Dynamic recrystallization (DRX) is critically discussed based on the relative grain size model, that is DRX can be controlled by the ratio of initial grain size (D_o) and the stable DRX grain size (Ds). Modeling the new grain evolution and computer simulation of the stress-strain curves lead to almost the same results reported in the literature. In grain coatsening. the grain size chagnes according to the equation, (Dn/Ds) = (D_n-1/D_s)~(1/4). Here D n is the grain size developed in the nth cycle of DRX. In contrast, grain refinement takes place accompanied by necklace recrystallization, in which the grain size evolved in each strand is equal to a stable size Ds. Such DRX behaviors can result from the fact that the main nucleation sites for DRX are at or near grain boudnaries, which are serrated in the early stages of deformation. When grain boundary sliding or shearing takes place at such serrated boundaries, high local strain or orientation gradients are evolved along the such serrated boudnaries, high local strain or orientation gradients are evolved along the boudnaries and so bulging frequently takes place at the serrated portions. Successive strands os necklace DRX also can develop in coarse grain interiors according to the same process, because grain boudnary sliding occurs predominantly in the fine grained necklace regions. Finally, a limitation where DRX coccurs and also new grain evolution takes place even at low temperatures is discussed and compared with high temperature DRX described above.
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