Solid-State Phase Transformation Kinetics and Mechanisms in Additively Manufactured Ti–6Al–4V Studied Using In-Situ High-Energy Synchrotron X-ray Diffraction

Additive manufacturing enables new processing domains due to its rapid cooling rates and variety of novel parameters, but their effects on the Ti–6Al–4V solid-state phase transformation and consequent mechanical properties are not well understood. Optimal processing requires accurate models of these thermomechanical processes that are parameterized by high-fidelity kinetic and mechanistic data. In this work, these data are acquired with in-situ synchrotron x-ray diffraction during the β → α/α′ transformation in controlled isothermal and continuous cooling experiments that provide time evolution of α phase fraction ( \({f}_{\alpha }\) $f_{\alpha }$ ) and β phase V concentration ( \({V}_{\beta }\) $V_{\beta }$ ). Equilibrium, time-temperature-transformation, and continuous cooling diagrams are constructed for both \({f}_{\alpha }\) $f_{\alpha }$ and \({V}_{\beta }\) $V_{\beta }$ . The in-situ determination of \({V}_{\beta }\) $V_{\beta }$ shows that completion of the structural transformation precedes V redistribution for isothermal temperatures ≤ 865 °C and all continuous cooling rates, indicating that diffusional processes alone cannot be responsible for the transformation. A diffusional–displacive mechanism is proposed by thermally activated nucleation of α lath subunits and their displacive growth, which is consistent with observations. These data both inform the selection of novel processing routes and alloy compositions for materials with improved mechanical and functional properties, as well as provide valuable calibration data for microstructure simulation.