The alloys of the ternary system Zn-Al-Cu are characterised by dimensional changes that occur on ageing in conjunction with certain phase transformations and changes in solid solubility [1]. The phase transformation that is suggested to be responsible is the four phase reaction

that occurs below 273°C [2]. The average density of the products of this reaction was hypothesised to be less than that of the reactants resulting in an increase in volume. Previous work [3] had indicated that the lattice parameters of the zinc structure cell varied considerably on ageing and this change was attributed to the change in solubility of substitutionally dissolved copper. On ageing it is hypothesised that copper tends to come out of solution in zinc, contributing to the total increase in volume. This copper may come out as ε-phase which may react to form T'-phase with a resulting change in volume.