Dynamic in situ heating and post mortem transmission electron microscopy studies, including high-resolution electron microscopy, were 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 so-called massive mode dominated the kinetics of this composition invariant transformation, also observed was 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. The 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 was identified, and the synergistic role of the nucleation interfaces of the massively transformed τ-phase was considered.

Grain Boundary Mediated Displacive–Diffusional Formation of τ-Phase MnAl. J.M.K.Wiezorek, A.K.Kulovits, C.Yanar, W.A.Soffa: Metallurgical and Materials Transactions A, 2011, 42[3], 594-604