Microstructure and phase morphology during thermochemical processing of α2-based titanium aluminide castings

Changes in the microstructure, volume fraction and distribution of phases during different stages of thermochemical processing of Ti-25Al-10Nb-3V-1Mo (at.%) castings were investigated. Up to 14.5 at.% (0.35 wt.%) of hydrogen was introduced into the material by gas charging at temperatures between 650 and 980 °C for times up to 20 h. The material was subsequently dehydrogenated by vacuum annealing at 650 °C for 48 h. Investment cast Ti-25Al-10Nb-3V-1Mo alloy, hot isostatically pressed (HIP) at 1175 °C at 260 MPa for 6 h, was used as the starting material. The microstructure of the as-HIP material consists of α2, B2 and orthorhombic phases. The α2 phase exists in equiaxed, Widmanstäten and cellular morphologies. The B2 phase is observed mainly along α2α2 boundaries. Some α2 Widmanstäten plates also contain very fine orthorhombic phase in a plate-like morphology. Hydrogenation of the material modified the microstructure; however, the morphology of the α2 and B2 phases did not change. Furthermore, hydride precipitation and a higher volume fraction of the orthorhombic phase were observed compared with the as-HIP material. Following dehydrogenation, the hydrogen level in the material was found to be less than 0.1 at.% (0.0025 wt.%). Transmission electron microscopy of the dehydrogenated material did not reveal the presence of hydride precipitates; however, the high volume fraction of the orthorhombic phase was found to persist following dehydrogenation.