The mechanism for the self-accommodation microstructure of α variants during phase transformation of the Zr-2.5Nb alloy

Although variant selection during the phase transformation of zirconium (Zr) alloys has been studied extensively, studies on the formation mechanism of microstructural characteristics related to alpha variant selection remain limited. The formation mechanisms of the self-accommodation morphology and inter-variant boundary characteristics of alpha variants in homogenized Zr-2.5Nb alloy cooled by water quenching (WQ), furnace cooling (FC), and air cooling (AC) were systematically investigated from the perspective of local strain during phase transformation. The alpha variants exhibited triangular morphologies in both the WQ and AC samples, and a colony morphology in the FC sample. Further, there were five types of inter-variant boundaries: Type I <0 0 0 1>/10.53 degrees, Type II <1 1 <(2)over bar> 0>/60 degrees, Type III <<(1.377)over bar> (1) over bar 2.3770.359>/60.83 degrees, Type IV <<(10)over bar> 5 5 (3) over bar>/63.26 degrees, and Type V <1 <(2.38)over bar> 1.380>/90 degrees. The proportion of Type II is up to 98 % in the AC sample and 57.9 % in the WQ sample; the Type I was very low in all three samples; and a high proportion of the Type V was observed in the FC sample (23.6 %). The self-accommodation morphology of alpha variants is closely related to the equivalent strain (epsilon(VM)) during the variant selection. Theoretical calculations indicated that, for a specific 2-variant combinations, there were always one or more 3-variant combinations with a lower epsilon(VM) than the 2-variant combinations. A lower epsilon(VM) contributes to the presence of 3-variant combinations, which forms a triangle morphology. The formation of inter-variant boundaries is determined by the type and frequency of variants as well as the epsilon(VM) of the 2-variant combinations. The order of the mean values of epsilon(VM) for the five types of boundaries was Type II (0.0757), Type III (0.0859), Type IV (0.1012), Type V (0.1112), and Type I (0.1307). That is, Type II is the easiest and Type I is the most difficult, which resulted in a very high fraction of Type II and a very low fraction of Type I in the WQ, AC, and FC samples. The presence of a high fraction of Type V in the FC sample was related to the type and fraction of each variant. (c) 2025 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.