Mechanical Properties of β-Annealed Ti-5Al-5V-5Mo-3Cr: An Attempt to Define Critical Fracture Properties On the Basis of Microstructural Features

The yield and fracture stresses for Ti-5553 after β annealing were measured in tension and in compression. A linear dependence was found to apply between the yield and fracture stresses with β grain size, in accordance with the Hall-Petch relation. The mode of deformation and fracture was by localized planar slip, which leads to transgranular crack formation along the {011} slip planes. In addition to the yield and fracture stresses, the ductility was found to increase with decreased β grain size, which is due to the limiting effect of the β grain boundaries to the localized slip. TEM analysis of the microstructure revealed the presence of a nano-scale precipitate, possibly ω phase, suggesting that the slip localization process occurs as dislocations cut though the fine precipitates.

A plausible explanation for the initiation of transgranular cracks along the planes of localized slip is presented in terms of the formation and growth of microvoids. It is proposed that microvoids stem from dilatational effects which arise out of extreme core field stresses at dislocation pile ups.

Finally, unlike the behavior of many engineering alloys, the yield and fracture stresses were found to be more or less the same in tension and in compression. Such finding indicates that the Tresca yield criterion is an accurate predictor of yield behaviour. As the process from the onset of yield to fracture is through very limited slip, the Tresca fracture envelope in two dimensional stress space is determined to be just a slightly expanded yield envelope.