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

Abstract

The mechanisms governing the reverse martensite (α′) to austenite (γ) transformation (α′→γ) and the effect of prior precipitation on the austenite reversion are investigated for martensitic Fe–Mn alloys containing 5 and 10wt.% Mn and their age-hardenable variants with the addition of 1wt.% Pd, respectively. Dilatometric experiments employing heating rates between 0.5 and 200Kmin−1, atom-probe tomography measurements on continuously heated specimens and thermo-kinetic simulations were performed. On fast heating (200Kmin−1), the α′→γ transformation appeared in a single stage and can be regarded as a partitionless and interface-controlled reaction. In comparison to the binary alloys, the transformation 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 2Kmin−1, splitting of the α′→γ 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.