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 and
bas slip bursts associated with slip localisation. In contrast, the addition
of trace O interstitials is beneficial, reducing the magnitude of bas slip
bursts and increasing the homogeneity between basal and prismatic 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.Comment: Revised after revie