# Characterization of Invar Syntactic Foams Obtained by Spark Plasma Sintering
## 2 Materials and Methods  
Invar alloyed powder (64 Fe-36 Ni) was used to prepare the foams. The Invar powder was obtained from Ni and Fe elemental powder by mechanical alloying in a planetary ball mill (Frich Pulverisette 6-FRITSCH GmbH, Oberstein, Germany) in an argon atmosphere for 16 h, according to the reference [23]. The average particle size of the Invar powder was \(-\)300 \(\upmu\)m, with the following parameter values: D90 = 199.4 \(\upmu\)m, D50 = 54.3 \(\upmu\)m, D10 = 24.8 \(\upmu\)m. Figure 1 shows the morphology of the Invar (Figure 1a) and NiCrSiB (Figure 1b) powders used in this study. It can be observed that the Invar particles have an irregular shape with rounded edges, and the NiCrSiB superalloy particles are spherical. To generate the pores in the Invar structure, a quantity of 20% nickel-based super-alloy hollow sphere particles (NiCrSiB) obtained by gas spraying from the liquid phase were used. The NiCrSiB powder was produced in the home-made atomization equipment within the Department of Materials Science and Engineering of the Technical University of Cluj-Napoca. The particle sizes of the superalloy powder were in the range of 500-700 \(\upmu\)m. Table 1 presents the chemical composition and theoretical density of the raw powders. Both powders were analyzed by optical microscopy (Optika Microscopes, Ponteranica, Italy) and X-ray diffraction (Inel Equinox 3000 diffractometer, Artenay, France, operating with Co K\({}_{\alpha}\) radiation; \(\lambda_{\mathrm{Co}}\) K\({}_{\alpha}\) = 0.1790300 nm). The diffraction patterns were recorded in the angular range of 20 = 20-100\({}^{\circ}\). In order to compute the average crystallite sizes, the Scherrer relationship was calculated [24]. The porosity was evaluated using optical images of the hollow spherical NiCrSiB particles, embedded in epoxy resin, sanded, polished and etched with 4% Nital (Figure 2a-c). The polishing was carried out with a machine from Buehler (IL, USA), using a cloth disc with alumina suspension (1 \(\upmu\)m), and the platen wheel speed was 250 rpm. The thickness of the particle walls was in the range of 6-55 \(\upmu\)m, and the porosity was in the range of 60-80%. The mixture of Invar powders with 20 vol % hollow spherical particles of the NiCrSiB superalloy (Invar/20%NiCrSiB) was homogenized for 15 min in a Turbula-type spatial homogenizer (3D). During mixing, the segregation of empty superalloy particles was observed. To ensure a more homogeneous distribution of the particles in the Invar powder, less than 1% ethanol was added. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Powders**} & \multicolumn{4}{c}{**Chemical Composition [wt.\%]**} & \multicolumn{2}{c}{**Theoretical**} \\ \cline{2-7}  & **Ni** & **Fe** & **Cr** & **Si** & **B** & **Density [g/cm\({}^{3}\)]** \\ \hline Invar & 36 & 64 & - & - & - & 8.22 \\ Superalloy NiCrSiB & 70.19 & 12.34 & 7.08 & 6.73 & 3.66 & 6.72 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition and theoretical density of used powders. Figure 1: Optical images of Invar powder (**a**) and NiCrSiB superalloy (**b**). The spark plasma sintering (SPS) of the 80%Invar + 20%NiCrSiB powder mixture was carried out in a cylindrical graphite mold with a diameter of 15 mm, at 600 \({}^{\circ}\)C, with a holding time of 10 s. During SPS, the applied pressure was only 5 MPa to ensure contact between the powder particles and to avoid deformation of the NiCrSiB hollow particles. For the SPS process, homemade equipment was used. A JEOL-JSM 5600 LV (Tokyo, Japan) scanning electron microscope (SEM) coupled with an energy-dispersive X-ray (EDX) spectrometer UltimMAX65 (Oxford Instruments, Aztec software, version 4.2, High Wycombe, UK) was used for the investigation of sintered Invar/20%NiCrSiB samples. The sintered samples were also investigated by X-ray diffraction. The effect of the addition of 20% NiCrSiB superalloy on the coefficient of thermal expansion (CTE) up to 400 \({}^{\circ}\)C was studied by dilatometric analysis using an Ulbricht-Weiss dilatometer.