# Effect of Heat Treatment on Microstructure and Creep Behavior of Fe-40 Ni-24 Cr Alloy
## 3 Result and Discussion
### Microstructure Evolution during Creeping
Precipitation of Cr\({}_{23}\)C\({}_{6}\) carbides can strengthen the grain boundaries and retard the dislocation movement and prevent grain boundary sliding and dynamic recrystallization [55]. Due to the carbide precipitates being harder and more brittle than the matrix of material, distribution along the grain boundaries affects the high temperature creep performance, ductility and strength. Continuous carbide films along the grain boundaries were not desired due to the continuous rupture paths causing brittleness. Cr\({}_{23}\)C\({}_{6}\) carbides can precipitate as films, globules, platelets, lamellae and cells. Figure S8, presented in the Supplementary Materials,  
Figure 10: (**a,b**) FESEM images and and corresponding EDX spectra of the alloy from (**c**) spectrum 1, (**d**) spectrum 2, (**e**) spectrum 3 and (**f**) spectrum 4 of Fe-40 Ni-24 Cr creep test at 800 \({}^{\circ}\)C under 100 MPa stresses. shows the XRD patterns of the samples after creep test at 800 \({}^{\circ}\)C and 100 MPa of stress. Figure S9, presented in the Supplementary Materials, shows the FESEM micrographs supported with EDX elemental mapping analysis of Fe-40 Ni-24 Cr alloy after undergoing a creep test at 900 \({}^{\circ}\)C. Figure S10 shows the rupture surface of alloy Fe-40 Ni-24 Cr after undergoing a creep test at 900 \({}^{\circ}\)C (presented in the Supplementary Materials). Figure 13 shows the FESEM micrographs on Fe-40 Ni-24 Cr after undergoing a creep test at 900 \({}^{\circ}\)C; 100 MPa. TiN, NbC, Ti(C,N) and (Nb,Ti)C precipitates were observed after the creep test. They are insoluble at temperatures below the melting point. These precipitates tend to be located at the grain boundaries and austenitic matrix. The precipitates' elemental composition was analyzed by EDX analysis. Square shaped precipitates were identified as TiN, as shown in EDX spectrum 1. The blocky precipitate was identified as NbC due to the high atomic % in Nb and C elements. EDX spectrum 3 indicated the presence of Ti(C,N) particles due to the high atomic % in Ti, C and N elements. EDX spectrum 4 was identified as (Nb,Ti)C precipitates. After the high- temperature test, chromium-rich carbide (Cr\({}_{23}\)C\({}_{6}\)) precipitates tend to form along the grain boundaries. Cr\({}_{23}\)C\({}_{6}\) carbides can form by heat treatment or high-temperature service at a temperature between 760 \({}^{\circ}\)C and 982 \({}^{\circ}\)C. Cr\({}_{23}\)C\({}_{6}\) carbides can form either from soluble carbon in the matrix or degeneration of primary MC type carbides, where M is titanium, niobium or tungsten, and tantalum [56]. Cr\({}_{23}\)C\({}_{6}\) carbides can precipitate as films, globules, platelets, lamellae and cells. Continuous  
Figure 11: (**a**) TEM images and corresponding EDX spectra of the alloy from (**b**) spectrum 1 and (**c**) spectrum 2 of the heat-treated alloy at 1200 \({}^{\circ}\)C after creep test at 800 \({}^{\circ}\)C under 100 MPa stresses. Figure 12: FESEM micrographs with EDX elemental mapping analysis of heat-treated sample at 1200 \({}^{\circ}\)C after creep test at 800 \({}^{\circ}\)C under 100 MPa stresses, (**a**) inner grains, (**b**) necking region and (**c**) grain boundary precipitates. carbide films along the grain boundaries were not desired due to the continuous fracture paths, which would cause brittleness. The carbides can hinder grain boundary sliding and migration during high-temperature service [58; 59; 60]. Figure S11, presented in the Supplementary Materials, exhibits the X-ray diffraction patterns of the sample after undergoing a creep test at 900 \({}^{\circ}\)C and 100 MPa of stress.