Formation Mechanism of New Grains in Dynamic Recrystallizaion based on Local Orientation Analysis

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In order to clarify the mechanism of new grain formation in dynamic recrystallization, the texture formation in γ-TiAl, nickel and copper during dynamic recrystallization (DRX) in compression was studied in relation to the microstructural evolution and grain boundary analysis. Compression tests were conducted at various temperatures and true strain rates ; the corresponding logarithmic value of Zener-Hollomon parameter lnZ ranged from 12.2 to 31.7. The behavior of texture formation was different, depending on the deformation conditions and the kind of materials.In low Z deformation conditions, DRX proceeded mainly through the process in which new grains are generated by strain induced migration of grain boundaries. In this case, the behavior of texture formation was different in γ-TiAl and in nickel and copper, texture developed sharply in γ-TiAl (the maximum pole density PMAX= 10) but it was not formed in nickel and copper (PMAX less than or equal 2). The difference in texture formati … More on was concluded to arise from the difference in the frequency of twin formation during grain boundary migration. That is, recrystallized grains change their orientation towards the stable orienctation for compression ({032) in γ-TiAl) and the texture, having this orientation as a main component develops, sharply when twinning scarcely occurs, while no texture develops when twinning occurs very frequently as in nickel and copper.In high Z deformation conditions, DRX proceeded through the nucleation and growth of new grains in the vicinity of grain boundaries where the deformation occurred severely and inhomogeneously. In this case, no notable difference in texture formation was found in each material ; the texture having a main component in or near the stable orientation for compression could certainly be observed though it was not sharp (PMAX=3-4).Both the value of PMAX and the frequency of twinning changed with the value of lnZ, but any discontinuous shift was not observed in their variation. This indicates that, with increasing lnZ, the behavior of texture formation changes gradually from that for low Z conditions to that for high Z conditions, depending on the proportion of the two processes of DRX, the bulging of grain boundaries and the nucleation and growth of new grains. Less