Effect and Evolution of Oxide Film in the HDH-Ti Powder Surface on Densification Behavior During Sintering

In this work, we used hydride-dehydride Ti (HDH-Ti) powders (with the oxygen levels of 0.17 and 0.51 wt pct, respectively) combined with microstructural characterization and thermodynamic analysis to reveal the evolution of surface oxide film and associated densification mechanism during sintering. The results show that the oxide film in the powder surface could start to be dissolved above 500 °C during sintering. Besides, the dissolution behavior of the oxide film was investigated via thermodynamic and kinetic analysis. The high-oxygen Ti-0.51O powders exhibited a lower onset temperature of sintering necking, a higher β phase transus temperature and a lower densification rate within the β phase zone, compared with the low-oxygen Ti-0.17O powders. Fundamentally, this is attributed to the high-concentration gradients of vacancy defect and oxygen atom within the oxide film driven by the gradual oxide-film dissolution to promote the surface diffusion, subsequently leaving the solute oxygen atoms to hinder the following α -to- β phase transition. Due to the grain boundary pinning and diffusional activation energy increase, the higher oxygen atoms dissolved in the Ti matrix delay the bulk and grain boundary diffusion rates, and thus disfavors the final densification at the high-temperature sintering stage. This work affords opportunities to understand the densification mechanism of Ti powder sintering involved with the oxide film in the powder surface, and thus help to benefit the final properties of sintered parts.