Mechanism of phase transformation in MnAl-C alloys

Abstract

The phase transformations from the ε- to τ-phases for alloys with the composition of Mn53.6-Al46.4 doped with carbon were investigated using SEM with EDS, optical microscopy and XRD, where the ferromagnetic τ-phase was clearly visualized by suspending a ferromagnetic colloid fluid. When the quenched specimen of the ε-phase was annealed at 600°C, the aluminum rich γ2-phase occurs first at grain boundaries and after that the τ-phase grows inside grains around the γ2-phase. These results indicate that the precipitation of the γ2-phase leads an increase in Mn concentration in the τ-phase and the growth of the τ-phase occurs from grain boundaries when it reaches at the eutectoid concentration (55 at%Mn). Our observations that the nucleation and growth processes occur first from the grain boundaries, no change of the concentration between the ε- and resulting τ-phases suggest that this phase transformation is the massive type one.