Microstructural evolution and phase transformation kinetics of MnBi alloys

The low temperature phase (LTP) compound of MnBi is a promising, rare-earth metal free, and permanent magnetic material. However, it is difficult to obtain pure LTP MnBi, which results from a peritectic reaction between Mn and Bi atoms. In this study, the microstructural evolution and phase transformation kinetics of MnxBi100-x (x = 50, 55, 60) alloys were systematically investigated. Choosing an appropriate Mn content (Mn55Bi45) was found to promote the formation of the LTP MnBi. The results of the phase transformation kinetics indicated that the nucleation, growth and soft impingement processes are involved in the phase transformation process. In addition, in the initial stage of the LTP phase transformation, it was shown that the diffusional transformation was governed by two- and three-dimensional nucleation and growth, as well as a single mechanism of diffusion-controlled growth, which describes the entire transformation. At the final stage of the phase transformation, diffusion controlled growth, with soft impingement effects, dominates the process. The phase transformation kinetics analysis was confirmed with the X-ray diffraction and scanning electron microscopy results. This work implies that the local activation energy, Ec(α), and local Avrami exponent, n(α), can be applied to characterize the phase transformation behaviors of the LTP MnBi alloy.