Microstructural changes during superplastic deformation of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel

Abstract

The superplasticity of Fe-24Cr-7Ni-3Mo-0.14N duplex stainless steel after solution treatment at 1350 degrees C followed by 90% cold rolling was investigated at 850 degrees C with a strain rate ranging from 10(-3) to 10(-1) s(-1). The microstructure of duplex stainless steel consists of matrix gamma phase having low angle boundaries and sigma phase as second phase particles before the deformation at 850 degrees C. The constituent phases in the duplex stainless steel were found to be changed following alpha --> alpha + gamma --> alpha + gamma + sigma --> gamma + sigma through phase transformation during the deformation at 850 degrees C. A maximum elongation of 750% was obtained at 850 degrees C with strain rate of 3.16 x 10(-3) s(-1). The low angle grain boundaries were changed into high angle grain boundaries by dynamic recrystallization of gamma phase at an early stage of deformation. 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 with increasing strain; thus the low angle grain boundaries were transformed into high angle grain boundaries suitable for sliding by the dynamic recrystallization during the deformation. The grain boundary sliding assisted by dynamic recrystallization is considered a controlling mechanism for superplastic deformation at 850 degrees C. (C) 1999 Elsevier Science S.A. All rights reserved.