Novel transformation pathway and heterogeneous precipitate microstructure in Ti-alloys

Materials with certain heterogeneous microstructures have been shown to hold a synergistic combination of strength and ductility. In this study, we demonstrate novel transformation pathways for creating such heterogeneous microstructure in Ti-alloys by integrating thermodynamic databases with phase field simulations. The results show that the concentration modulations at different length scales produced by (a) precursory spinodal decomposition in the parent phase and (b) interdiffusion in multi-layers having different alloy compositions can generate effectively hierarchical and gradient alpha + beta two-phase microstructures, with a mixture of fine alpha precipitate regions and alpha precipitate-free-zones or coarse alpha precipitate regions. The novel microstructures produced include "inverted globular alpha" bi-modal microstructures and gradient microstructures with controlled spatial gradients in particle size and number density of alpha precipitates. This study may shed light on how to design novel hierarchical and gradient two-phase microstructures with tunable size and density of precipitates as well as the length scale of their spatial heterogeneity for desired properties. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.