镁合金中形变孪晶与析出相交互作用的原位定量透射电镜研究

The relatively low strength and ductility as well as their strong deformation anisotropy limit the large-scale application of Mg alloys. The low nucleation stress and propagation stress of {10-12} deformation twinning are considered to be one of the major origins. Therefore, to understand the twinning mechanism then to find out an effective strategy that can reduce the twinability is one of the key question of the research of Mg alloys. Although the precipitates can significantly strengthen the dislocation dominant plasticity, the strengthening effects on deformation twinning haven’t been completely understood. Recently, basal/prismatic interfaces are found to be widely existed on the {10-12} twin boundary in Mg and its alloys, and the migration of such interfaces can also result to the twin boundary propagation. However, effects of this new mechanism on the precipitate strengthening hence on the mechanical behavior of Mg alloys are still unclear. Based on this background and the research advantages of our group, we aim to systematically study the interaction between the precipitate and the {10-12} twin boundary, to understand the dynamics of twin boundary migration from the micro/nano-scales. We choose pure Mg and typical commercial Mg alloys as model materials, combining the in situ TEM quantitative mechanical testing technique, high resolution imaging and EELS. We hope this project will fuel the design of the new strengthening strategy in Mg alloys.

镁合金在室温下的强度和塑性较差，且具有显著的变形各向异性，其根源之一在于镁中的{10-12}形变孪晶在极低的应力下即可形核和长大。因此，探索控制{10-12}孪晶形核与长大的要素对镁合金的强韧化设计十分重要。人们已经熟知析出相可以有效地强化以位错滑移变形为主的材料，但对其与孪晶的相互作用的认识尚不充分。最近的研究表明镁中的{10-12}孪晶界上存在大量的基面/柱面界面，通过这种界面的迁移也可实现基体和孪晶的转换，但目前尚不清楚这种新机制如何与析出相交互作用以及如何影响镁合金的宏观力学性能。基于上述研究现状和本研究团队的优势，本项目拟选取纯镁和典型的镁合金为研究对象，以原位、定量的力学测试为主要手段，配合消球差成像和电子能量损失谱方法，在微纳尺度下系统地研究并揭示镁合金中{10-12}孪晶与析出相交互作用的机理，阐明孪晶界迁移的动力学，期望为镁合金的强韧化设计提供理论基础和方法论的指导。

镁及镁合金在结构材料轻量化中有重要应用潜力。然而，镁的力学性能较差，具体表现为强度和塑性低，这制约了其规模化应用。镁的力学性能的不足与其内在的塑性变形机制密切相关。本研究采用原位电镜纳米力学测试技术，发展了3D图像重构技术，在微纳尺度下深入研究了镁及镁合金的形变孪晶和位错滑移行为，解答和澄清了多个困扰本领域的科学难题和争议，为新一代镁合金的开发提供理论基础和方法论的指导。主要创新性研究成果如下。.（1）系统研究了镁中单个{10-12}孪晶界的迁移动力学。研究发现，镁中{10-12}孪晶界可以在循环加载下往复迁移，对应于孪生和退孪生过程；孪晶界形貌可以在孪晶界的迁移过程中连续变化；孪晶界的迁移应力和速率存在拉压不对称性，这种不对称性可能来自于基体和孪晶体中不同的缺陷环境以及孪晶界本身的结构特点。.（2）系统研究了镁合金中析出相形貌对孪晶行为的影响。研究发现，离散的颗粒状和棒状析出相对形变孪晶的抑制作用有限，带来的强化效果也有限；片层和网状析出相对形变孪晶的抑制效果最明显，带来的强化效果最佳。本结果一方面对高强镁合金的设计提供了重要的实验依据，同时也发展了一种判断镁合金强塑性和析出相强韧化效果的快速判据。.（3）澄清了有关决定镁塑性的位错的长期争议，为开发高塑性镁合金带来新启发。申请人采用原位TEM测试和3D图像重构，证明了镁的<c+a>位错可在两类锥面上滑移，是有效的塑性变形载体。该成果为完善镁的塑性变形理论提供了重要实验数据，为镁合金开发带来新启发。