单晶镁中拉伸孪晶扩展行为的研究

Magnesium {10-12} twinning process consists of nucleation, propagation and growth. Due to the extremely fast propagating speed and the complex stress state that propagation involved, it is difficult to access this stage for further study by the traditional way. In a previous work, the applicant found that the spherical nanoindentation could trigger the formation of single {10-12} twin when indented on {10-10} crystal plane. The crop up of the daughter twin causes the tensile strain along the <c> axis of the parent. It is assumed that if a contraction force is applied along the <c> axis on the single crystal Mg right before the nanoindentation test, the twin growth can be restricted. This leads to an extension of the twin propagation period, allowing one for further investigation. So the present project proposes to study the twin propagation in single crystal pure Mg as well as the aged single crystal alloys Mg-6Zn and Mg-9Al by imposing an external force along the <c> axis initially and inducing twinning by the spherical nanoindentation on {10-10} crystal plane. The advanced characterization and the crystal plasticity finite element method are combined to investigate the dynamic of the twin propagation and the precipitation hardening effect. This work will provide a promising way to study the twin propagation.

镁中{10-12}拉伸孪晶的形成包括形核、扩展和长大三个阶段，由于扩展速度极快且所处应力状态复杂，传统实验方法很难捕捉，导致对拉伸孪晶扩展行为的认知还很不清楚。申请人前期研究发现，利用球形纳米压痕可引发单一{10-12}孪晶的形成，导致母体沿<c>轴方向的拉伸应变。因此，如果以单晶为研究对象，在进行压痕实验前给材料施加沿<c>轴方向的原位辅助压缩应力，就可实现抑制孪晶的长大进而延长孪晶扩展的时间，有助于深刻认识孪晶扩展行为。本项目拟以单晶纯Mg及时效态单晶镁合金为研究对象，预先沿<c>轴方向施加原位辅助压缩应力，在{10-10}晶面使用压痕诱发孪晶，通过显微表征和数值模拟的方法研究孪晶扩展行为，构建孪晶扩展和形核、生长的本构关系，揭示孪晶扩展动力和析出强化机制，建立孪晶扩展动力数学模型。该研究可为孪晶扩展行为的认知提供新方法，丰富镁合金变形和强化理论，具有重要的学术意义和工程价值。

{10-12}拉伸孪晶是镁最重要变形机制之一，强化拉伸孪晶已成为调控镁力学性能的有效方式。{10-12}拉伸孪晶的形成过程包括形核、扩展和长大三个阶段，然而由于通过宏观变形的方法很难将这三个阶段区分开，导致对拉伸孪晶的认识不全面。本项目采用球形纳米压头在镁单晶{10-10}晶面进行压痕测试引发单一孪晶的方法，通过给单晶c轴方向施加辅助压缩应力，结合原位压痕测试、原子力显微镜和电子背散射衍射等技术，明晰了拉伸孪晶形核、扩展和长大过程，揭示了球形压头下孪晶的形貌特征。系统研究了温度对拉伸孪晶形核、扩展和长大应力的影响，探明了基体中热激活非基面位错对协调孪晶变形的作用。通过显微表征结合晶体塑性有限元仿真的方法，构建了孪晶形核、扩展和长大应力的数学模型。最后研究了多晶镁合金中第二相对孪晶的强化作用。通过本项目的研究丰富和发展孪晶变形的理论体系，为利用孪晶提升镁合金的塑性变形和力学性能提供理论指导。