Interaction of austenite reversion with precipitation/dissolution during aging in a medium Mn steel alloyed with Cu, Ni and Al

Abstract Controlling the amount of retained austenite and precipitate particles allows for tailoring of mechanical properties in medium Mn steel alloyed with Cu, Ni and Al. The austenite reversion and precipitation/dissolution occur simultaneously during aging treatment at 550 °C being lower than Ac1. The degree of austenite transformation depends on the diffusion of austenite stabilizing elements (Mn, Cu and Ni), and the dissolution of the precipitate particles promotes the diffusion. Small angle neutron scattering (SANS) measurements were performed at the China Spallation Neutron Source (CSNS) to investigate the precipitation/dissolution behaviors. A large number of precipitate particles form at the aging time of 1 h, and then the volume fraction of the particles continuously decreases, and the average particle size has little change as holding time increases, indicating the dissolution of the precipitate particles. Atom probe tomography and transmission electron microscopy were used to study the type, composition and distribution of the precipitate particles. The particles contain Cu and NiAl-type precipitates, which have BCC and B2 structures, respectively. The characterization and modeling of austenite reversion kinetics indicate that austenite transformation is mainly controlled by the diffusion of Cu, which is promoted by the dissolution of Cu particles. This work offers significant insights towards an austenite-precipitate cooperative design for controlling the mechanical properties of the steel.