Superplastic Deformation Mechanisms of Monolithic Intermetallics

Abstract

Superplastic deformation hehavior and microstructural evolution were investigated using typical monolithic intermetallics, L1 2 ordered Co 3 Ti (Co-22mol%Ti), B2 ordered Fe 3 Si (Fe-14mol%Si) and Fe 3 Al (Fe-28mol%Al), and D0 3 ordered Fe 3 Si (Fe-18mol%Si). Superplastic elongation in Co 3 Ti polycrystals increases with decreasing initial grain size. A grain boundary sliding-based mechanism is found to be responsible for the superplastic deformation. B2 ordered Fe 3 Si and D0 3 ordered Fe 3 Si polycrystals, which deform by and slip, respectively, exhibit superplasticity even though their initial grains are coarse. Dynamically recrystallized (DRX) grains with an appropriate size (e.g., of about 40 μm for Fe-18Si), which contain subgrains, are evolved during superplastic deformation. The formation of finer DRX grains less than 10 μm in deformation at low temperatures and high strain rates does not lead to large elongation because of high flow stress. Fe 3 Si single crystals also exhibit superplasticity, and subgrain structure is evolved without inducing DRX. Thus, superplasticity in Fe 3 Si polycrystals is closely related to the formation of subgrain structures. It is found that there are two types of superplasticity in monolithic intermetallics. At a temperature range where superplasticity appears, Fe 3 Si and Fe 3 Al single crystals have low yield stress due to easy glide and climb motion of dislocations, while Co 3 Ti single crystals have high yield stress due to positive temperature dependence of flow stress. Among intermetallics which have been found to show supcrplasticity, FeAl and Fe 3 (Al, Si) belong to the former type, and Ni 3 Al and Ni 3 (Si, Ti) do to the latter type. Based on the obtained results on superplasticity in monolithic intcrmctallics together with reported references, superplastic deformation mechanisms will be discussed.