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

研究内容大致分为以下两个方面：(1)镁合金热变形过程中的应变诱导细化研究了AZ31镁合金在623K至423K温度范围内的多向锻造工艺。在一道又一道的降温过程中，通过改变加载方向，MDF的累积应变达到5左右。这种组织变化的特征是，在低应变时，许多相互交叉的扭结带演化，随后它们的数量增加和取向错误，最终在高应变下形成充分发展的细晶。降低温度的MDF可以加速细晶组织的演化，改善塑性加工性能。在423K下，累积应变为4.8时，形成的平均晶粒度为0.3μm。(2)镁合金热变形后的等时退火行为平均晶粒尺寸(D)随温度的变化可分为三个温区，即(A)450K时晶粒长大的孕育期，(B)473~523K时的快速粗化，(C)T&GT；550K时的正常晶粒长大。由此可见，在整个区域内，晶粒的生长是连续发生的。而形变织构即使在高温下退火后也几乎没有变化。结果表明，热变形镁合金的退火过程主要受伴随回复的晶粒长大控制，即连续静态再结晶。

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).