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 β′-Cu4Ti precipitates are dispersed finely and grown continuously in grains without discontinuous transformation into the stable β-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 β′-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 β′-Cu4Ti. Conversely, the rapid decrease in the hardness of poly-crystal specimens is due to the discontinuous formation of coarse β-Cu4Ti, that accompanies the consumption of fine β′-Cu4Ti precipitates.