Isothermal transformation of β-phase in Cu-rich Cu-Al-Sn alloys

Abstract

Abstract Microstructural changes resulting from isothermal decomposition of the β-phase have been studied in Cu-rich binary Cu-Al and ternary Cu-Al-Sn alloys containing up to 3 at.% Sn at temperatures from 873 to 673 K. Results are presented as TTT diagrams. The decomposition occurs in several stages, each of which involves the establishment of metastable equilibrium between β and one or more of the product phases α β1 and γ2. Addition of Sn has been shown to increase the stability of the ordered β1-phase in relation to β. In alloys containing more than 2 at.% Sn, the β1 emerges as a stable phase. At low Sn concentrations β1 is metastable. An important new finding is the existence of three-phase equilibrium microstructure containing α β1 and γ2. Increasing addition of Sn alters the morphology of β1 from rosettes to dendrites and finally to Widmanstätten needles.