Composite structure of α phase in metastable β Ti alloys induced by lattice strain during β to α phase transformation

The β to α phase transformation of Ti alloys progresses in a displacive-diffusive mixed-mode. The associated transformation strain has important influence on the resultant microstructure. In this work, the microstructural features of α precipitates in a metastable β Ti alloy, Ti-7333, were thoroughly investigated. Special attention was paid to the intragranular α for the advantage of a stress-free transformation environment. Results show that the constituents of each α precipitate is not single. Two kinds of nano-sized α domains exist. One is situated on the broad face of the major α precipitate (named interface α), and the other goes through the major α (termed penetrating α). The interface α is related with the β matrix by the Burgers orientation relationship (BOR) and with the major α by a 60°/〈112¯0〉_α rotation. The nucleation of such α particles is induced by the largest shear strain generated by the formation of the major α. They act as stress-associated sympathetic nuclei of the neighboring α precipitates and eventually contribute to the formation of the triangular α cluster. The penetrating α does not obey the BOR with the β matrix but is related with the major α by a 60° rotation around another 〈112¯0〉_α axis. The nucleation of such α is induced by the largest normal strain generated by the formation of the main α plate. This work provides comprehensive information on the displacive characters of the β to α transformation and their impact on the microstructure of metastable β Ti alloys.