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Elemental segregation is a ubiquitous phenomenon in additive-manufactured (AM) parts due to solute rejection and redistribution during the rapid solidification process. Using electron microscopy, in situ synchrotron X-ray scattering and diffraction, and thermodynamic modeling, we reveal that in an AM nickel-based superalloy, Inconel 625, unwanted δ-phase precipitates grow on a much shorter time scale than in wrought alloys (hours versus hundreds of hours). The root cause for this behavior is localized elemental segregation that results in local compositions outside the bounds of the allowable range set for wrought alloys. Our in situ small angle scattering experiments reveal that platelet-shaped δ phase precipitates grow continuously and preferentially along their lateral dimensions during stress-relief heat treatment, while the thickness dimension reaches a plateau very quickly. No nucleation barrier was observed in the in situ XRD experiments. An activation energy for the growth of δ phase was found to be (131.04 ± 0.69) kJ mol-1. We further demonstrate that a subsequent homogenization heat treatment can be effective both in homogenizing the AM alloy and in removing the deleterious δ phase. The methodology established here can be extended to elucidate the phase evolution during heat treatments in a broad range of AM materials.
additive manufacturing, metal, kinetics, phase evolution, precipitates, synchrotron, X-ray scattering, X-ray diffraction, in situ