Influences of Environment in Heat Treatment and Pre-Aging at Room Temperature on Age-Hardening of Ti–Mo Alloys

The influences of environment in solute heat treatment and aging, and pre-aging at room temperature on the age-hardening behavior of Ti-14 and 20 mass% Mo alloys at 623 K were studied by Vickers hardness measurements. The solution heat treatments were performed in vacuums of 1.3 mPa and 8 Pa. The atmosphere, a pure argon current and a vacuum were used as the environments during aging. The periods of pre-aging were 1.6×10⁵ s for the short pre-aging and 1.0×10⁶–3.7×10⁷ s for the long one. The specimens which were solution heat treated in each vacuum and pre-aged for 1.6×105 s showed a two-stage hardening. The first stage seemed to be due to a product formed by the reaction of quenched-in vacancies, interstitial atoms and metallic atoms, and the second stage was due to the formation of aged omega phase as reported previously. The age-hardening behavior of the specimen pre-aged for a short period was more active than that of the specimen pre-aged for a long period in spite of the solution heat treatments in either vacuum. The role of quenched-in vacancies in the age-hardening behavior was very important in Ti-20 mass% Mo alloys in connection with the pre-aging at room temperature. The higher the oxygen content in the environments during aging, the greater became the hardness values of the surface layer and the interior of Ti-14 mass% Mo alloys.