Creep and ductility in an Al-Cu solid-solution alloy

Abstract

High-temperature creep was investigated in an Al-3 wt pct Cu alloy at temperatures in the range of 773 to 853 K and at a normalized shear stress range extending from 10^-5 to 7 × 10^-4. The results show the presence of three distinct regions. In region I (low stresses), the stress exponent is 4.5 and the activation energy is 155 kJ/mole. In region II (intermediate stresses), the stress exponent is 3.2 and the activation energy is 151 kJ/mole. In region III (high stresses), the stress exponent is 4.5 and the activation energy is 205 kJ/mole. Creep curves obtained in the three regions exhibit a normal primary stage, but the extent of the stage is less pronounced in region II than in regions I and III. The creep characteristics in regions I and II, along with the values of the transition stresses between the two regions, are in conformity with the prediction of the deformation criterion for solid-solution alloys. While the advent of region III (high stresses) correlates well with dislocation breakaway from a solute-atom atmosphere, the creep characteristics in this region are not entirely consistent with any of the existing high-stress creep mechanisms. The plot of elongation to fracturevs initial strain rate at 853 K exhibits two peaks at strain rates of 1 × 10^-4 and 6 × 10^-4 s^-1. The first peak (1 × 10^-4 s^-1) is attributed to the variation of the stress exponent for creep in the alloy with strain rate, and the second peak (6 × 10^-4 s^-1) appears to reflect the effect of solute drag on dislocation velocity.