Effect of natural ageing on the strain-rate sensitivity of flow stress in high-purity aluminium polycrystals

Strain-rate-cycling experiments between 4.6 x 10(-3) s(-1) and 4.6 x 10(-4) s(-1) were performed with 99.996% aluminium polycrystals of mean grain-dia meter 0.25, 0.36, and 0.47 mm, which had been annealed at 500 degrees C and aged for six months at room temperature prior to deformation. Similar stud ies were also carried out with 500 degrees C annealed but unaged aluminium specimens of the same purity and grain size for comparison. It is observed that yield stress, ultimate tensile stress, fracture stress, and strain-rat e sensitivity Delta sigma of a given flow stress a in the range 30 to 70 MP a are not influenced by the grain size in both aged and unaged aluminium. H owever, natural ageing significantly enhances the yield stress as well as t he strain-rate sensitivity Delta sigma of a given flow stress sigma by 30% while ultimate tensile stress and fracture stress remain unaffected. Migrat ion of point defects, e.g. vacancies, residual gaseous and metallic impurit y atoms etc., to the cores of edge dislocations during ageing seems to be r esponsible for the increase in yield stress and strain-rate sensitivity of the flow stress. The analysis of the Delta sigma/sigma data in terms of Fel tham's single barrier model of plastic flow shows that the intrinsic height of the thermally activable energy barrier (3.4 eV) evaluated for aged alum inium is comparable with that (4.2 eV) for unaged aluminium, and dislocatio n-dislocation intersection is the rate process of plastic flow.