Reverse α' → γ transformation mechanisms of martensitic Fe-Mn and age-hardenable Fe-Mn-Pd alloys upon fast and slow continuous heating

The mechanisms governing the reverse martensite (alpha') to austenite (gamma) transformation (alpha' -> gamma) and the effect of prior precipitation on the austenite reversion are investigated for martensitic Fe-Mn alloys containing 5 and 10 wt.% Mn and their age-hardenable variants with the addition of 1 wt.% Pd, respectively. Dilatometric experiments employing heating rates between 0.5 and 200 K min(-1), atom-probe tomography measurements on continuously heated specimens and thermo-kinetic simulations were performed. On fast heating (200 K min(-1)), the alpha' -> gamma transformation appeared in a single stage and can be regarded as a partitionless and interface-controlled reaction. In comparison to the binary alloys, the transfoimation temperatures of the Pd-containing steels are considerably increased, due to precipitates which act as obstacles to migrating austenite/martensite interfaces. For low heating rates of 0.5 and 2 K min(-1), splitting of the alpha' -> gamma transformation into two consecutive stages is observed for both the binary and the ternary alloys. With the assistance of thermo-kinetic simulations, a consistent description of this phenomenon is obtained. The first transformation stage is associated with the decomposition of the martensite matrix into Mn-rich and Mn-deficient regions, and the austenite formation is dominated by long-range diffusion. In the second stage, the austenite reversion mechanism changes and the Mn-depleted regions transform in a predominantly interface-controlled mode. This is corroborated by the results for the ternary alloys. The precipitates mainly impede the austenite formation in the second stage, which occurs over a considerably wider temperature range compared to the binary alloys. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.