Abnormal austenite-ferrite transformation behaviour in substitutional Fe-based alloys

The gamma-->alpha phase transformation behaviours of Fe-Co and Fe-Mn alloys were systematically investigated by dilatometry and Differential Thermal Analysis (DTA). Two kinds of transformation kinetics, called normal and abnormal, were recognized for the first time and classified according to the variation of the ferrite formation rate. These transformation characteristics were observed for both isothermally and isochronally conducted annealing experiments. A transition, from abnormal to normal transformation kinetics, occurs for Fe-1.79at.%Co when successive heat treatment cycles are executed, which contrasts with Fe-2.26at.%Mn for which only normal transformation kinetics occurs after each of all successive heat treatment cycles. A possible mechanism for the appearance of abnormal transformation kinetics is given, which is based on the austenite grain size. Light microscopical analysis indicates a repeated nucleation of ferrite in front of the migrating gamma/alpha interface. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.