The importance of fracture toughness evaluation for additively manufactured metals

Abstract

Yield strength and fracture toughness are key properties of structural materials used by designers for engineering applications, whereas secondary properties like ductility are not generally used in quantitative design calculations. However, since ductility is usually measured along with strength in the same tensile test, it is often used as an indication of the fracture toughness even though these properties are usually not directly proportional. As a result, many researchers initially, and only, screen new materials and processing routes for good combinations of strength and ductility rather than evaluating fracture toughness independently. This can result in poorly informed decisions being made early in the development stages for structural applications where materials with combinations of high strength and good fracture toughness are required. In this article, we evaluate the correlation between fracture toughness and ductility of Al-, Fe-, and Ti-based alloys that were processed using conventional methods as well as by additive manufacturing (AM) routes. We highlight that the correlation between fracture toughness and ductility is very weak, which in the case of AM materials is attributed to their often-metastable microstructures, their layer-by-layer mesostructures, and their anisotropic failure characteristics that lead to pronounced variability in the fracture toughness data for materials with similar ductility. Our findings suggest that an independent assessment of fracture toughness alongside the tensile properties (strength and ductility) is required to correctly optimize the AM processing of structural materials and provide engineers with the properties required for the design of parts and components for structural and safety-critical applications.