Titanium alloys are used in the aerospace industry due to their high strength, low density, and corrosion resistance. Despite all of these virtues their use is hampered by a failure mechanism called cold dwell fatigue (CDF), which can reduce the lifetime of susceptible alloys by a factor of 10.

The plastic deformation leading to CDF failure is caused by creep at stresses as low as 60% of the yield stress and at temperatures as low 15% of the melting point of Ti. This 'cold creep' is the result of the planar glide of dislocations in the hexagonal close-packed or alpha-phase of Ti. Slip is the primary deformation mechanism for Ti and its alloys, so there is an inherent interest in the slip behaviour of alpha-Ti.

Motivated by this I computed the entire first-order pyramidal gamma-surface for alpha-Ti using density functional theory (DFT) simulations. I found one low energy and one high energy stacking fault with energies of 163 and 681 mJ/m2, respectively. In contrast with previous suggestions, I found no stable stable stacking fault at (c + a)/2. I computed this surface using two different pseudopotentials with 4 and 12 valence states, showing that the 4 state pseudopotential adequately reproduces the results of the 12 state pseudopotential.

At the time of writing the mechanism controlling cold creep, and hence CDF is unknown. One observation for helping to understand CDF is that the Ti alloy Ti-6Al-2Sn-4Zr-2Mo is highly susceptible to CDF failure but Ti-6Al-2Sn-4Zr-6Mo is not. The main difference between these alloys is 4 weight percent Mo. Motivated by this observation and using DFT simulations I investigated a hypothesis that Mo traps vacancies in alpha grains thus reducing the creep relaxation that exacerbates the conditions leading to CDF failure. Based on my results I found no support for this hypothesis.