镁合金晶体塑性-相场动态耦合循环塑性模型研究

For lightweight magnesium alloy components, twinning-detwinning will occur under alternating cyclic loading, and twins interact with dislocations. Therefore, it is difficult to establish a mathematical model to accurately describe the relationship between micro-deformation mechanism and macro-mechanical behavior. In this project, the twinning-detwnning physical mechanism is considered, and the dynamic coupled crystal plasticity and phase field cyclic plasticity model of magnesium alloy is established. Extruded ZK60 wrought magnesium alloy is chose for this study. The mechanism of plastic deformation is quantitatively studied by experimental method. The tensile twin nucleation criterion based on dislocation density and intergranular orientation difference is proposed. The phase field model is established to describe the twin nucleation, propagation, growth and detwinning processes. The dislocation density based full field crystal plasticity model is used to describe the slip deformation mechanism. The crystal plasticity model and the phase field model are dynamically coupled by the variable transferring, and the integrated crystal plasticity and phase field cyclic plasticity model of magnesium alloys is established. By cyclic tension-compression experiments under different cyclic loading conditions, the dynamic coupling model is verified and improved. The quantitative relationship between dislocation density, twin volume fraction and macroscopic cyclic plasticity characteristic is studied, which lays the foundation for the establishment of cyclic plasticity constitutive theory of magnesium alloys.

交变循环载荷下，轻质镁合金零部件会发生反复的孪生-去孪现象，且孪生又与位错发生动态交互作用，故很难建立数学模型准确描述微观变形机制与宏观力学行为之间的关系。本项目拟重点考虑孪生-去孪的物理机制，建立镁合金晶体塑性和相场动态耦合的循环塑性模型，研究微观动态演变的孪生和位错对宏观应力应变的定量贡献：选用挤压ZK60镁合金，通过实验方法量化研究塑性变形机理；提出基于位错密度和晶间取向差的拉伸孪晶形核判据，建立相场模型来描述孪晶形核、扩展、长大和去孪过程；采用基于位错密度的全场晶体塑性模型来描述滑移的变形机制；通过变量传递将晶体塑性模型和相场模型进行动态耦合，建立镁合金的晶体塑性和相场动态耦合循环塑性模型；针对不同加载条件下拉压循环变形实验，验证和完善动态耦合模型，重点研究位错密度、孪生体积分数与宏观循环塑性特性的定量关系，为建立镁合金循环塑性本构理论奠定基础。

镁合金塑性变形机制复杂，且不同变形模式之间交互作用，故很难建立数学模型准确描述微观变形机制与宏观力学行为之间的关系。本研究建立了一种晶体塑性-相场法耦合全场模型，该模型可定量地、可视化地仿真研究位错滑移、孪晶的形核和长大等过程，以及位错、孪晶、晶界等之间的交互作用，是在介观尺度下深入研究镁合金的复杂变形机理的有力工具。这对于完善镁合金循环塑性变形理论，提升和调控镁合金的强韧性能，进而推广镁合金的应用有着重要意义。同时，指明了耦合模型需要进一步进行完善的方向，更完善的全场模型可以为在介观尺度下深入研究镁合金的复杂变形机理奠定更坚实的基础。.首先实验研究了强织构挤压ZK60镁合金塑性变形机理，结果表明，单向拉伸以位错滑移机制为主，单向压缩以拉伸孪生变形机制为主，4%应变幅值下的循环变形过程中，在压缩半圈，塑性变形以拉伸孪生为主，在拉伸半圈，塑性变形以去孪为主。残余拉伸孪晶随加载周次的增加，累积并不明显，第21圈时残余孪晶的体积分数仅为3%。.其次，本研究首次建立了一种适用于镁合金多晶体弹塑性变形的晶体塑性-相场法耦合全场模型，在耦合模型中，采用了基于位错密度的晶体塑性模型来描述位错滑移诱导塑性变形行为以及应力、应变的不均匀分布行为；利用孪晶形核概率模型描述了镁合金中{10"1" ̅2}拉伸孪晶在多晶体晶界处的形核行为；在相场模型中，基于Ginzburg-Landau理论建立了孪晶模型，用以描述拉伸孪晶的扩展和长大行为，采用双势垒函数和指示函数，将相场动力学方程转化为变分不等式，极大地提升了数值计算效率。.最后，基于该耦合全场模型，通过对镁合金的塑性变形行为和循环变形行为的仿真研究，揭示了微观组织与宏观力学性能之间的联系，研究清楚了外力作用下拉伸孪生的扩展和长大过程，研究结果表明孪晶尖端应力集中和孪晶界附近的应力释放是孪生不均性长大的主要原因；几何协调因子是决定孪晶是否能穿越晶界的主要因素。