AZ31镁合金拉伸扭折带结构的产生及交互作用机制
Generation and Interaction Mechanism of Tension Kink Band in AZ31 Magnesium Alloy
通讯作者: 隋曼龄,mlsui@bjut.edu.cn,主要从事材料物理与力学多尺度性能调控的显微结构的研究
责任编辑: 肖素红
收稿日期: 2019-05-07 修回日期: 2019-08-02 网络出版日期: 2019-11-29
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Corresponding authors: SUI Manling, professor, Tel: (010)67396644, E-mail:mlsui@bjut.edu.cn
Received: 2019-05-07 Revised: 2019-08-02 Online: 2019-11-29
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作者简介 About authors
周博,男,1988年生,博士生
利用TEM结合SAED花样,对室温轧制变形AZ31镁合金中拉伸扭折带结构及交互作用的形貌和晶体学特征进行了系统的研究。在镁合金塑性变形过程中,当外力不利于常见的孪晶形成及位错滑移时,扭折带作为一种补充的变形方式可以继续协调hcp结构的拉压不对称特性,对材料宏观塑性有着重要的影响。结果显示,hcp结构在与基面呈小角度的拉应力作用下,会形成基面位错对,并向相反方向运动,进而形成以{10
关键词:
The deformation structures, such as deformation twins, dislocations and kink bands, play an important role in the plasticity of magnesium alloys during the deformation process. However, due to the complexity of hcp structure, the deformation structures of the magnesium alloys, especially the interactions between deformation structures are still not well understood. Thus, it is of great scientific significance to study the microstructure of magnesium alloys, especially to characterize their structural characteristics of the interaction areas, which plays a significant role in understanding the structure and performance relationships of magnesium alloys. In this work, a combination of TEM and SAED pattern was applied to study the interaction mechanism associated with different kinds of deformation structures in Mg-Al-Zn (AZ31) alloy. When the applied external force is not beneficial for deformation twins and dislocations, kink bands act as a supplementary deformation mode to coordinate the asymmetry of hcp structure. According to crystallographic analysis, it is found that under the action of tensile stress nearly lie on basal plane in hcp structures, the basal dislocation pairs form and move to the opposite directions, forming tension kink band with the interface of {10
Keywords:
本文引用格式
周博, 隋曼龄.
ZHOU Bo, SUI Manling.
寻求一种具有轻质性、高效性、环境友好性等特点的工程材料是新世纪材料科学领域发展的重点。作为一种典型的轻质有色合金,镁合金具有密度低、储量大等特点,受到了研究者广泛的关注[1,2,3,4]。在满足力学性能的条件下,相比于钢铁、钛合金等其它金属,镁合金能大幅度降低材料重量[5,6,7]。然而,由于镁合金在室温变形条件下独立的滑移系少,难以满足Von Mises准则[8],造成其室温塑性能力差的特点,严重限制了镁合金大规模应用。此外,镁合金hcp的结构特点,使得其在变形过程中存在明显的各向异性,无法启动足够的滑移系来调节c轴方向的应力。形变孪晶作为一种可以调节c轴方向应力的变形方式,对镁合金塑性变形过程起着非常重要的作用。在镁合金塑性变形过程中,最常见的孪晶为{10
本工作对商用AZ31镁合金在室温变形条件下进行轧制变形,利用透射电子显微镜(TEM)结合选区电子衍射(SAED)花样对拉伸扭折带的形貌特征和晶体取向进行细致的观察,分析微观结构的交互作用对材料塑性的影响规律。
1 实验方法
实验选择商用AZ31镁合金板材,主要化学成分(质量分数,%)为:Al 3.3,Zn 0.58,Mn 0.27,Si 0.095,S 0.018,P 0.003,Mg余量。机械加工切取尺寸为20 mm×15 mm×10 mm的长方体块材。为了消除初始织构对实验结果的影响,在375 ℃下对样品进行12 h的再结晶退火,使样品达到完全再结晶状态。在室温下对再结晶样品进行2~3道次轧制,变形速率为10-3 s-1,变形总量为10%;取平行于样品轧制方向(RD)和轧面法线方向(ND)切取金属薄片,采用水磨金刚石砂纸进行打磨至厚度为50 μm,并用特定模具冲压制备出直径为3 mm的圆片,样品制备方法如图1所示。采用电解双喷结合离子减薄的办法制备TEM样品,具体制备条件见文献[22]。TEM样品的观察和分析采用配备球差校正器的Titan environmental TEM进行,点分辨率可达到0.068 nm。
