# Solid-State Transformation of an Additive Manufactured Inconel 625 Alloy at 700 \({}^{\circ}\)[PERSON]
## 3 Results and Discussion
Second, we observed a monotonic increase in scattering intensity between \(\approx\)4 \(\times\) 10\({}^{-4}\) A\({}^{-1}\) and \(\approx\)0.01 A\({}^{-1}\), notably with two Guinier regions near 2 \(\times\) 10\({}^{-3}\) A\({}^{-1}\) and 8 \(\times\) 10\({}^{-3}\) A\({}^{-1}\), respectively. Since the in situ XRD data and ex situ SEM images only show the \(\delta\) phase precipitates, we attribute this scattering signal to the \(\delta\) phase. Previous microscopic studies have established that the \(\delta\) phase precipitates are platelets with two characteristic sizes [10; 21; 47], which is in agreement with the observation of two Guinier regions in the scattering data. Lastly, the SAXS data shown in the inset are simple extensions of the high \(q\) power-law slope in the USAXS data; i.e., the SAXS data contain no additional information, which indicates that no additional nm-sized precipitates formed during this heat treatment. We constructed a scattering model to describe the scattering data based on these observations, as illustrated in Figure 7a. Using the USAXS data acquired at 630 min into the in situ experiment as an example, this model consists of two components. The first component is the scattering baseline, which is obtained on the same sample volume at room temperature before the heat treatment. The second component represents the excess scattering originating from the \(\delta\) phase precipitates. As established previously [21], we described this excess scattering using an analysis approach analogous to the unified small-angle scattering method with two scattering levels [48]. Together, this two-component model describes the in situ SAXS data well through the entire data sequence. Figure 6: In situ SAXS data of AM IN625 acquired during isothermal heat treatment at 700 \({}^{\circ}\)C for 10.5 h. The main plot and the inset show the USAXS and SAXS data, respectively. The acquisition time is color-coded following the time arrow. The USAXS intensity is slit-smeared. Figure 7b shows the evolution of the mean thickness (minor dimension) and diameter (major dimension) of the \(\delta\) phase precipitates at 700 \({}^{\circ}\)C as a function of time. The thickness and diameter demonstrate a similar trend, with an initial rapid increase followed by a gradual increase. By the end of the heat treatment, the mean thickness and diameter are 34 \(\pm\) 2 nm and 154 \(\pm\) 7 nm, respectively. These values are significantly smaller than the values acquired from AM IN625 after 10 h at 870 \({}^{\circ}\)C, where the mean thickness and diameter are 52 \(\pm\) 5 nm and 961 \(\pm\) 94 nm, respectively [21], again pointing to significantly slower precipitation kinetics at 700 \({}^{\circ}\)C. In the context of typical residual stress heat treatment, after a one-hour heat treatment at 870 \({}^{\circ}\)C, the mean thickness and diameter are 45 \(\pm\) 4 nm and 424 \(\pm\) 40 nm, respectively [21]; after a two-hour heat treatment at 800 \({}^{\circ}\)C, the mean thickness and diameter, depending on the build condition, range between 61 nm to 77 nm and 416 nm to 634 nm, respectively [24]. In other words, a stress relief heat treatment at 700 \({}^{\circ}\)C for as long as 10 h results in \(\delta\) phase precipitates significantly smaller than those developed during typical residual stress heat treatment of AM 625. Figure 7: **(a)** An illustration of the SAXS model used in this work. The data were acquired at 630 min during the heat treatment at 700 \({}^{\circ}\)C. The overall scattering comprises two parts: (1) scattering baseline and (2) excess scattering from the \(\delta\) phase precipitates. **(b)** The time-dependent evolution of the mean diameter (major dimension) and thickness (minor dimension) of the \(\delta\) phase precipitates.