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

The beta to alpha phase transformation of Ti alloys progresses in alpha displacive-diffusive mixed-mode. The associated transformation strain has important influence on the resultant microstructure. In this work, the microstructural features of alpha precipitates in a metastable Ti alloy, Ti-7333, were thoroughly investigated. Special attention was paid to the intragranular a for the advantage of a stress-free transformation environment. Results show that the constituents of each alpha precipitate is not single. Two kinds of nano-sized a domains exist. One is situated on the broad face of the major alpha precipitate (named interface alpha), and the other goes through the major a (termed penetrating alpha). The interface a is related with the matrix by the Burgers orientation relationship (BOR) and with the major a by a 60 /(1120)alpha rotation. The nucleation of such a particles is induced by the largest shear strain generated by the formation of the major alpha. They act as stress-associated sympathetic nuclei of the neighboring a precipitates and eventually contribute to the formation of the triangular alpha cluster. The penetrating a does not obey the BOR with the beta matrix but is related with the major a by a 60 degrees rotation around another (11 (2) over bar0), axis. The nucleation of such a is induced by the largest normal strain generated by the formation of the main a plate. This work provides comprehensive information on the displacive characters of the beta to alpha transformation and their impact on the microstructure of metastable beta Ti alloys. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.