晶界取向差对镁合金孪晶形核与变体选择的影响规律及其应变协调机理

Twinning is an important strain accommodation mechanism for Mg alloys. A deep understanding of twinning mechanism is important to enhance the room-temperature workability of Mg alloys and hence to extend their applications. However, currently it is still not very clear about the criteria of twin activation and variant selection owning to the complicated deformation mechanism of Mg alloys. Considering the fact that deformation twins likely nucleate from grain boundaries in Mg alloys, this project proposed to study the strain compatibility between twins and slip on grain boundaries and hence to reveal the physical nature of twin nucleation. To achieve this goal, in-situ EBSD, FIB and HRTEM techniques will be employed to investigate the interactions of twin-twin, twin-slip and between twin/slip and grain boundaries. Based on the experimental investigation and statistical analysis, the effects of grain boundary misorientation, grain orientation and loading direction on twin nucleation could be revealed and the underlying mechanism of strain compatibility between twins and slip will be clarified. Moreover, combined with crystallographic calculation and simulations, the collaborative effects of Schmid factor and geometrical compatibility factor on twin nucleation will be revealed and then the criteria of twin variant selection and their application limit will be obtained. This project will help to develop the plastic working mechanics for Mg alloys and provide theoretical guidance for the development of low-cost material preparation and processing technologies.

孪生是协调镁合金塑性变形的重要机制，深入研究孪生变形机理，对于改善镁合金的室温塑性、推广镁合金材料的广泛应用具有重要意义。然而，由于镁合金变形机制的复杂性，目前尚缺少孪生启动与变体选择的有效准则。因此，针对镁合金变形孪晶在晶界形核的特点，本项目提出，从分析晶界两侧孪晶、滑移之间的应变协调关系入手，揭示孪晶形核的物理本质。为此，本项目拟利用In-situ EBSD、FIB、HRTEM等微观分析手段，原位观察与分析镁合金孪晶-孪晶、孪晶-滑移，以及孪晶、滑移与晶界之间的交互作用，通过实验观察与统计分析，揭示晶界取向差、晶粒取向及载荷方向对孪晶形核的影响规律及其应变协调机理，结合晶体学计算与模拟，探明Schmid因子与几何协调因子对孪晶形核的协作关系，从而明确镁合金孪晶变体选择准则及其适用条件。本项目有助于完善镁合金的塑性加工理论，为发展低成本的镁合金材料制备加工技术提供理论依据。

孪生是协调镁合金塑性变形的重要机制，深入研究孪生变形机理，对于改善镁合金的室温塑性、推广镁合金材料的广泛应用具有重要意义。然而，由于镁合金变形机制的复杂性，目前尚缺少孪生启动与变体选择的有效准则。因此，针对镁合金变形孪晶在晶界形核的特点，本项目提出，从分析晶界两侧孪晶、滑移之间的应变协调关系入手，揭示孪晶形核的物理本质，这有助于完善镁合金的塑性加工理论，为发展低成本的镁合金材料制备加工技术提供理论依据。. 在项目的实施过程，申请人选用具有典型基面织构和纤维织构的两种镁合金材料，系统研究了单轴拉伸、单轴压缩、平面压缩/轧制、三点弯曲，以及显微压痕过程中的孪生行为特征，揭示了孪晶对、孪晶带、复杂孪晶共存等新型的孪晶特征及其变体选择机理。通过引入与晶界取向差相关的几何协调因子，更好的解释了各种变形条件下的非施密特孪晶、孪晶对及孪晶链等行为，并提出了一种基于晶界特征的复合Schmid因子，可以更有效的预测孪晶变体启动和选择行为。项目深入研究了拉伸-拉伸二次孪晶的晶体学特征以及变体选择机理，理论分析了几何协调因子与晶界取向差之间的相关性。提出了一种自动判定二次孪晶变体的算法，该算法可以在未知初次孪晶取向的情况下判定二次孪晶变体，并且可以推算出初次孪晶的晶体学取向。. 项目研究成果发展了镁合金孪晶变体选择的判定准则，对利用孪晶调控镁合金的织构和性能具有重要的理论指导意义，项目有关几何系统因子的系统研究，对发展适用于镁合金的晶体塑性模型提供了重要依据，新提出的复合Schmid因子耦合了宏观与局部的载荷影响，为判断或预测孪生行为提供了一种新的更为有效的材料参数。

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