Synthesis of an intermetallic powder with a hard magnetic phase τ-MnAl by calcium thermal reduction

This study investigates the synthesis parameters of MnAl(C) powders using the calcium-thermal reduction of aluminum oxide (Al₂O₃) and manganese oxide (MnO), aiming to produce Mn-Al alloys within the compositional range of the metastable hard magnetic phase (τ-MnAl). Pure graphite was used as the carbon source for the alloying. This study explored the impact of Al₂O₃/MnO ratio, CaH₂ content, and compaction method. Hydrothermal purification with acetic acid yielded irregularly shaped Mn-Al powders (20–50 μm). Analysis of the Mn/Al compositions indicated that a 50–100 % excess of the initial aluminum was needed to obtain an alloy composition laying within τ-MnAl phase. The powder obtained in air-cooled container had small τ-phase content. Approximately 50 % τ-MnAl was achieved by conducting the reaction in pre-evacuated steel containers at 1250 °C for 2 h, followed by water quenching from 950 °C. This resulted in a low-cost powder with a magnetization of 50 A m²/kg and a coercive force of 38 A/m. While the formation of the τ-phase is promising, its limited fraction (≈50 %) significantly reduces the magnetic performance. This currently makes the material unsuitable for additive manufacturing or mechanical grinding followed by compaction. Further optimization is required to increase the τ-phase fraction before considering practical applications.