  The dislocation structures appearing in highly mis-oriented soft/hard grain
pairs in near-alpha titanium alloy Ti6242Si were examined with and without the
application of load holds (dwell) during fatigue. Dislocation pile-up in a soft
grain resulted in internal stresses in an adjacent hard grain which could be
relaxed by dislocation multiplication at localized Frank-Read sources, a
process assisted by the provision of a relaxation time during a load hold. The
rate of this process is suggested to be controlled by <c+a> pyramidal
cross-slip and <a> basal junction formation. A high density of <a> prism
pile-ups is observed by dual slip, together with the nucleation of edge
dislocations in the soft grain of a highly mis-oriented grain pair, increasing
the possibility of cracking. The stress concentration developed by such
pile-ups is found to be higher in dwell fatigue (single-ended pile-ups) than in
LCF (double ended). Analytical modelling shows that the maximum normal stress
produced on the hard grain in dwell fatigue by this pile-up would be
near-basal, ~2.5 degree to (0002). This provides support for the dominant
hypothesis for the rationalisation of dwell fatigue crack nucleation in Ti
alloys, which derives from the Stroh pile-up model, and elaboration of the
underlying dislocation phenomena that result from load shedding and lead to
basal faceting.
