The formation and tensile behaviour of alpha-case in fine-grained Ti-6Al-4V

The formation mechanisms of α-case in Ti-6Al-4V were examined, and the effects of α-case on the material under monotonic tensile loading at room temperature, elevated temperatures of 300°C - 600°C, and superplastic form- ing (SPF) temperature of 920°C were investigated. Regular arrays of cracks were found to occur in the α-case in most instances, which were analysed by light optical microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) to gain an understanding of the mechanisms which were operating.
The impact of various material textures on the fracture behaviour were considered, both to provide insight in to the differences in fracture morphol- ogy which were observed, and to widen the applicability of the research. Crystallographic orientation was found to be an important aspect of crack behaviour, impacting the extent of each phase of fracture.
Both oxygen and nitrogen were utilised to form the α-case, and the be- haviour of the material was found to differ between the two. This was particularly pronounced when they were introduced during SPF, where the flow stress in the material was found to dramatically alter with the presence of oxygen or nitrogen. The absorption rates of these elements were inves- tigated with thermal gravimetric analysis (TGA), where it was found that the interaction between them could be a factor in the absorption rate and therefore formation of α-case.
The lack of a robust definition for α-case has been discussed, and the need for an understanding of the term which reflects developments in the field has been emphasised. A simple test for the presence of “effective α-case” has been proposed, which would allow rapid determination of the minimum machining depth to remove the affected material.