应变诱发镁合金中时效析出相沿基体六方相锥面和密排基面竞争析出的相场法模拟研究

The aging precipitates in magnesium alloy are less and in shape of thin disk lied on the (0001) close packed crystal plane of Mg matrix so that they can not effectively blocked the dislocation slip plane. Therefore, the strength of magnesium alloys generally is much low than that of the aluminum alloys. Experiments show that the phase such as MgZn2 of aging precipitates can form along the pyramidal plane of magnesium crystal, and it can improve the strength of the magnesium alloy significantly. However, the mechanism and control of pyramidal precipitation has not reported in literature. The experimental is cost and time-consuming and simulation is a new trend to solve the problem. It is suggested in this application for the first time that the lessons from practical strain induced lamellar microstructure in titanium alloy and steel strengthening by strain induced phase transformation, a strain induced aging treatment in magnesium alloys is needed to control the formation of a large number of the pyramidal precipitates in order to break through the bottleneck of low strength of magnesium alloy. We will simulation the microstructure of precipitates competitively formed along the basal and pyramidal Mg crystal planes observed in a real magnesium alloy. We next will change the conditions to simulate the microstructure to find out mechanism of the pyramidal precipitation. Nevertheless, we can find out the optimal control parameters such as aging temperature, applied strain field during aging and alloy elements to most introduce pyramidal precipitates in addition of little experiment. We will eventually give a new high temperature deformation of magnesium alloy with a stain aging treatment to realize our promise of significant high strength. This research will result in important academic progress and give an important innovation guide for the magnesium industry.

镁合金中的时效析出相少且都是沿Mg基体的（0001）密排面析出，不能有效封锁位错滑移面，因此导致了镁合金时效强化效果远比铝合金差。实验得出，镁合金中有部分例如MgZn2等时效析出相可沿镁的晶体锥面析出，能提高镁合金的强度。但是，沿锥面析出的机理和数量的控制还没有研究报道。实验的成本和耗时巨大，模拟解决是国际的新热点。本申请首次提出：借鉴钛合金中应变诱发全片层组织和钢的应变诱发相变强韧化的实践，在镁合金中引入应变诱发时效的方法，控制形成大量锥面析出相，突破镁合金强度低的应用瓶颈。本研究将模拟再现观察得到的镁合金中锥面和密排基面竞争时效析出的显微组织，进而改变条件模拟得出锥面析出的机理，计算锥面析出的合金成分、应变的影响、合适的应变温度，辅之少量实验确定最佳应用工艺，给出一个新高温形变镁合金的应用范例的证明。本研究将学术重要进步，并对镁合金行业进步给出重要创新指导意义。

本项目的性质是发展计算机模拟技术解决材料产业重大问题。经过4年的研究努力，已经全面完成了计划书提出的目标。在模拟技术进步方面：建立了外力场下，镁合金相场法显微组织演变的模拟模型，确定了应力诱发析出相（Mg2Sn）在外力场下，析出变体取向对于固定的外应力方向不同时，各变体的应变能的不同表达矩阵函数；采用颗粒复合体Khachaturya微观应变能计算公式，首次提出以旋转单晶AZ31镁合金刚度矩阵表达不同取向晶粒的本征应变矩阵，并将其引入相场模型，成功得到了外应力场下镁合金显微演变条件下的ODF织构图。在解决形变镁合金提高强度的研究方面：模拟发现了ＡＺ３１镁合金应力诱发析出相（Mg2Sn）形成的动力学条件，发现了应力时效热处理工艺可以改变析出相惯析面的规律，实现了利用这个建议的工艺方法改变时效相沿密排面析出成为沿母相锥面析出的项目构想，对镁合金强韧化有重要的参考价值。模拟发现单向压下轧制的应力大于400MPa时，在动态再结晶充分消除塑性变形能驱动晶粒长大的各向异性条件下，也就是模型中忽略塑性形变储能的条件下，单向应力会使AZ31镁合金在晶粒长大过程中形成较强的<0001>基面织构，并且形成织构的强度会随着基体中第二相颗粒含量的增加而降低。模拟还发现第二相粒子细化晶粒，存在临界尺寸的现象，小于该临界值，粒子尺寸越大，对晶粒生长的钉扎作用越大；大于该临界值后，粒子的尺寸越大，对晶粒长大的钉扎作用越小。在AZ31镁合金中，该临界尺寸基本是500—800nm。这些模拟结果有效推动了颗粒复合体显微组织设计的概念和技术，课题组将初步设计结果扩大应用在陶瓷颗粒增强铁基复合材料的开发和钛合金显微组织控制性能优化均取得明显效果。