Microstructural development in MnAl-base permanent magnet materials: New perspectives

The MnAl-base permanent magnets continue to loom on the horizon as potentially important technological materials. Marginal enhancement of their technical magnetic properties and improved processing could markedly change the relationship of these materials to the current market. MnAl-base magnets derive their attractive properties essentially from the formation of a metastable tau phase ( L1(0)) after cooling from the high-temperature E-phase (hcp) field. For many years the conventional wisdom has been that the tau phase forms via a displacive or martensitic transformation. In this paper the results of a new electron microscopy investigation into the nature of the epsilon -> tau transformation and microstructural development are reported. It is shown that over the temperature range 450-700 degrees C the transformation involves a compositionally invariant, diffusional nucleation and growth process rather than a shear mechanism. Importantly, elucidating the mechanism of the phase transformation has important practical implications since a diffusional mechanism involving the migration of incoherent or partially coherent interfaces points to very different thermomechanical processing routes and alloying strategies. (C) 1997 American Institute of Physics.