Mechanical properties and nanostructures in a duplex stainless steel subjected to equal channel angular pressing

The nanostructure and resultant mechanical property were studied in an UNS S32304 duplex stainless steel subjected to equal channel angular pressing. The successive refinement of original grains was observed to happen in dual phases, i.e., austenite and ferrite, during 4-pass pressing. Both phases exhibit similar lamellar features of microstructures by the formation of dislocation boundaries and their misorientation evolution with increasing strains. The deformation twinning and martensitic transformation as well were observed to happen in austenite. The transversal spacing of elongated grains is 79 nm in austenite and 130 nm in ferrite, respectively. After annealing at 973 K for 20 min, the elongated grains tend to become equiaxed, and the average size of nanograins increases to 108 nm in austenite and 235 nm in ferrite, respectively. The tensile tests show that the yield strength of the dual-phase nanostructure is 1460 MPa after 4-pass pressing, as compared to 403 MPa of its coarse-grained counterpart, but with uniform elongation of merely 2%. After annealing at 973 K, the uniform elongation increases to 7% with the yield strength of 1100 MPa. In addition, different stages of strain hardening were observed in various microstructures during tensile testing, and the corresponding plastic responses were discussed.