图1
图1
样品制备方法
Fig.1
Sample preparation methods
(a) sample dimensions and rolling direction (RD, ND and TD are the rolling direction, normal direction and transverse direction of the sample, respectively)
(b) schematic of the rolled sample prepared for TEM specimen
2 实验结果
2.1 扭折带之间的交互作用
在对镁合金变形结构的TEM研究中,观察到许多像衬度不同的板条结构,通过SAED分析可以确定这些板条状结构并不是孪晶板条,而是扭折带结构。对其中一个产生扭折带板条结构的晶粒进行了各个区域连续的TEM明场像拍摄,并将42张明场像拼接组合成大视场图像,如图2所示。很明显,该晶粒在轧制变形的外力作用下,并不利于孪晶的形成,而是形成了大量的2组不同方向的扭折带,扭折带的顶端呈现出细而尖的形态。在图2中,K1、K3、K5、K7、K9为一组近似平行的扭折带,而K2、K4、K6为另一组近似平行的扭折带。值得一提的是,在比较宽的K5扭折带中还观察到2条{10
图2
图2
高密度扭折带区域的TEM像
Fig.2
TEM image of the high density kink bands (Two sets of approximately parallel kink bands: one group is K1, K3, K5, K7 and K9, while the other group is K2, K4 and K6)
在图2中可以看到2个尺寸存在明显差异的扭折带发生交互作用,其中K2宽度(1.43 μm)约为K1宽度(486 nm)的3倍,扭折带交互作用区域的TEM分析,如图3所示。为了研究扭折带的晶体学取向特征及交互作用,采用SAED对扭折带的取向进行分析,拍摄方向为基体的[1
图3
图3
扭折带交互作用区域的TEM分析
Fig.3
TEM analyses of the interaction of kink bands
(a) TEM image of the interaction area (Subscript M indicates matrix)
(b~e) SAED patterns of different areas in Fig.3a
2.2 扭折带与{10 1}孪晶之间的交互作用
在镁合金变形过程中,还观察到了{10
图4
图4
{10
Fig.4
TEM analyses of interaction of {10
(a) TEM image of the interaction area (The dashed blue lines and green lines indicate the kink band interface and the twin boundary, respectively; SFs—stacking faults)
(b~e) SAED patterns of different areas in Fig.4a (Subscript T indicates twin)
3 分析讨论
在镁合金变形结构中,虽然扭折带结构并不如位错滑移及孪生普遍,但其对研究材料的塑性同样具有非常重要的意义。在上述的2种现象中都观察到了2个变形板条交叉排列,并且扭折带界面均为{10
图5为拉伸扭折带形成机理示意图。在hcp结构晶胞中,最容易产生的变形结构为基面<a>位错[9]。外部拉力作用在接近沿Mg基体基面方向时,会产生大量<a>位错对(如正刃位错和负刃位错),并在应力的作用下克服异号位错相互吸引的阻力,各自向相反方向运动,如图5a所示,进而使得Mg基体的基面以<1
图5
图5
拉伸扭折带形成机理示意图
Fig.5
Schematics of the tension kink band formation mechanism
(a) generation of dislocation pairs in initial structure (F indicate the force direction)
(b) formation of K1 (α1 is the angle between the interface of K1 and the basal plane of matrix. The inset shows the orientation between the matrix and kink band under the external force)(c) interaction of K1 and K2 (α2 is the angle between the interface of K2 and the basal plane of matrix)
在图4中观察到扭折带与{10
图6
图6
拉伸扭折带与{10
Fig.6
Schematics of deformation structures
(a) interaction between tension kink band and {10
表1 不同变形结构在外力作用下的Schmid因子
Table 1
Deformation structure | Force direction | Slip plane | Slip direction | Schmid factor |
---|---|---|---|---|
(10 | [ | (10 | [10 | 0.349 |
(10 | [ | (10 | [10 | -0.056 |
Kink band | [ | (0001) | [1 | 0.209 |
4 结论
在AZ31镁合金塑性变形过程中,当外加应力的取向不利于孪晶等变形结构的产生时,扭折带就会作为一种特殊的结构来调节局域的应力集中,孪晶与扭折带以及扭折带之间的交互作用对材料宏观塑性的影响同样重要。当镁合金的晶体结构受到沿基面拉伸应力的作用时,会形成拉伸扭折带,而且是以{10