## 3 Results
### Microstructures
#### 3.4.3 Microstructure after Creep Rupture
(**d**) Diffraction pattern taken at the [112] zone axis showing the reflections from Ti(C, N) and M\({}_{23}\)C\({}_{6}\), which were used to take the DF images in (**b,c**). Base Metal Close to Rupture Surface (Region 3 in Figure 6)  
Figure 16 provides the TEM bright field images taken from the base metal region close to the fracture surface at three different reflections, namely, g = 11\(\overline{1}\) (a), g = 1\(\overline{1}\)1 (b), and g = 111 (c). It can be seen that the dislocation structure in the form of dislocation loops is well aligned with the <110> {111} slip systems. These dislocation loops were produced due to the interaction between dislocation and precipitates via the Orowan dislocation-precipitate bypass mechanism [17]. Cross slips were also observed to take place, indicating that screw dislocations move from one slip plane to other {111} planes during creep deformation. The bypassed precipitates in the {111} slip planes were identified as Ti(C, N) with a size of about 10 nm. Figure 17 shows the distribution and morphology of M\({}_{23}\)C\({}_{6}\) and Ti(C, N) precipitates in the deformed Alloy 800H base metal section of the weldment specimen. They appeared in three main manners. The first manner was block-shaped M\({}_{23}\)C\({}_{6}\) (arrowed in Figure 17a) segregated at the grain boundary and having a size of hundreds of nanometers. The second was cube-shaped M\({}_{23}\)C\({}_{6}\) clustered adjacent to the grain boundary and also precipitated along the slip planes or dislocations. These had a size of about 50 nm and contained Moire fringes, as shown in the enlarged area in Figure 17b. The amount of these M\({}_{23}\)C\({}_{6}\) gradually decreased towards the grain interiors. The third was cube-shaped M\({}_{23}\)C\({}_{6}\) clusters that consisted of large blocky M\({}_{23}\)C\({}_{6}\) (hundreds of nanometers in size) and smaller ones (about 50 nm in size and containing Moire fringes), as shown in Figure 17c. Figure 17d provides an enlarged TEM BF image showing small M\({}_{23}\)C\({}_{6}\) about 50 nm in size (arrowed in (d)) containing Moire fringes and Orowan dislocation loops with Ti(C, N) precipitates. High-Temperature Ageing Structure in Incoloy 800H (Region 4 in Figure 6)  
Figure 18 reveals the microstructure of the un-deformed base metal. Cr-rich M\({}_{23}\)C\({}_{6}\) with Moire fringes and nano-sized Ti-rich Ti(C, N) carbides were the two predominant precipitates. Both types of precipitates tended to appear along the pre-existing dislocation within the microstructure. The absence of an Orowan dislocation loop surrounding the Ti(C, N) indicated that these carbides precipitated as a result of thermal ageing without interaction with the dislocation movement. This is in contrast to the observation in the gauge section presented in Figures 16 and 17d. Figure 16: TEM BF images showing the Orowan dislocation loops and the cross slips, taken at three different reflections. (**a**) g = 11\(\overline{1}\). (**b**) g = 1\(\overline{1}\)1. (**c**) g = 111. Fractography  
Figures 19-21 provide SEM micrographs of the fracture surface from the Alloy 800H BM specimens and the weldment specimens with Inconel 625 and Haynes 230 filler materials. The SEM micrographs reveal obvious surface oxidation on the fracture surface of all specimens due to prolonged exposure in air during the tests. The surface oxidation appears to be more significant in the weldment specimens compared with the BM specimens. This  
Figure 17: (**a–d**) TEM BF images of base metal in the necked region close to the fracture surface (**a**), M\({}_{23}\)C\({}_{6}\) near grain boundary (**b**), M\({}_{23}\)C\({}_{6}\) clusters with dislocation networks (**c**), and small M\({}_{23}\)C\({}_{6}\) containing Moiré fringes and Ti(C, N) precipitates with Orowan dislocation loops (**d**). Note that the arrows show the block-shaped M\({}_{23}\)C\({}_{6}\) segregated at the grain boundary.