Development of High Strength Wrought Magnesium Alloy by Multidirectional Forging under Decreasing Temperature Conditions

这项研究大致分为以下两类。(1)镁合金热锻过程中应变致晶粒细化研究了镁合金AZ31在多向锻造（MDF）中，温度从623K降至423K时的晶粒细化。随着温度的逐次降低，加载方向发生变化，MDF的累积应变达到5左右。其结构变化表现为：在低应变时，形成许多相互交叉的扭结带，然后扭结带的数量和取向错误增加，最终在高应变时形成完全发育的细晶粒。随着温度的降低，中密度纤维板的晶粒细化速度加快，塑性和易性得到改善。在累积应变为4.8、423K时形成的晶粒平均尺寸为0.3 μm。结果表明，在MDF条件下，晶粒细化是由一系列变形引起的连续反应引起的；本质上类似于连续动态再结晶（cDRX）。(2)镁合金热变形后的等时退火行为平均晶粒尺寸(D)的温度依赖关系可分为3个温度区域，即(a) <450 K时晶粒生长的潜伏期，(b) 473 ~ 523 K时晶粒快速粗化，(c) tbb0 ~ 550 K时晶粒正常生长。然而，即使在区域(a)，单位面积的细粒数也显著减少。因此，可以得出结论，整个地区的粮食生长是连续的。而在高温退火后，变形织构几乎没有变化。结果表明，热变形镁合金的退火过程主要受晶粒长大和恢复的控制，即连续静态再结晶（cSRX）。

The research falls roughly into the following two heads.(1)Strain-Induced Grain Refinement in Mg Alloy during Hot ForgingGrain refinement of a magnesium alloy AZ31 was studied in multi-directional forging(MDF) with decreasing temperature from 623K to 423K. The MDF was carried out up to cumulative strains of around 5 with changing the loading direction during decreasing temperature from pass to pass. The structural changes can be characterized by the evolution of many mutually crossing kink bands at low strains followed by increase in their number and misorientation, finally resulting in a fully developed fine-grains at high strains. MDF with decreasing temperature can accelerate the evolution of much finer grains and the improvement of plastic workability. An average grain size of 0.3 μm is formed at an accumulative strain of 4.8 and at 423K. It is concluded that grain refinement under MDF conditions occurs by a series of deformation-induced continuous reactions; that is essentially similar to continuous dynamic recrystallization(cDRX).(2)Isochronal Annealing Behaviors of Mg Alloy After Hot DeformationTemperature dependence of the average grain size (D)is categorized into three temperature regions, i.e. (a)incubation period of grain growth at<450 K, (b)rapid grain coarsening at 473-523 K, and (c)normal grain growth at T>550 K. The number of fine grains per unit area, however, is reduced remarkably even in region (a). It is concluded, therefore, that grain growth takes place continuously in the whole regions. While, deformation texture scarcely changes even after annealing at high temperatures. It is concluded that the annealing processes operating in the hot deformed magnesium alloy is mainly controlled by grain growth accompanied with recovery, that is continuous static recrystallization(cSRX).