Achieving high strength and high ductility in 304 stainless steel through bi-modal microstructure prepared by post-ECAP annealing

The 304 austenitic stainless steel (ASS) undergone equal-channel angular pressing (ECAP) was annealed to achieve a strength-ductility balance through bi-modal microstructure (BMS) tailoring. The coarse grained ASS was first refined down to the nano-grained (NG) level by ECAP for eight passes. The NG microstructure with high dislocation density offers higher strength but lower ductility. The recrystallization curve of NG ASS exhibits three distinct regions including the recovery region below 650 °C, the recrystallization region from 650 °C to 800 °C, and the grain growth region above 800 °C. The results of microstructure observation and tensile test show that the annealing NG ASS at 750 °C for 30 min produces the BMS consisting of 62 v/v% ultrafine grains with average size 350 nm and 38 v/v% coarser grains with average size 1.4 µm. Consequently this BMS ASS presents high strength and high ductility with yield strength 725 MPa, tensile strength 930 MPa, uniform elongation 34.5% and total elongation 40.6%. The present study provides an easy route to enhance the mechanical properties of face center cubic metal and alloys, and thus is of wide application potential.