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

Abstract

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.