Microstructural strengthening and toughening mechanisms in Fe-containing Ti-6Al-4V: A comparison between homogenization and aging treated states

Abstract

Microalloying of Ti–6Al–4V alloy by Fe addition has attracted interest as a promising way to improve castability and comprehensive mechanical performance. The mission of this work is twofold by employing the experimental examination and the phenomenological analysis, (1) to investigate the effect of Fe addition on the microstructure features and mechanical properties of the Fe-containing Ti–6Al–4V (TC4-xF) alloys subjected to casting and homogenization treatment, and (2) to unveil the critical microstructure features in homogenization, hot-worked and aging treated alloys, respectively, that benefit the yield strength and the fracture toughness. Experimental observations evidence that the addition of 0.5 wt.% Fe is most effective in enhancing the tensile ductility and the mode I fracture toughness. Further Fe addition up to 0.7–0.9 wt.% results in plateau values of yield and ultimate strengths with some fluctuations. Phenomenological analyses screen out the microstructural strengthening and toughening determinants which exhibit distinct sensitivities on Fe content under different processing conditions. The solid solution strengthening is confirmed as the primary effect that governs the yield strength of the homogenization treated TC4-xF alloys, followed by the refined size of colony and α lamella, so does it for the hot-worked and the aging-treated alloys. The strengthening effect of Fe could be further promoted by hot-working but impaired by a prolonged annealing time or a lowered cooling rate. The type of crack propagation path and the α morphology are discerned to play their own leading roles in different cases to influence the performance of fracture toughness. A long crack propagation distance that traverses broad α/β lamellae embraces a high crack propagation resistance and gives rise to enhanced fracture toughness. The experimental results enrich the dataset of microstructure features and mechanical properties of Ti–6Al–4V relevant alloys. While upon the phenomenological analysis, the discovered microstructural strengthening and toughening factors provide deeper mechanism insights into the mechanical behaviors of Fe-modified Ti-6Al-4 V alloys and are of the technical importance to future machine-learning of microstructure-property relationship.