Functionally Gradient Material Fabrication Based on Cr, Ti, Fe, Ni, Co, Cu Metal Layers via Spark Plasma Sintering

Abstract

The paper presents a method of obtaining functionally graded material (FGM) of heterogeneous (layered) type based on joined metals Cr-Ti-Fe-Co-Ni-Cu using spark plasma sintering (SPS) technology. The structure, elemental and phase composition of FGM obtained on the basis of joined metals with different values of the temperature coefficient of linear expansion (CTLE) were studied by SEM, EDS and XRD methods with regard to the phase states of the alloy system. Based on the Vickers microhardness data, the evaluation of the mechanical characteristics of FGM in the whole sample body and locally at the contact boundaries of the joined metals was carried out. The results of the study are new and represent a potential for FGM, as well as functionally graded coatings (FGC), which have special physical, chemical and mechanical properties and are highly demanded for the manufacture of structures and products for industrial applications.

1. Introduction

In view of the emergence of new production technologies, materials science is actively moving towards advanced composites with a special set of performance features and properties unusual for traditional materials. In this context, a new class of composite materials, referred to as functional-gradient materials (FGM) [1], which can also be embodied as functional-gradient coatings (FGC) [2], has attracted considerable attention from the scientific community due to inherent special physical, chemical and mechanical characteristics and properties.

FGM combines the characteristics and properties of two or more materials. The structure of FGM can be of two types: homogeneous, when heterogeneous materials are distributed equally within each other throughout the entire volume, and heterogeneous (layered), when various materials are interconnected at a specific boundary of their contact. FGM is characterized by a gradient in the chemical and physical nature of the constituent materials, i.e., mainly the composition and structure of the composite change throughout its volume, which leads to a change in the properties of the entire system. Regarding the composition, monophasic FGMs are distinguished (rare), in which the miscible materials are soluble in each other in the range of conditions of their homogenization and chemical composition with the formation of one phase and multiphase (widely used)—the change in the chemical composition of one material in another is clearly traced throughout the volume, which leads to a significant gradient of properties. However, it should be noted that the formation of these phases depends on the conditions of obtaining and processing FGM, in particular, on the modes of heat treatment and cooling rate [3].

Potential applications for FGMs include energy, radiation technology, mechanical engineering, optoelectronics, semiconductors, biosystems, cutting tools and more [4]. These applications require resistance to wear, heat, mechanical shock, hardness, radiation, corrosion, etc. The combination of these properties in one material without losing the actual functionality of the constituent materials is only possible with FGM.

The majority of FGMs, which attract attention in the scientific community, are related to the aerospace industry. In this industry, reliability and safety are of primary importance. These materials have such properties as a high strength-to-weight ratio, high-temperature resistance and corrosion resistance. In particular, developments of thermal barriers for descent spacecraft [5,6], made based on the SiC/C system, and the fabrication of electromagnetic energy absorbers and accumulators for military aviation and rocket systems are known. The development of solar receivers of TiC/Mo composition for space energy [7,8], heat-protective coatings [5,9] ZrO₂-NiCrAlY for gas-turbine engines [9], and machining tools [4] based on metal carbides systems resistant to thermomechanical loads. FGMs are of separate applied interest as bonded composites, e.g., metal–ceramic type, for gas turbine engines and reentry vehicles, which are produced by diffusion welding, e.g., Al₂O₃ with nickel superalloy [10].

An important problem of FGMs, particularly of the layered type, is their delamination or volume failure due to the big difference in the physicochemical nature of the bonded materials. In particular, heterogeneous materials have a difference in the coefficient of linear thermal expansion (CTLE). In the case of heating and cooling during the manufacture of FGM materials or the operation of products based on them, strong internal mechanical stresses in the volume arise, which leads to mechanical failure [11]. In this regard, obtaining heterogeneous FGMs is a difficult technological task. Even more problematic is the connection of materials with different melting temperatures. The solution is found using different ways, including soldering of materials or diffusion sintering using intermediate bonding layers of materials with intermediate CTLE. These layers play the role of a damping base and compensate internal stresses at the point of contact of dissimilar materials being joined, thereby maintaining the integrity of the entire product [12,13,14]. In addition, a key role in the manufacturing process of FGMs or FGPs is played by their production technologies, which are now available in a wide variety [1,4,15]. In this case, one should highlight the high potential of spark plasma sintering (SPS) technology [16,17,18,19,20,21,22,23,24], which is associated with the uniqueness of the heating mechanism when combining dissimilar materials in the process of FGM production [25]. The established advantages of SPS over conventional technologies are low sintering temperatures, short sintering cycle time (minutes), low power consumption (about 1/5 of GP) [26], homogeneous heating of the material, control of the temperature gradient, no sintering additives and plasticizers, sintering powders with a wide grain size distribution, achieving maximum material density (up to 100% of the theoretical), one-step sintering and cleaning the particle surface under the influence of current. In addition, high-speed heating provides the formation of unique physical and chemical characteristics and properties of the obtained materials. It is possible to synthesize materials with the preservation of initial grain size and microstructure, formation of different porosity, and achieving high density and structural strength of compacts. Synthesis of metastable materials with preservation of this state is provided. Phase transformation control is achieved due to the exclusion or initiation of solid-phase chemical interactions during material processing. Due to the high-speed heating by pulsed electric current, high local heating in the contact areas of the joined material boundaries is achieved, and a high rate of atomic diffusion occurs without overheating the entire volume of the heated billet [11,25]. In this regard, it is possible to exclude a number of negative effects, in particular, to reduce the effect of internal stresses and to preserve the integrity of the product.

