New insights into the phase transformations to isothermal ω and ω-assisted α in near β-Ti alloys

For multicomponent near-beta alloys, we have investigated the mechanisms responsible for the beta-to-omega and omega-to-alpha phase transformations upon isothermal ageing at 573 K. Experimental evidence from atom probe tomography and aberration-corrected high-resolution transmission electron microscopy indicates that the formation of isothermal omega involves a structural reconstruction assisted by nanoscale spinodal decomposition of the beta matrix, prior to the specific chemistry change required to form omega, rather than a mixed-mode process with structure and chemistry changes occurring simultaneously as has been previously suggested. First, incommensurate embryonic omega evolve via a displacive mechanism within Molean regions created by second-order coherent spinodal decomposition of the beta matrix. The subtle spinodal decomposition in beta and chemistry of embryonic omega are carefully analysed by an advanced atom probe data analysis algorithm. When the size of embryonic omega exceeds a critical value, commensurate isothermal omega forms through the exit of the other alloying solutes. O-rich regions present at the isothermal omega/beta interface provide potent sites for the formation of alpha. The concurrent compositional partitioning of solutes in omega and alpha indicates the transformation from omega to alpha involves both a rapid lattice reconstruction at the omega/alpha interface and a slow Al diffusion at the alpha/beta, therefore a mixed-mode displacive-diffusive process. This study provides novel experimental evidence to understand the much-disputed transformation processes and elucidate the mechanisms responsible for these important phase transformations. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.