High-temperature transformation pathways for metastable ferromagnetic binary Heusler (Al–55 at.%Mn) alloy

The present manuscript probes the decomposition behaviour of the ferromagnetic binary metastable Heusler alloy in the Al–Mn system. The ferromagnetic ordered tetragonal phase (L10, popularly known as τ phase), which is metastable in nature, was synthesised from the melt through a copper mould chill casting process using 10-mm diameter mould. Isothermal and non-isothermal heat treatments were carried out to understand the decomposition sequence of τ phase at high temperatures. The metastable phase undergoes, on heating, a eutectoid-like decomposition of \( \tau \to \beta {\text{Mn}} + \gamma_{2} \), where \( \gamma_{2} \) is the intermetallic phase Al8Mn5. Both the phases are present in the equilibrium diagram. The two phases at higher temperature further transform to a single phase e having a hexagonal structure. We show through in situ X-ray diffraction studies that \( \gamma_{2} \) transforms to a disordered bcc phase (\( \gamma_{\text{bcc}} \)) prior to the transformation to the e-phase. The overall sequence of phase decomposition during heating can be given as \( \tau \to \tau + \beta \to \beta + \gamma_{2} \to \beta + \gamma_{\text{bcc}} \to \varepsilon \). The high-temperature \( \gamma_{\text{bcc}} \) phase cannot be retained through quenching experiment and its existence can only be established by in situ studies.