The number of studies on SPS application for the formation of coupled-type FGM is limited, and for a wide range of systems, it is completely absent. In particular, the literature does not present studies on the coupling of metals with very different melting temperatures and CTLE values to obtain FGMs. In this regard, the present work investigated a method of obtaining FGM of the heterogeneous (layered) type consisting of metals with different coefficients of thermal linear expansion (Cr—5.1 × 10⁶ °C⁻¹, Ti—7.7 × 10⁶ °C⁻¹, Fe—11.3 × 10⁶ °C⁻¹, Co—12 × 10⁶ °C⁻¹, Ni—13.4 × 10⁶ °C⁻¹, Cu—16.7 × 10⁶ °C⁻¹ [27,28] using SPS, including the study of physical, chemical and mechanical characteristics concerning to the phase states of the FGM alloy system. The results of the study are new and represent a prospect for the FGM and FGC on their basis in the fabrication of structures and products for industrial applications.

2. Materials and Methods

Reagents. The following metal powders were used for the FGM composite: Cr, Ti, Fe, Co, Ni and Cu (pure 99.9%, Sigma-Aldrich, St. Louis, MI, USA). Powders were initially stirred in a 700 rpm planetary mill for 10 min for each mixture. The balls made of tungsten carbide (WC) with diameters of 10 mm and 5 mm were used as grinding bodies. A total of 50 g of powder was milled in the presence of 15 balls (total mass of 125 g).

Methods of FGM composite fabrication. The FGM composite was obtained using a spark plasma sintering machine SPS-515_S installation (Dr. Sinter*LAB^TM, Saitama, Japan) in two consecutive steps:

(1)

Consolidation of Cr and Ti powders into a two-layer Cr-Ti compact by pouring them layer by layer into a graphite mold and heating at 1200 °C for 5 min, with a pressing pressure of 24.5 MPa;

(2)

Consolidation of Cr-Ti and Fe, Co, Ni and Cu (in the order from the bottom to the top vertically) powders in a graphite mold at 900 °C for 5 min at pressing pressure of 24.5 MPa.

The sintered powder was placed in a cylindrical shape graphite mold, pressed (20 MPa), then the blank was placed in a sintering vacuum chamber (6 Pa). Sintering was carried out with a pulse current heating rate of 100 °C/min and cooling time of 40 min. The characteristic of the pulsed current in On/Off mode was 12/2 packets (pulse packet duration 39.6 ms/pause 6.6 ms). In order to prevent the consolidated powder from sticking to the mold and plungers, as well as to easily extract the obtained compound, 200 µm thick graphite foil was used. The process temperature was monitored using an optical pyrometer IR-AHS (Hitachi, Tokyo, Japan) focused on a hole located in the middle of the plane of the mold’s outer wall with a depth of 5.5 mm. The layered FGM sample had a cylindrical shape with a diameter of 15.3 mm and a height of 10 mm.

Characteristics of the research methods. Particle size distribution was determined on a particle size analyzer Analysette-22 NanoTec/MicroTec/XT “FRITSCH” (Idar-Oberstein, Germany). Scanning electron microscopy (SEM) was carried out on a CrossBeam 1540 XB “Carl Zeiss” (Jena, Germany) equipped with the add-on for energy-dispersive spectral analysis (EDX) “Bruker” (Ettlingen, Germany). Vickers microhardness (HV) was determined at 0.2 N load on a microhardness tester HMV-G-FA-D “Shimadzu” (Tokyo, Japan). The data were systematized by 10 or more points measured randomly locally in each layer and at their contact boundaries. Based on the results of the measurements obtained, the box-and-whiskers diagram for all the layers and all interfaces was plotted. X-ray diffraction analysis (XRD) of the samples was carried out on a multipurpose X-ray diffractometer D8 Advance “Bruker AXS GmbH” (Karlsruhe, Germany), CuKα radiation, Ni-filter, average wavelength (λ) 1.5418 Å, range of angles 10°–80°, scanning step 0.02° and spectra registration rate—5°/min. For XRD, samples were prepared based on two metal components that correspond to the boundary layers (Cu-Ni, Ni-Co, Co-Fe, Fe-Ti and Ti-Cr). Powders were initially stirred in a 700 rpm planetary mill for 10 min for each mixture and then sintered under SPS conditions at 900 °C for Cu-Ni, Ni-Co, Co-Fe, Fe-Ti and 1200 °C for Ti-Cr (heating, exposure, pressing and cooling regimes were similar to those for FGM production). XRD imaging was performed from a cross-polished section for each sample (each metal pair).

