Microstructure, phase composition and hardness evolution in 316L stainless steel processed by high-pressure torsion

A 316L stainless steel was processed by high-pressure torsion (HPT) to evaluate the grain refinement and phase transformation. The initial material was essentially a single phase gamma-austenite with a coarse grained microstructure of similar to 42 mu m but the grain size was reduced to similar to 45 nm after 10 turns of HPT. In addition, there was a phase transformation and the initial gamma-austenite transformed initially to epsilon-martensite and finally to alpha'-martensite with increasing strain. The dislocation density increased to an exceptionally high value, of the order of similar to 10(16) m(-2), in the main alpha'-martensite phase after 10 HPT revolutions. The formation of the multiphase nanocrystalline microstructure yielded a four-fold increase in hardness to reach an ultimate value of similar to 6000 MPa. The Hall-Petch behaviour of the HPT-processed alloy is compared directly with coarse-grained materials. (C) 2016 Elsevier B.V. All rights reserved.