusp in the orientation-dependent interface energy of a twinned bicrystal. This interface appears in several published twin structures and for instance accommodates the large deviations of twin interfaces from (-1012) planes reported recently [Zhang et al., Scr. Mater. 67 (2012) 862].
DOI:10.1016/j.scriptamat.2013.02.023      URL     [本文引用:]
[64] Wang J, Yadav S K, Hirth J P, Tomé C N, Beyerlein I J.Mater Res Lett, 2013; 1: 126
The propagation of deformation twins in hexagonal-close-packed metals is commonly described by a conventional glide-shuffle mechanism. The widely accepted convention is that this process is also responsible for twin nucleation, but lacks direct confirmation. Using atomistic simulations, we identify an unconventional pure-shuffle mechanism for the nucleation of (1炉012) twins, which then grow through the conventional glide-shuffle mechanism entailing the glide of twinning disconnections. The pure-shuffle nucleation of twins at grain boundaries can be ascribed to a high-stress concentration and pre-existing grain boundary dislocations.
DOI:10.1080/21663831.2013.792019      URL     [本文引用:]
[65] Wang J, Liu L, Tomé C N, Mao S X, Gong S K.Mater Res Lett, 2013; 1: 81
The (104012) twin boundaries experimentally observed in hexagonal-close-packed metals are often serrated rather than fully coherent. These serrated coherent twin boundaries (SCTBs) consist of sequential (104012) coherent twin boundaries and parallel basal–prismatic planes serrations (BPPS). We demonstrated that the formation of BPPS is geometrically and energetically preferred in the SCTBs, and an SCTB thus migrates by glide and climb of twinning dislocations, combined with atomic shuffling. Particularly, the climb mechanism, combined with the density and the height of BPPSs in the SCTBs, could be crucial in controlling twinning and de-twinning, and twinning-associated hardening.
DOI:10.1080/21663831.2013.779601      URL     [本文引用:3]
[66] Shan Z W.JOM, 2012; 64: 1229
In this article, our most recent progress on applying a unique quantitative transmission electron microscope deformation technique on micronanoscaled metal pillars will be reviewed. We found that single-crystal pillars fabricated through focused ion beam always contain high density of defects. However, if the sample size is small enough, then both face-centered-cubic metals and body-centered-cubic metal pillars can experience 'mechanical annealing,' i.e., a phenomena referring to the reduction of dislocation density in the deforming volume, when dislocation generation is outweighed by dislocation annihilation through the free surface. We also found that when the sample size was reduced below 1 m or so, stress saturation and deformation mechanism transition occurred in a hexagonal-close-packed Ti alloy. Unlike crystalline materials, metallic glasses do not allow the presence and movement of dislocations or deformation twinning. However, we demonstrated the metallic glasses also follow the well-established tenet for crystalline materials: i.e., smaller is stronger and can reach its theoretical elastic limit under appropriate testing conditions. In addition, for the tested size regime, we found that high-energy electron beam has no obvious effect on the mechanical properties of materials with metallic bond. However, for materials with covalent bond and ionic bond, significant electron beam effects have been confirmed.
DOI:10.1007/s11837-012-0436-8      URL     [本文引用:1]
[67] Liu B Y, Li B, Shan Z W.In: Hort N, Mathaudhu S N, Neelameggham N R, Alderman M eds., Magnesium Technology 2013, San Diego: John Wiley & Sons, Inc., 2013: 107
The Magnesium Technology Symposium, which takes place every year at the TMS Annual Meeting & Exhibition, is one of the largest yearly gatherings of magnesium specialists in the world. Papers are presented in all aspects of the field, ranging from primary production to applications to recycling. Moreover, papers explore everything from basic research findings to industrialization. Magnesium Technology 2011 covers a broad spectrum of current topics, including alloys and their properties; cast products and processing; wrought products and processing; forming, joining, and machining; corrosion and surface finishing; ecology; and structural applications. In addition, you ll find coverage of new and emerging applications in such areas as biomedicine and hydrogen storage.
DOI:10.1002/9781118663004.ch16      URL     [本文引用:1]
[68] Liu B Y, Wang J, Li B, Lu L, Zhang X Y, Shan Z W, Li J, Jia C L, Sun J, Ma E.Nat Commun, 2014; 5: 3297
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[69] Liu B Y, Wan L, Wang J, Ma E, Shan Z W.Scr Mater, 2015; 100:86
Boundaries created through basal-prismatic transformation in submicron-sized single crystal magnesium have been investigated systematically using in situ transmission electron microscopy. We found that these boundaries not only deviated significantly from the twin plane associated with { 1 0 1 2 } mathContainer Loading Mathjax twin, but also possessed a non-planar morphology. After the sample was thinned to be less than 90 nm, aberration-corrected scanning transmission electron microscopy observation found that the basic components of these boundaries are actually terrace-like basal-prismatic interfaces.
