3.2. Heat Treatment
Inconel 718 is typically purchased as annealed forging, billet, rod bar, plate, and stress-relieved conditions. This material is then fabricated in its most malleable condition. After fabrication, it could be heat-treated as required in terms of specifications according to the applicable specification.
For most applications, Inconel 718 is used in precipitation hardened (aged) condition, combining rupture life, rupture ductility, and impact strength, etc. This alloy is hardened by the precipitation of secondary phases (e.g., γ′ and γ″) into the metal matrix [
21]. The specific process of aging treatment is as follows: solution at 720 °C for 8 h, then furnace cool to 620 °C at the rate of −50 °C/h for 2 h and continue aging for another 8 h at this temperature. The total treatment is 18 h followed by air cooling. The dynamic recrystallization (DRX) temperature is about 950 °C, according to the time-temperature-transformation diagram [
25].
3.3. Microstructure
Metallographic analysis of Inconel 718 was investigated before and after heat treatment. Over 90% of fatigue life in metals is consumed in initiation in HCF and VHCF range at room temperature in general, which is highly influenced by the microstructure [
1,
5]. Leica
® DM ILM inverted light microscope (OM, Wetzlar, Germany) with Baumer
® TXG50c progressive scan camera (Friedberg, Germany) and Jeol
® JSM-6010Plus scanning electron microscope (SEM, Tokyo, Japan) were conducted to investigate the microstructure (phase, grain size, and precipitations).
The metallographic sample was sliced with an Al2O3 cut-off blade using a low feed speed of 0.01 mm/s. It was then polished with SiC emery paper of 80 to 2000 grit, and finished with 1/4 μm diamond paste for 30 min. All these steps were completed under water cooling to avoid increase in temperature. Afterwards, the sample was etched with homemade Kalling’s II reagent (also known as waterless Kalling) to reveal the grain boundary in less than 10 min after polishing, in order to avoid the passivity of the smooth surface.
OM micrographs of Inconel 718 are shown in
Figure 2. Heterogeneity can be seen in the microstructure of the annealed material. It contains a “fine grain band” (
Figure 2a) consisting of a large amount of fine grains up to hundreds, and several “non-recrystallized” zones (
Figure 2b). The length of these zones can exceed 100 μm, while the width is nearly 20 μm (
Figure 2c). Size of precipitate particles is nearly 10 μm, close to the size of base γ grains (
Figure 2d). “Fine grain band” and “non-recrystallized” grains disappear after aging treatment (
Figure 2e). A twin boundary can still be found in the micrograph, with the length less than 10 μm (
Figure 2f).
SEM micrographs and EDS maps of annealed and aged Inconel 718 are shown in
Figure 3.
Figure 3a,b show the same zone of annealed material under secondary electrons (SEI) and back-scattered electrons (BEC) modes. It is clear that the large non-recrystallized grains include several twins. The maximum twin boundary is nearly 50 μm, which seems to be the potential weakness initiation for fatigue failure.
The strength of superalloy Inconel 718 comes from coherent solid-state precipitates, which comprise small amounts of γ′ (Ni
3Ti/Al), but are mostly γ″ (Ni
3Nb), producing coherency strains in the γ FCC matrix. The γ′ has a unique morphology characterized by round particles that can be smaller than 200 Å, while γ″ is an ellipsoid with a length 5~6 times its thickness. The resolution of SEM is not sufficient to observe them, and transmission electron microscope (TEM) is needed to observe these morphologies [
8,
9]. Precipitate particles are lack of elements Ni, Cr, and Fe, which are the main elements of Inconel 718. The lighter particle is rich in Nb and Mo, and darker particle is rich in Ti and Al, as seen in
Figure 3c.
Vickers micro-hardness tests with 25 g force loading and 10 s dwell time (HV
m25/10) were used on grains and precipitations. Six independent measurements were applied and the results of 90% confidence intervals are listed in
Table 2. The “Non-recrystallized” zone is softer than the “fine grain band” zone due to the lack of grain boundary. Precipitate particles are much harder than the basic matrix. Thermal expansion coefficients for inclusions or precipitations are far lower than the surrounding matrix. Apparent residual stress would develop in the boundaries during heat treatment [
26]. These boundaries become another potential area for crack initiation. Fatigue life is reduced by several orders of magnitude when inclusion size increases for some superalloys [
27].
“Fine grain band” and “non-recrystallized” grains disappear after aging treatment (
Figure 3d). Non-strengthened orthorhombic phase δ (Ni
3Nb) is incoherent with the γ matrix. Globularity and lenticular-shaped δ distributes in the FCC matrix. Needle-wafer-shaped δ nucleates at the grain boundary (
Figure 3e). It is used to control grain size in wrought material and seems to be also important for notch ductility [
28]. Grain boundary δ allows to identify the boundary between different grains, as well as the size of grains.
Electron back-scattered diffraction (EBSD) images of annealed and aged Inconel 718 are shown in
Figure 4. Grain orientation spread (GOS) is calculated as the average deviation of the orientation of each pixel in the grain from the average orientation for the grain [
29]. Most of the grains with well-developed high-angle grain boundaries are characterized by no internal structure and uniform orientation. Several yellow and orange points appear in the center of the “fine grain band” zone (
Figure 4b). This reveals that a high level of internal residual strain exists in these grains which are not sufficiently developed, becoming a potentially dangerous zone for fatigue crack initiation. For aged material, most areas are displayed in blue, since residual strain was relieved during aged treatment (
Figure 4d). Histograms of grain size are shown in the images of
Figure 4c,f. Grain diameter distributes in the range of 2~130 μm for annealed material, while all grains are smaller than 50 μm after aging treatment. The average is 24.6 μm, which is consistent with ASTM E112 grain size number 8 [
30].
Directly aged high-quality (DAHQ) is a special type of Inconel 718 developed for advanced turbine discs. The δ phase not only exists in the grain boundary, but also diffuses into the base γ matrix in this edition of 718. The proportion and size of δ phase could be important factors determining fatigue behavior [
21,
31], which will be examined in a future study. In DAHQ Inconel 718, the grain diameter distribution is in the range of 4~10 μm, which is consistent with ASTM 10~12 [
30]. Physical properties of these three materials are listed in
Table 3.