# 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 inverse pole figure (IPF) orientation map shows some minor color contrast within the grain due to the creep test. 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 8c. The IPF map shows a random distribution of grain orientation and directions. The grain's average misorientation (GAM) map is shown in Figure 8d. The GAM map is the average misorientation between the neighboring pair of measurement points within the grain. The misorientation angle was taken from 0\({}^{\circ}\) to 10\({}^{\circ}\). The grains showed a large misorientation angle between 1\({}^{\circ}\) and 2\({}^{\circ}\). The kernel average misorientation (KAM) map is a set of points of prescribed size surrounding the interest scans point. A specific tolerance angle value from 0\({}^{\circ}\) to 4.0\({}^{\circ}\) was selected, and a 0.8\({}^{\circ}\) misorientation angle gave the highest KAM number count. The misorientation developed inhomogeneously due to the deformation. The KAM result also showed that the misorientation tends to be larger near the boundaries and decreases evidently towards the center of the original grains, as shown in Figure 8e. Figure 9 shows the TEM micrograph of the selected specimen (heat-treated at 1200 \({}^{\circ}\)C) after undergoing creep at 800 \({}^{\circ}\)C and 100 MPa and interrupted at 110.7 h. The TEM analysis focused on the center area of the creep specimen. The selected area diffraction (SAD) pattern of the grain (matrix) is shown in Figure 9a,b. From the spots indexing, the zone axis determined was [1 \(\overline{1}\) 0] direction (parallel to the electron beam). Figure 9 shows a random distribution of precipitates in the matrix. Precipitates that are uniformly distributed can retard the dislocations to glide through the grains and form microvoids at the grain boundaries. The TEM micrographs described the evolution of the dislocation structure during the creep test. A large number of dislocations were found in the matrix, and they were randomly distributed within the matrix. The dislocations piled up at the precipitates inducing the formation of a low-angle grain boundary (Figure 9c). Figure 9d shows some dislocations were pinned by the nano-precipitates. Straight segments dominated the dislocations' character. The nano-scale precipitates were known to contribute  
Figure 7: (**a**) TEM image of the selected area diffraction (SAD), (**b**) SAD area, (**c**) recrystallized grain boundary, (**d**) recrystallized grain boundary (higher resolution), (**e**) precipitation in the matrix, (**f**) dislocation pile-up around the precipitates and (**g**) dislocation pile-up at the precipitate (higher resolution) of the heat-treated sample at 1200 \({}^{\circ}\)C after creep test at 800 \({}^{\circ}\)C under 100 MPa stresses. to the precipitation strengthening. Sessile dislocation loops can be observed in Figure 9e. Discontinuous chains of chromium-rich (Cr\({}_{23}\)C\({}_{6}\)) carbide precipitates were found along the grain boundaries, as shown in Figure 9f. Cr\({}_{23}\)C\({}_{6}\) carbides formed from the soluble carbon in matrix or the degeneration of the primary MC type of carbides during hot deformation, where M is titanium, niobium or tungsten, and tantalum [56]. These fine carbides can effectively pin at the grain boundaries and inhibit the formation of dynamic recrystallization. Figure 9g shows the block shape of Cr\({}_{23}\)C\({}_{6}\) carbide particles at the grain boundary, which interact with dislocations and may lead to grain boundary serration. The selected area diffraction (SAD) pattern of the Cr\({}_{23}\)C\({}_{6}\) particles located along the grain boundary and within the matrix is shown in Figure 9h-k. From the spot indexing, the zone axis for the grain boundary particle was determined as [0 1 \(\overline{1}\)], while the zone axis for the matrix particle was [1 \(\overline{1}\) 0].