Partial isothermal sections of the copper-rich corner of the al-cu-zn system at 200ºc and 240ºc

Abstract

The Al-Cu-Zn alloy system is well-used because it has demonstrated many useful properties. Currently, Al-Cu-Zn alloys are primarily used as shape memory alloys. Other applications are in catalysis, in electronics and in the manufacture of metal matrix composites. They have also shown great potential to replace the brittle and high-cost Bi and Ni-based solders in applications of up to 350ºC. The phase equilibria of this system have been the subject of extensive theoretical and experimental assessment since early 20th Century. Some of the features assessed are the liquidus surface, isothermal sections and thermodynamic descriptions using calculation of phase diagrams method. The isothermal sections at 200ºC and 240ºC are incomplete at their copper-rich corners and form the basis for this research. In this work, 10 ternary alloys with compositions around the copper-rich corner were cast in induction furnaces and each cut into two, sealed in vacuum ampoules and homogenised at 370ºC for 10 days. Then, one set was annealed at 200ºC, and the other set at 240ºC for 28 days. All samples were ground and polished for metallographic examination. Finally, the microstructures of the alloys were characterised using scanning electron microscopes and an energy-dispersive X-ray spectrometer. Partial isothermal sections at 200ºC and 240ºC were plotted up to 85 at.% Zn and 55 at.% Al. Eight phases were identified at both 200ºC and 240ºC, namely: the binary phases (αCu), γ Cu9Al4, η2 CuAl, γ Cu5Zn8, ζ2 Cu11Al9 and ε CuZn4, and the ternary phases τ Cu5Zn2Al3 and τ׳ Cu3ZnAl4. The γ phase was found to exist as two distinct phases, that is, γ Cu9Al4 and γ Cu5Zn8. The α2 Cu4Al, δ Cu8Al5 and β CuZn phases were not observed in any alloy. The ranges of composition of the τ Cu5Zn2Al3, τ׳ Cu3ZnAl4 and ε CuZn4 phases found in this work at 200ºC, and the solubility limit of Al in ε CuZn4 at 240ºC differed with those reported in earlier studies.