Dwell fatigue, the reduction in fatigue life experienced by titanium alloys
due to holds at stresses as low as 60% of yield, has been implicated in several
uncontained jet engine failures. Dislocation slip has long been observed to be
an intermittent, scale-bridging phenomenon, similar to that seen in earthquakes
but at the nanoscale, leading to the speculation that large stress bursts might
promote the initial opening of a crack. Here we observe such stress bursts at
the scale of individual grains in situ, using high energy X-ray diffraction
microscopy in Ti-7Al-O alloys. This shows that the detrimental effect of
precipitation of ordered Ti_3Al is to increase the magnitude of rare pri<a> and
bas<a> slip bursts associated with slip localisation. In contrast, the addition
of trace O interstitials is beneficial, reducing the magnitude of bas<a> slip
bursts and increasing the homogeneity between basal and prismatic <a> slip.
This is further evidence that the formation of long paths for easy basal plane
slip localisation should be avoided when engineering titanium alloys against
dwell fatigue.