We test a hypothesis to explain why Ti-6242 is susceptible to cold dwell fatigue (CDF), whereas Ti-6246 is not. The hypothesis is that, in Ti-6246, substitutional Mo-atoms in α-Ti grains trap vacancies, thereby limiting creep relaxation. In Ti-6242, this creep relaxation enhances the loading of grains unfavourably oriented for slip and they subsequently fracture. Using density functional theory to calculate formation and binding energies between Mo-atoms and vacancies, we find no support for the hypothesis. In the light of this result, and experimental observations of the microstructures in these alloys, we agree with the recent suggestion (Qiu 2014, 6075-6087. (doi:10.1007/s11661-014-2541-5)) that Ti-6246 has a much smaller susceptibility to CDF because it has a smaller grain size and a more homogeneous distribution of grain orientations. We propose that the reduction of the susceptibility to CDF of Ti-6242 at temperatures above about 200° is due to the activation of 〈+〉 slip in 'hard' grains, which reduces the loading of grain boundaries.

Using first principles methods we calculated the entire γ-surface of the first-order pyramidal planes in α-titanium. Slip on these planes involving dislocations with c + a-type Burgers vectors is one means by which α-titanium polycrystals may supplement slip on prism planes with a-type Burgers vectors to maintain ductility. We find one low energy and one high energy stacking fault with energies of 163 mJ/m 2 and 681 mJ/m 2 respectively. Contrary to previous suggestions [1, 2], we do not find a stable stable stacking fault at (c + a)/2.

Point defects, generalized stacking faults (gamma surfaces) and dislocations all play a fundamental role in the plasticity of titanium. We have begun a programme of work to simulate these defects using a variety of methods from empirical potentials to density functional theory. We have calculated the gamma surfaces on the basal, prism and first order pyramidal planes to identify possible stable stacking faults and dissociation reactions of �a� and �c + a� dislocations.

No abstract