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 $\alpha'$ martensitic microstructures, with high strength but poor ductility and toughness. These properties may be modified by heat treatments, whereby the $\alpha'$ phase decomposes into equilibrium $\alpha+\beta$ structures, while possibly conserving microstructural features and length scales of the $\alpha'$ 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$^{\circ}$C up to the alloy $\beta$-transus temperature (995$^{\circ}$C). Computational simulations rely on an experimentally-informed computationally-efficient phase-field model. Experiments confirmed that as-built microstructures were fully composed of martensitic $\alpha'$ laths. During martensite decomposition, nucleation of the $\beta$ phase occurs primarily along $\alpha'$ lath boundaries, with traces of $\beta$ 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 $\alpha+\beta$ phases may be complete at 650$^{\circ}$C provided sufficient annealing time, visible morphological evolution of the microstructure was only observed for $T\geq\,$700$^{\circ}$C, without modification of the prior-$\beta$ grain structure.