Anisotropy in cyclic behavior and fatigue crack growth of IN718 processed by laser powder bed fusion

This study aims at characterizing the role of strong anisotropy in fatigue crack growth for a superalloy processed by additive manufacturing. The choice has been made to investigate two specimens exhibiting significantly different crystallographic textures, representative of possible variations of microstructures within a real part. Both cyclic mechanical behavior and fatigue crack growth have been investigated at high temperature. As a consequence of the oriented microstructure, high levels of anisotropy of elastic and viscoplastic behaviors were observed. These behaviors have been modeled by the combination of orthotropic elasticity, a Hill's criterion and linear and non-linear kinematic hardening laws. To assess fatigue crack growth driving forces, in the context of complex crack shape, finite element analysis was proceeded taking care of 3D crack shape description. J was derived from G-theta, this method being convenient for anisotropic material. In most cases, fatigue cracks were found to grow preferentially at the vicinity of grain boundaries, regardless of the crystallographic orientation. An intrinsic fatigue crack growth rate related to this intergranular mechanism has been determined. On the other hand, mostly transgranular crack growth were observed when the loading was applied in the processing direction. In this case, the crystallographic texture appears to be the main factor governing the fatigue crack growth rate.