# Solid-State Transformation of an Additive Manufactured Inconel 625 Alloy at 700 \({}^{\circ}\)[PERSON]
## 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, M\({}_{23}\)C\({}_{6}\), \(\sigma\), \(P\), and \(\delta\) are thermodynamically stable equilibrium phases. \(\delta\), especially, is stable over a wide temperature range from below 600 to \(\approx\)1200 \({}^{\circ}\)C, depending on the mass fraction of Nb. We have previously established that significant microsegregation in the interdendritic region exists in the as-fabricated 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 \(\approx\)2% to \(\approx\)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. Figure 2 shows the SEM images of AM IN625 under four different conditions (Figure 2a, as-fabricated, Figure 2b, one hour at 700 \({}^{\circ}\)C, Figure 2c, 2d h at 700 \({}^{\circ}\)C, and Figure 2d, one hour at 800 \({}^{\circ}\)C), with the imaged surfaces parallel to the build direction. The dendritic microstructure is visible in all four images. An EDS analysis of the as-fabricated specimen reveals that the interdendritic regions are enriched in Nb and Mo, and the dendritic regions are enriched in Ni and Cr. The effects of different heat treatment conditions on the microstructures are subtle, with one-hour heat treatment at 700 \({}^{\circ}\)C leading to no visually observable differences under the measurement conditions. In contrast, a prolonged heat treatment at 700 \({}^{\circ}\)C promotes the formation of precipitates of a platelet morphology near the interdendritic regions. The morphology of this phase is consistent with previous observation of \(\delta\) phase, with the nucleation of the \(\delta\) phase being more favorable with a higher concentration of Nb and Mo [33]. Figure 1: A steady-state Nb isopleth section of IN625 phase diagram constructed assuming a composition of Ni-20.7 Cr-8.83 Mo-0.72 Fe-0.35 Ti-0.28 Al-0.18 Co-0.13 Si-0.04 Mn-0.01C (mass%). Figure 2: Microstructure in AM IN625 under four different conditions (**a**) as-fabricated, (**b**) after a one-hour heat treatment at 700 \({}^{\circ}\)C, (**c**) after a 24 h heat treatment at 700 \({}^{\circ}\)C, and (**d**) after a one-hour heat treatment at 800 \({}^{\circ}\)C. The imaged surfaces are all parallel to the build direction. Red arrows in (**c**,**d**) highlight the platelet \(\delta\) phase precipitates. A one-hour heat treatment at 800 \({}^{\circ}\)C leads to a similar change to the microstructure with the formation of \(\delta\) phase precipitates, as observed in Figure 2c (24 h at 700 \({}^{\circ}\)C). We note that in both Figure 2c,d, the \(\delta\) precipitates have comparable sizes. The difference in the heat treatment duration suggests that the precipitation kinetics of the \(\delta\) phase precipitates is much accelerated at 800 \({}^{\circ}\)C, compared with 700 \({}^{\circ}\)C, which is consistent with the TTT diagrams [33; 36] previously constructed for AM IN625. This slowdown in precipitate growth can be significant for residual-stress relief heat treatment.