Diffusional-displacive transformation in a metastable β titanium alloy and its strengthening effect

Diffusional-displacive transformations are generally associated with mechanical properties of materials such as high strength. Understanding structure evolution of these transformations is of great interests from both physics and application perspectives. By combining atomic resolution electron microscopy, energy dispersive spectroscopy and first principles calculations, a continuous beta ->alpha ''->alpha transformation process has been quantitatively characterized at the atomic level in a metastable beta-Ti alloy during aging treatment. The transformation is revealed to develop by a novel mechanism involving continuous structural and compositional changes towards the equilibrium assisted by compositional fluctuation in the beta matrix. Moreover, the product phase induces a precipitate-matrix lattice mismatch, thus produces a coherency strain field surrounding the precipitates. The coherent strain field contributes significantly to the increasing hardness of the alloy after aging. These results have great potential for tailoring thermomechanical treatment routes and improving mechanical properties of materials. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.