Metallurgy of the magnetic τ-phase in Mn-Al and Mn-Al-C

The known information on Mn-Al(-C) permanent magnets that has accumulated since 1958 is reviewed critically and combined with new measurements to provide a definitive description of the metastability and the conditions of formation of tetragonal τ-Mn1.1Al0.9 and its decomposition into the equilibrium phases. Metastable τ forms readily because its high-temperature precursor, close-packed hexagonal Ε, is readily undercooled well below its equilibrium eutectoid decomposition temperature as a result of the delayed nucleation of the stable β-Mn solid solution. The off-stoichiometric composition of binary τ is connected with the composition, 55 at.% Mn, of the metastable eutectoid reaction Ε &rarr γ2 + β, where the maximum undercooling of Ε is achieved and second phases are absent.