Effects of Graphite Addition on Structure and Properties of CrCuFeNiTiAl₁ High-Entropy Alloys

Abstract

In this study, the CrCuFeNiTiAl₁ equiatomic alloy was used as a base, which was modified by adding graphite in proportions of 0.5, 1.0, 2.5, and 5.0 mol%. The samples were obtained by powder metallurgy and sintering at 1200 °C for 2 h in a furnace with a protective argon atmosphere. Structural characterization was performed by XRD. A microstructural evaluation was conducted by SEM. The best mechanical microhardness and compressive strength results were obtained in the samples with the lowest amounts of graphite (238 μHV and 1000 MPa, respectively). The density values showed that samples with low amounts of graphite had better densification, lower porosity, and finer structural characteristics than those with graphite percentages higher than 1 mol%. The XRD studies determined the formation of a mixture of crystalline structures composed of FCC due to the presence of Cu, Ni, and Al metals; BCC due to Fe and Cr metals; and HCP due to Ti, and the formation of a Cr₇C₃ compound. SEM analysis showed the formation of cracks and porosity due to the formation of carbides.

1. Introduction

High-entropy alloys are characterized by having an equiatomic combination of multiple elements in their composition. Unlike conventional alloys, which usually have one or two main elements, high-entropy materials contain four or more elements in similar atomic proportions [1]. The idea behind high-entropy materials is to create a composition of elements that provides a crystalline structure with dislocations without following a specific pattern. High configurational entropy can generate beneficial effects, such as increased microstructural stability, higher mechanical strength and toughness, and hardness properties [2,3,4,5,6]. The Cantor alloy (CoCrFeMnNi) is a high-entropy multicomponent alloy first developed by Cantor et al. in early 2004 [7]. The Cantor alloy exhibits high mechanical hardness, good ductility, corrosion resistance, thermal stability, and low degradation at high temperatures compared to conventional/traditional alloys [8]. The CoCrFeMnNi alloy is widely considered an exemplary high-entropy material, and is stable as a single solid solution at all temperatures below its melting point [9]. Hence, from Cantor’s alloy, many more have been developed with some variations in the alloying elements, giving rise to a large number of alloys with different characteristics [10,11] for instance, the CoCrFeNiAl_X alloy results in a multicomponent material with promising thermo-mechanical properties. Recent studies have indicated that its combination of strength and ductility results from the complex interplay between multiple strengthening mechanisms created by varying the aluminum concentration in the alloy [12,13,14]. Since the macroscopic properties of this type of alloy depend, to a large extent, on interatomic interactions, it is convenient to probe the local structure and structural disorder around each element using specific techniques [15]. The conventional way to develop a new material is to select the main component based on some primary property requirements, and use alloying additions to impart secondary properties. This strategy has resulted in many successful multicomponent materials with remarkably good physical properties for engineering uses, such as toughness, fatigue resistance, hardness, and fracture toughness [16]. Different metallurgical processing routes, such as forging and casting, can be used to obtain this type of alloy. It is also possible to opt for more sophisticated methods that provide greater control over the microscopic and macroscopic structure of the alloy, such as direct energy deposition, binder jetting, spark plasma sintering, and powder metallurgy. The disorder caused by multiple elements tends to inhibit the formation of brittle phases by randomly dispersing atoms of each element, leading to increased strength and toughness. So far, several HEAs with outstanding properties have been processed by various methods, including the synthesis of CuCrFeNiTiAl_X by mechanical alloying [14]; the production of CrCuFeNiTiAl1C_x by powder metallurgy, of the alloys Al₂CoCrFeNiSi and Al₂CoCrCuFeNi_1.5Ti by laser cladding, and of AlCoCrCuFeNi₁, AlCoCrCuFe, and CuCoFeNiTix by laser surface alloying [17]; the fabrication of CuCoFeNiTix [18] and AlCrFeNi₂Ti_0.5 [19] by electric arc melting; and the synthesis of Fe₂₀Cr₂₀Mn₂₀Ni₂₀Co₂₀ by induction melting [20]. On the other hand, some studies have investigated the effect of carbon additions on the microstructure and mechanical properties of high-entropy alloys [21,22,23]. In these research studies, it has been reported that phase transformations due to graphite content strongly influence the final characteristics of the alloys. The objective of this work is to study the effect of graphite addition in different concentrations on the phase evolution, microstructure, and mechanical properties of the high-entropy CrCuFeNiTiAl₁C_X alloy synthesized by the powder metallurgy route. This alloy is considered a unique multicomponent alloy due to the characteristics it can adopt if manufactured by the powder processing route. By presenting a structure without a specific order, the alloy acquires properties of each of its components, thus improving its physical characteristics [24].

