Formation and 3D morphology of interconnected α microstructures in additively manufactured Ti-6Al-4V

Abstract

Three-dimensional characterization methods, such as 3D electron backscatter diffraction (3D-EBSD), have been used to reveal phase transformation and microstructural evolution mechanisms in multi-phase materials such as steel or titanium alloys. While 3D techniques have enabled many findings in steels, fine dual phase microstructures in titanium alloys such as the basketweave structure have been challenging to resolve. Now, advances in 3D-EBSD methods using sectioning with a plasma focused ion beam have allowed in-depth analyses of fine α microstructures. We apply 3D-EBSD to investigate the microstructures formed in Ti-6Al-4V by electron powder bed fusion (E-PBF) using different scanning strategies. Basketweave, acicular, and colony microstructures are produced from linear, Dehoff, and random scanning strategies, respectively. Different types of 3D interconnectivity were revealed in each microstructure including within clusters of platelets in the basketweave microstructure, within a grain boundary allotriomorph in the acicular microstructure, and between platelets in colonies. These observations are discussed in terms of the formation mechanisms of interconnectivity, including sympathetic nucleation, impingement, and morphological instability. Morphological instability was found to potentially play a role in both the basketweave and colony structures while the interconnectivity in the acicular structure likely forms via sympathetic nucleation or impingement. This information allows for a more complete description of the phase evolution of Ti-6Al-4V during thermal cycling in E-PBF than previously available and represents new insights into the complex branching reported in different titanium microstructures.