Phase equilibria and Heusler phase stability in the Cu-rich portion of the Cu–Al–Mn system

Results pertaining to the phase equilibria between the phases α (A1), β (A2, B2 or L21(D03)) and γ (γ-bronze type), and the two-stage order–disorder transition and decomposition reaction A2–B2–L21 in the ternary system Cu–Al–Mn are reported. Ternary isothermal section diagrams at 800, 700, 600 and 550°C have been constructed using Energy Dispersion X-ray Spectrometry (EDX) analysis results, and it is found that the β single-phase region in the Cu–Al system is very significantly widened on increasing the Mn content. The critical temperatures (Tc) of the A2–B2–L21 order–disorder transitions, determined by Differential Scanning Carolimetory (DSC) analysis are found to be strongly dependent on the Al content rather than on the Mn content. It is confirmed by DSC measurements and TEM-EDX analysis that a miscibility gap island between Cu3Al and Cu2AlMn phases exists in the L21 phase region. The second order ordering reaction between D03 and L21 structures has also been detected by X-ray diffraction. The stability of the bcc β phase is discussed in terms of atomic and magnetic ordering.