## 4 Discussion  
### High-Temperature Creep Deformation and Mechanisms  
The Alloy 800H weldments with Inconel 625 filler and Haynes 230 filler materials exhibit a strengthened property in both high-temperature tensile and creep behaviors. This is mainly because of the higher hardness and apparent YS in the gauge section, which consists mainly of FZ and HAZ with a small volume factor of the unaffected base metal. The higher apparent 0.2% YS in the weldment caused a lower ratio of the applied stress to the apparent yield stress, about 0.40 versus 0.68, in the weldment specimens compared with the base metal specimen when the same stress of 80 MPa was applied in the current study. In the deformation mechanism maps developed by [PERSON] et al. [18], three creep mechanisms were presented: low-temperature power-law, high-temperature power-law, and Coble creep in Alloy 800H with an average grain size of 100 \(\upmu\)m, at test temperatures of 750 to 1020 \({}^{\circ}\)C and test stresses of 14.1 to 105 MPa. In this study, high-temperature power-law dislocation creep was likely to be the dominant mechanism in the Alloy 800H base metal (HAZ and unaffected base metal in the gauge section) under the current test conditions (stress and temperature). Moreover, the dislocation bypass on the precipitates by bowing around them (Orowan mechanism) or cutting through them (shearing mechanism) was an important contributor to facilitate dislocation slips in addition to the dislocation climb. The physical-based model based on the climb and Orowan mechanisms has been widely developed for creep rate and lifetime prediction [19; 20; 21]. [PERSON] et al. [22] developed a high-temperature creep model by considering the coexistence of climb, cutting, and Orowan mechanisms during creep processes, and the contribution of these mechanisms were considered to be dependent on the precipitate size and distribution relative to the slip plane. The considerable amounts of Orowan dislocation loops and cross slips in the Alloy 800H base metal material of the weldment specimens have provided strong evidence of the dislocation power-law creep and of the coexistence of climb, cutting, and Orowan mechanisms under the current test conditions. In the current study, both the base metal and the weldment specimens show a significantly longer tertiary creep stage than the primary and secondary stages, as shown in Figure 5. In particular, the secondary creep stage is relatively short. It took only 60 h (13%  
Figure 21: (**a**) SEM micrograph of fracture surface in the WM specimen with the Haynes 230 filler material, (**b**,**c**) fracture micrograph with a higher magnification. The arrow in (**c**) indicates an intergranular crack. of the total time) in the primary and secondary stages, and 407 h in the tertiary stage until final rupture in the base metal material. In comparison, the weldment specimen took about 30% of the total time to reach the tertiary stage. Tertiary creep is a stage characterized by an increased creep rate due to a high recovery rate. It is often associated with metallurgical changes, including coarsening of precipitate particles and formation of voids, ultimately leading to failure [23]. It is likely that the competitive process between the significant precipitation and growth of Ti(C, N) and M\({}_{23}\)C\({}_{6}\) played a role in the prolonged tertiary creep stage in the studied 800H and its weldment. On the one hand, the precipitation and growth of these carbides could have increased the creep resistance as obstacles of dislocation movement; on the other hand, it also caused the initiation and growth of the voids due to the growth of these precipitates, which will promote voids, cavities, and cracking especially at the grain boundaries. The latter process is aggressively more prevalent than the former with increasing creep time, so that the creep strain rate increases gradually at the tertiary stage.