EFFECTS OF ALLOYING ON SLIP INTERMITTENCY AND LOCALISATION IN α-Ti

Cold dwell fatigue exemplifies the importance of time-dependent, spatially localised slip in understanding the performance of α+β titanium alloys for jet engine applications. Typically present in the hcp α phase of such alloys are interstitial oxygen (at a few hundred wppm) and aluminium (at a few wt.%). Both are known to promote spatial localisation of slip within α grains, through strong oxygen-dislocation core interactions and due to crystallographic ordering of Al to form Ti3Al α2 domains. The effects of these solutes on the time intermittency of slip have not been well established, however, leaving an open question regarding their contributions to dwell fatigue crack initiation. Here, we study model alloys Ti-7Al-0.05O and Ti-7Al-0.25O (wt.%) in crystallographically disordered and ordered states to isolate the effects of interstitial O and ordering of Al. Using in situ high-energy X-ray diffraction microscopy, the time-dependent behaviours of different slip systems in hundreds of grains were tracked during creep loading. Post-deformation dislocation configurations were characterised to provide complementary spatial and temporal pictures of slip phenomena. Our results demonstrate that, despite superficially similar effects on spatial localisation of slip at the grain scale, oxygen and aluminium have notably dissimilar effects on time intermittency of slip in hcp titanium.