基于畸变强化的变形镁合金加工各向异性理论预测研究

The anisotropic behavior is the key restriction for the processing and application of wrought magnesium alloys. Concerning on the anisotropic behavior, more attention has been paid to weaken this effect by microstructure investigation. The anisotropic behavior, however, cannot be eliminated. Therefore, it is an urgent scientific issue to understand the evolution of anisotropic behavior and predict it quantitatively for the improvement of processing technology of wrought magnesium alloys by the utilization of anisotropy. The evolution of yield surface shape, also known as distortional hardening, is the latest achievement in the field of finite strain elasto-plasticity theory. More importantly, the anisotropic behavior of materials (e.g., steels and Al alloys) can be fully characterized and predicted precisely by the constitutive modeling based on distortional hardening. Therefore, in this project, the novel constitutive model with distortional hardening will be built for wrought magnesium alloys, the initial and subsequent yield surfaces will be investigated, the relationship between the evolution of microstructure and evolution of yield surface shape during plastic deformation will be analyzed. Consequently, the evolution of anisotropic behavior of wrought magnesium alloys will be demonstrated and predicted theoretically, which will benefit the improvement of processing technology by the utilization of anisotropic behavior. Our work will be helpful for the development of finite strain elasto-plasticity theory as well as the improvement of processing technology for wrought magnesium alloys.

各向异性行为是制约变形镁合金加工应用最关键的因素之一。针对各向异性行为的研究，人们往往关注于显微组织调控进而弱化该行为，始终无法完全消除各向异性。因此，了解各向异性行为演化规律，定量预测该行为，是利用各向异性改善变形镁合金加工制备工艺所急需解决的科学问题。屈服面形状演化，即畸变强化机制，是近年来大变形弹塑性力学理论最新的研究进展，基于该机制构建的本构模型可以准确表征并定量预测各向异性行为，并已成功应用于钢铁材料、铝合金等。因此，本项目选取变形镁合金作为研究对象，构建新型畸变强化本构模型，研究初始与后继屈服面，分析其塑性变形过程中显微组织演变与屈服面形状演化之间的关系，揭示具有普遍意义的变形镁合金各向异性行为演化规律，进而理论预测其各向异性行为，为利用该行为优化加工制备工艺建立基础。本研究对于发展大变形弹塑性力学理论具有重要意义，同时对于优化变形镁合金加工制备工艺具有重要的工程应用价值。

各向异性行为是制约变形镁合金加工制备过程最关键的因素。本项目突破传统等向强化、随动强化理论，系统创建了畸变强化本构模型，通过屈服面形状变化，从根源上直接表征了变形镁合金各向异性行为。项目取得的主要研究结果如下：1）实验研究了AZ31镁合金轧制板材、挤压棒材各向异性变形行为，研究发现，初始屈服面呈现明显的各向异性，同时兼具拉-压不对称性行为，后续单轴加载过程中，屈服面形状随塑性变形发生变化，加载路径改变影响屈服面形状演化；2）基于Feigenbaum & Dafalias模型，对耗散不等式中关于等向强化和畸变强化分解，并引入权重系数，研究表明，该权重系数可以控制材料的强化方式；3）采用双轴拉伸、双轴压缩实验数据，验证新型畸变强化本构模型，研究发现，等向强化与畸变强化权重系数与加载方式有关；4）澄清了屈服面与等塑性功面的异同，研究表明，针对加载路径相关的各向异性材料，屈服面能有效表征加载路径对各向异性演化的影响；5）探讨了AZ31镁合金在拉伸-扭转复合加载下显微组织演变，研究发现，预拉伸之后扭转过程中基面滑移是主要变形机制，由预拉伸加载引起的剪切方向织构弱化导致预拉扭后剪切屈服应力低于初始剪切屈服应力。加载路径不同导致变形机制不同，进而影响各向异性行为演化。上述结果已陆续发表，经同行评议，重要结果发表在 Int. J. Plast (国际塑性杂志，中科院SCI大类分区1区，影响因子：5.7，金属材料、力学领域权威期刊之一) 2篇，并获得国际同行广泛关注和正面引用，该研究为变形镁合金塑性加工过程中金属各向异性流动规律探索奠定了基础。