Precipitate coarsening parameters for gas induced semi-solid cast 7075-T6 Al alloy determined by SAXS measurements

Abstract

The particle radii of precipitates in the gas induced semi-solid (GISS) cast 7075-T6 Al alloys, for ageing times ranging from peak ageing to over ageing, were measured by small-angle X-ray scattering (SAXS) technique. The measured precipitate radii increased with ageing time and temperature, and data were well fit by the Lifshitz–Slyozov–Wagner (LSW) coarsening process along with a modified Arrhenius equation for temperature dependency. The activation energy for the coarsening process was estimated at 1.19 eV, which is close to that of the wrought 7075 Al alloy. This energy is in the range of activation energies of Zn, Mg and Cu for their diffusion in Al. The precipitate coarsening rate of this alloy appears to be diffusion limited. The fine precipitates of the GISS 7075-PA Al alloy coarsened slowly at $63\,\hbox {nm}^{3}\,\hbox {h}^{-1}$ rate, at the elevated $200{^{\circ }}\hbox {C}$ temperature, while rapid $216\,\hbox {nm}^{3}\,\hbox {h}^{-1}$ coarsening was found in the wrought 7075-T651 Al alloy.