Massive transformation and the formation of the ferromagnetic L10 phase in manganese-aluminum-based alloys

Manganese-aluminum alloys in the vicinity of the equiatomic composition exhibit an attractive combination of magnetic properties for technological applications, including bulk permanent magnets and thin-film devices. The technical magnetic properties derive from the formation of a metastable L1(0) intermetallic phase (tau-MnAl) characterized by a high, uniaxial magnetocrystalline anisotropy with an "easy" c-axis. Carbon is generally added to stabilize the tetragonal tau phase with respect to the stable phases in the system. The magnetic hysteresis behavior of the Mn-Al-C genre of permanent magnet alloys is extremely sensitive to the microstructure and defect structure produced during the formation of the tau phase (L1(0)) within the high-temperature epsilon phase (hcp). In this study, modern metallographic techniques, including high-resolution electron microscopy (HREM), have been applied to elucidate the nature of the phase transformation and the evolution of the unique microstructure and defect structure characterizing the structural state of the ferromagnetic tau phase. It is concluded that the metastable tau phase is the product of a compositionally invariant, diffusional nucleation and growth process or massive transformation. The massive product nucleates preferentially at the grain boundaries of the parent 8 phase and is propagated by the migration of incoherent interphase interfaces. The interphase interfaces are revealed to be faceted on various length scales. It is concluded that this faceting is not a feature of the bicrysiallography of the parent and product phases. The high density of lattice defects within the tau phase, generated by the phase transformation, is attributed to growth faults produced during atomic attachment at the migrating interfaces. Classical nucleation theory has been applied quantitatively to the grain-boundary nucleation process and was found to be consistent with the observed time-temperature-transformation (TTT) behavior. Analysis of the growth kinetics gives an DeltaH(D) value of 154 kJ mol(-1) for the activation energy of the transboundary diffusional process controlling boundary migration.