On the potential mechanisms of β to α′ + β decomposition in two phase titanium alloys during additive manufacturing: a combined transmission Kikuchi diffraction and 3D atom probe study

Abstract

This paper focuses on the mechanisms of phase transformations in additively manufactured Ti–6Al–4V during cooling. In particular, the goal is to understand if the imposed thermal cycles during fabrication results in the complete transformation of the α′ to the β phase or the sub-transus decomposition of α′ (martensite) to α + β. To this effect, samples fabricated using electron beam melting and laser-directed energy deposition techniques were analyzed using atom probe tomography (APT) in conjunction with correlative transmission Kikuchi diffraction (TKD). While the composition measurement using APT shows partitioning of vanadium into the β phase, the crystallographic analysis suggests evidence of a shear-induced transformation. Despite the pronounced differences in the processing conditions, both of the additive manufacturing techniques lead to similar partitioning of vanadium to the β phase. Calculations using THERMOCALC and DICTRA show that under the time and temperature regimes of additive manufacturing the microstructure could develop by the decomposition reaction of α’ → α+β.