Age-hardening behavior of a single-crystal Cu-ti alloy

The microstructure and hardness of a single-crystal Cu-4 mol% Ti alloy aged at 723 K are studied, and compared with those for conventional poly-crystal specimens. Only the metastable beta'-Cu4Ti precipitates are dispersed finely and grown continuously in grains without discontinuous transformation into the stable beta-Cu4Ti laminates as found in poly-crystal specimens, because of lack of grain boundaries. Hardness increases with aging and reaches a maximum at approximately 12 h, which is attributed to the fine dispersion of beta'-Cu4Ti. On further aging, hardness decreases more gradually in single-crystal specimens than in poly-crystal specimens. The gradual decrease in hardness for single-crystal specimens is due to the gradual growth of beta'-Cu4Ti. Conversely, the rapid decrease in the hardness of poly-crystal specimens is due to the discontinuous formation of coarse beta-Cu4Ti , that accompanies the consumption of fine beta'-Cu4Ti precipitates. (C) 2014 Elsevier B.V. All rights reserved.