Phase field modeling of the Ti–6Al–4V solid-state transformation using synchrotron in-situ heat treatment calibration and validation

Abstract

Ti–6Al–4V is a titanium alloy commonly used for its balance of strength, ductility, and heat treatability resulting from its versatile β → α/α′ solid-state phase transformation. Redistribution of V and Al solute species during the transformation is crucial to determining the resulting microstructure and associated mechanical performance, but their concentration evolution in microstructure prediction models has not yet been validated experimentally. This study predicts the Ti–6Al–4V solid-state diffusional transformation using the phase field (PF) method and compares the α phase fraction (\({f}_{{\alpha }}\)), β phase V concentration (\({V}_{\beta }\)), and β phase Al concentration (\({\text{Al}}_{\beta }\)) with in-situ synchrotron X-ray diffraction measurements and thermodynamic calculations to perform model calibration and validation. Equilibrium \({f}_{{\alpha }}\), \({V}_{\beta }\), and \({\text{Al}}_{\beta }\) from isothermal simulations are between experimental and theoretical values, and continuous cooling simulations show increasing accuracy of \({f}_{{\alpha }}\) and \({V}_{\beta }\) predictions at lower cooling rates. It is observed that V diffuses into the β phase through α lath tips during isothermal transformation at temperatures ≤ 875 °C, as opposed to uniformly across the α/ β interface at higher temperatures, suggesting that the relative α lath growth and V diffusion rates influence the transformation behavior. Limitations of the model in accurately predicting the transient variation of \({f}_{{\alpha }}\) are related to the nucleation mechanism and model dimensionality, and recommendations are made for further refinements. The model is assessed to be successful for predicting V redistribution, and the study overall deepens insights into mechanisms of the β → α transformation, informs PF model calibration and development, and elucidates process optimization for Ti alloys.

Graphical abstract