Ductility improvement mechanism of pure titanium with excessive oxygen solid solution via rapid cooling process

Abstract

Oxygen solid solution has been well known to have a high hardening effect on Ti and its alloys, while it also imposes a serious embrittlement behavior. This study investigated the effect of a rapid cooling process by water quenching(WQ) after heat treatment on the microstructures and mechanical properties of powder metallurgyα PM-titanium Ti materials with dissolved 0.94 mass-oxygen-O-to clarify their ductility improvement mechanism. The water quenching was applied to PM Ti extruded rod after iso-thermal annealing at 1173∼1373 K. Tensile test results at the ambient temperature indicated that the elongation to failure of the quenched ones at 1223 K and 1273 K was 12.5% and 19.1%, respectively. They are significantly higher than that of as-extruded Ti specimen-6.0%. The Ti-0.94 mass% O materials via the above WQ treatment mainly consisted of equiaxed α-Ti grains, and some martensite phases were also detected at the grain boundaries. They were formed from βphases during phase transformation during WQ due to a lower oxygen content in pre-β phases compared to pre-α phases. The pre-β phase also had a high concentration of dislocation after tensile test. The significantly increased elongation was due to a lower oxygen content phase formation assuming an important role to improve the plastic deformation ability.