Influence of Cr additive on the threshold stress for superplastic flow in Al-33%Cu alloy

The effect of Cr additive on the threshold stress in region I superplastic behavior of AI-Cu eutectic alloy is investigated using high-purity alloys w ith various Cr contents ranging from 0.002 to 0.26 mass%. The threshold str ess increased largely with increasing Cr content up to 0.05 mass% and exhib ited a tendency toward a possible saturation. It decreased with increasing temperature and grain size. The temperature dependence was similar to that of grain boundary segregation of impurity atoms. Transmission electron micr oscopy did not reveal the presence of fine precipitates at the grain and ph ase boundaries. Therefore, it was proposed that segregating Cr atoms are pi nning grain boundaries, inhibiting grain boundary migration that accompanie s grain boundary sliding and bringing about the threshold stress.