Precipitation Behavior During Aging in α Phase Titanium Supersaturated with Cu

Age hardening of Ti-2.3 mass pct Cu (Ti-2.3Cu) at 673 K to 873 K (400 °C to 600 °C) after solution treatment at 1063 K (790 °C) was observed. The relationship between precipitates formed during aging and changes in hardness was investigated. During aging at 673 K (400 °C), the hardness increased rapidly up to 200 hours, and subsequently increased more slowly up to 1000 hours. At 873 K (600 °C), the hardness began to decrease immediately. Transmission electron microscopy showed that fine disk-shaped precipitates of 20 to 40 nm in diameter grew in the α phase. It is concluded that these precipitates interacted with dislocations and increased the hardness. At 873 K (600 °C), precipitates of 1 µm in length and Ti₂Cu particles of 200 nm in length were observed. The decrease in hardness may have resulted from the precipitate formation decreasing the concentration of Cu in the α phase. Bright/dark contrast of the three atomic layers and small atomic shift of the hcp structure were observed in the atomic resolution imaging of the precipitates. This suggests that the precipitates are not just Cu-enriched zones and have structures with similar periodicity to the Ti₂Cu phase, which is thermally stable at those aging temperatures.