2. Materials and Methods

In this work, CrCuFeNiTi equiatomic alloys were fabricated with the addition of 1% mol of Al and different amounts of graphite. For this purpose, elemental powders of Al, Cr, Cu, Fe, Ni, and Ti (SkySpring Nanomaterials, Inc., Houston, TX, USA), with purity greater than 99% and sizes of 1–2 μm, and graphite (Alfa Aesar, Haverhill, MA, USA, 99.9%, 1 μm) were mechanically processed with a SPEX 8000M high-energy mill (Shibang Industry & Technology Group Co., Ltd., Shanghai, China) in a hardened steel container with 13 mm balls as milling media and an inert Ar atmosphere. The milling ball-to-powder ratio (in wt.%) was kept at 5 to 1 for all experimental runs. Milling was conducted in an argon atmosphere at 600 rpm for 6 h. To control the particle size distribution and segregation of the powder particles during the ball milling action, 1 mL of CH₃OH was used as a process control agent. The studied system was CrCuFeNiTiAl₁C_X, where x = 0, 0.5, 1, 2.5, 5 mol%; the remaining metals were added in equimolar fractions, with the exception of aluminum, of which, as mentioned above, only 1% mol was added. The graphite contents were studied because, according to reports [21,22], moderate carbon levels in HEAs have shown the best properties. The samples were labeled as 1, 2, 3, 4, and 5 for graphite content of x = 0, 0.5, 1.0, 2.5, and 5.0 mol%, respectively. After the grinding process, the samples were passivated for 24 h in a glove box filled with an Air–Argon mixture. With the help of a uniaxial press (Montequipo 9T, Tlalnepantla de Baz, Mexico), the resulting powder from the grinding stage was compacted into cylindrical green samples at 258 MPa for 10 min. Before the sintering process, the five alloy samples were placed in a ceramic quartz tube separated by quartz pads, as shown in Figure 1; this was performed in order to avoid component diffusion during sintering.

Samples were sintered in an electrical furnace (Carbolite Gero, Hope Valley, UK) with an argon atmosphere at 1200 °C for 2 h, using a heating speed of 10 °C/min. Before characterization of the sintered samples, their surfaces were ground with SiC sandpaper and then polished using diamond suspension. In order to observe the structure and microstructure of the alloy, an electrochemical attack on the surface of the samples was performed. To remove oxides, a 10% oxalic acid solution in distilled water was used for microetching. Electrochemical etching was carried out as follows: A power source was used to apply an electrical discharge on the surface of the samples with the help of a mm diameter electrode, using 6 V and 10 A for 12 s. Shimadzu SALD-201V laser diffraction equipment (Kyoto, Japan) was used to measure the particle size distribution. Prior to measurement, 0.1 g of powder was added to 100 mL of distilled water and subjected to an ultrasonic bath for 15 min to thoroughly disperse the powder in the water. The density and open porosity of the samples were determined using the Archimedes method in agreement with the ASTM B962-17 standard [25]. The crystalline phases of the sintered alloys were determined using X-ray diffraction analysis (XRD) under CuKα radiation, performed on a DRX D8 Advance (Bruker, Billerica, MA, USA), scanning from 20 to 120° with a step size of 0.016° and a 10 s exposure time, and interpreted with the X’Pert Highscore Plus PANalytical software (Highscore 4.0) using patterns in the ICDD PDF2 database. The microstructural features of the HEAs were analyzed using a Zeiss Axio optical microscope (ZEISS, Oberkochen, Germany) and an FEI Quanta 200 FEG scanning electron microscope (FEI, Lausanne, Switzerland) equipped with a Shimadzu EDX-LE for the chemical analysis of the samples. The working distance was 5.6 mm using a voltage of 10 kV. Micrographs were taken at 200, 500, and 1000 magnifications using a secondary-electron detector, and EDS mappings were obtained at 500x for each sample. Microhardness was determined in agreement with the ASTM E384–16 standard [26]. In this case, ten measurements were performed, and each indentation was made at a spacing distance of at least 5 times the size of each print; the used load was 9.8 N for 15 s. These measurements were performed with a microhardness tester (Wilson Instruments Model S400, Buehler, Chicago, IL, USA). The compressive strength was evaluated with a WP 300 Gunt Universal Material Tester (GUNT Gerätebau, Barsbüttel, Germany), equipped with a 20 kN load cell, with a head displacement rate of 10 mm/min and 2 cm diameter cylindrical samples.

