Insights into formation and stability of $\mathrm{\tau-MnAlZ_{x}}$ (Z = C and B)

The τ-phase MnAl alloys are promising candidate for rare earth free permanent magnets. In this study, In order to better understand the MnAl ε$\rightarrow$$\tau$ phase transition mechanism in a continuous cooling process and metastable MnAl τ-phase high temperature stability, $Mn_{0.54}Al_{0.46}$, $Mn_{0.55}Al_{0.45}C_{0.02}$ and $Mn_{0.55}Al_{0.45}B_{0.02}$ alloys were systematically studied by in situ synchrotron X-ray powder diffraction (SR-XRD). The relationship between $\tau$-phase formation tendency and different cooling rates of $Mn_{0.55}Al_{0.45}C_{0.02}$ was investigated. Besides, the high temperature stabilities of undoped $\tau$-MnAl and carbon/boron doped $\tau$-MnAl were studied. Differential thermal analysis (DTA) was also employed to study the phase transformation as well. The research results show that a high cooling rate of 600 °C/min leads to a 50/50 wt% mixture of ε- and $\tau$-phase; almost pure $\tau$-phase was obtained when cooled at a moderate cooling rate of 10 °C/min; while for a slow cooling rate of 2 °C/min, the $\tau$-phase partially decomposed into $\beta$ and $\gamma$2 phases. No intermediate ε’-phase was observed during the ε$\rightarrow$$\tau$ phase transition during the experiments. For the boron and carbon doped $\tau$-MnAl, the 800 °C high temperature stability experiments reveal that C stabilizes the $\tau$-MnAl while doped B destabilises the tetragonal structure and it decomposes into $\beta$- and $\gamma$2-phases