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

Abstract

Abstract 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 ɛ-phase. Crystallographic correspondence is transmitted from the ɛ- to the τ-phase during the whole transformation sequence. The intermediate long-range ordered ɛ′-phase nucleates within the ɛ-matrix. As ɛ′-domains evolve, elastic strains increase at the ɛ′-interfaces until shear occurs in ɛ′-domains to reduce the transformation strains. Stacking faults appear between ɛ′-domain boundaries. Polytypes are the result of a shuffling of the stacking faults within ɛ′-domains. Finally, the ɛ′-domain boundaries are overcome by the collective motion of transformation dislocations which form twinned τ-plates. The ɛ′-domain boundaries are nucleation sites for successive transformation steps and limit the spatial extension of the transformation for the intermediate steps.