# Effect of Heat Treatment on Microstructure and Creep Behavior of Fe-40 Ni-24 Cr Alloy
## 3 Result and Discussion  
### Microstructure  
The microstructure of the as-received alloy (Figure 1a) showed a fully austenitic equiaxed matrix with a few annealing twins across the grains. An annealing twin will form during cooling due to transformation and rearrangement from unstable to stable crystal structure. The atoms reposition themselves and the resulting formation of stacking faults in a deformed crystal. Annealing twins show a mirror image of the structure existing across a boundary. The microstructure of heat-treated Fe-40 Ni-24 Cr alloy at a temperature of 950 \({}^{\circ}\)C to 1250 \({}^{\circ}\)C for 3 h followed by rapid water quench is shown in Figure 1b-h. Heat-treated specimens also showed the austenite matrix fully with annealing twin (red arrows) and precipitates (yellow arrows). The grain size was observed from the optical image to increase with the increase of heat treatment temperature from 950 \({}^{\circ}\)C to 1150 \({}^{\circ}\)C, where the grain size of the as-received Fe-40 Ni-24 Cr alloy was 53 \(\upmu\)m. The as-received specimen's grain size was smaller than the specimen that had been heat-treated at 950 \({}^{\circ}\)C due to the higher heat treatment temperature. Similarly, the as-received specimen's grain size was smaller than the specimen that had been heat-treated at 1000 \({}^{\circ}\)C due to the short soaking time. Abnormal grain growth is observed in higher heat treatment temperature. Due to a decrease in the amount of carbide precipitates, the Zener pinning effect on the high-temperature grain growth is lowered, causing coarsening in the grains. This was proved by the sudden abnormal grain growth on heat-treated specimens at 1200 \({}^{\circ}\)C, and 1250 \({}^{\circ}\)C, respectively. FESEM images of the as-received Fe-40 Ni-24 Cr and the solution-treated Fe-40 Ni-24 Cr at 1200 \({}^{\circ}\)C also further confirmed that grain growth takes place after solution treatment due to the movement of grain boundaries, with large grains consuming the smaller grains (see Figure S3 presented in the Supplementary Materials). Figure 2a,b shows the SEM image of Fe-40 Ni-24 Cr alloy. The images display four different types of precipitates. Fe-40 Ni-24 Cr alloy contains 0.441 wt% niobium, 0.029 wt% titanium, 0.206 wt% nitrogen and 0.048 wt% carbon. Hence, Fe-40 Ni-24 Cr alloy tends to form NbC, (Nb,Ti)C, TiN and Ti(C,N) precipitate during a high-temperature casting. These precipitates' elemental composition was determined by EDX analysis (recorded in atomic %), as shown in Figure 2c-f. Precipitates with irregular shapes were identified as niobium (Nb,Ti)C. The blocky precipitate observed in spectrum 1 indicates the presence of NbC due to the high atomic % of niobium and carbon (Figure 2c), while spectrum 2 indicates the presence of (Nb,Ti)C (Figure 2d). Spectrums 3 and 4 in Figure 2e,f showed high atomic % in terms of titanium, carbon and nitrogen elements, which indicated the presence of TiC and Ti(C,N) particles with a cubical shape. Even at high soaking temperatures, these precipitates do not easily dissolve during heat annealing. [PERSON] et al. [PERSON] et al. (2019) reported that TiN and Ti(C,N) precipitates formed during solidification from the melt and could not easily be dissolved at high soaking heat annealing temperatures. All precipitates were found along the grain boundaries as well as in the matrix, and they were formed during the  
Figure 1: Optical images of Fe-40 Ni-24 Cr (**a**) As-received and heat-treated samples at different temperatures: (**b**) 950 \({}^{\circ}\)C, (**c**) 1000 \({}^{\circ}\)C, (**d**) 1050 \({}^{\circ}\)C, (**e**) 1100 \({}^{\circ}\)C, (**f**) 1150 \({}^{\circ}\)C, (**g**) 1200 \({}^{\circ}\)C and (**h**) 1250 \({}^{\circ}\)C. Note: Annealing twin (red arrows) and precipitates (yellow arrows). solidification process from the melt. Similar observations were reported by [PERSON] et al. [42], [PERSON] [43] and [PERSON] et al. [44].