# Effect of Heat Treatment on Microstructure and Creep Behavior of Fe-40 Ni-24 Cr Alloy
## 3 Result and Discussion
### Creep Characteristic  
Figure 3a-d illustrates the creep strain of heat-treated Fe-40 Ni-24 Cr alloy at a temperature 800 \({}^{\circ}\)C and initial stress of 100 MPa. The specimen heat-treated at 1200 \({}^{\circ}\)C, 1225 \({}^{\circ}\)C and 1250 \({}^{\circ}\)C showed longer secondary creep than the specimen heat-treated at 1050 \({}^{\circ}\)C. The specimen heat-treated at 1050 \({}^{\circ}\)C showed limited primary creep and immediately entered the secondary and tertiary creep stage, as shown in Figure 3a. The creep curves for four different grain sizes--62 \(\upmu\)m (HST1050/heat-treated at 1050 \({}^{\circ}\)C), 158 \(\upmu\)m (HST1200/heat-treated at 1200 \({}^{\circ}\)C), 183 \(\upmu\)m (HST1225/heat-treated at 1225 \({}^{\circ}\)C) and 312 \(\upmu\)m (HST1250/heat-treated at 1250 \({}^{\circ}\)C)--were evaluated in this research. At the primary stage, a sudden increase in creep strain value is referred to as instantaneous creep strain due to the sudden load application, as shown in Figure 3b,c. The specimen with the smallest grain size, heat-treated at 1050 \({}^{\circ}\)C, had the highest grain boundary per area and gave the smallest instantaneous value, 0.016% strain. This was followed by 0.032% strain for the specimen heat-treated at 1200 \({}^{\circ}\)C, 0.088% strain for the specimen heat-treated at 1225 \({}^{\circ}\)C and 0.096% strain for the specimen heat-treated at 1250 \({}^{\circ}\)C. The grain boundaries can retard the dislocation movement from the sudden stress application. Figure 3d shows the logarithmic scale plot of the strain rate for heat-treated Fe-40 Ni-24 Cr alloy. The graphs clearly show that the creep rate decreased to a minimum value before rapidly increasing at the end of the creep test. Figure 2: (a,b) FESEM images of as-received Fe-40 Ni-24 Cr alloy and (c–f) EDX spectrum of as-received Fe-40 Ni-24 Cr alloy. The minimum value of the curves was known as the minimum steady-state creep rate. Figure 3e shows the minimum strain rate value versus grain size. The relationship between strain rate and grain size can be written as follows: y = 0.0081x\({}^{-1.139}\), where y is grain size and x is minimum strain rate [49; 50; 51; 52]. The minimum strain rate value decreased from grain size 62 to 183 \(\upmu\)m and slightly increased from 183 to 312 \(\upmu\)m. For the small grain size in the specimen range, 62 \(\upmu\)m (heat-treated at 1050 \({}^{\circ}\)C) to 183 \(\upmu\)m (heat-treated at 1225 \({}^{\circ}\)C), the increase of the minimum creep rate value with the decrease of grain size was due to the grain boundary sliding mechanism. Grain boundary sliding is a deformation mechanism of materials that includes the displacement of grains against each other. Grain boundary sliding is a mechanism that occurs in polycrystalline materials at a temperature 0.4 times and above the absolute melting temperature. This strain rate mechanism agreed with [PERSON]'s model [49], which stated cavitation could form from grain boundary sliding and dislocation pile up at the grain boundaries and particles. Small grain size has more ratio of grain boundaries surface area per unit volume. These cause more cavitation to form at the grain boundaries in the smaller grain size specimens than the larger grain size specimens due to grain boundary sliding. Grain boundary sliding leads to cavitation and is often accompanied by a diffusion process. Grain boundary sliding can also cause cavitation  
Figure 3: Creep curve of heat-treated Fe-40 Ni-24 Cr at temperature 800 \({}^{\circ}\)C and initial stress of 100 MPa; (**a**) total strain versus time (**b**) instantaneous creep strain and (**c**) instantaneous strain versus grain size and (**d**) strain rate versus time (**e**) minimum creep rate versus grain size. (**f**) Creep rupture time versus grain size of heat-treated Fe-40 Ni-24 Cr after creep test at 800 \({}^{\circ}\)C and 100 MPa. at the particles located at the grain boundary.