Open AccessArticle

Characterization of Invar Syntactic Foams Obtained by Spark Plasma Sintering

Argentina Niculina Sechel

^1,*

Călin-Virgiliu Prică

Traian Florin Marinca

Florin Popa

Loredana-Maria Baglaevschi

¹,

Gyorgy Thalmaier

and

Ioan Vida-Simiti

^1,2

Department of Materials Science and Engineering, Technical University of Cluj-Napoca, Muncii Ave. 103-105, 400641 Cluj-Napoca, Romania

Romanian Technical Sciences Academy, 26 Dacia Boulevard, 010413 Bucharest, Romania

Author to whom correspondence should be addressed.

Appl. Sci. 2025, 15(6), 2932; https://doi.org/10.3390/app15062932 (registering DOI)

Submission received: 23 January 2025 / Revised: 20 February 2025 / Accepted: 5 March 2025 / Published: 8 March 2025

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Figure 1
Optical images of Invar powder (a) and NiCrSiB superalloy (b). "> Figure 2
Optical images of the wall thickness (g) of some NiCrSiB superalloy particles; (a) g ≅ 6 μm, (b) g ≅ 26 μm, (c) g ≅ 54 μm. "> Figure 3
Photo images of spark plasma sintered Invar/20%NiCrSiB syntactic foam sample. "> Figure 4
SEM images of Invar/20%NiCrSiB composite foam at different magnifications: 35× (a), 100× (b), 1000× (c) and 5000× (d). "> Figure 5
EDX maps of element distributions of Invar/20%NiCrSiB syntactic foam – mixed elements map distribution (a), map of the Fe (b), map of the Ni (c), map of the Cr (d), map of the Si (e), map of the B (f) and map of the C (g). "> Figure 5 Cont.
EDX maps of element distributions of Invar/20%NiCrSiB syntactic foam – mixed elements map distribution (a), map of the Fe (b), map of the Ni (c), map of the Cr (d), map of the Si (e), map of the B (f) and map of the C (g). "> Figure 6
SEM image and EDX line scan of Invar/20%NiCrSiB sample. "> Figure 7
XRD diffraction patterns of Invar 16 h milled powders (a), NiCrSiB hollow particles (b) and Invar/20%NiCrSBi spark plasma sintered composite foam (c). "> Figure 8
Elongation variation (Δl) as a function of temperature for syntactic foam Invar/20% NiCrSiB and for Invar [<a href="#B17-applsci-15-02932" class="html-bibr">17</a>]. ">

Versions Notes

Abstract

This study presents the synthesis of sintered composite foams based on the Invar alloy (64Fe-36Ni), using hollow spherical particles from a nickel superalloy (NiCrSiB) in order to generate porosity. The Invar powder was obtained by mechanical alloying (MA), and the NiCrSiB hollow spherical particles were incorporated into the composite at 20 vol %. The sintering was realized using the spark plasma sintering (SPS) process in an argon atmosphere at 600 °C and 5 MPa, with 10 s holding time. The porous structures were structurally characterized by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The coefficient of linear thermal expansion (CTE) of the Invar/NiCrSiB syntactic foams was found to be 2.52 × 10⁻⁶ °C⁻¹ in the 25–150 °C temperature range and 19.68 × 10⁻⁶ °C⁻¹ in the 150–400 °C range.

Keywords:

Invar syntactic foams; spark plasma sintering; coefficient of linear thermal expansion

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 ceramic particles with high porosities have been studied, including cenospheres [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-36wt.% 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 °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 cenospheres and hollow glass particles, in which the Invar matrix is obtained from iron and nickel elemental powders [9,10]. J. Weise et al. [10] studied the effect of the Ni and Fe particle size on the compressive and tensile strength of syntactic Invar foams with 5% glass microspheres, obtained by metal injection molding (MIM). The influence of the deformation speed of extruded and sintered Invar foams with 5% and 10% glass microspheres on mechanical behavior was also investigated by L. Peroni et al. [11]. In the literature, there are relatively few studies regarding the consolidation of syntactic foams with Invar matrices by spark plasma sintering; therefore, this study aims to complete the gap in knowledge in this field.

The aim of this work is to obtain syntactic foams of Invar with NiCrSiB superalloy hollow spherical particles through specific powder metallurgy techniques, and to study the influence of the addition of NiCrSiB on the thermal expansion coefficient of Invar. Superalloy powders from the Ni–Cr–Si–B(±C) system, obtained by gas atomization, are used as deposited layers, especially to increase the corrosion and oxidation resistance at high temperatures [21,22]. The CTE value of the NiCrSiB superalloy foams, obtained by sintering of the hollow spherical particles (similar to those used in this research), is approximately 18.2 × 10⁻⁶ °C⁻¹ in the temperature range of 25–400 °C [20].

