Effect of Thermomechanical Processing on the Dwell Fatigue Behaviour of Near Alpha Titanium Alloy IMI 834

In this work, the dwell fatigue sensitivity of the alloy IMI 834 has been investigated as a function of microstructures obtained through three different thermomechanical processing routes. The different thermomechanical process routes led to three different product forms of IMI 834, namely, forged billet, hot rolled bar and forged cylinder. All the product forms had bimodal microstructure, however, with different microstructural features and microtextural characteristics. It was found that the Dwell Fatigue Debit (DFD), which is defined as the ratio of number of cycles to failure in fatigue to number of cycles to failure in dwell fatigue, was highest in the forged cylinder followed by forged billet and then the hot rolled bar. The DFD parameter was directly related to the volume fraction of primary α grains. Furthermore, EBSD characterization revealed that the spatial distribution of texture components was different, which could further account for the difference in DFD. The texture components are responsible for the lowest fatigue and dwell fatigue lives of the forged billet. On the other hand, the presence of these components as hard-soft grain interfaces in the forged cylinder is responsible for its highest dwell fatigue debit. This study emphasizes the importance of microtexture in the dwell fatigue behaviour of titanium alloys.