| In this study, the transformation behavior of MnAl-based ferromagnetic alloys was investigated. The low-cost and availability of the Mn-Al base metals along with their high mechanical strength, machineability and high magnetic energy product (BH) per unit weight make these materials attractive candidates for permanent magnet applications. These alloys derive their magnetic properties from the metastable L10 τ-phase, which generally appears towards the Mn-rich side of the near equiatomic composition. The magnetic properties of these materials are strongly influenced by the microstructure and characteristic defect structure of the τ-phase. The τ-phase exhibits a unique defect structure, which includes twins, stacking faults, anti-phase domain boundaries and dislocations. Understanding the true nature of defect generation is necessary in order to be able to develop processing techniques to enhance and optimize the properties of these materials.; The τ-phase derives from a phase mixture of &egr;(hcp) and &egr; ′(B19) phases through various heat treatment processes. Controversial mechanisms are reported in the literature regarding the nature of the &egr; + &egr;′ → τ transformation. Phase transformation mechanisms that are displacive and those involving a massive transformation have been reported. In this study, the true nature of the τ-phase formation was investigated experimentally by utilizing techniques such as transmission electron microscopy (TEM), high-resolution electron microscopy (HREM) and in-situ TEM heating experiments. It was shown that both of the transformation modes, i.e. massive and displacive mechanisms, can operate and result in τ-phase formation. The atomic nature of the displacive transformation was studied in detail to elucidate the viability of transformation of a two-phase mixture into a single phase through a shear transformation. In the absence of stress, the massive mode was shown to dominate microstructural evolution in bulk materials.; The isothermal nucleation and growth kinetics of the massive transformation were quantitatively studied and compared with similar alloy systems. The nature of growth interfaces, which are believed to play a major role on the generation of the defect structure of the τ-phase, and planar defects were extensively studied by electron microscopy. Models for defect generation were developed to account for the extraordinary defect density in the τ-phase. |
