Phase Equilibrium of the AuMn-Cu2MnGa System

Abstract

Phase equilibrium and phase constitution of the Au-Cu-Ga-Mn quaternary system at 873 K was studied along the AuMn-Cu2MnGa (=CuMn0.5Ga0.5) both of which are ferromagnetic shape memory alloys. Three alloys were investigated: AuMn, Cu2MnGa and Au0.5Cu0.5Mn0.75Ga0.25 (AuMn:CuMn0.5Ga0.5=1:1). It was found that 1 ordered bct phase with the axis ratio c/a=0.948 was formed in the AuMn alloy due to the Mn reduction by evaporation during melting. The CuMn0.5Mn0.5 alloy was composed of MgZn2-type Laves phase and Cu9Al4-type  phase. Besides, the following three phases existed in the Au0.5Cu0.5M0.75Ga0.25 alloy: Laves,  and 2 ordered bct with c/a=1.041. The c/a value of the ordered bct phase is decreased by Cu and/or Ga substitution in AuMn. The values of electron atom ratio (e/a) calculated based on chemical compositions for the  phases were 1.61 and 1.68, which were close to the ideal value of 1.62 (=21/13). Based on the results, ordered bcc  (transforming into bct at lower temperature), and Laves are judged to be equilibrium phases at 873 K along the AuMn-Cu2MnGa.