Evolution of microstructure and elastic modulus along a Ti–6Al–4V to Ti–35Nb–7Zr–5Ta compositionally graded alloy

A compositionally graded alloy, transitioning from Ti-6Al-4V to Ti-35Nb-7Zr-5Ta, with nine intermediate compositions, was fabricated using laser-directed energy deposition, as a candidate system for stiffness-matched biomedical implants. This study focuses on microstructural evolution across layers and interfaces, while reducing the effects of thermal history - an aspect largely unexplored in prior investigations on graded alloys. Microstructural characterization of the as-deposited condition revealed a gradual transition from HCP α + BCC β phases near the Ti-6Al-4V end to a fully β phase near the Ti-35Nb-7Zr-5Ta end, consistent with XRD analysis and CALPHAD predictions. Post-processing β-solutionization and quenching led to a partial dissolution of the cellular structure, and a phase evolution from martensitic α’ to α” and eventually to a fully β microstructure, across the gradient from Ti-6Al-4V to Ti-35Nb-7Zr-5Ta. EBSD analysis indicated an absence of strong crystallographic texture, with β grains elongated along the build direction. Elastic modulus measurements exhibited a decreasing trend from 0% to 80% TNZT, followed by an increase toward the 100% TNZT composition. TEM investigations revealed the presence of metastable ω and O’ phases in the 80% and 100% TNZT layers, which may be the potential reason for this non-uniform trend in modulus.