DOI:10.1016/j.scriptamat.2014.12.020      URL     [本文引用:20]
[70] Liu B Y, Shan Z-W, Ma E.In: Singh A, Solanki K, Manuel M V, Neelameggham N R eds., Magnesium Technology 2016, Nashville: John Wiley & Sons, Inc., 2016: 199
Summary The mechanism of magnesia production was investigated experimentally in reversion reaction process in vacuum. The results showed that the reversion reactions were exothermic reaction, the gas-phase reversion will commence as soon as a saturated gas mixture was cooled down. MgO and Mg 2 C 3 were detected in the condensing tower by Mg vapors and CO during reversion reaction process below 1373K. The reversion reaction indicated that Mg and CO was not only occurred in MgO, but also yielded got Mg 2 C 3 . The main reaction was 6Mg(g)+4CO= Mg 2 C 3 +4MgO+C. A high purity metal magnesium product was obtained in the the vacuum distillation process. The calculated peak value of reversion reaction ratio was less than 9%. Contrast the condensing product and purity magnesium distillation product, the condensing product had a filament crystallization and loose structure. But the purity magnesium product had a lump structure and good crystalline feature.
DOI:10.1002/9781119274803.ch14      URL     [本文引用:7]
[71] Zhang X Y, Li B, Wu X L, Zhu Y T, Ma Q, Liu Q, Wang P T, Horstemeyer M F.Scr Mater, 2012; 67: 862
In deformation twinning, twin boundaries (TBs) should coincide with the twinning plane. Here we show that the TBs of the most common twinning mode in hexagonal close-packed metals, {1 0 (1) over bar 2}< 1 0 (1 1) over bar >, may not lie on the {1 0 (1) over bar 2} twinning plane. Examinations using transmission electron microscopy (TEM) reveal that the TBs in Co and Mg deviate significantly from the {1 0 (1) over bar 2} plane. High-resolution TEM confirms that the incoherent TBs entirely depart from the twinning plane with a magnitude greater than 45 degrees.
DOI:10.1016/j.scriptamat.2012.08.012      URL     [本文引用:2]
[72] Tu J, Zhang X Y, Wang J, Sun Q, Liu Q, Tomé C N.Appl Phys Lett, 2013; 103: 051903
Using high-resolution transmission electron microscopy, we characterized the structures of { 10 1 04 2 } deformation twin boundaries (TBs) in cobalt and found that TBs consist of { 10 1 04 2 } coherent twinning boundaries and basal-prismatic interfaces (BPs or PBs). According to these structural features, the propagation of { 10 1 04 2 } twins cannot be simply accomplished through the glide-shuffle mechanism of twinning dislocations. Correspondingly, two migration mechanisms of such TBs are proposed based on dislocation theory. In addition, abundant basal stacking faults are observed experimentally to connect with TBs, and this phenomenon is ascribed to the release of residual dislocations resulting from the migration of the BPs and PBs.
DOI:10.1063/1.4817180      URL     [本文引用:2]
[73] Sun Q, Zhang X Y, Ren Y, Tu J, Liu Q. Scr Mater, 2014; 90-91: 41
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[76] Jian W W, Cheng G M, Xu W Z, Yuan H, Tsai M H, Wang Q D, Koch C C, Zhu Y T, Mathaudhu S N.Mater Res Lett, 2013; 1: 61
We report high-resolution transmission electron microscopy (HRTEM) observation of a high density of dislocations with edge components (6510m) in nanocrystalline (NC) body-centered cubic (bcc) Mo prepared by high-pressure torsion. We also observed for the first time of the 05 and pure edge dislocations in NC Mo. Crystallographic analysis and image simulations reveal that the best way using HRTEM to study dislocations with edge components in bcc systems is to take images along zone axis, from which it is possible to identify 05 pure edge dislocations, and edge components of 05 and mixed dislocations. The pure edge dislocations can only be identified from zone axis. The high density of dislocations with edge components is believed to play a major role in the reduction of strain rate sensitivity in NC bcc metals and alloys.
DOI:10.1557/jmr.2012.403      URL     [本文引用:1]
[77] Li B, Yan P F, Sui M L, Ma E.Acta Mater, 2010; 58: 173
<h2 class="secHeading" id="section_abstract">Abstract</h2><p id="">We present transmission electron microscopy (TEM) observations of stacking faults (SFs) and their interactions with pyramidal dislocations, in plastically deformed polycrystalline pure magnesium. We have observed well-defined fringes as well as streaking in diffraction patterns, typical of SFs. The basal SFs are decorated by a large number of dark speckles, which are created by the interaction with pyramidal dislocations that have both 〈<em>c</em>〉 and 〈<em>a</em>〉 components as revealed by our contrast analysis. The SFs do not appear to result from the splitting of unit dislocations, as the SFs are relatively wide and no dislocation nodes were observed. By tilting the specimen systematically inside TEM, the SFs and the associated dislocations in Mg are found to exhibit a rich variety of features in terms of their morphology and diffraction contrast.</p>
DOI:10.1016/j.actamat.2009.08.066      Magsci     URL     [本文引用:]
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关键词(key words)
Mg
形变孪晶
基面-柱面界面
强度
合金设计

Mg
deformation twinning
basal/prismatic interface
strength
alloy design

作者
单智伟
刘博宇

SHAN Zhiwei
LIU Boyu