Insights into formation and stability of τ-MnAlZx (Z = C and B)

- The MnAl ε → τ phase transition studied by in situ synchrotron X-ray diffraction.
- A cooling rate of 10 °C/min yields pure τ-phase when cooled from high temperature.
- No intermediate ε'-phase was observed during the ε-τ-phase transition process.
- C doping stabilizes the τ-phase whereas B doping destabilises the τ-phase.

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→τ phase transition mechanism in a continuous cooling process and metastable MnAl τ-phase high temperature stability, Mn0.54Al0.46, Mn0.55Al0.45C0.02 and Mn0.55Al0.45B0.02 alloys were systematically studied by in situ synchrotron X-ray powder diffraction (SR-XRD). The relationship between τ-phase formation tendency and different cooling rates of Mn0.55Al0.45C0.02 was investigated. Besides, the high temperature stabilities of undoped τ-MnAl and carbon/boron doped τ-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 τ-phase; almost pure τ-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 τ-phase partially decomposed into β and γ2 phases. No intermediate ε'-phase was observed during the ε→τ phase transition during the experiments. For the boron and carbon doped τ-MnAl, the 800 °C high temperature stability experiments reveal that C stabilizes the τ-MnAl while doped B destabilises the tetragonal structure and it decomposes into β- and γ2-phases.

The ferromagnetic τ-phase is metastable and advanced synthesis protocols are needed to produce the material in high purity. In this study, the ε → τ phase transition was investigated by in situ synchrotron radiation X-ray powder diffraction (SR-XRD). A continuous cooling process was employed in combination with different cooling rates to gain insight on the ε → τ phase transition. Furthermore, the effect of doping elements (boron and carbon) on the high temperature stability of the ferromagnetic τ-phase was systematically studied. The relationship between phase composition and different cooling rates shows the importance of appropriate synthesis protocols in order to obtain the metastable τ-phase. The results show that a moderate cooling rate of 10 °C/min yields a pure ferromagnetic τ-phase at the end of the cooling process, and that no intermediate ε'-phase was observed during the experiments.