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

Abstract

The phase transformation in two modes, including both displacive and massive growth of τ-phase from ε-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 ε→τ 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 ε-phase can be transformed into τ-phase via displacive mode while the remaining two-thirds or more via massive mode. In bulk τ-phase, most τ-nanocrystals formed via displacive mode are distributed in the matrix of large τ-grains that formed via massive mode. The typical massive growth rate of the τ-phase is 8–60 nm/s, while the displacive growth rate is low. A more complete understanding of the ε→τ phase transformations in the MnAl-based magnets was provided in this work, based on which the annealing process for ε→τ was optimized and thus high purity τ-phase with high saturation magnetization was obtained.

High Coercivity in MnAl Disc Prepared by Severe Plastic Deformation

“…We attribute this grain refinement to the growth of τ‐phase from numerous nuclei in the matrix of ε‐phase and the formation of β‐phase due to decomposition of the meta‐stable τ‐phase. The phase transformation from ε → τ in MnAl‐based alloys has two modes, i.e., displacive mode and massive mode . The τ‐nanocrystals with dimensions up to 40 nm formed via displacive mode are distributed evenly in the matrix of large τ‐grains that formed via massive mode .…”

“…The phase transformation from ε → τ in MnAl‐based alloys has two modes, i.e., displacive mode and massive mode . The τ‐nanocrystals with dimensions up to 40 nm formed via displacive mode are distributed evenly in the matrix of large τ‐grains that formed via massive mode . Usually, τ‐phase grows from numerous nuclei with random orientations, and thus the dimensions of one grain of the τ‐phase is very limited and is smaller than that of the ε‐phase in average.…”

“…Most of the previous studies on MnAl‐based alloys were focused on achieving a higher fraction of τ ‐phase and thus a higher saturation magnetization ( M s ), understanding the ε → τ phase transformation mechanism, and enhancing the magnetic performance, particularly the coercivity, via various methods . So far high‐purity τ‐phase has been prepared and there is not much space for improving M s . Currently, the ε → τ phase transformation mechanism is also well‐known and the ε → τ process can be controlled by optimized annealing conditions .…”

“…So far high‐purity τ‐phase has been prepared and there is not much space for improving M s . Currently, the ε → τ phase transformation mechanism is also well‐known and the ε → τ process can be controlled by optimized annealing conditions . The main challenge remaining unsolved lies in that the coercivity of the MnAl‐based alloy produced by various methods is relatively low, and this has largely restricted the development of high performance MnAl magnets for applications .…”

Phase transformation and magnetic properties of MnAl powders prepared by elemental-doping and salt-assisted ball milling

“…The phase transformation in this low temperature range is a diffusionless process, during which a so-called displacive phase transformation mode involving co-operative movements of atoms that are not hindered by high energy barriers occurs. 13,14 The displacive mode of ε→τ for MnAlFe, MnAl, MnAlSi 0.5 , and MnAlSi is maintained at temperatures up to T m ∼628, 700, 771, and 781 K, respectively. The T m was determined by approaching the M-H curves in Fig.…”