Introduction

    Inconel®625 (IN625) is a nickel-based solid-solution superalloy with a Ni-Cr matrix strengthened by Nb/Mo solutes [1]. IN625 features high strength, high fracture toughness, and good corrosion resistance and finds many applications in marine and energy industries, for example, turbine engine components, fuel and exhaust systems, and chemical processing components. IN625 also has excellent weldability and resistance to hot cracking. These characteristics make IN625 a primary alloy in the recent advancement of various additive manufacturing (AM) technologies [2–7], where only a few existing alloys out of more than 5500 alloys in use today meet the stringent printability criteria imposed by AM [8].

     Printability represents an inherent and fundamental challenge to AM. One central issue related to this challenge is the build-up of residual stress during the rapid solidification and subsequent thermal cycles with localized cooling rates as high as 1 × 106 ◦C/s to 1 × 107 ◦C/s [9]. For example, neutron diffraction measurements on AM IN625 have demonstrated that within a single component, the residual stress variation can be as significant as 1 GPa [6,10]. Residual stresses of this magnitude can lead to part distortion, introduce fatal defects, and adversely affect the fabricated part’s mechanical properties and performance [11,12]. While several strategies have been developed to reduce the residual stress introduced during the fabrication processes, such as optimizing the scan pattern [13,14] or heating the base plate [15], stress-relief heat treatments still represent the most common and reliable approach to mitigating residual stress.

    Another ubiquitous phenomenon associated with AM is microsegregation [16,17]. In conventional manufacturing processes, macrosegregation manifests as compositional variations on length scales ranging from millimeters to centimeters or even meters [18]. The finite size of the melt pool in AM creates much more localized microsegregation, mainly due to the difference in the solubility of alloying elements in the liquid phase and solid matrix phase. In nickel-based superalloys such as IN625, microsegregation leads to a high concentration of refractory elements, for instance, Mo and Nb, near the interdendritic regions [19]. A distribution coefficient k, defined as the mass-concentration ratio between those of the dendrite center and the interdendritic region, describes the degree of elemental segregation. In IN625 welds, the k values for Mo and Nb are typically 0.95 and 0.50, respectively [20]. In AM IN625 fabricated using powder laser-bed fusion (PLB-F), thermodynamic simulations predicted the k values for Mo and Nb to be approximately 0.3 and 0.1, respectively [19]. In other words, AM fabrication can lead to a more localized and more extreme elemental segregation when compared with the traditional welding processes. 

     The need to relieve residual stress and the presence of microsegregation can generate an unfavorable situation for microstructural control and optimization. AM IN625 serves as a good example because it has local compositions well outside the standard compositional range for IN625, rendering the as-fabricated part not being IN625 everywhere despite both the powder composition and the average nominal composition being within the standard [21]. A stress-relief heat treatment at 870 ◦C for one hour, as recommended by the AM machine manufacturer [22], is highly effective in relieving the residual stress. However, it also introduces a significant amount of large δ phase precipitates, which is a phase that negatively affects the performance of IN625. An alternative stress-relief heat treatment at 800 ◦C for two hours proves effective in reducing the residual stress, too. However, it still creates sizable δ phase precipitates with major dimension exceeding 600 nm. A separate strategy is to completely remove the microsegregation using a hightemperature homogenization heat treatment. For example, a heat treatment at 1150 ◦C for one hour completely homogenizes the alloy. However, this heat treatment promotes grain growth and can be both challenging and costly to implement for industrial-scale large parts due to the time required for the temperature to equilibrate as well as the high annealing temperature required.

    These complicating factors contribute to an industrial need to investigate the feasibility of using lower temperature stress-relief heat treatments. To understand the microstructural responses of AM IN625, in this study, we investigate the solid-state transformation kinetics of an AM IN625 alloy at 700 ◦C primarily using synchrotron-based in situ scattering and diffraction methods. Specifically, we use X-ray diffraction to monitor the phase transformation kinetics and small-angle X-ray scattering to evaluate the morphological changes in the precipitates. In contrast to most studies of the effect of heat treatment on nickel-based superalloys, where experimental evidence is mainly gathered from microscopy and from in-house X-ray diffraction data, synchrotron measurements probe a fixed and significantly larger sample volume through in situ experiments that allow the annealing kinetics to be unambiguously determined. Such results are also more statistically representative. The kinetics results from the same sample volume are elucidated with thermodynamic predictions by CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) methods.