特定取向柱状晶镁合金优异塑性与孪生及孪生演进主导的形变模式

The Mg alloy with special orientated grain and flat grain boundary was prepared through the adjustment of the process of the directional solidification and the chemical composition and the microstructures including the preferential growth direction and the structure of grain boundary was characterized. Starting from the columnar crystal growth orientation and external load, the compression twin and secondary or even higher twinning was investigated on the aspect of the physical mechanism and the many twin system gradually operating criterion was proposed. The deeper analyses of the evolution of {10-11} compression twin and {10-11}-{10-12} double twin during the deformation process was performed and the deformation mode of the gradual, alternate, and continued operating motivated by the interaction of the compression twin/ compression twin and compression twin/dislocation was investigated. By the analyzing of the flat grain boundary of the column grain and its influence on the dislocation and twin, it is revealed that the deformation is dominated by compression twinning and double twinning, which is the mechanism for the improvement of the room temperature plasticity of the polycrystal Mg alloy with the special orientated grains. To know the physical nature of the plastic improvement of Mg alloy by the special orientated column grain does not only enrich the theory of the fundamental plastic deformation of Mg alloy, but also offer the theoretical basis and novel ideal for developing Mg alloy with the high formability.

本课题采用定向凝固技术，通过调控定向凝固工艺参量和合金成分，制备出具有特定生长取向、晶界平直的柱状多晶镁合金，表征其组织（择优生长取向、晶界结构）特征。进一步从柱状晶生长取向和外载荷入手，从物理机制的层面上研究压缩孪生及二次甚至高次孪生、多孪生系统渐次开动准则。深入研究{10-11}压缩孪晶及{10-11}-{10-12} 双孪晶在变形过程中的演化，研究压缩孪晶/压缩孪晶、压缩孪晶/位错间相互作用促使变形模式渐次、交替、持续开动机制，研究柱状晶平直晶界结构及其对位错和孪生的影响，揭示压缩孪生和双孪生主导、多孪生系统渐次开动形变模式下的特定生长取向柱状多晶镁合金室温塑性提升机制。认知特定生长取向柱状晶组织提升镁合金塑性的物理本质，不但丰富了镁合金塑性变形基础理论，也为开发可成型性高的镁合金提供理论依据与新思路。

铸造镁合金由于组织粗大，晶界应变协调性差，导致其强度、尤其是塑性变形能力较差，难以满足航空和航天、汽车等领域不断发展对于轻型复杂结构铸件的旺盛需求。因此，设计、调控铸造镁合金晶粒形貌、晶体取向、充分改善晶界应变协调性就成为高性能铸造镁合金开发的关键课题。定向凝固是在合金凝固过程中采取强制手段使晶体按所设计结晶取向生长，获得柱状晶的一种技术。柱状晶具有类似双晶体的几何特征，形变时晶界约束条件可由等轴晶的5个减少到3个，因而从理论上预期减少晶界约束的柱状晶对于室温下仅有2个独立滑移系的HCP镁合金晶界应变协调性改善将更具有深刻意义。因此，本项目开展了镁合金定向凝固的研究。通过定向凝固工艺参量调控，研究了Mg-xGd(x=0~16%)、Mg-Gd-Y(-Er)合金晶体生长取向与力学性能相关性，研究了准晶增强Mg-Zn-Y合金和LPSO增强Mg99.2Zn0.2Y0.6合金晶体生长取向、第二相形态与分布及其力学性能，研究了不同取向生长柱状晶晶界应变协调机制。结果表明，柱状晶生长取向对其拉伸性能起决定性作用。<112(_)2>（<224(_)5>）方向生长的柱状晶，因基面<a>滑移SF＞0.40、启动{101(_)2}拉伸孪生协调应变，故具有低的屈服强度和高的延伸率。<112(_)0>方向生长的柱状晶，因基面<a>滑移SF＜0.1、启动{101(_)1}压缩孪生协调应变，故具有高的屈服强度和较低的延伸率。<112(_)0>取向生长的LPSO增强Mg99.2Zn0.2Y0.6合金的屈服强度达230MPa，延伸率为6.17%。但<112(_)0>取向生长的准晶增强Mg-4.78Zn-0.45Y-0.14Zr合金屈服强度为180MPa，延伸率高达52%；这与其拉伸形变时晶界两侧启动高密度“压缩孪晶对”、“压缩孪晶对“参与协调晶界应变密切相关。其研究结果为开发新型高强塑镁合金提供了一种新思想、新手段。