Experimental Study of Phase Equilibria in the System Cu-Al-Sn

Phase equilibria in the Cu-rich corner of the ternary system Cu-Al-Sn have been re-investigated. Final equilibrium microstructures of 20 ternary alloy compositions near Cu₃Al were used to refine the ternary phase diagram. The microstructures were characterized using optical microscopy (OM), x-ray diffraction (XRD), electron probe microanalysis and transmission electron microscopy. Isothermal sections at 853, 845, 833, 818, 808, 803 and 773 K have been composed. Vertical sections have been drawn at 2 and 3 at% Sn, showing β₁ as a stable phase. Three-phase fields (α + β + β₁) and (β + β₁ + γ₁) result from β → α + β₁ eutectoid and β + γ₁ → β₁ peritectoid reactions forming metastable β₁ in the binary Cu-Al. With the lowering of temperature from 853 to 818 K, these three-phase fields are shifted to lower Sn concentrations, with simultaneous shrinkage and shifting of (β + β₁) two-phase field. The three-phase field (α + β + γ₁) resulting from the binary reaction β → α + γ₁ shifts to higher Sn contents, with associated shrinkage of the β field, with decreasing temperature. With further reduction of temperature, a new ternary invariant reaction β + β₁ → α + γ₁ is observed at ~813 K. The β disappears completely at 803 K, giving rise to the three-phase field (α + β₁ + γ₁). Some general guidelines on the role of ternary additions (M) on the stability of the ordered β₁ phase are obtained by comparing the results of this study with data in the literature on other systems in the systems group Cu-Al-M.