3. Results

3.1. Particle Size Distribution

Figure 2 shows the particle size distribution for each system. In this figure, it can be seen that the samples with 0.5 C, 1.0 C, and 2.5% C presented a trend of sizes that ranged from 3 to 5 μm. The sample without the dopant element presented particle sizes from 0.5 to 0.8 μm, while the sample with 5% C presented diametric ranges from 6 to 9.5 μm, indicating the formation of agglomerates of the powder used for the alloy; this is corroborated by the image in Figure 3, where images of samples with different graphite contents are presented. In these images, it can be observed in the sample with C that there is strong agglomeration and lamination of the metallic particles. Certainly, graphite is a material with very good lubricating characteristics, which favors particle size reduction. However, graphite is also an excellent electrical conductor, which causes electrostatic forces to dominate over gravitational forces as the particle size is reduced. This leads to particle agglomeration due to the strong electrostatic attractions on their surface.

3.2. Density

Figure 4 shows the density graphs and porosity measurements of the sintered samples. These plots show that the density reached in the alloys is very low, since samples 1 and 2 are the ones that best densified, reaching approximately 80% of their relative density. It is also observed that as the alloy’s graphite content increases, the alloy’s relative density tends to decrease with a consequent increase in the porosity of the alloy. The graphite in the alloys plays a very important role in the densification of the alloys, since it has already been observed that milling tends to make the particles agglomerate. Commercially chemically purified graphite normally contains a large amount of chemically occluded gases, which, during sintering, are desorbed, generating porosity in the material. In addition, the graphite reacts at high temperatures, as can be seen in the Ellingham diagram [27], where the free energy is lower as the temperature increases, generating gaseous products (CO and CO₂). The generation of these gases generates porosity and, with it, a decrease in the density of the samples; the higher the concentration of graphite, the more this effect is intensified.

3.3. Structure

Figure 5 shows the diffraction patterns of the five samples after the sintering stage. In this figure, for the alloys with 0 and 0.5% mol C at 39.3, 42.1, 51.3 and 76.2°, we can observe reflections, which are normally related to the formation of some FCC solid-solution phases due to the presence of copper, nickel, and aluminum; on the other hand, at 32.3, 37.2, 63.1, 78.4, and 99.1° the solid-solution BCC phase appears due to chromium and iron present in the alloy. The figure shows that from 1% mol C in the diffraction patterns at 2 theta degree angles of 71.4°, 82.6°, and 93.2°, a compact hexagonal structure appears, which corresponds to the formation of carbides in the alloys, as well as to titanium characteristic reflections. The carbides formed are mainly of the Cr₇C₃ type. On the other hand, the diffraction patterns also show that the intensity of the FCC phase increases as the C content increases in the alloy, while the BCC phase tends to decrease in intensity. Due to the high electronegativity of C, this is a strong carbide former, mainly with Cr, Fe, and Ti [28]. This is because the phases formed between C and Fe, and C and Ti, are FCC. Moreover, C forms carbides with Cr. Both Cr and Fe are BCC elements, and when forming carbides, the presence of the BCC phase should tend to decrease, increasing consequently the presence of the FCC phase and generating the HCP phase, which is in accordance with other authors’ reports [23]. It can be seen that the alloys have a mixture of different crystalline structures, causing microstructural disorder, which is expected for a high-entropy alloy.

3.4. Microstructure

Figure 6 shows a comparison of the microstructure of the five alloys observed by optical microscopy. In these images, it can be seen that the microstructure is characterized by the presence of grains with a similar size distribution that do not follow a specific pattern and are microstructurally disordered due to the number of elements contained in the alloys; nevertheless, it can be seen that upon increasing the amount of the dopant element, cracks and pores are observed. Higher porosity is observed in the samples with more than 1.0 mol % C, with cracks reaching 200 μm, thus confirming the observation made in the density measurements that as the C content increases, the density decreases; this, in turn, increases the porosity, a situation that occurs due to the agglomeration of particles because of the presence of C and the abnormal growth of the microstructure during sintering. The presence of cracks is likely to affect the mechanical properties of the alloys. The images also show a diversity of shades on the surface of the alloys due to the different phases present in the alloys, which, in turn, is indicative of the entropy present in the alloys.