3. Results

In the experiment, the heterogeneous FGMs were obtained according to the SPS technology using the initial powders of metals (Cr, Ti, Fe, Co, Ni, Cu) of different fractional compositions (Figure 1). The average particle size of the powder fractions was as follows: 20 and 45 μm for chromium (Figure 1a); 65 μm for titanium (Figure 1b); for iron, the largest fraction of powder particles occupied the size range of ~100 μm (Figure 1c); 14 μm for cobalt (Figure 1d); 15 μm with nano- and submicron size particles for nickel (Figure 1e); and 50 μm with the presence of nano component for copper (Figure 1f).

The results of cross-section microscopy of the FGM sample revealed that the defects (cracks) in the sample volume and at the contact boundary of the joined metals are absent (Figure 2). This indicates that the differences in the size of the initial particles presented above (Figure 1) do not significantly affect the structure of the obtained sample. The metals’ sintering proceeds uniformly both in the volume of the layers and at the boundary of their contact. The elemental composition of the layers of joined metals, according to the EDS data, is uniform over the entire contact area of their surfaces. The morphology of the obtained metal alloys is homogeneous, and the structure is close to monolithic without the presence of open and closed pores. Connections at the contact boundaries in all metal pairs Ni/Cu, Co/Ni, Fe/Co, Cr/Fe and Ti/Cr are uniform, without defects and metal melt zones. Contact zones in the pair Ni/Cu, Co/Ni and Ti/Cr do not have sufficiently defined boundaries, which is due to the formation of a uniform melt, as well as, probably, the tendency to the mutual dissolution of these metals in each other. According to EDS analysis (Figure 2), metal diffusion at the contact boundary is absent in Ni/Cu, Co/Ni, Fe/Co and Cr/Fe alloys and is only slightly observed for Cr in the Ti/Cr system.

The strength characteristics of the FGM sample were evaluated by determining the Vickers microhardness. The micro-homogeneity of the sample was studied, and a box-and-whiskers diagram was plotted (Figure 3). The data were systematized by 10 or more points measured randomly locally in each layer and at the boundaries of their contact. Statistical processing of the data shows that the hardness varies between 50 and 500 HV depending on the particular metal. It can be seen that the microhardness in the area of the contact boundary of Cu-Ni, Ni-Co and Co-Fe alloys corresponds to their average value. On the contrary, in the case of Fe-Ti and Ti-Cr alloys, the microhardness values in the area of metal contacts are much higher as compared to pure metals, which is probably caused by the formation of intermetallic phases.

According to XRD data (Figure 4), the composition of Cu-Ni, Ni-Co, Co-Fe and Fe-Ti alloys is characterized by pure metal phases for each alloy pair. The Ti-Cr alloy includes the metallic titanium phase and the intermetallic phase TiCr₂, which was assumed above based on of microhardness data of the sample.

In order to confirm the XRD data, including the formation of intermetallic phases in the studied alloy systems, one should consider the literature data on the phase diagrams of the systems. For this purpose, the systematization of the mentioned phase diagrams of the alloys under study was carried out. The Cu-Ni system is characterized by the formation during crystallization of a continuous series of solid solutions (Cu, Ni) with a face-centered cubic structure (Figure 5a) [29]. Co and Ni are infinitely soluble in each other and also form a series of solid solutions (Figure 5b) [30]. The liquidus and solidus temperatures differ by only a few degrees, so they merge into one line. The Fe-Co system phase diagram is shown in (Figure 5c) [31]. The transformation temperature decreases to room temperature in alloys with 91–93 at.% Co. The literature suggests the existence of superstructures based on the compositions Fe₃Co and FeCo₃, and in the case of recrystallization of the alloy, three more intermediate phases, Co₃Fe, FeCo and Fe₃Co, may exist. Additional research is needed to finally establish the nature of the equilibria in this system. The presence of intermetallic phases was not detected in the composition of the Fe-Co layer in the obtained FGM volume.

