Formation of a metastable ferromagnetic τ phase during containerless melt processing and rapid quenching in Mn-Al-C alloys

Abstract

Solidification of selected Mn‐Al‐C alloys during containerless levitation and rapid quenching has yielded the first report for a ferromagnetic metastable τ (L1₀) phase formed directly from the melt. Complete solidification to τ phase was interrupted by the competitive evolution of an equilibrium ε phase during recalescence. The amount of undercooling required to produce the metastable ferromagnetic τ phase in a Mn_0.55Al_0.433C_0.017 alloy during solidification was estimated as approximately 470 K based on differential thermal analysis results. When the alloy carbon content was increased to 3.4 at. % (i.e., Mn_0.55Al_0.416Co_0.034 alloy), a transition in structure development occurred so that the samples exhibited γ₂ phase formation as well as τ and ε phases. Both microstructural observations and x‐ray diffraction examination were used to guide the interpretation and the analysis of solidification pathways. The attainment of the high liquid undercooling required to nucleate the metastable τ phase from the melt may be facilitated by containerless processing.