## 3 Results and Discussions
### Microstructures of the Alloys after Creep Tests  
The microstructures of the creep-tested samples were observed to understand the effects of the introduced Laves phase particles and their precipitation routes on the creep rates of the 9 Cr-based ferritin alloys. Figure 7 shows BSE images of the 0.09 Hf and the 0.33 Hf alloys after creep tests interrupted in the early stage of their week creep acceleration regimes under the condition of 650 \({}^{\circ}\)C/60 MPa. No significant microstructural change from their initial states was observed in the low-magnification micrographs in both the specimens, as seen in Figure 7a,b. High-resolution observations under an ECCI condition in the 0.09 Hf alloy show a high density of fine precipitates and dislocations which are bulged between the precipitates (Figure 7c). This observation with an EDS analysis (see Figure 8) suggests that fine Fe\({}_{2}\)Hf phase particles were precipitated from the supersaturated ferrite matrix during the creep test, and the fine precipitates (hereafter called CP particles) act to retard the motion of dislocations from the transient creep stage through the early stage of the weak creep acceleration stage. The high-magnification micrograph for the 0.33 Hf alloy demonstrates that the fine distribution and size of IP particles almost remain until the early acceleration creep regime (Figure 7d). A smaller number of dislocations were visible in the observation under an ECCI condition in the 0.33 Hf alloy sample showing the creep curves of the alloys obtained under the conditions indicated. The 0.09 Hf alloy shows the longest creep life under the condition of 650 \({}^{\circ}\)C/60 MPa while the 0.33 Hf alloy shows the longest life under the condition of 700 \({}^{\circ}\)C/30 MPa. A clear stress dependence was found, as seen in a Larson-Miller-type plot for the present results in Figure 4; the 0.09 Hf alloy shows the longest creep life at higher stress conditions whereas  
Figure 5: Creep rate vs. time curves in a logarithmic time scale for the creep tests shown in Figure 3: (a) 650 \({}^{\circ}\)C/60 MPa [11] and (b) 700 \({}^{\circ}\)C/30 MPa. Figure 6: Creep rate vs. time curves in a liner time scale for the creep tests shown in Figure 3: (a) 650 \({}^{\circ}\)C/60 MPa [11] and (b) 700 \({}^{\circ}\)C/30 MPa. the 0.33 Hf alloy shows the longest creep life at lower stress conditions. The creep data are summarized in Table 2.  
Figure 8 shows STEM bright field images and EDS Hf maps of the samples corresponding to Figure 7. It can be seen that CP particles are Hf-rich and are formed not only on dislocations within grains/subgrains but also at grain boundaries (sub-boundaries) in the 0.09 Hf alloy, and the amount and size of the particles are somewhat larger at the boundaries than on the dislocations (Figure 8a,c). In the 0.33 Hf alloy, a high density of dislocations is visible in the ferrite grains and the dislocations are bulged between IP particles (Figure 8b). It should be noted that the size and spatial distribution are more uniform for IP particles in the 0.33 Hf alloy than CP particles in the 0.09 Hf alloy. Figure 9 shows BSE images of the alloys after creep rupture under the condition of 650 \({}^{\circ}\)C/60 MPa. Low-magnification micrographs taken from the gauge length of the alloys exhibit equiaxed ferritic grains, which indicates that recovery and recrystallization took place extensively in the late stage of the creep tests in the alloys. High-magnification observation demonstrates that CP particles in the creep-ruptured sample are less dense and larger compared to the creep-interrupted sample, which suggests that particle coarsening occurred in the late stage of the creep tests. Coarsening of IP particles is also observed in the 0.33 Hf alloy, but their density is still high and higher than that of CP particles in the 0.09 Hf alloy, as seen in Figure 9c,d.