Figure 7 shows photographs of the microstructure of the alloys taken under an optical microscope at 500x magnification, where the presence of porosity in the alloys is more clearly visible. Here, it is observed that as the graphite content increases, the porosity increases in quantity and size. From an image analysis, it was determined that the porosity contents are 17, 18, 22, 27, and 33% for the samples with 0, 0.5, 1, 2.5, and 5% graphite in them, respectively, which is in accordance with the results of the porosity measurements made using the Archimedes method.

3.5. Mapping

Figure 8 shows some SEM images and their corresponding mappings to verify the spatial distribution of the alloy components. Elemental mapping indicated that after sintering, CrCuFeNiTiAl₁C_X alloys are composed of a multi-phase microstructure, as can be seen in the figure, where the zones have different levels of contrast in the microstructure. In the mapping of spatial regions, a good distribution of alloying elements was observed, indicating the formation of high-entropy alloys. From these results, it can be inferred that the microstructure of this alloy was composed of a high-Cu, -Fe, and -Ni solid solution (light region); a dark region composed of Ti and Cr; and dispersed chromium carbide phases.

3.6. Energy-Dispersive X-Ray Spectroscopy (EDS)

The microstructure and the chemical composition of the different prepared CrCuFeNiTiAl₁C_X alloys were characterized by SEM and EDS analysis, as shown in Figure 9. In this figure, the microstructures with different phases can be observed and distinguished by their different levels of contrast, indicating variation in their chemical composition. On the right side of each image, the corresponding spectrum is shown, which presents the elemental constituents of the alloys and a chart with the results of the chemical analysis. These analyses, in all cases, specify that the resulting chemical composition was close to the hypothesized composition. Thus, the X-ray diffraction results and the observations made in the mappings are confirmed. Thus, the chemical composition is related to the presence of these phases in the microstructure, and the chemical analyses suggest the formation of alloys between the used elements, which, in turn, form a solid solution and, consequently, have high entropy. In all alloys, the microstructure shows a non-homogeneous grain distribution due to the presence of the different phases that make up the sample. As the C content in the alloys increases, grain size growth is also observed; for higher C contents, some dark zones can be observed, indicating the formation of porosity and carbides in these sample areas.

3.7. Mechanical Properties

3.7.1. Microhardness

Figure 10 shows the microhardness for each alloy. The best microhardness value of 238 HV is obtained in the sample that does not contain C, while the sample with 5% mol C presents the lowest microhardness value of 137 HV. According to this graph, with increasing C content in the alloy, the microhardness of the alloy tends to decrease. Although, an increase in the hardness of the alloys could be expected due to the formation of carbides [24], it can be inferred from these results and the observations made in the previous analyses that the formation of porosity and cracks in the microstructure of the alloys, which occur due to agglomeration of the powders during milling and abnormal grain growth during sintering, impede the full hardening of the sample. Also, it has been reported in the literature [23] that with increases in carbon in alloys, phase transformations from BCC to FCC and HCP have a significant influence on the decrease in the mechanical properties of the alloys, which is in good agreement with the behavior observed in this study.

3.7.2. Compression Strength

Figure 10 shows the compressive strength values. Similarly to what was found in the hardness measurements, in this graph, it can be observed that the sample that obtained the best compressive strength value is the sample that does not contain the dopant element, while the one that contains the highest amount of C is the one that presents the lowest strength value. From all of this, it can be concluded that graphite is not a component that benefits the mechanical properties of this type of alloy. As already mentioned, this is due to the agglomeration of the powders during milling, the grain growth during sintering, and the formation of carbides and porosity in the last stage of manufacture.

4. Conclusions

High-entropy alloys were successfully fabricated by powder metallurgy. However, from the results obtained, it can be deduced that the use of C is not recommended to increase the mechanical properties of this type of alloy. Based on the experimentation and analysis carried out on the alloys, the following conclusions were obtained:

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With increasing C, there is agglomeration of the metal particles, which causes the generation of large agglomerates and, consequently, abnormal grain growth during sintering.

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The structure consists of cubic phases centered on the body and faces for samples with 0 and 0.5% C, while for higher C contents, a compact hexagonal structure appears due to the formation of carbides, mainly of chromium (Cr₇C₃).

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The microstructure is characterized by the presence of grains with a similar size distribution that do not follow a specific pattern and are disordered due to the number of elements contained in the alloy. Also, increasing the amount of the dopant element causes cracking and pore formation.

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Elemental mapping indicated that the samples with sintered CrCuFeNiTiAl₁C_X alloys are formed of a multi-phase microstructure, as demonstrated by the zones having different levels of contrast in the microstructure.

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The mechanical properties (microhardness and compressive strength) are negatively affected as the C content of the alloy increases.