Probing nanoscale variant distribution in a heterogenous α/β titanium alloy

Abstract

In titanium alloys, engineering highly heterogeneous α-phase precipitate microstructure—comprising micron-scale primary α and nano-scale secondary α precipitates—offers a promising strategy to bypass the strength-ductility trade-off. Beyond classical microstructure descriptors (e.g., volume fraction, size, shape, orientation, coherency state, and spatial distribution), the variant distribution of precipitates crucially governs mechanical properties. However, the variant distribution behavior remains poorly understood for super-refined α precipitate ($<100\text{nm}$). For the first time, this work systematically investigates the size-dependent variant distribution of α precipitates in an α/β Ti alloy by integrating Transmission Kikuchi Diffraction (TKD) characterization and crystallographic analysis. Notably, inter-variant misorientation is quantified using misorientation axis analysis rather than conventional angular thresholds. A precipitate size-dependent variant distribution is identified, where the spatial correlation and occurrence frequency of α variants linked by the Type B misorientation axis-angle pair ${\left[\bar{1}2\bar{1}0\right]}_{\alpha }/60.0\text{°}$ strengthens as precipitate size decreases. These experimental observations are validated through phase-field simulations that generate α precipitates with varying sizes and number densities. Mechanistic analysis reveals that finer α precipitates, formed via an ω-assisted nucleation mechanism, amplify elastic interactions that promote strain-induced correlated nucleation, driving the stronger preference of Type B inter-variant correlations. The study provides crucial insights into the distribution and characteristics of the super-refined α phase in titanium alloys and may contribute to the understanding of deformation mechanism of the highly heterogeneous precipitate microstructure.