The novelty of the research presented in this paper is in the synthesis of a syntactic foam comprised of a metal matrix (Invar), with the pore-forming agent being NiCrSiB superalloy hollow spherical particles (also a metal). This metallic foam would combine the characteristic property of Invar (low CTE) with the high temperature stability of the superalloy, while also having a low density.

2. Materials and Methods

Invar alloyed powder (64Fe-36Ni) 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 µm, with the following parameter values: D90 = 199.4 µm, D50 = 54.3 µm, D10 = 24.8 µm. 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 µm. 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_α radiation; λ_Co K_α = 0.1790300 nm). The diffraction patterns were recorded in the angular range of 2θ = 20–100°. 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 μm), and the platen wheel speed was 250 rpm. The thickness of the particle walls was in the range of 6–55 μm, 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.

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 °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 °C was studied by dilatometric analysis using an Ulbricht-Weiss dilatometer.

The density was measured using the following process. SPS cylindrical samples (Figure 3) were weighed, the volume of sample was computed and the density (ρ_s in g/cm³) was determined as the ratio between the mass and volume of the samples using the following relationship:

ρ_{s} = \frac{m}{V} = \frac{4 \cdot m}{π {\cdot d}^{2} \cdot h} [\frac{g}{{c m}^{3}}]

(1)

where m is the sintered sample mass (g), V is the sintered sample volume (cm³), d is the sample diameter (cm) and h is the sample height (cm). The value of the Invar/20%NiCrSiB composite’s sintered density was calculated to be 3.13 g/cm³.

The relative density of the spark plasma sintered foams (ρ_r) was computed using the following relationship:

ρ_{r} = \frac{ρ_{s}}{ρ_{t}}

(2)

where ρ_s is the sintered density, and ρ_t is the theoretical density of the composite foam (g/cm³).

The theoretical density of the composite foam (ρ_t in g/cm³) was determined using the mixture relationship rule, as shown in Equation (3). The theoretical density of the NiCrSiB superalloy is 6.72 g/cm³ and that of the Invar alloy is 8.22 g/cm³.

ρ_{t} = \sum f_{i} \cdot ρ_{i}

(3)

where f_i is the volume fraction and ρ_t is the theoretical density of both components of the Invar syntactic foam.

The porosity values (P) were computed with the following relationship:

{P = (1 - ρ}_{r}) \cdot 100

(4)

The porosity values of the sintered Invar/20%NiCrSiB foam were found to be 40%.

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.

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 μm. 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 Ni₃Si, Cr₂Ni₃ and Ni₂B 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. The peaks of the chemical compounds from the NiCrSiB superalloy powder were no longer highlighted, possibly due to their small amount (below the detection limit of diffractometer), as well as the fact that the spherical NiCrSiB particles are hollow. The mean sizes of the Invar crystallites, computed with the Scherrer formula, increased from 14 nm (mechanically alloyed powder) to 30 nm (600 °C spark plasma sintered material).

Invar/20%NiCrSiB foams were subjected to dilatometric analysis by heating up to 400 °C. The variation in sample elongation (Δl) with increasing temperature are graphically rendered in Figure 8 and compared with results obtained for (SPS) sintered Invar from a previous study [17]. In the case of syntactic foams, an insignificant change in elongation can be observed up to a temperature of 150 °C. In the temperature range of 150–400 °C, elongation increases continuously and proportionally with temperature. This suggests that the Invar/20%NiCrSiB porous structure is characterized by two different coefficients of thermal expansion (CTE), α₁ for the 25–150 °C and α₂ for the 150–400 °C temperature range.

The values of the coefficient of thermal expansion (α) were computed using the following relationship:

α = \frac{1}{l_{o}} \cdot \frac{Δ l}{Δ T}

(5)

where l_o is the original sample length, Δl is the sample elongation and ΔT is the temperature range.

Table 2 shows the CTE values of the Invar/20% NiCrSiB spark plasma sintered foam.

Based on the data presented in Figure 8 and Table 2, it can be concluded that the addition of hollow spherical NiCrSiB particles in the Invar matrix leads to an increase in the coefficient of thermal expansion, from 0.6 × 10⁻⁶ °C⁻¹ of the reference Invar [17] to 2.52 × 10⁻⁶ °C⁻¹, up to 150 °C.

4. Conclusions

The syntactic foams of Invar/20%NiCrSiB were successfully obtained by spark plasma sintering from mechanically alloyed Invar powder and gas atomization-produced NiCrSiB hollow particles.

A porosity of 60% was obtained for the samples sintered at 600 °C with a 10 s holding time. Also, the average crystallite sizes of Invar were maintained in the nanoscale range, around 30 nm. The density was as low as 3.13 g/cm³.

