Effect of Natural Ageing on the Strain-Rate Sensitivity of Flow Stress in High-Purity Aluminium Polycrystals

Abstract

Strain-rate-cycling experiments between 4.6 × 10 -3 s -1 and 4.6 × 10 -4 s -1 were performed with 99.996% aluminium polycrystals of mean grain-diameter 0.25, 0.36, and 0.47 mm, which had been annealed at 500°C and aged for six months at room temperature prior to deformation. Similar studies were also carried out with 500 °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-rate sensitivity Δσ of a given flow stress σ in the range 30 to 70 MPa are not influenced by the grain size in both aged and unaged aluminium. However, natural ageing significantly enhances the yield stress as well as the strain-rate sensitivity Δσ of a given flow stress σ by 30% while ultimate tensile stress and fracture stress remain unaffected. Migration of point defects, e.g. vacancies, residual gaseous and metallic impurity atorns etc., to the cores of edge dislocations during ageing seems to be responsible for the increase in yield stress and strain-rate sensitivity of the flow stress. The analysis of the Δσ/σ data in terms of Feltham's single barrier model of plastic flow shows that the intrinsic height of the thermally activable energy barrier (3.4 eV) evaluated for aged aluminium is comparable with that (4.2 eV) for unaged aluminium, and dislocation-dislocation intersection is the rate process of plastic flow.