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

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.

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.

1 { }形变孪晶的异常行为

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”.

2 经典的孪晶定义

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

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

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.

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

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

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

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

4.2 BP界面的发现

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

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

4.3 晶胞重构

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)

5 结语与展望

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.

Abstract

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 twinning under compressive loading, resulting in a comparably low compressive yield stress. In contrast, the basal slip and 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 ∼265 to ∼160 MPa, irrespective of the amount of pre-deformation strain. The latter value of ∼160 MPa nearly coincides with the compressive yield strength. The lattice reorientation of 86.3° 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.

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

Abstract

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 〈a〉 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–twin configurations were modeled using crystal plasticity finite element models. The contraction twin was predicted to form an internal extension twin , resulting in the commonly observed “double twin” sequence. The extension twin is suggested to inhibit thickening of this double twin by loss of twin–matrix coherency. Extension twins were predicted to retain their coherency and thus thicken.

Abstract

Mechanisms for 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 c/a, is along when , but along the opposite direction when . 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.

Abstract

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 〈c〉 and 〈a〉 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.

Magsci     URL     [本文引用:] [78] Bere A, Chen J, Hairie A, Nouet G, Paumier E.Phys Status Solidi, 2004; 241B: 2482 [本文引用:1]

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