## 4 Discussion
### Precipitate Evolution during High-Temperature Crep  
The precipitates of the studied materials exhibited significant evolution and changes after long-term exposure at high temperature and stress. Carbides (MC or Ti(C, N), M\({}_{23}\)C\({}_{6}\), and M\({}_{6}\)C) have been commonly observed in the microstructure of FZ, HAZ, and base metals, with significant changes in their amount and morphology evident across different conditions: as-received, as-welded, and after creep. The thermal exposure and creep load led to a substantial change in the compositions and increase in morphological variations of carbides.  
The study reveals significant changes in the types and compositions of precipitates in the FZ of weldment specimens after creep tests. In both as-weld and post-creep conditions, the FZ of the Inconel 625 filler weldment exhibited an intermetallic Laves phase. A Laves phase is a hexagonal close-packed intermetallic compound with an AB\({}_{2}\)-type structure. The 'A' atoms, such as Fe, Ni, or Cr, and 'B' atoms, including Mo, Nb, or Si, play crucial roles in the formation of a Laves phase [27]. The Laves phase forms in Inconel 625 during solidification such as in a welding process, because the carbon content of Inconel 625 is generally sufficiently high to promote the formation of both carbides and intermetallics at the end of solidification [27; 28; 29]. The Laves phase also continues to precipitate during the prolonged heat treatment in Inconel 625 according to the TTT diagram [27]. Laves phases are extremely hard and not plastically deformable at room temperature. Therefore, the Laves phase has been considered to have a detrimental effect on the hardening of a material and encourages brittleness.  
In addition, a \(\delta\) phase was observed after the creep deformation in the FZ of the Inconel 625 filler WM specimen. The \(\delta\) phase initiates precipitation after approximately 20 h of heating within the temperature range of 650-980 \({}^{\circ}\)C in Inconel 625 [27]. The \(\delta\) phase is Nb- and Ni-enriched and has an acicular morphology that provides a distinctive needle-like or rod shape. The Laves particles are very similar in morphology to the blocky, irregularly shaped MC and M\({}_{23}\)C\({}_{6}\) carbides. Like the Laves phase, \(\delta\) particles formed during creep are also detrimental and reduce the ductility and toughness of the material. They can cause stress concentration and become a favorable location for the initiation and propagation of cracks. The solid solution strengthening can be significantly weakened, because he precipitations of these phases could deplete the solute atoms that are contributing in solid solution hardening and, therefore, reduce the strength of materials; corrosion resistance can also be reduced as a result of the solid solution depletion [27].  
In the current study, whereas the formation of \(\delta\) and Laves phases in the Inconel 625 filler material potentially introduces brittleness in the fusion zone, their effect on creep resistance is mitigated in comparison to the less creep-resistant Incoloy 800H base metal. The failure, therefore, occurs in the base metal rather than in the fusion zone. In contrast, the weld metal in the fusion zone of Haynes 230 did not exhibit deleterious \(\delta\) and Laves phases. Principal phases precipitated from solid solution were of the same types before and after creep. They were all carbides, i.e., Ti(C, N), Cr- and Mn-rich M\({}_{23}\)C\({}_{6}\) carbides, and W-, Mo-, Nb-, Si-, Ti-, and Al-rich M\({}_{6}\)C carbides. The observation that the weldment with Haynes 230 filler material maintained the same types of precipitates before and after creep, particularly carbides, suggests better phase stability compared to the Inconel 625 filler material.