METALLURGY OF THE MAGNETIC TAU-PHASE IN MN-AL AND MN-AL-C

The known information on Mn-Al(-C) permanent magnets that has accumula ted since 1958 is reviewed critically and combined with new measuremen ts to provide a definitive description of the metastability and the co nditions of format tion of tetragonal tau-Mn1.1Al0.9 and its decomposi tion into the equilibrium phases. Metastable tau forms readily because its high-temperature precursor, close-packed hexagonal epsilon, is re adily undercooled well below its equilibrium eutectoid decomposition t emperature as a result of the delayed nucleation of the stable beta-Mn solid solution. The off-stoichiometric composition of binary tau is c onnected with the composition, 55 at.% Mn, of the metastable eutectoid reaction epsilon --> gamma(2) + beta, where the maximum undercooling of epsilon is achieved and second phases are absent.