The Fe-Ti phase diagram (Figure 5e) [32] shows that titanium stabilizes the volume-centered cubic lattice modifications of iron and promotes the wedging out of the face-centered cubic modifications. Thus, the transition between δ- and α-Fe is continuous with temperature changes, and the region of δ-Fe existence is practically impossible to distinguish. Two intermetallic compounds are formed in the alloys of the system: TiFe₂ and TiFe. The first of these compounds crystallize with an open maximum at 1427 °C and has a fairly wide area of homogeneity within 10 at.% at 1300 °C. With decreasing temperature, this area narrows insignificantly. Compound TiFe is formed by peritectic reaction at 1317 °C; the area of its homogeneity does not exceed ~4 at.%. The ultimate solubility of titanium in α-Fe does not exceed 9.8 at.%. TiFe₂ + α-Fe eutectic crystallizes at 1289 °C, and titanium solubility in γ-Fe at 1100 °C is ~1.0 at.%. On the titanium side at 1085 °C, the TiFe + β-Τi eutectic crystallizes, and the eutectic point is located at ~71 at.% Ti. The maximum solubility of iron in β-Ti reaches 22 at.%. At about 590 °C, the eutectoid reaction occurs; β-Ti ↔ TiFe + a-Ti. iron solubility in α-Ti is negligible. The maximum solubility of iron in α-Ti at the eutectoid temperature is 0.44 at. %, decreasing at 400 °C to 0.34 at. %. The solubility of titanium in a-Fe reaches 3.08 at. % at 900 °C and decreases to 1.86 at. % at 600 °C. The TiFe compound melts congruently at ~1500 °C. The above-described confirms the assumption made by us about the formation of intermetallic phases in system Fe-Ti in the structure of the obtained FGM which microhardness is considerably above pure metals, in spite of the fact that their presence has not been defined by XRD (Figure 4). The state diagram of the Cr-Ti system [33] (Figure 5e) indicates that Cr and Ti are unlimitedly soluble in each other. As the temperature decreases, the alloys crystallize with the formation of a continuous series of solid solutions (βΤi, Cr). On the solidus and liquidus curves, there is a minimum at 1410 °C and a content of 44 at.% Cr. Intermediate phases with the structure of Laves phases are formed near the composition TiCr₂. The high-temperature modification γTiCr₂ forms congruently from (βΤi, Cr) at 1370 °C. At temperatures below 1270–1275 °C, this modification transforms into the medium-temperature modification βTiCr₂. The homogeneity interval of the βTiCr₂ phase is 64–66 at.% Cr at 1220 °C. The low-temperature modification αTiCr₂ is formed by a peritectoidal reaction from βTiCr₂ and (βΤi, Cr) at 1220 °C and has a homogeneity interval of 63–65 at.% Cr. The presented confirms the earlier conclusion of the formation of the intermetallic phase TiCr₂, determined by XRD data (Figure 3).

The paper presents a technological method for obtaining coupled FGM composites, taking into account the exclusion of problems related to delamination and fracture caused by different physical and chemical nature of metals, including due to significant differences in CTLE, for various tasks of the current industry, including obtaining FGM coatings.

4. Conclusions

The paper studies the method of obtaining FGM of the heterogeneous (layered) type consisting of metals with different CTLE (Cr—5.1–10⁶ °C⁻¹, Ti—7.7 × 10⁶ °C⁻¹, Fe—11.3 × 10⁶ °C⁻¹, Co—12 × 10⁶ °C⁻¹, Ni—13.4 × 10⁶ °C⁻¹, Cu—16.7 × 10⁶ °C⁻¹) using SPS technology, including the study of physicochemical and mechanical characteristics with regard to the phase states of the FGM alloy system. It has been established by SEM and EDS methods that regardless of the fractional composition of the initial powders in the SPS conditions, the production of FGM with a strong and integral metal bonding of Cr-Ti-Fe-Co-Ni-Cu is achieved. Connections at the contact boundaries of all metal pairs Ni/Cu, Co/Ni, Fe/Co, Cr/Fe and Ti/Cr in the composition of FGM are uniform, and defects (cracks) are absent. Diffusion of metals is revealed only insignificant for Cr in pair of system Ti/Cr because of the high solubility of metals in each other. It has been determined that the Vickers microhardness in the contact area of Cu-Ni, Ni-Co and Co-Fe alloys corresponds to their average value, and for Fe-Ti and Ti-Cr, it is much higher than for pure metals, which is caused by the formation of intermetallic phases. The presence of the intermetallide phase TiCr₂ for the Ti-Cr system is confirmed by XRD data, and the presence of intermetallide phases for TiFe₂ and TiFe for the Fe-Ti system under SPS conditions is predicted by the phase diagrams. The results of the study are new and present a prospect for FGM fabrication and FGC on their basis in the manufacture of structures and products for industrial applications.