The Role of Microstructure Inhomogeneity in Ti-6Al-4V Forging on Fracture Toughness Behavior

Microstructure development and its control in dual-phase titanium alloy (Ti-6Al-4V) are challenging as they involve a complex processing route. Any inhomogeneity in microstructure or texture results in variation in the mechanical properties. Such inhomogeneity develops during the deformation step of the processing route and may persist in the final forged product. In this study, a processing route for bimodal microstructure development has been proposed to understand microstructural inhomogeneity and its effect on mechanical properties. Electron backscatter diffraction analysis suggests the formation of local microtexture (or macrozone) during the lamellar breakdown. Following the hot deformation stage, recrystallization annealing treatments were performed for different soaking time periods. A homogenous bimodal microstructure is achieved at a higher annealing time of 4 h or more. The local texture weakening at a higher annealing time is due to recrystallization. Further, fracture toughness (KIc) tests were performed on bimodal microstructure with lamellar fractions 31 and 67%, respectively, to understand the influence of morphology on the crack resistance. It was found that the bimodal microstructure with a higher lamellar fraction has weaker texture intensity and enhances fracture toughness.