Superplasticity-like behavior of ultrafine-grained austenitic steel 321

Hot rolled commercial metastable austenitic steel 321 with strongly elongated thin delta-ferrite particles in its microstructure was the object of investigations. Ultrafine-grained (UFG) microstructure in steel 321 was formed by Equal Channel Angular Pressing (ECAP) at 150 oC and 450 oC. When heating the UFG steel specimens, the nucleation of sigma-phase particles blocking the grain boundary migration was observed. The maximum elongation to failure (~250%) was achieved at the deformation temperature 750 oC. The process of superplastic deformation of the UFG steel 321 is controlled by simultaneous grain boundary sliding and power-law creep. The contribution of each process depends on the grain growth rate in the superplasticity regime as well as on defect accumulation on the grain boundaries. The fracture of the UFG steel 321 specimens has a cavitational character - an intensive formation of large elongated pores at the non-metallic particles as well as of the submicron pores at the sigma-phase particles in the course of superplastic deformation were observed.