Microstructural change during superplastic deformation of δ-ferrite/austenite duplex stainless steel

Abstract

Superplasticity of a 25 pct Cr-6.5 pct Ni-3 pct Mo-0.14 pct N δ/γ duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation. Two large superplastic elongations are obtained at temperatures around 1323 K in δ/γ duplex phase region and 1173 K where σ phase particles precipitate dynamically at a strain rate of ~10⁻³ s⁻¹. During deformation in the higher temperature region, fine Widmanstätten γ particles coarsen and coarse γ grains formed during the prior treatments are broken into spherical particles, resulting in a homogeneous dispersion of γ particles within the σ-ferrite matrix. The dynamic recrystallization of soft σ-ferrite matrix occurs locally in the region where the strain reaches some critical value, and the final microstructure consists of equiaxed σ and γ grains. In the case of lower temperature deformation, a eutectoid decomposition of δ-ferrite into γ and σ phases occurs. The relatively soft γ grains which are severely deformed by hard σ particles recrystallize dynamically, and these processes lead to the γ/σ equiaxed duplex structure. The extremely large superplasticity of this alloy can mainly be explained in terms of the above microstructural change during deformation.