Transformation kinetics of the ferromagnetic alloy Mn-Al-C

Mn 1-x-y Al x C y forms for 0.42 \leq x \leq 0.47 and 0 \leq Y \leq 0.021 a metastable tetragonal τ-phase which is ferromagnetic. Its unaxial magnetic anisotropy makes it an interesting candidate for permanent magnetic applications. The metastable τ-phase is produced from the high temperature equilibrium e-phase in two steps; an ordering of the hexagonal e-phase to the e'-phase, followed by a martensitic transformation to τ. It can be obtained either by cooling at a rate of 0.5 K/sec or by quenching and subsequently isothermally annealing between 700 K and 1000 K. At these temperatures the low diffusion rate favours the formation of the intermediate τ-phase rather than a decay into the equilibrium phases β-Mn and γ, since the former requires only small displacements of the atoms. To investigate the kinetics of the occuring transformations we established the time-temperature-transformation diagram of the alloy with the composition Mn 0.538 Al 0.446 C 0.016 . It was homogenized for 2 hours at 1375 K, quenched and subsequently isothermally annealed at temperatures between 700 K and 1000 K for various times. The quantitative phase analysis were performed by means of x-ray diffraction and stereological analysis of the microstructure. The addition of carbon increases the stability of the τ-phase allowing longer annealing times for the formation of single phase samples. The time-temperature-transformation diagram gives the range for appropriate heat treatment and serves as a basis for the search for other alloying elements to further improve the stability of theτ -phase.