Grain Boundary Mediated Displacive–Diffusional Formation of τ-Phase MnAl

Dynamic in situ heating and postmortem transmission electron microscopy studies, including high-resolution electron microscopy, have been performed for a near equiatomic composition of a Mn-Al-base permanent magnet alloy to investigate the mechanisms of the transformation of hexagonal close-packed ε-phase to a chemically ordered tetragonal τ-phase with a face-centered cubic-related L1₀ structure. Although a “massive” mode dominates the kinetics of this composition invariant transformation, we also observed the genesis of a morphologically plate-like τ-phase by partial dislocation glide, described as Shockley-type partials with respect to the L1₀ structure, and a transformation mode exhibiting displacive characteristics. The sources for the transformation dislocations facilitating the formation of the morphologically plate-like τ-phase were associated with the interfaces between the parent ε- and product τ-phase produced by the “massive” transformation mode. Our experiments revealed diffusional and displacive features of the mechanism accomplishing the formation of τ-MnAl with plate-like morphology. Hence, a hybrid displacive–diffusional mechanism has been identified, and the synergistic role of the nucleation interfaces of the massively transformed τ-phase has been discussed.