In situ Observation of Phase Transformation in MnAl(C) Magnetic Materials.

The phase transformation in two modes, including both displacive and massive growth of tau-phase from epsilon-MnAl(C), was observed by in situ transmission electron microscopy. The exact temperature range for different phase transformation modes was determined by magnetic measurements. The displacive growth of epsilon ->tau in Mn54Al46 (or Mn54Al46C2.44) occurs at temperatures below 650 K (or 766 K), above which both modes coexist. One-third or less of the tau-phase can be transformed into tau-phase via displacive mode while the remaining two-thirds or more via massive mode. In bulk tau-phase, most tau-nanocrystals formed via displacive mode are distributed in the matrix of large tau-grains that formed via massive mode. The typical massive growth rate of the tau-phase is 8-60 nm/s, while the displacive growth rate is low. A more complete understanding of the epsilon ->tau phase transformations in the MnAl-based magnets was provided in this work, based on which the annealing process for epsilon ->tau was optimized and thus high purity tau-phase with high saturation magnetization was obtained.