Mechanism of impact toughness enhancement obtained by globularization of αGB phase for selective laser melted Ti–6Al–4V alloy

Abstract

The low impact toughness of selective laser melted (SLM) Ti–6Al–4V (Ti64) is a main factor limiting its widespread application in engineering. In this work, we've greatly improved its toughness by modifying the morphology of the α phase via annealing. Specifically, the as-built sample had a toughness of 14.87J/cm2. Annealing at 800 °C for 4 h boosted the toughness to 25J/cm2. Notably, annealing at 950 °C for 4 h increased the toughness by a remarkable 90 %, reaching an impressive 36.88J/cm2, which is comparable to its wrought counterpart. Particularly, 950 °C annealing induces the globularization of αGB phase (α phase along prior β grain boundary) while maintains the lamellar morphology of the α phase within the prior β grain. This mixed structure significantly outperforms the full lamellar structure in the 800 °C annealed sample in terms of toughness. The mechanism of toughness enhancement differs from the traditional one where thick lamellar α phase improves the toughness by increasing the crack path's tortuosity. Our study indicates that more deformation twins are activated in 950°C-annealed samples in impact test, which effectively absorb additional impact energy and improve toughness. In the 800°C-annealed sample, only a limited number of grains with specific orientations can activate deformation twins. Whereas in the 950°C-annealed sample, a broader range of grain orientations demonstrated the capability to activate deformation twins. The mechanism for twin activation in the 950°C-annealed sample is proposed. It is associated with equiaxed αGB, inducing significant interface deformation accompanied by stress concentration. This ultimately leads to the formation of the deformation twins.