Small-angle scattering from GP zones in Al-Cu alloy

It is well known that Guinier Preston (GP) zones form in Al-Cu alloys upon solutionizing and artificial aging, which are extensively used in commercial practice. It is well established that GP zones are discshaped precipitates, i.e. disks of clusters of copper atoms in the FCC aluminium matrix. These disks have coherency strain fields in aluminium that give the alloy its high yield strength. The formation of GP zones in the supersaturated aluminium matrix is thought to be heterogeneous nucleation and growth. Some authors have believed that the formation of GP zones is by spinodal decomposition of the supersaturated Al-Cu solid solution. The main objective of the present work is to test whether spinodal decomposition is responsible for the formation of GP zones in Al-Cu alloy. The experimental alloy AA2219 was selected for its high copper content (Al-6%Cu-0·2%Zr). After solutionizing and artificial aging, the aging curve was plotted and small-angle scattering experiments were carried on the powdered samples as a function of time during artificial aging. Small-angle scattering data were analysed, and evidence has been obtained for the occurrence of spinodal decomposition as the mechanism responsible in the early stages of formation of GP zones.