DYNAMIC EVOLUTION OF MICROSTRUCTURES IN SUPERPLASTIC NI3AL

High temperature deformation and microstructural evolution were studied in tension using single phase polycrystalline Ni3Al doped with boron. The samples with grain sizes from 3.0 to 18 mu m were rapidly H-2 gas-quenched immediately after straining in order to examine deformed structures and to elucidate the mechanism of superplastic deformation. A stress peak appears at a strain of less than 0.1 more remarkably with increasing initial grain size, and with increasing strain rate or decreasing temperature. Dynamic recrystallization (DRX) initiates along prior grain boundaries at near the peak stress and develops toward grain interiors on further deformation, leading to the evolution of necklace structures. A rapid decrease in flow stress after the peak is mainly due to grain boundary sliding at interfaces between DRX grains. In contrast, work hardening takes place at low and moderate strains for samples with smaller initial grain sizes, when they are deformed at lower strain rates or higher temperatures. Under these conditions, grain boundary sliding occurs predominantly accompanied with grain coarsening due to DRX. Superplastic elongation appears in specimens exhibiting no significant behavior of work softening or work hardening. It is concluded that work softening or hardening behavior depends sensitively on the relative difference between the initial grain size and the stable DRX grain size evolved at high strains, and high temperature deformation of Ni3Al can be controlled by grain boundary sliding assisted by DRX.