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) alpha-titanium (Ti) materials with dissolved 0.94 mass% oxygen (0) to clarify their ductility improvement mechanism. The water quenching was applied to PM Ti extruded rod after isothermal annealing at 1173 similar to 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 alpha-Ti grains, and some martensite phases were also detected at the grain boundaries. They were formed from beta phases during phase transformation during WQ due to a lower oxygen content in pre-beta phases compared to pre-alpha phases. The pre-beta 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.