Insights into formation and stability of tau-MnAlZ(x) (Z = C and B)

• Published in: Journal of alloys and compounds 2017-01, Vol.692, p.198
• Main Authors: Fang, Hailiang, Cedervall, Johan, Casado, Francisco Javier Martinez, Matej, Zdenek, Bednarcik, Jozef, Ångstrom, Jonas, Berastegui, Pedro, Sahlberg, Martin
• Format: Article
• Language: English
• Subjects: In situ; Permanent magnet; Phase stability; Phase transition; Powder diffraction; Thermal analysis
• ISSN: 1873-4669; 0925-8388
• DOI: 10.1016/j.jallcom.2016.09.047

The tau-phase MnAl alloys are promising candidate for rare earth free permanent magnets. In this study, In order to better understand the MnAl epsilon ->tau phase transition mechanism in a continuous cooling process and metastable MnAl tau-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 tau-phase formation tendency and different cooling rates of Mn0.55Al0.45C0.02 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 degrees C/min leads to a 50/50 wt% mixture of epsilon- and tau-phase; almost pure tau-phase was obtained when cooled at a moderate cooling rate of 10 degrees C/min; while for a slow cooling rate of 2 degrees C/min, the tau-phase partially decomposed into beta and gamma(2) phases. No intermediate epsilon'-phase was observed during the epsilon ->tau phase transition during the experiments. For the boron and carbon doped tau-MnAl, the 800 degrees 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.