Understanding the role of microstructure on creep anisotropy in additively manufactured IN718

Abstract

This study investigates the predominant influence of microstructural features resulting from the additive manufacturing process and subsequent heat treatment on the creep rupture behaviour of laser powder bed fusion additively manufactured Inconel 718 (LPBF-AM-IN718) with respect to different build orientations. Two build orientations were produced through an LPBF process followed by stress relief annealing at 980 °C (15 min). Subsequently, the heat treatment (HT) cycle involving solution treatment at 1080 °C followed by conventional double ageing treatment was employed. Detailed microstructural characterization was performed both along and normal to the build direction using SEM, EBSD, TEM and STEM-EDS, revealing the presence of precipitate-free zones adjacent to the grain boundaries. Creep characterization of the two differently oriented specimens in the temperature range of 625–675 °C and at 500–750 MPa indicate a superior creep resistance of vertical build (loading along the build direction) specimens compared to horizontal build (loading normal to the build direction) specimens. The correlation of heat-treated microstructures, creep deformation behaviour and comprehensive post-deformation microstructural characterization illustrates that damage accumulation occurs via grain boundary cavitation predominantly along the transverse grain boundaries and is rationalized to be the dominant softening mechanism in HB specimens. In contrast, the triple point cavitation leading to damage initiation and subsequent propagation was recognized as the damage mechanism in VB specimens. This study emphasizes that creep anisotropy in AM IN718 alloys is not solely due to a single microstructural feature; instead, it results from the complex interplay among various factors, including grain morphology, crystallographic texture and microsegregation.