Analysis of ferrite growth retardation induced by local Mn enrichment in austenite created by prior interface passages

A series of new cyclic phase transformation dilatometric experiments has been designed to investigate systematically and in detail the newly discovered "growth retardation stage" during the final austenite ferrite transformation, and to provide further evidence for the existence of residual Mn spikes in austenite created during prior cyclic phase transformations. The magnitude of growth retardation increases with increasing Mn concentration, and is absent in the binary Fe C alloy. New experiments also prove that growth retardation does not occur at a particular temperature but when the moving austenite ferrite interface hits the residual Mn spike. The magnitude of growth retardation is proportional to the number of prior temperature cycles in the cyclic phase transformations, and there is no growth retardation in experiments with only one temperature cycle. The fact that the growth retardation can be observed by dilatometry implies that the interface moved back and forth in a retraceable manner. The classical local equilibrium model can still qualitatively predict the newly observed features of growth retardation, while the paraequilibrium model does not work. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.