Influence of predeformation on ageing in an Al-Zn-Mg alloy - I. Microstructure evolution and mechanical properties

We investigate the ageing kinetics of a ternary Al-Zn-Mg alloy, and the cou pling between prestraining and subsequent ageing. The precipitation sequenc e is investigated using DSC. The microstructure is followed using TEM and S mall-Angle X-ray Scattering, and the hardening resulting from precipitation studied by hardness measurements. The effect of prestraining depends signi ficantly on the parameters of the thermomechanical treatment, and has to be considered in terms of competition between homogeneous precipitation and h eterogeneous precipitation on dislocations. The effect of prestraining on p eak hardening is controlled by the nucleation of the hardening eta' phase, and therefore depends critically on the heating rate to the ageing temperat ure. The effect of prestraining on overageing is an increase in overall coa rsening kinetics due to the combination of faster precipitate coarsening on dislocations and dissolution of homogeneous precipitates followed by solut e diffusion towards dislocations. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.