Influence of alternate grain boundary α on the development of Widmanstätten microstructure in TC21 Ti alloy

Abstract The high fracture toughness of a dual-phase Ti alloy is determined by its lamellar microstructure, such as the Widmanstatten α (αGBw) formed in prior β grains. Therefore, it is fundamental to understand the formation mechanism of αGBw colonies in the alloy. In this work, the influence of alternate grain boundary α (αGB) precipitates on the subsequent formation of αGBw on either side of β grain boundaries was examined in TC21 alloy. The crystallographic relationships among the αGB, adjacent β grains and αGBw colonies were analyzed systematically. It was revealed that the alternate αGB precipitates can maintain Burgers orientation relationship (BOR) with the same β grain. In such case, the microstructure on the Burgers side is determined by αGBw colonies with the same orientation of the parent αGB. However, there is no clear trend for the selection of αGBw variants on the non-Burgers side. When alternate αGB precipitates select different sides as Burgers grains, there are predominantly two αGBw orientations in the vicinity of grain boundaries. Moreover, the double BOR variants of αGB were preferred at some special grain boundaries. Then, the formation of αGBw on either side of grain boundaries was determined by the double BOR αGB rather than by the other general αGB variants. Based on the present results and some recent reports, the mechanism for the evolution of distinct microstructures in dual-phase Ti alloys was proposed.