Microstructure evolution and mechanical properties of near-α Ti-8Al-1Mo-1V alloy at different solution temperatures and cooling rates

Nine solution-ageing treatments with different solution temperatures (915 °C, 960 °C, 1010 °C) and cooling media (water quenching, oil cooling, air cooling) were firstly conducted on the rolled near-α Ti-8Al-1Mo-1V alloy. The microstructure evolution behavior and various mechanical properties of this alloy were studied. The results show that solution temperature can greatly influence microstructure features. By increasing solution temperature, the volume fraction of primary α phase continuously decreased, the original shapeless primary α phase was sectioned and gradually showed equiaxed shape, and the β grain kept growing until its grain boundary reached the biggest sectional area of primary α particles (pinning effect). Due to the proper mixture of primary α and lamellar α phase, Ti-8Al-1Mo-1V alloy can achieve the best combination of tensile properties, thermal exposure properties and creep properties after being solution-treated at 1010 °C followed by oil cooling. Based on above results, high solution temperature is necessary to the application of Ti-8Al-1Mo-1V alloy. Thus a comprehensive study on the microstructure evolution behavior of Ti-8Al-1Mo-1V alloy at various precisely controlled cooling rates on cooling down from 1010 °C is of great significance. The results show that: with decreasing cooling rate on cooling down from 1010 °C, the volume fraction and the diameter of equiaxed α as well as the thicknesses of the grain boundary α and the lamellar α continuously increased. Equiaxed α shows an evolution process of precipitation, growth, mergence of adjacent equiaxed α and mergence between equiaxed α and lamellar α. Obvious changes of equiaxed α only happened at the cooling rates below 15 °C/s. However, for grain boundary α and lamellar α, the mutant range is “below 40 °C/s”. The exponential decay formula perform well in quantitatively characterizing the microstructure features-cooling rate relations.