# Effect of Heat Treatment on Microstructure and Creep Behavior of Fe-40 Ni-24 Cr Alloy
## 3 Result and Discussion
### Microstructure Evolution during Creeping  
The rupture surface of alloy Fe-40 Ni-24 Cr after undergoing a creep test is shown in Figure 5a-e. The rupture surface shows dimples characteristic of micro-void coalescence. The micro-void formed and coalesced to form a micro-crack. Figure 5a,b show only dimple rupture mode due to the small grain size. However, Figure 5c-e show a mix between intergranular and dimple rupture mode. A dimple type of rupture surface indicates a ductile rupture manner. Ductile rupture is characterized by a large amount of deformation calculated in the percentage of elongation. The percentage of elongation for the as-received specimen was 40%. The percentage of elongation for the heat-treated specimen was 51% for the specimen heat-treated at 1050 \({}^{\circ}\)C, 40% for the specimen heat-treated at 1200 \({}^{\circ}\)C, 31% for the specimen heat-treated at 1225 \({}^{\circ}\)C and 30% for the specimen heat-treated at 1250 \({}^{\circ}\)C, as shown in Figure 5f. The results show that the total elongation significantly decreased with an increase in grain size. The results are in agreement with the critical length criterion for intergranular creep rupture that [PERSON] et al. [24] found to be applicable in Ni-Cr wrought alloy and 316 stainless steel. The grain size dependence of creep elongation is essentially due to the effect of grain size in the tertiary stage of the creep test. The tertiary creep stage involved the growth and inter-linkage of cracks and led to final rupture. In the fine grain (heat-treated at 1050 \({}^{\circ}\)C), there are a large number of triple junctions. A crack will form at the triple junction with low aspect ratio (length/width) compared to coarse grain size. By increasing the grain size (heat-treated at 1200 \({}^{\circ}\)C, 1225 \({}^{\circ}\)C and 1250 \({}^{\circ}\)C), the crack density decreased. The stress concentration at the crack tip is higher for coarse grain due to the aspect ratio for cracks increasing with an increase in the grain size. The cracks can therefore readily grow through the triple point boundaries and critical crack length is attained faster. This results in lower in ductility with increasing grain size. A large number of triple junctions in fine grain can limit the crack propagation. The specimen heat-treated at 1200 \({}^{\circ}\)C was selected to undergo electron backscatter diffraction (EBSD) analysis to better understand the microstructure after undergoing a high temperature creep test. Figure 6 shows the EBSD results of the HST1200 specimen, which focused on the rupture necking area. Figure 6a shows the pattern quality (PQ) image. The PQ image generally showed a grayscale map because every point on the map is assigned a brightness based on the pattern quality for every point. They appeared similar to an SEM image. Grain boundaries were observed as dark linear features due to the low-quality pattern. The addition of grain coloring into the pattern quality map resulted in a rainbow-colored map, as shown in Figure 6b. Each color represents different grains. Figure 5: SEM images of rupture surface of Fe-40 Ni-24 Cr after creep test at 800 \({}^{\circ}\)C and 100 MPa; (**a**) as-received and heat-treated sample at different temperatures: (**b**) 1050 \({}^{\circ}\)C, (**c**) 1200 \({}^{\circ}\)C, (**d**) 1225 \({}^{\circ}\)C and (**e**) 1250 \({}^{\circ}\)C; (**f**) percentage of elongation versus grain size of heat-treated alloy. At the rupture boundary tip, recrystallized grains can clearly be observed. The specimen showed intergranular rupture mode along the recrystallized grains' boundaries. Figure 6c shows the inverse pole figure (IPF) orientation map. For cubic phases, the full dark red, dark blue and dark green represent [001]\(\gamma\), [111]\(\gamma\) and [101]\(\gamma\) axes directions, respectively, as shown in Figure 6c. Due to the recrystallized grains appearing as a new equiaxed grain that does not undergo deformation, recrystallized grains showed no color contrast within the recrystallized grains [12; 31].