Acceleration or Suppression of α-Phase Precipitation Using Isothermal ω Phase in Ti-20 at.pct Nb Alloy

Homogeneous precipitation of a fine α phase in the β matrix of Ti alloys is a promising method for obtaining a highly strengthened Ti-based alloy. Isothermal ω particles are known to be the nucleation sites for fine α-phase precipitation, but an understanding of the kinetics of α-phase formation on isothermal ω particles is still lacking. This study aimed to reveal the effect of isothermal ω particles on α-phase precipitation onset time. Two-step isothermal aging of a Ti-20 at.pct Nb alloy after solid solution treatment at 1273 K (1000 °C) was carried out. The first step of the aging at 633 K (360 °C) involved the formation of isothermal ω particles in the β matrix. This was followed by a second aging step at 673 K, 723 K, and 773 K (400 °C, 450 °C, and 500 °C) for α-phase precipitation. Suppression of α-phase nucleation on the isothermal ω particles occurred at 673 K (400 °C), whereas acceleration of α-phase nucleation on the isothermal ω particles was observed at 723 K and 773 K (450 °C and 500 °C). Thermodynamic stability of the isothermal ω particles and solute partitioning were controlling factors for the α-phase precipitation kinetics.