Identifying microstructural features that control fracture in additive materials

A unique fractographic technique, FRASTA, was applied to additively manufactured IN718 J-R fracture test specimens to determine the microstructural failure mechanism. The results clearly showed that the material failed by nucleation, growth, and coalescence of microfailures in the highly stressed process zone in advance of the macrocrack front. Microfailure initiation sites were pinpointed on the fracture surfaces, enabling those weak spots to be examined metallographically and with EDX to identify and characterize the responsible defect or microstructural feature. The microfailure activity in a specimen processed differently was distinctly different, showing the effects and importance of processing conditions and providing an explanation of mechanical property variability in additively manufactured materials. The improved understanding of the microstructural failure process in additive materials achievable with FRASTA can guide development of more effective processing protocols and, ultimately, provide a route for achieving additive materials with improved and consistent fracture properties. Moreover, the microfailure evolution data provide a basis for computational model development and suggest a procedure for determining fracture toughness from fracture surfaces.