Effect of Microstructural Characteristics on Fracture Toughness in Direct Energy Deposited Novel Ti-6Al-4V-1Mo Alloy

Meeting the damage tolerance requirements for engineering-grade titanium alloys pose a significant challenge in achieving high fracture toughness in direct energy deposition (DED) titanium alloys. This work primarily investigated the relationship between the microstructure and the fracture toughness of DED new Ti-6Al-4V-1Mo alloy. Two types of microstructures were designed via two process strategies: high-line energy density (HE) and low-line energy density (LE). Relative to LE samples, HE samples possess larger-sized microstructural characteristics (coarser grain boundary α (α_GB), larger α colonies, and coarser α laths). Less α/β phase boundaries were formed by coarser α laths in the HE samples, increasing the movement of dislocations, resulting in tensile strength decreasing from 1007.1 MPa (LE) to 930.8 MPa (HE) and elongation increasing from 10.8% (LE) to 15.7% (HE). Also, HE samples exhibited an excellent fracture toughness of 114.0 MPa m^1/2, significantly higher than that of LE samples (76.8 MPa m^1/2). An analysis of crack propagation paths was conducted to investigate the factors contributing to toughening. The primary factor enhancing toughness is the frequent obstruction of cracks by coarse α_GB and large α colonies in HE samples. Particularly, the pretty large-angle deflections induced by the superposition effect of coarse α_GB and large α colonies play a vital of significant role. These factors induced the long and tortuous high-energy pathways, which resulted in ultimately improved fracture toughness. The discovered microstructural toughening mechanisms can serve as a reference for future studies involving titanium alloys, offering insights on how to enhance fracture toughness by achieving similar characteristics.