Study On Phase Equilibrium And Interdiffusion Behavior In The Low Cu Part Of The Al-Zn-Cu Ternary System

The Al-Zn-Cu ternary system is one of practically important systems among Al-based light alloys, and there is inconsistent acknowledge on miscibility gap of fcc solid solution in this system. If the miscibility gap of fcc solid solution can be correctly obtained, there is practical importance for understanding precipitation of Al alloys containing Zn and Cu during earlier aging after quenching and controlling microstructure and properties of these alloys. In this paper, miscibility gap of fcc solid solution in the Al-Zn-Cu ternary system has been systematically investingated by combining experimental study with thermodynamical calculation; The effect of Cu addition on interdiffusion behavior in Al-Zn alloy has been analysized by Matano method on the basis of experimental measurement.The boundary trend of α₁/(α₁+α₂) has been successfully determined for miscibility gap of fcc solid solution in the low Cu part between 277 and 351 ℃ by means of diffusion couple-EPMA technique. It is determined that the solubility of Zn in both α₁ and α₂ phase decreases with the increasing of Cu; At the same time, thepartitioning ratios- K_Cu^{α₂/α₁} are remarkably more than 1, i.e. Cu is mainly distributed inthe α₂ phase. It suggests that miscibility gap of fcc solid solution appears divergent, transforming continuously from Al-Zn side to Al-Cu side.It is also found that miscibility gap of fcc solid solution still exists above 351 ℃ in the low Cu part of the Al-Zn-Cu ternary system. The Zn content in the α₁ phase decreases and the Zn content in the α₂ phase increases with the increasing of Cucontens; At the same time, the partitioning ratios- K_Cu^{α₂/α₁} are still markedly more than1. It means that the existing temperature of miscibility gap of the Al-Zn system is improved due to the addition of Cu. It is concluded from experimental results that the miscibility gap of fcc solid solution is divergent to form connective tunnel space, which is in agreement with thermodynamic calculation, but not convergent to form bell-shaped space as considered previously.Besides, it is found that the partitioning ratios- K_Cu^β/α₂ are clearly more than 1, i.e. Cu is mainly distributed in the β phase, when equilibrium composition between α₂(α)...