Micromechanical behavior and thermal stability of a dual-phase α plus α′ titanium alloy produced by additive manufacturing

In order to improve the tensile properties of additively manufactured Ti-6Al-4V parts, specific heat treatments have been developed. Previous work demonstrated that a sub-transus thermal treatment at 920 degrees C followed by water quenching generates a dual-phase alpha+alpha' microstructure with a high work-hardening capacity inducing a desirable increase in both strength and ductility. The present study investigates the micromechanical behavior of this alpha+alpha' material as well as the thermal stability of the metastable alpha' martensite. To that end, annealing of the alpha+alpha' microstructure is performed and the resulting microstructural evolution is analyzed, along with its impact on the tensile properties. A deeper understanding of the micromechanics of the multiphase microstructure both before and after annealing is achieved by performing in-situ tensile testing within a SEM, together with digital image correlation for full-field local strain measurements. This approach allows the strain partitioning to be quantified at a microscale and highlights a significant mechanical contrast between the two phases. In the alpha+alpha' microstructure, the alpha' phase is softer than the a phase, which is confirmed by nanoindentation measurements. Partial decomposition of the martensite during annealing induces a substantial hardening of the alpha' phase, which is attributed to fine-scale precipitation and solution strengthening. A scale transition model based on the iso-work assumption and describing the macroscopic tensile behavior of the material depending on the individual mechanical behavior of each phase is also proposed. This model enables to provide insights into the underlying deformation and work-hardening mechanisms. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.