In-situ observation of microstructurally small fatigue crack initiation and growth behaviors of additively-manufactured alloy 718

To reveal the effects of microstructural features on fatigue property, microscopic fatigue crack initiation and growth behavior were successively observed for rolled plate and additively-manufactured nickel base 718 alloy. Three types of additive manufacturing (AM) methods: powder bed Electron Beam Melting (EBM), Selective Laser Melting (SLM), and wire-fed Electron Beam Additive Manufacturing (EBAM) were adopted for specimen preparation to create various microstructures for comparison. The results showed that the fatigue lives of the EBM and EBAM specimens, which have a robust (001) texture, were shorter than those of the rolled plate and SLM specimens. This fatigue life difference could not be explained by the defects, because the fatigue cracks in the part of EBAM specimen were initiated along a slip plane. On the other hand, acceleration of the early stage fatigue crack growth rate (FCGR) was found in the EBM and EBAM specimens. Cross-sectional electron back scatter diffraction (EBSD) observations also showed that early stage cracks initiated and grew along the {111} slip plane; but in the EBAM specimens, the cracks continuously propagated straight to an adjacent grain due to their (001) texture. It thus appears that the low-angle grain boundaries which tend to be generated as a result of the (001) texture do not act as barriers to the penetration of dislocations; instead, they accelerate the shear mode fatigue crack growth rate in the early stages and decrease fatigue life.