# Characterization of Invar Syntactic Foams Obtained by Spark Plasma Sintering
## 1 Introduction  
Metallic foams (metallic structures with high porosity) are of high scientific interest mainly due to their low density and unique mechanical and physical properties, such as shock, vibration and sound absorption capacity and thermal conductivity [1, 2]. Depending on the type of pores (closed or open), metallic foams find their applicability in various fields of technology, as functional or structural materials [1, 2, 3]. The most intensively studied materials used to obtain metallic foams are based on aluminum, due to their low density and low processing temperature. Thus, these materials have been applied in the production of light structures used in the automotive, aerospace or construction industries [1, 3, 4]. Besides the type of material used to manufacture the foams, their properties are given by their relative density, type of pores/cells and the shape, size and distribution of the pores in the structure [1, 2, 4]. The characteristics of metallic foams are determined by their production technique; in general, these are classified according to the state of aggregation of the material, i.e., liquid, solid or vapor phase elaboration methods [1, 2, 4, 5]. In order to optimize the strength and thermal properties of metallic foams, many researchers are currently oriented towards the development of foams from composite materials [3, 6]. A special category of high-porosity composite materials is represented by syntactic foams, in which the pores are generated inside the matrix by using hollow spherical particles or particles with high internal porosity as \"reinforcing elements\" [4, 6, 7, 8, 9]. In the case of syntactic foams reinforced with a metallic matrix, different types of ceramicparticles with high porosities have been studied, including nanospheres [9], glass [10,11], expanded perlite [12], expanded clay [13] and different natural/volcanic rocks [14]. Also, metallic hollow particles have been used in several studies to generate porosity in a metal matrix (steel [3], iron [8] and a Ni-based superalloy [7]). The most common methods used for obtaining syntactic foams are infiltration from liquid state for matrices with a low melting point (aluminum [6,8], magnesium [14]) and solid phase processing, particularly for ferrous alloys [4]. The performance of syntactic foams is mainly determined by the nature of the components, the compatibility of the matrix and particles that create the pores, the interface bond, but also by the particle morphology (size, wall thickness) and volume fraction [3,4,6,7,13]. Although the Invar36 alloy (Fe-36 wt.% Ni) has been widely used on an industrial scale due to its thermal stability and low thermal expansion coefficient (CTE) in a wide range of temperatures (0-250 \({}^{\circ}\)C), it continues to be investigated [11,15,16,17,18,19]. Studies on Invar aim to optimize its processing in order to obtain a favorable structure with the lowest possible CTE value [16,19]. The austenitic structure of Invar leads to both a low mechanical machinability and a low mechanical strength. Thus, much of the research is oriented towards overcoming these disadvantages by designing new Invar-based composites [17] or by applying/optimizing the manufacturing technologies [15,18,19]. On the other hand, a reduction in the density of Invar by creating porosity in the structure would be a great advantage in several fields, including aerospace [9,19]. It is necessary to point out that the CTE does not depend on the material porosity [1,16,20]. Thus, Invar-based cellular structures can find new applications, due to the combination of the specific properties of syntactic foams and their dimensional stability at high temperatures. Several researchers have focused on the study of sintered syntactic foams, reinforced with nanospheres and hollow glass particles, in which the Invar matrix is obtained from iron and nickel elemental powders [9,10]. [PERSON] et al.