# Effect of Heat Treatment on Microstructure and Creep Behavior of Fe-40 Ni-24 Cr Alloy
## 3 Result and Discussion
### Microstructure Evolution during Creeping
Figure 10a,b shows the FESEM analysis on Fe-40 Ni-24 Cr after undergoing creep test 800 \({}^{\circ}\)C and 100 MPa. Niobium carbide (NbC), niobium-titanium carbide ((Nb,Ti)C), titanium carbonitride (Ti(C,N)) and titanium nitride (TiN) precipitates were observed after creep test. They are insoluble at temperatures below the melting point. These precipitates  
Figure 8: EBSD analysis on the heat-treated sample at 1200 \({}^{\circ}\)C underwent a creep test at 800 \({}^{\circ}\)C under 100 MPa stresses; (**a**) pattern quality (PQ) map, (**b**) pattern quality grains map, (**c**) inverse pole figure (IFF) map, (**d**) grain average misorientation (GAM) map and (**e**) kernel average misorientation (KAM) map. Figure 9: (**a**) TEM image of the selected area diffraction (SAD), (**b**) SAD area, (**c**) dislocations and precipitates distribution, (**d**,**e**) dislocation at the precipitates, (**f**) carbide precipitate along grain boundary, (**g**) dislocation pile up at the carbide precipitate, (**h**) TEM image of the SAD on grain boundary carbide, (**i**) SAD area on grain boundary carbide, (**j**) TEM image of the SAD on matrix precipitate and (**k**) SAD area on matrix precipitate of the heat-treated sample at 1200 \({}^{\circ}\)C after creep test at 800 \({}^{\circ}\)C under 100 MPa stresses. tend to be located at the grain boundaries and austenitic matrix. These precipitates' elemental composition was analyzed by EDX, as shown in Figure 10c-f. Spectrum 1 indicates the presence of NbC from the high atomic % of niobium and carbon. The cubical-shaped particle shown in Figure 10c was identified as TiN. Spectrum 2 (Figure 10d) shows the major atomic % in titanium and nitrogen elements. The irregular-shaped precipitates were identified as (Nb,Ti)C based on Spectrum 3 (Figure 10e). Spectrum 4 in Figure 10f shows high atomic % in titanium, carbon and nitrogen elements and indicates the presence of Ti(C,N) particles. Besides (NbC), (Nb,Ti)C, Ti(C,N) and TiN precipitates, chromium-rich (Cr\({}_{23}\)C\({}_{6}\)) carbide precipitates also formed after high temperature creep. Due to the small size of the Cr\({}_{23}\)C\({}_{6}\) precipitates (nanometer), they can only be detected using TEM analysis supported with EDX. Cr\({}_{23}\)C\({}_{6}\) tend to precipitate along the grain boundaries and enhance the stress rupture properties of the material. The optimum distribution of Cr\({}_{23}\)C\({}_{6}\) carbides existed as a discontinuous chain along the grain boundaries. The continuous thin strip Cr\({}_{23}\)C\({}_{6}\) can accelerate the propagation of micro-cracks along the grain boundaries [57]. The carbides can hinder grain boundary sliding and migration during high temperature service. A block-shaped particle at the grain boundaries as shown in Figure 11a was identified as chromium rich (Cr\({}_{23}\)C\({}_{6}\)) carbides. EDX analysis spectrum 1 (Figure 11b) shows a high concentration of chromium element. Spectrum 2 in Figure 11c shows chromium-rich niobium carbide located in the matrix. Figure S6 shows the cross-sectional FESEM images of the recrystallized grains at the creep necking region. FESEM image and elemental mapping were as shown in Figure S7, presented in the Supplementary Materials. Figure 12a shows EDX elemental mapping of selected specimens heat-treated at 1200 \({}^{\circ}\)C (heat-treated at 1200 \({}^{\circ}\)C) after undergoing a creep test at 800 \({}^{\circ}\)C and 100 MPa. Distribution of Cr\({}_{23}\)C\({}_{6}\) along the grain boundaries can clearly be observed. Mapping results showed a random distribution of titanium (Ti) and niobium (Nb)-rich particles in the matrix. Figure 12b shows EDX elemental mapping of the selected specimen (heat-treated at 1200 \({}^{\circ}\)C) at the necking region. Mapping results also showed the distribution of Cr\({}_{23}\)C\({}_{6}\) along the grain boundaries and Ti and Nb-rich particles in the matrix. A discontinuous chain of Cr\({}_{23}\)C\({}_{6}\) carbides along the grain boundaries can clearly be observed in Figure 12c.