Martensite decomposition kinetics in additively manufactured Ti-6Al-4V alloy: In-situ characterisation and phase-field modelling

Additive manufacturing of Ti-6Al-4V alloy via laser powder-bed fusion leads to non-equilibrium α^′ martensitic microstructures, with high strength but poor ductility and toughness. These properties may be modified by heat treatments, whereby the α^′ phase decomposes into equilibrium α+β structures, while possibly conserving microstructural features and length scales of the α^′ lath structure. Here, we combine experimental and computational methods to explore the kinetics of martensite decomposition. Experiments rely on in-situ characterisation (electron microscopy and diffraction) during multi-step heat treatment from 400 ^∘C up to the alloy β-transus temperature (995 ^∘C). Computational simulations rely on an experimentally-informed computationally-efficient phase-field model. Experiments confirmed that as-built microstructures were fully composed of martensitic α^′ laths. During martensite decomposition, nucleation of the β phase occurs primarily along α^′ lath boundaries, with traces of β nucleation along crystalline defects. Phase-field results, using electron backscatter diffraction maps of as-built microstructures as initial conditions, are compared directly with in-situ characterisation data. Experiments and simulations confirmed that, while full decomposition into stable α+β phases may be complete at 650 ^∘C provided sufficient annealing time, visible morphological evolution of the microstructure was only observed for T≥700 ^∘C, without modification of the prior-β grain structure.