A study on the austenite-to-ferrite phase transformation in binary substitutional iron alloys

The massive transformation from austenite to ferrite (γ→α) in binary substitutional Fe–X alloys, where X represents successively about 1 or 2 at.% of Co, Cu, Mn, Cr or Al, is experimentally investigated by means of dilatometry. The resulting transformation curves have been modelled by an interface-controlled growth model, taking the characteristics of the austenitic microstructure into account. The assumption of an Arrhenian temperature dependence of the interface mobility, defined as the ratio between the interface velocity and the driving force, is found to give a consistent picture of the observations with an activation energy for atoms crossing the interface of 140 kJ mol−1. The spurious presence of interstitial nitrogen is shown to have a significant effect on the mobility, as large as a factor 8 at concentration levels on the order of 10−4.