The interface between Invar and NiCrSiB had a diffusive character, with the mutual diffusion distance being approximately 2.5 μm.

From the dilatometry analyses, it was concluded that the thermal expansion coefficient of the Invar/20%NiCrSiB syntactic foam obtained by SPS remained approximately equal to that of Invar in the temperature range of 25–150 °C (2.52 × 10⁻⁶ °C⁻¹). In the temperature range of 150–400 °C, the CTE suddenly increased to 19.68 × 10⁻⁶ °C⁻¹. From this point of view, the Invar/20%NiCrBSi syntactic foam has the advantage of keeping its CTE low up to 150 °C along with its low density (3.13 g/cm³).

Author Contributions

Conceptualization, A.N.S. and C.-V.P.; methodology, A.N.S.; software, C.-V.P., F.P. and T.F.M.; validation, A.N.S., C.-V.P. and I.V.-S.; formal analysis, F.P. and T.F.M.; investigation, A.N.S., L.-M.B., C.-V.P.; resources, I.V.-S. and G.T.; data curation, A.N.S. and L.-M.B.; writing—original draft preparation, A.N.S. and C.-V.P.; writing—review and editing, A.N.S. and C.-V.P.; visualization, F.P., T.F.M., L.-M.B., G.T. and I.V.-S.; supervision, A.N.S.; project administration, A.N.S.; funding acquisition, A.N.S., G.T. and I.V.-S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Optical images of Invar powder (a) and NiCrSiB superalloy (b).

Figure 2. Optical images of the wall thickness (g) of some NiCrSiB superalloy particles; (a) g ≅ 6 μm, (b) g ≅ 26 μm, (c) g ≅ 54 μm.

Figure 3. Photo images of spark plasma sintered Invar/20%NiCrSiB syntactic foam sample.

Figure 4. SEM images of Invar/20%NiCrSiB composite foam at different magnifications: 35× (a), 100× (b), 1000× (c) and 5000× (d).

Figure 5. EDX maps of element distributions of Invar/20%NiCrSiB syntactic foam – mixed elements map distribution (a), map of the Fe (b), map of the Ni (c), map of the Cr (d), map of the Si (e), map of the B (f) and map of the C (g).

Figure 6. SEM image and EDX line scan of Invar/20%NiCrSiB sample.

Figure 7. XRD diffraction patterns of Invar 16 h milled powders (a), NiCrSiB hollow particles (b) and Invar/20%NiCrSBi spark plasma sintered composite foam (c).

Figure 8. Elongation variation (Δl) as a function of temperature for syntactic foam Invar/20% NiCrSiB and for Invar [17].

Table 1. Chemical composition and theoretical density of used powders.

Powders	Chemical Composition [wt.%]					Theoretical Density [g/cm³]
Powders	Ni	Fe	Cr	Si	B	Theoretical Density [g/cm³]
Invar	36	64	-	-	-	8.22
Superalloy NiCrSiB	70.19	12.34	7.08	6.73	3.66	6.72

Table 2. The CTE values of Invar/20%NiCrSiB spark plasma sintered foam.

Syntactic Foam	Temperature Range, ΔT (°C)	α (×10⁻⁶ °C⁻¹)
Invar/20%NiCrSiB	25–150	2.52
Invar/20%NiCrSiB	150–400	19.68

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Sechel, A.N.; Prică, C.-V.; Marinca, T.F.; Popa, F.; Baglaevschi, L.-M.; Thalmaier, G.; Vida-Simiti, I. Characterization of Invar Syntactic Foams Obtained by Spark Plasma Sintering. Appl. Sci. 2025, 15, 2932. https://doi.org/10.3390/app15062932

AMA Style

Sechel AN, Prică C-V, Marinca TF, Popa F, Baglaevschi L-M, Thalmaier G, Vida-Simiti I. Characterization of Invar Syntactic Foams Obtained by Spark Plasma Sintering. Applied Sciences. 2025; 15(6):2932. https://doi.org/10.3390/app15062932

Chicago/Turabian Style

Sechel, Argentina Niculina, Călin-Virgiliu Prică, Traian Florin Marinca, Florin Popa, Loredana-Maria Baglaevschi, Gyorgy Thalmaier, and Ioan Vida-Simiti. 2025. "Characterization of Invar Syntactic Foams Obtained by Spark Plasma Sintering" Applied Sciences 15, no. 6: 2932. https://doi.org/10.3390/app15062932

APA Style

Sechel, A. N., Prică, C.-V., Marinca, T. F., Popa, F., Baglaevschi, L.-M., Thalmaier, G., & Vida-Simiti, I. (2025). Characterization of Invar Syntactic Foams Obtained by Spark Plasma Sintering. Applied Sciences, 15(6), 2932. https://doi.org/10.3390/app15062932

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