Quantification of the strain-induced promotion of τ-MnAl via cryogenic milling

Ferromagnetic τ-MnAl is a promising material for permanent magnetic applications due to its high density-compensated energy product; however, the transformation pathway from its high-temperature parent ϵ-phase remains poorly understood. In this work the evolution of phase composition, microstructure, chemical order and magnetic attributes accompanying the direct (i.e., without annealing) transformation of ϵ-MnAl to τ-MnAl has been quantified as a function of time spent processing via mechanical milling at cryogenic temperatures. These results provide unique and enabling information to better define the processing window for obtaining and optimizing metastable τ-MnAl from the quenched-in ϵ-phase. It is revealed that moderate microstrain level promotes the direct formation of τ-MnAl with a high degree of chemical order, attributed to a previously reported displacive shear mechanism. Higher microstrain levels degrade the chemical order in the τ-phase and attributed to antiferromagnetically coupled Mn–Mn nearest neighbor pairs that donate a complicated evolution of the saturation magnetization. These results provide useful insights for the development of future processing protocols for the manufacture of τ-MnAl as a permanent magnetic material. - Highlights: • Microstrain directly contributes to the promotion of the ϵ-to-τ transformation. • Cryomilling enhances microstrain, thereby promoting τ-phase formation. • Cryomilling also enhances chemical disorder, resulting in Mn–Mn pair formation. • AF-coupled Mn–Mn pairs likely cause complicated evolution of M_2τ with cryomilling.