Phase Transformations in Mn-Al and Mn-Bi Magnets by Repeated Heat Treatment

A stepped heating cycle in a tube furnace at 1000 degrees C for 1 h followed by holding at 400 degrees C for 1 h directly increases the amount of ferromagnetic tau-phase of manganese-aluminum (Mn-Al). Repeating this heating regime for 2 more times, further increase in the amount and uniform distribution of the tau-phase is obtained at the expense of the epsilon-phase. The magnetization and coercivity are increased after each heating cycle, and the effect is more pronounced when 2% carbon (C) is added to Mn-Al. When the similar heat treatment cycle is applied to manganese-bismuth (Mn-Bi), the ferromagnetic alpha-phase is detected with larger magnetizations, but substantial Mn and Bi are segregated from the alloy. This solid-state diffusion in tube furnace offers a facile route to improve rare-earth-free magnets.