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 degrees C followed by 90% cold rolling was investigated at 850 degrees C with a strain rate ranging from 10(-3)-10(-1)s(-1). The microstructure of duplex stainless steel consists of a matrix gamma phase having low angle grain boundaries and a sigma phase as second phase particles before the deformation at 850 degrees C. It is well known that the constituent phases in duplex stainless steel is changed following alpha-->alpha+gamma-->alpha+gamma+sigma-->gamma+sigma through phase transformation during deformation at 850 degrees C. The final microstructure of duplex stainless steel consisted of 70 vol.% of gamma and 30 vol.% of the sigma phase. A maximum elongation of 750% was obtained at 850 degrees C with a strain rate of 3.16x10(-3)s(-1). The dislocation density within matrix gamma grains was low and a significant strain-induced grain growth was observed during the deformation. The misorientation angles between the neighboring gamma 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 degrees C. The grain boundary sliding assisted by dynamic recrystallization is considered to be a controlling mechanism for superplastic deformation at 850 degrees C.