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.