2. Materials and Methods

2.1. Material Fabrication and Sample Preparation 

    within the allowed composition range specified by the ASTM Standard for Additive Manufacturing Nickel Alloy UNS N06625. The vendor-supplied compositions are listed in Table 1. The fabrication parameters include a Nd:YAG laser operated at 195 W, a scanning speed at 800 mm/s, and a hatch spacing of 100 µm. During the fabrication, the melt-pool width varied between 105 and 115 µm. More details about the fabrication can be found elsewhere [19].  

    Table 1. Measured composition of the virgin IN625 feedstock powders used in this work as provided by the vendor-supplied data sheet and determined following ASTM E1019 standard as well as the allowable range of composition of IN625. The testing relative uncertainty for elements with mass fraction between 5 and 25% is ±5% of the value, for elements with mass fractions between 0.05% and 4.99% is ±10% of the value, for elements with mass fractions less than 0.049% is ±25% of the value.

2.2. Ex Situ Scanning Electron Microscopy (SEM) 

     We used scanning electron microscopy (SEM) to perform ex situ microstructural examinations of the as-fabricated and heat-treated samples. The JEOL S-7100F (JEOL, Ltd., Akishima, Tokyo, Japan) field emission SEM is equipped with an Oxford X-MAXN (Oxford Instruments Plc., Abingdon, UK) energy-dispersive X-ray spectrometry (EDS) detector. We operated the SEM at 15 kV.

   To evaluate the effect of heat treatment on the microstructure of IN625, we encapsulated IN625 specimens in evacuated ampoules and performed heat treatments at 700 ◦C and 800 ◦C. We polished the SEM specimens following standard metallographic procedures, etched the surface with aqua regia, and performed the microstructural analysis with SEM. For this characterization, the imaged sample surfaces are parallel to the build direction, allowing microstructural information of the dendritic and interdendritic regions to be captured.

2.3. In Situ Synchrotron Small Angle X-Ray Scattering and X-Ray Diffraction

    We performed synchrotron-based, in situ ultra-small-angle X-ray scattering (USAXS), small-angle X-ray scattering (SAXS), and XRD measurements at the USAXS facility at the Advanced Photon Source, Argonne National Laboratory, U.S.A. [23]. The in situ USAXS and SAXS monitor the morphology changes during a solid-state transformation induced by heat treatment. Within its detection limits, the in situ XRD provides information regarding the nature of the solid-state transformation. Combined, USAXS, SAXS, and XRD cover a continuous scattering q range from 1 × 10−4 Å−1 to ≈6.5 Å−1 . Here, q = 4π/λ sin(θ), where λ is the X-ray wavelength and θ is one-half of the scattering angle 2θ. More details about this setup can be found elsewhere [24]. 

    For this study, we used monochromatic X-rays at 21 keV (λ = 0.5904 Å). The X-ray flux density at the sample is in the order of 1013 mm−2 s −1 . The as-fabricated sample was mechanically polished to ≈50 µm in thickness. We used a Linkam 1500 thermal stage to control the temperature. After an initial measurement at room temperature, we performed a 10.5 h isothermal hold at 700 ◦C, with a heating rate from room temperature to the target temperature at 200 ◦C per min. The data acquisition times for USAXS, SAXS, and XRD are 90 s, 30 s, and 60 s, respectively, leading to a measurement time resolution of ≈5 min. The spatial dimensions of the gauge volume area were 0.8 mm × 0.8 mm for USAXS and 0.8 mm × 0.2 mm for SAXS and XRD.

2.4. Thermodynamic Calculations

     superalloys [25,26]. To compare to the experimentally observed precipitation events, we calculated the precipitation kinetics using the TC-PRISMA module [27–29]. This module is based on the Langer–Schwartz theory [30] and Kampmann–Wagner numerical methods [31,32] and calculates the nucleation, growth, and coarsening of precipitates in a multicomponent and multiphase system by integrating thermodynamic and diffusion information provided by CALPHAD descriptions. The simulation output includes the time-dependent evolution of the particle size distribution, number density, mean radius, and volume fraction. More details about the CALPHAD calculations can be found elsewhere [33].

3. Results and Discussion

Figure 1 shows an equilibrium Nb-isopleth for the powder composition listed in Table 1. In addition to the FCC matrix, MC, M23C6, σ, P, and δ are thermodynamically stable equilibrium phases. δ, especially, is stable over a wide temperature range from below 600 to ≈1200 ◦C, depending on the mass fraction of Nb. We have previously established that significant microsegregation in the interdendritic region exists in the asfabricated IN625 due to solute rejection caused by the difference in solubility in liquid and solid phases [19,34]. CALPHAD-based solidification simulations predicted by the Scheil– Gulliver model and by DICTRA using finite-element-analysis thermal-model predictions as input suggests extreme microsegregations of alloying elements of Mo and Nb. For example, the predicted Nb mass fraction ranges from ≈2% to ≈22% between secondary dendritic cores, which is well beyond the allowable range of Nb of between 3.15% and 4.15% (Table 1). Previous synchrotron SAXS measurements demonstrated that the microsegregation is concentrated near the interdendritic centers on a scale of 10 nm [35], which is consistent with model predictions [19]. This type of extreme microsegregation effectively renders the as-fabricated IN625 part not within the spec of IN625 in all places, resulting in unintended and deleterious solid-state transformations in this alloy.