Effect of stress reduction ratio on the creep behaviour of Al

Abstract

Stress-reduction and uninterrupted experiments have been conducted on Al in the temperature range 573–923 K; in performing stress-reduction experiments, stress-reduction ratios (τ₁-τ₂)/τ₁, (where τ₁ is the initial shear stress and τ₂ is reduced shear stress) ranging from 0.05 to 0.75 were used. The stress exponent inferred from stress-reduction experiments, n∗, was compared with that obtained from uninterrupted tests, n. On the basis of this comparison three regions of behaviour, depending on temperature, were identified. At high temperatures (T≥673K), n∗ = n regardless of the value of the stress-reduction ratio. At intermediate temperatures (600K<T<673K), n∗ = n for large reduction ratios (≤0.2), whereas n∗>n for small reduction ratios (≤0.15); for small reductions, the lower the reduction ratio, the larger the discrepancy between n∗ and n. At low temperatures (T<600K), n∗ > n for all stress-reduction ratios used (whether small or large). It is suggested that the discrepancy between n∗ and n at intermediate temperatures (small reductions) and low temperatures may arise from the segregation of impurity atoms at sub-boundaries and their interference with sub-boundary migration after a stress reduction. Experimental support for this suggestion is presented.