# Effect of Heat Treatment on Microstructure and Creep Behavior of Fe-40 Ni-24 Cr Alloy
## 3 Result and Discussion
### Creep Characteristic
Due to the small grain size having more grain boundary surface area per unit volume, these caused the grain boundaries to react as dislocation sources. Dislocations pile up at the grain boundaries and form cavities at the boundaries. The cavity is often formed in more spherical rather than plate-like or lenticular shapes and tends to grow larger under the grain boundary sliding process. For the large grain size in the specimen range, 183 \(\upmu\)m (heat-treated at 1225 \({}^{\circ}\)C) to 312 \(\upmu\)m (heat-treated at 1250 \({}^{\circ}\)C), the results show a slight increase in minimum strain rate with an increase in grain size dominant by the dislocation climb phenomenon. At high temperature and stress, dislocation climb controlled the power-law creep. Dislocation creeps (dislocation glide and dislocation climb) involve the vacancies or interstitial diffusion mechanism, and the dislocation glide movement overcomes the barrier by being thermally assisted where \(10^{-5}<\sigma/\text{G}<10^{-2}\) and T \(>0.4\text{T}_{\text{m}}\), as referred to in the deformation mechanism map. Dislocation climb is a dislocation motion mechanism. It allows an edge dislocation to move perpendicularly out of its lattice slip plane. The dislocation climb mechanism involves atom movement from the edge dislocation line to fill the vacancy. Smaller grain size has a greater ratio of grain boundaries surface area per unit volume; more grain boundaries react as barriers for dislocation mobility. When a dislocation is brought to a halt by an obstacle (precipitate) and barrier (grain boundary), the dislocation requires energy to overcome the obstacle and become mobile again [49; 50; 51; 52]. Dislocation climb requires a high temperature for an atom to dislocate from its lattice. Due to the specimen heat-treated at 1225 \({}^{\circ}\)C having smaller grain size with more ratio of grain boundaries surface area per unit volume compared to specimen heat-treated at 1250 \({}^{\circ}\)C, the grain boundaries can retard the dislocation mobility, and more energy is required for the dislocations to overcome the barriers and continue to glide. Figure 3f shows the creep rupture time versus grain size. The creep rupture time for a grain size of 62 \(\upmu\)m (heat-treated at 1050 \({}^{\circ}\)C) was 76 h, for 158 \(\upmu\)m (heat-treated at 1200 \({}^{\circ}\)C) it was 166 h, for 183 \(\upmu\)m (heat-treated at 1225 \({}^{\circ}\)C) it was 193 h and for 312 \(\upmu\)m (heat-treated at 1250 \({}^{\circ}\)C) it was 239 h. The graph shows that the creep rupture time increases with grain size. The creep rupture time was affected by the steady-state creep rate results. As the steady-state creep rate decreases with grain size, the creep rupture time increases with the increase in the grain size. Figure 4a illustrates the creep strain versus time curve of heat-treated Fe-40 Ni-24 Cr at temperature 900 \({}^{\circ}\)C and the initial stress of 100 MPa. All the creep curves show limited primary creep and immediately enter the secondary and tertiary creep stage due to high temperature and applied stress. The creep curves represented 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)--which were evaluated in this research. A sudden jump in strain value due to the sudden application of load bar (stress) is referred to as instantaneous creep strain, as shown in Figure 4b,c. The specimen with the smallest grain size has the highest grain boundary per area. Due to grain boundaries that can retard the dislocation movement from the sudden stress application, the specimen heat-treated at 1050 \({}^{\circ}\)C with the smallest grain size had the lowest instantaneous value, 0.056% strain.