Deformation mechanism and ductile fracture behavior in high strength high ductility nano/ultrafine grained Fe-17Cr-6Ni austenitic steel

Tensile deformation and ductile fracture behavior of nano/ultrafine grained (Nano/UFG) steel were studied in this investigation. The results indicate that deformation induced martensite transformation (DIMT) is the active deformation mechanism in coarse grained (CG) steel because of the low stacking fault energy. With grain size decreasing to Nano/UFG scale, deformation twining (DT) and DIMT become both the active deformation mechanism. This is because with grain size decreasing to a critical value (several hundred nanometers in this paper), the critical stress for nucleating a partial dislocation becomes smaller than that for nucleating a perfect dislocation, leading to the nucleation of deformation twins. Lüders deformation also contributes greatly to the plastic deformation in Nano/UFG steel. High fractions of low angle grain boundaries and DIMT during tensile process are believed to be responsible for Lüders deformation. Decreasing grain size to nano/ultrafine grain scale also greatly influences the fracture behavior. The voids in Nano/UFG steel are much smaller than that in CG steel. This is because high fraction of grain boundaries provide enough nucleation sites for microvoids in Nano/UFG steel. Besides, deformation twining leads to the formation of line-up of voids in Nano/UFG steel.