# Characterization of Invar Syntactic Foams Obtained by Spark Plasma Sintering
## 3 Results and Discussion  
Microstructural analysis of the spark plasma sintered Invar/20%NiCrSiB foam was carried out on metallographic samples using SEM, as shown in Figure 4. The samples were prepared by sanding, polishing and etching with Nital 4%. It can be observed that the particle distribution of NiCrSiB is inhomogeneous (Figure 4a), and some of the thin-walled hollow superalloy particles are deformed, possibly due to the sanding process. The formation of sintering necks between the particles of the Invar matrix and between the particles of Invar and superalloy can also be observed (Figure 4b). Figure 4c,d (SEM images at high magnification) show discontinuities in sintering necks between Invar and NiCrSiB superalloy particles. The porosity of the Invar/20%NiCrSiB spark plasma sintered foams is of a bimodal type. Besides the pores of the empty NiCrSiB particles, there are also pores between the particles of the Invar matrix, as well as between the particles of Invar and the superalloy (Figure 4a,b). Also, in Figure 4c the presence of pores in the former Invar particles and the dendritic structure of the superalloy particles can be observed. The EDX analysis of the spark plasma sintered foams at the Invar-NiCrSiB interface highlighted the presence of alloying elements of the superalloy (Cr, Si, B) in the Invar matrix (Figure 5d-f). Also, carbon (C) was detected in the sintered foam structure, as shown in Figure 5g. C was found in greater amounts in the Invar matrix than in the NiCrSiB superalloy. The use of the graphite mold during spark plasma sintering, along with the high porosity of Invar/20%NiCrSiB, facilitated carbon diffusion into the foam structure. Although elements such as boron and carbon are difficult to highlight by EDX, for a qualitative assessment of the distribution mode in the sintered samples, their distribution maps are presented in Figure 5f,g to highlight the sintering bonds between the two powders. A relatively homogeneous distribution of boron can be observed both in the superalloy and in the Invar matrix (Figure 5f), while carbon is present in a higher content in the Invar matrix (Figure 5g). The silicon-rich areas in Figure 5e can be attributed to residual silicon carbide particles trapped in the pores of the sintered foam from the sanding process. Figure 4: SEM images of Invar\(/20\%\)NiCrSiB composite foam at different magnifications: \(35\times\) (**a**), \(100\times\) (**b**), \(1000\times\) (**c**) and \(5000\times\) (**d**). Figure 5: \(Cont\). The diffusive character of the Invar-NiCrSiB interface can also be observed through results obtained by line scan local chemical analyses Figure 6 shows an SEM image of the interface between the Invar matrix and a hollow NiCrSiB particle, and the variation in the concentration profile of the elements. It can be observed that the Fe content gradually decreases at the edge of the Invar particle. In contrast, the Ni content increases, since this element is present in higher quantities in the NiCrSiB powder. Therefore, it can be concluded that at the interface there is mutual diffusion, both of Fe in the wall of the NiCrSiB hollow particle and of Ni in the Invar matrix. This observation is valid for other elements (Si, Cr, C). The mutual diffusion distance is approximately 2.5 \(\upmu\)m. The existence of this elemental diffusion sustains the physical and chemical bond formed between particles upon sintering. Figure 7 shows the X-ray diffraction patterns of both the starting powders and sintered foams (Invar and NiCrSiB superalloy). In the diffraction pattern of the Invar powder, only its characteristic peaks are present. The X-ray diffraction pattern of NiCrSiB shows the characteristics peaks of the Ni-based solid solution and the peaks of the Ni\({}_{3}\)Si, Cr\({}_{2}\)Ni\({}_{3}\) and Ni\({}_{2}\)B chemical compounds. In the case of the sintered foams, only the characteristic maxima of Invar and the solid solution of iron in nickel with the CFC structure (which is present in the starting superalloy powders) were identified, respectively.