A multi-scale microscopic study of phase transformations and concomitant α/β interface evolution in additively manufactured Ti-6Al-4V alloy

Abstract

Ti-6Al-4V alloy produced by laser-based powder bed fusion (LPBF) technique consists of an α′-martensite phase which is usually brittle and hence required, microstructure optimization by heat treatment to improve ductility. In this study, detailed microstructure analysis in the as-built condition and its evolution after sub-transus annealing was investigated. On annealing, the α′-martensite decomposes into α- and β-phases with the evolution of the α/β interface. Considering that the composition and microstructure play a crucial role in deciding the mechanical properties and service performance, a multi-scale correlative microscopy approach involving Transmission Electron Microscopy (TEM), Transmission Kikuchi Diffraction (TKD), and Atom Probe Tomography (APT) was employed. In the as-built condition, predominant segregation of β-stabilizers at the defect site especially along grain boundaries was observed; and these act as potential nucleation sites for β-phase during subsequent annealing. Low temperature annealing (750ᵒC) transforms the V-rich regions into the β-phase in the form of nanoscale precipitates. However, as the annealing temperature increases above 800ᵒC, the β-phase adopts a rod-like morphology. Comparing α-stabilizers, it is realized that β-stabilizers have a twofold greater contribution to elemental partition. Further, the composition and width of the α/β interface significantly impacts the mechanical properties of the alloy. Annealing at 750 °C and 850 °C resulted in significant improvement in ductility by 57.5% and 62.7% respectively without much of a trade-off in strength (>1 GPa). The strength and ductility are seen to decline when annealing temperature is raised above 850 °C due to a rise in compositional fluctuation and broadening of the α/β interface.