Preferential formation of screw dislocations within the α phase after the grain/interface boundary emission during elevated temperature dwell of an aero-grade Ti-6Al-4V alloy

Abstract

The current work examines the dislocation emission arrays within the α phase of a Ti-6Al-4V alloy, subjected to short-duration dwell performed in tension at about 96 % of the yield stress at a temperature of 300 °C. The arrays formed through dislocation emission from the α/α grain boundaries and α/β interfaces were examined post-mortem using transmission electron microscopy. As expected, the above grain/interface boundaries proved to be the major dislocation sources during the applied dwell loading. There was a systematic tendency to preferentially generate screw dislocations either through progressive altering the dislocation lines after emission or via direct emission. In the former case, after achieving the screw character, the dislocations tended to cross slip out of the emission planes. Screw dislocations were directly emitted as sequentially formed, expanding dislocation loops exhibiting long straight screw-type segments parallel to the migration direction. The dislocation emission character appeared to be governed by the local conditions around the emission sites. The observed planes of dislocation emission arrays, together with the corresponding Burgers vectors, revealed the operation of prismatic as well as 1st and 2nd order pyramidal <a> glide, and the Schmid factor values were closely followed. Some emission planes were irrational, and dislocations seemed to be able to cross slip from these planes to the high Schmid factor glide planes. The current results provide direct evidence of the evolution of dislocation structures during dwell loading at elevated temperatures and will contribute to better understanding of the reported thermal alleviation of dwell fatigue in titanium alloys.