Dynamic grain evolution of a magnesium alloy AZ31 was studied in compression at 673 K (0:73T m ) by optical and SEM/OIM microscopy. The flow curve shows rapid hardening accompanied by a stress peak at a relatively low strain (" p ¼ 0:12), followed by strain softening and then a steady state flow stress at high strains. Fine grains evolved at corrugated grain boundaries at around " p and developed rapidly during strain softening, finally leading to a full structure of equiaxed fine grains. Such characteristics of new grain evolution and flow behavior are apparently similar to those in conventional, i.e. discontinuous, dynamic recrystallization (DRX). On the other hand, kink bands were observed frequently near corrugated grain boundaries and also in grain interiors, even around " p . The misorientation of the boundaries of the kink bands increases rapidly during strain softening and approaches a saturation value of around 43 at high strains. The average size of the regions fragmented by kink bands is almost the same as that of the new grains. It is concluded, therefore, that new grain evolution in this alloy is controlled by a deformation-induced continuous reaction, i.e. continuous DRX.

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New grain evolution taking place during superplasticity was studied by means of tensile tests as well as metallographic observation for a unrecrystallized coarse-grained 7075 aluminum alloy. Grain boundary sliding (GBS) frequently takes place even on the layered high angle boundaries (HABs) parallel to the tensile axis and brings about rotation of subgrains near the HABs and subsequently in grain interiors. The misorientations of (sub)grain boundaries evolved in the pancaked grains increase accompanied by a randomization of the initial texture, followed by development of new grains with HABs. This indicates that unrecrystallized and pancaked grain structure developed by cold rolling is an important prerequisite not only for the appearance of superplasticity, but also for the dynamic evolution of new fine grains. It is concluded that the mechanism of new grain evolution can be a deformation-induced continuous reaction, that is continuous dynamic recrystallization (CDRX). A model for CDRX is discussed in detail comparing with previous several models.

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Annealing behaviors of hot-deformed magnesium alloy AZ31 were studied at temperatures of 373 to 673 K by optical and SEM/EBSD metallographic observation. The hot-deformed alloy had fine-grained structures developed by grain fragmentation due to continuous dynamic recrystallization (cDRX). Temperature dependence of the average grain size (D) is categorized into three temperature regions, i.e. (a) an incubation period for 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). These lead to grain coarsening taking place continuously in the whole temperature regions. In contrast, the deformation texture scarcely changes even after full annealing at high temperatures. It is concluded that the annealing processes operating in hot-deformed magnesium alloy with cDRXed grain structures can be mainly controlled by grain coarsening accompanied with no texture change, that is continuous static recrystallization (cSRX).

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