Variant selection of α precipitates formed at β triple junctions in titanium alloy

Abstract

Understanding the orientation of the product phase during β to α phase transformation, i.e., the transformation texture is important for optimizing the microstructure and mechanical properties of titanium alloys. As the α phase prefers to nucleate from the triple junction of β grain boundaries (GBs), it is essential to understand the orientation preference, i.e., the variant selection of the α formed at the triple junction (denoted as the αTJ). Recent work indicated that in theory the αTJ can keep strict Burgers orientation relationship (BOR) with all the adjacent β grains for some special types of triple junctions. However, the experimental studies are very limited so far. In this work, a number of α precipitates formed at the triple junctions were analyzed. It was shown that at the triple junctions close to the special types, the observed αTJ obeyed the triple BOR criterion, i.e., tended to keep the BOR with all the adjacent β grains. However, at the triple junctions greatly deviated from the special types, the selection of the αTJ variants obeyed the double BOR criterion of one GB. Moreover, it was revealed that the special low angle GB, at which a double BOR α can in theory be possible, had no advantage for the selection of αTJ variants compared to the general high angle GBs forming the triple junction. This work sheds light on understanding the general rule governing the transformation texture in Ti alloys.