Grain Boundary Mediated Displacive-Diffusional Formation of [tau]-Phase MnAl

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 2011-03, Vol.42 (3), p.594
• Main Authors: Wiezorek, Jorg M K, Kulovits, Andreas K, Yanar, Cagatay, Soffa, William A
• Format: Article
• Language: English
• ISSN: 1073-5623; 1543-1940

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 L1^sub 0^ 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 L1^sub 0^ 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. [PUBLICATION ABSTRACT]