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

The superplasticity of Fe–24Cr–7Ni–3Mo–0.14N duplex stainless steel after solution treatment at 1350°C followed by 90% cold rolling was investigated at 850°C with a strain rate ranging from 10−3 to 10−1 s−1. The microstructure of duplex stainless steel consists of matrix γ phase having low angle boundaries and σ phase as second phase particles before the deformation at 850°C. The constituent phases in the duplex stainless steel were found to be changed following α→α+γ→α+γ+σ→γ+σ through phase transformation during the deformation at 850°C. A maximum elongation of 750% was obtained at 850°C with strain rate of 3.16×10−3 s−1. The low angle grain boundaries were changed into high angle grain boundaries by dynamic recrystallization of γ phase at an early stage of deformation. 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 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°C.