Precipitation Behaviors in Ti–2.3 Wt Pct Cu Alloy During Isothermal and Two-Step Aging

Time evolution of precipitates related to age-hardening in Ti–2.3 wt pct Cu alloys was investigated by electron microscopy. In isothermal aging at 723 K, the hardness increases continuously owing to precipitation strengthening, whereas in two-step aging where the aging temperature is switched from 673 K to 873 K after 100 hours, the hardness is found to drastically drop after the aging temperature switches. In isothermal aging, metastable and stable precipitates are independently nucleated, whereas characteristic V-shaped clusters of precipitates are observed during the two-step aging. It is revealed by atomic-scale observations that the V-shaped clusters are composed of metastable and stable precipitates and each type of precipitate has a different orientation relationship with the α phase: $$(10\bar3)$$ //(0001)α and $$[0\bar10]$$ // $$[11\bar20]_\alpha$$ for the metastable, and (201)// $$(1\bar103)_\alpha$$ and $$[0\bar10]$$ // $$[11\bar20]_\alpha$$ for the stable precipitates, respectively. The drop in hardness during two-step aging can be explained by a synergistic effect of decreased precipitation strengthening and solid solution strengthening.