Growth Behavior of β-Phase Grain and Influence of Its Grain Size on Hardness in Ti-Mo Alloys

The effects of molybdenum (Mo) on the growth behavior of beta-phase grain and hardness of a series of beta forged Ti-Mo alloys under different solution treatment temperatures and solution treatment times were studied with an emphasis on the relationship between beta-grain size and hardness. The average beta-grain sizes under different solution treatment conditions were measured and analyzed. The results show that the beta-grain size is not only relative to solution treatment temperatures and times but also relative to Mo contents from statistical results. The beta-grain size does not always increase with increasing of solution treatment temperature, a limit value of the grain size will appear in certain temperature region and certain Mo contents. When the Ti-4Mo alloy is solution treated at the range of 900 similar to 1050 degrees C, the activation energies for grain growth was 83.301 kJ/mol; and When the Ti-20Mo alloy is solution treated at the range of 750 similar to 900 degrees C, the activation energies for grain growth was 272.16 kJ/mol. The growth behavior of beta-phase grain is discussed in terms of grain growth thermodynamics and kinetics laws. Hardness results show that grain size has noticeable influence on hardness and hardness increases with decreasing of grain size.