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

## TL;DR

This study investigates how dwell holds influence dislocation behavior and crack initiation in near-alpha titanium alloy Ti6242Si during fatigue, highlighting the role of dislocation pile-ups and internal stress relaxation.

## Contribution

It provides new insights into dislocation mechanisms under dwell fatigue, emphasizing the effects of load holds on pile-up formation and crack nucleation in titanium alloys.

## Key findings

- Dislocation pile-ups cause internal stresses that facilitate crack nucleation.
- Dwell fatigue results in higher stress concentrations than LCF.
- Analytical models support the basal faceting hypothesis for crack initiation.

## Abstract

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.