On the plate-like τ-phase formation in MnAl-C alloys

A detailed description of the displacive transformation mode of the formation of plate-like τ-phase in MnAl (containing a small amount of carbon) is given. The plate-like τ-phase forms in a stepwise mode from the ε-phase. Crystallographic correspondence is transmitted from the ε- to the τ-phase during the whole transformation sequence. The intermediate long-range ordered ε′-phase nucleates within the ε-matrix. As ε′-domains evolve, elastic strains increase at the ε′-interfaces until shear occurs in ε′-domains to reduce the transformation strains. Stacking faults appear between ε′-domain boundaries. Polytypes are the result of a shuffling of the stacking faults within ε′-domains. Finally, the ε′-domain boundaries are overcome by the collective motion of transformation dislocations which form twinned τ-plates. The ε′-domain boundaries are nucleation sites for successive transformation steps and limit the spatial extension of the transformation for the intermediate steps.