High-Strain Rate Superplasticity and Role of Dynamic Recrystallization in a Superplastic Duplex Stainless Steel

Superplasticity of a 25Cr-7Ni-3Mo ferrite/austenite duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation, in order to clarify roles of dynamic recrystallization in the superplastic deformation. A microduplex structure consisting of fine ferritic subgrains and austenite particles (grain size: 1 μm) is obtained by annealing at 1273 K after 90% cold rolling of a fully ferrite structure. The steel with this fine-grained structure exhibits high-strain rate superplasticity at 1273 K in the ferrite/austenite two-phase region. A maximum elongation over 1700% is obtained at a strain rate of 1.7 x 10^-2/s. Even at a higher strain rate of 1.7 x 10^-1/s the elongation is near 1000%. Ferrite/ferrite boundaries are low-angle boundaries before the superplastic deformation. These low-angle boundaries are changed to high-angle ones by dynamic recrystallization of the ferrite matrix in the early stage of deformation. During the deformation after the recrystallization of the ferrite matrix, dislocations are scarcely observed in the whole of specimens. It is concluded that grain-boundary sliding is the dominant mode of the superplastic deformation, and that the role of the dynamic recrystallization is to make the fine structure suitable for the grain-boundary sliding in the early stage of deformation. A mechanism of the dynamic recrystallization is briefly discussed.