The role of dwell hold on the dislocation mechanisms of fatigue in a near alpha titanium alloy

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 localised 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 was observed with dual slip on two prism planes, together with edge dislocations on the third prism plane in the soft grain of a highly mis-oriented grain pair, increasing the pile-up stress. 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° 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.