Ductility Improvement Mechanism of Pure Titanium with Excessive Oxygen Solid Solution via Rapid Cooling Process

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 metallurgy (PM) α-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 isothermal 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 β phases 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.