## 3 Results and Discussion
### Morphology and Elemental Composition of Precipitated Phases  
According to previous studies, different types of precipitated phases are formed when alloying elements are added to steel. In addition to being the core of heterogeneous nucleation or as the particle point pinning the grain boundary to hinder the grain growth and achieve the purpose of refining the microstructure, these precipitated phases can also be distributed near the grain boundary to hinder the movement of dislocations and achieve the effect of strengthening the alloy [27]. Figure 10a,b shows that there are two forms in the precipitated phase of the alloy 2 specimen. One type forms in a \"chain-like\" pattern along the grain boundaries, while the other type appears as irregularly shaped spheres inside the grains. These carbides and nitrides distributed within grain boundaries and grains can effectively refine the microstructure of the invar alloy specimen, as shown in Figure 8. The EDS analyses of Figure 10d and Table 2 show that the main chemical composition in the precipitated phase inside the grains is Cr-Mo-V-N-C. V is a strong carbide-forming element, which is easy to combine with C to form VC. Moreover, there is a strong chemical affinity between N and V. Therefore, V can react with C and N to form carbon and nitrogen composite phases (NC, VC) [28; 29]. Cr and Mo are also carbide-forming elements and, therefore, can also react with C to form some carbides [30]. As can be seen from the EDS analysis in Figure 10f and Table 3, the precipitation phase at the grain boundaries contains a low content of Cr-Mo-V-C-N, indicating a low number of carbides and nitrides at the distributed grain boundaries. The reasons for this phenomenon include two main aspects: (1) Strong carbide-forming elements such as Cr, Mo, and V in the distributed grains have a strong trapping effect on C, which inhibits the diffusion of C, resulting in C elements that are not easy to aggregate at grain boundaries. (2) The precipitated phases at the  
Figure 9: The EBSD image quality mapping (**a**,**d**), inverse pole figures (**b**,**e**), as well as the grain boundary misorientation distributions (**c**,**f**) of the WAAM-fabricated invar alloy specimen: (**a**–**c**) alloy 1 specimen, (**d**–**f**) alloy 2 specimen.  
Figure 11 displays the element distribution inside the grain. In Figure 11a-e, the segregation of Cr, Mo, and Ni elements along the cell wall of the cellular substructure, and most of the precipitated phases within the grains are also distributed on the cell wall of the cellular substructure. In Figure 11k,l, the distribution position of V elements in the cell wall of the cellular substructure is highly coincident with C and N elements, indicating that V elements react with C and N elements to form some carbides and nitrides. In addition, the distribution positions of Cr and Mo basically coincide with C elements, and combined with the thermodynamic calculations in Figure 12, it can be seen that these carbides and nitrides are mainly VC, VN, Mo\({}_{2}\)C, Fe\({}_{20}\)Mo\({}_{3}\)C\({}_{6}\), and Cr\({}_{20}\)Mo\({}_{3}\)C\({}_{6}\). According to previous studies, there are high-density dislocations in the cell walls of the cellular substructure inside the austenitic steel grains. The elemental segregation and precipitation phases at the cell wall of the cellular substructure can effectively hinder the movement of dislocations, thereby improving the strength of the material [31; 32]. This is consistent with the results of the EBSD analysis in Figure 9.  
Figure 11: Elemental distribution analysis of precipitated phases inside the grains of alloy 2 specimen. (**a–f**) Distribution of elements at the cellular substructure. (**g–l**) Elemental distribution of precipitated phases at cellular substructure.  
Figure 12: Precipitation and transformation of the second phase, as calculated by FactSage.