Decomposition in Al-Zn alloys: Part I. Isothermal decomposition in an Al-22 at. pct Zn-0.1 at. pct Mg alloy

A small-angle X-ray scattering (SAS) study has been made on solution treated and isothermally annealed specimens of an Al-22 at. pct Zn-0.1 at. pct Mg alloy. The changes in peak position and integrated area of the SAS spectra with time and temperature indicate that de composition is nearly complete immediately after quenching, in agreement with the earlier interpretation that Gerold and Merz placed on the results of Rundman and Hilliard in the binary Al-22 at. pct Zn alloy. Furthermore, structural changes occurring during annealing are consistent with a coarsening or maturation of the fluctuations in the solution. The domi nant wavelength varies ast ^1/3 over a large time span and the temperature dependence of the coarsening process yields an activation energy of 94.2 kJ/mole. The effect of Mg is to re tard the formation of the equilibrium phases while having a small effect on the growth of the composition fluctuations during the coarsening process. Now on sabbatical leave to Caterpillar Tractor Co., Mapleton Plant, Mapleton, Ill. 61554.