Phase Evolution of Additively Manufactured Ni-base Alloy IN718 by Means of In-Situ Synchrotron X-ray Diffraction

Abstract

Additive Manufacturing (AM) of nickel-base superalloys has gained much attention as it enables the fabrication of complex structures. One AM-route is the powder-bed based Selective Laser Melting (SLM) process with a laser beam scanning over a thin powder layer and building a structure by selectively melting the powder according to a CAD-model. Due to high cooling rates (103-108 K/s) and depending on the processing parameters, microstructures and phases form which are distinct from those of conventionally cast materials. Subject of this study is the nickel-iron based superalloy Inconel 718 (IN718), which is used for high-temperature applications due to its excellent mechanical properties and its good corrosion resistance up to 650°C. IN718 consists of the solid-solution γ-matrix and is precipitation-strengthened through γ’’ and γ’. The γ’’-phase is metastable and transforms to the stable δ-phase after long-time exposure at high temperatures. Additionally, MC-Carbides and brittle Laves-phase can be present in the alloy. In this study phase analyses have been performed utilizing synchrotron X-ray diffraction (XRD) since it uses a high-energy monochromatic beam with a high coherency length, making it possible to detect small particles such as γ’’ and γ’ precipitates of about some 10 µm characteristic length [COZAR], which cannot be detected by conventional XRD. Further, the parallel beam allows for long working distances, which facilitates in-situ observations in a furnace. Phases in as-built SLM and solid solution treated cast material were first analyzed at room temperature. Further, the phase evolution of SLM material was monitored in-situ at different high temperatures (730°C, 800°C, 870°C and 900°C) for holding times between 30-85min. As a reference, the phase evolution at 800°C has been recorded for the cast material for 65min. In the cast sample γ-matrix, δ-phase and carbides were present at RT, and γ’’ starts to grow after 20min at 800°C. In contrast, in SLM-samples no δ-phase was found at RT, but Laves-phase is present, which dissolves at 870°C, while δ-phase starts to form at even lower temperatures. The SLM material displays texture effects, which are due to high cooling rates and thermal gradients. Additionally, intensity maxima associated with the SLM-matrix show an asymmetric broad peak shape, which might be explained by residual stresses and/or element segregations in the material. In conclusion, this study proves that synchrotron XRD enables in-situ phase analyses, providing an insight in the phase evolution of additively manufactured materials, which cannot be achieved using conventional laboratory methods.