Phase transformation kinetics and microstructural evolution of MnAl permanent magnet alloys

Abstract

In order to provide a better understanding about the formation of ferromagnetic τ phase in MnAl Rare-Earth-free permanent magnet alloy, the phase transformation kinetics of τ phase has been analyzed for Mn55Al45 alloy. The local activation energy Ec(α) was introduced to explore the change of energy in the process of phases transformation and its value is closely related to the transformation volume fraction. The local activation energy is low (about 175 kJ/mol) in the process of nucleation. However, the threshold value of energy will increase slowly and approaches a constant value (about 188 kJ/mol) when the grain-boundary-nucleated τ phase steadily grows into the parent ε-phase grains. With the exception of anomalous stage, the local Avrami exponents fall in the range of 2.0–4.0, which implies that two- and three-dimensional nucleation and grain growth controlled by diffusion play a crucial role in the whole transformation process. Meanwhile, transmission electron microscopy (TEM) morphology and the results of transformation kinetics are accordance with each other, indicating that the variable local activation energies Ec(α) and local Avrami exponents n(α) are applicable and correct in describing the τ-phase transformation behavior.