Phenomena and mechanism on superplasticity of duplex stainless steels

Abstract

The superplasticity of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel after being solution treated at 1350°C followed by 90% cold rolling was investigated at 850°C with a strain rate ranging from 10-3-10-1s-1. The microstructure of duplex stainless steel consists of a matrix γ phase having low angle grain boundaries and a σ phase as second phase particles before the deformation at 850°C. It is well known that the constituent phases in duplex stainless steel is changed following α→α+γ→α+γ+σ→γ+σ through phase transformation during deformation at 850°C. The final microstructure of duplex stainless steel consisted of 70 vol.% of γ and 30 vol.% of the σ phase. A maximum elongation of 750% was obtained at 850°C with a strain rate of 3.16xl0-3s-1. The dislocation density within matrix γ grains was low and a significant strain-induced grain growth was observed during the deformation. The misorientation angles between the neighboring γ grains increased as the strain increased, thus the low angle grain boundaries were transformed into high angle grain boundaries suitable for sliding by dynamic recrystallization during the deformation at 850°C. The grain boundary sliding assisted by dynamic recrystallization is considered to be a controlling mechanism for superplastic deformation at 850°C.