4.1. Severe IGO of AM samples
Alloy 625 can show internal oxidation of Al and Ti, according to the classic model of internal oxidation [25]. This is because Al and Ti form more stable oxides than chromia, and oxygen is soluble in the Ni-Cr matrix [26]. Furthermore, alloy grain boundaries are preferential sites for internal oxide nucleation due to the greater inward penetration of oxygen along boundaries and the lower degree of supersaturation needed to precipitate oxide [27]. Therefore, IGO is expected to be deeper than internal oxidation. What is notable is that the AM samples exhibited a more severe IGO than the conventionally-processed samples, which aligns with the results of previous studies [9, 11, 28]. Several proposed hypotheses to explain this behavior include (1) grain boundary misorientation, (2) minor elements, and (3) subsurface void formation. The following elaborates on and assesses these hypotheses.
(1) Grain boundary misorientation: In terms of grain character, grain boundaries with Σ3 coincident site lattice or twin boundaries display better resistance to corrosion, cracking, and sliding (creep) than grain boundaries with other misorientations [29]. These properties have been correlated to very low grain-boundary energies. Other boundary angles with low energy (such as Σ11, Σ19, and Σ27) have also exhibited similar properties, but not as consistently as Σ3 [30]. In the case of high-temperature oxidation, the lower oxygen diffusion along Σ3 grain boundaries contributes to their intergranular oxidation resistance [31–33].
It has been reported that as-built AM samples have a much lower proportion of Σ3 grain boundaries than wrought counterparts. Therefore, the less severe IGO of the wrought compared to the AM is expected. However, cast alloys do not have many Σ3 grain boundaries and their IGO is not that deep. After solution and annealing heat treatments, AM 625 still shows significant IGO following high-temperature oxidation despite having an increased amount of Σ3 grain boundaries compared to the as-built state [11, 34]. Further research is needed to understand the differences between the nature of the wrought and AM grain boundaries beyond the misorientation that makes the AM grain boundaries more susceptible to being oxidized.
(2) Minor elements: Regarding alloy compositional differences, Chyrkin et al. [11] reported that low Si content in AM 625 (0.08 wt.% in the AM to 0.27 wt.% in the forge) impedes the formation of a protective SiO2 beneath the chromia, thus increasing oxygen diffusion into the alloy and the IGO depth. The protective benefit of Si has been observed in other types of alloys as well [35]. In this study, Si content in the AM-processed alloys is consistently lower than that of wrought and cast, but the difference is one-third of what was reported by Chyrkin et al. [11], and the IGO severity is as strong. Despite the protective Si effect being found in model alloys made by casting, the nature of AM processing and its impact on the microstructure was not considered in this study.
(3) Formation of subsurface voids: Another distinctive characteristic of the oxidized AM samples' subsurface is the presence of voids. These voids can either be discrete or surrounding the IGO and are inferred to be Kirkendall voids [9, 28, 36]. During the chromia-scale formation, the outward flux of chromium from the subsurface is compensated by an inward flux of vacancies. These vacancies can accumulate and eventually lead to vacancy saturation in the subsurface, resulting in void formation. The LPBF sample may be prone to condense vacancies at the incoherent oxide/matrix interface along the grain boundaries [26], which would explain the open space associated with the IGO in the LPBF samples.
Another source for void formation comes from the initial porosity and interstitial oxygen in the as-built part. These pores may arise from trapped gas during atomization of the metal powder, or as a mesostructural defect during the additive manufacturing process. An analysis of the as-built parts found that the porosity is 0.12 ± 0.04% and 0.82 ± 0.11% for LPBF and DED, respectively. Although gas atomization and additive manufacturing processes were conducted in an argon atmosphere, oxygen traces may be present in the pores. The oxygen content, encompassing both interstitial oxygen and oxygen within the pores, in the LPBF and DED samples is significantly higher than in the wrought and cast alloys. This aligns with previous research that reported relatively high levels of interstitial oxygen in additively manufactured Ti alloys, Al alloys, and stainless steels [37–39]. Specifically, it has been shown that oxygen content increases from the AM process by comparing the powder composition with the built part (increasing up to 43%) [38, 39]. The higher oxygen content is likely due to a longer processing time and a higher surface area exposed to the atmosphere at melting temperature during the AM processes compared to conventional processes. In fact, a positive correlation was found between the oxygen content and the building rate, which is mainly attributed to the metal liquid residence time [39].
Interstitial oxygen is thought to play a role in forming a greater amount of IGO and the associated voids in the AM samples. A higher level of interstitial oxygen reduces the amount of oxygen needed for precipitate internal oxidation. Therefore, more internal oxidation is expected as the interstitial oxygen level increases. Furthermore, the greater internal oxidation leads to a higher generation of metallic vacancies in the subsurface. As the high-angle grain boundaries are the most incoherent interface in the subsurface, vacancy saturation usually occurs in them, which explains the voids associated with the IGO. In contrast, the cast and wrought alloys, for which interstitial oxygen was negligible, exhibit fewer and denser IGO.
4.2. Effect of severe IGO in crack initiation during fatigue
Multiple cracks perpendicular to the load axis were observed in all samples, except for the run-out sample (i.e., wrought alloy oxidized for 24h). The deepest cracks were associated with IGO and extended into the alloy further than the precipitate-free zone. The IGO can effectively act as a notch. Cracks can grow easily via intergranular oxidation due to the incoherent interface, compressive stresses induced by the oxide formation, and the abundant voids in the AM samples. It is informative to compare the critical toughness value (KIC) of the different constituents of the system. Alumina has a KIC range of 0.4-2.0 MPa.m1/2 [40], chromia KIC is 2.0 MPa.m1/2[40], and for Alloy 625 KIC is 7.1 MPa.m1/2 [41]. Since alumina is the least resistant to cracking, its formation increases the susceptibility of IGO to nucleate cracks during fatigue.
In addition, it is important to note that the crack extending through the IGO may widen in the precipitate-free zone due to an associated decrease in mechanical strength. For instance, Nb depletion in the subsurface due to the positive chemical interaction with Cr during oxidation causes a partial reduction of solution strengthening. Sudbrack et al. [15] reported that during high-temperature notch fatigue testing of oxidized Ni-based alloy ME3, the carbide-precipitate-free zone resulted in a decrease in fatigue life. This effect may be more pronounced during high-temperature testing due to the potential for creep damage [14].
Regarding crack propagation, the bulk-alloy microstructure defines the most likely path for crack growth. The bulk alloy undergoes temporal-related microstructural changes during oxidation exposure. Given the differences in initial microstructure among samples (e.g., segregation extent, grain size, cell size), the post-aging microstructure also varies (e.g., phase fraction, phase morphology, nature of grain boundaries), impacting the crack propagation stage differently [18].