Characterization of CoCrFeMnNi high-entropy alloy prepared by aluminothermal method

CoCrFeMnNi high-entropy alloys were prepared by the aluminothermal reaction method on copper and glass substrates, respectively. The effects of different substrates on the structure and hardness of the CoCrFeMnNi high-entropy alloys were investigated by XRD, SEM, EDS, and EPMA analysis methods and a micro-vickers hardness tester. The results showed that the phase composition of the CoCrFeMnNi high-entropy alloys prepared on both copper and glass substrates was the single-phase face-centered cubic solid solution. Compared with the copper substrate, the crystallinity of the CoCrFeMnNi high-entropy alloy prepared on the glass substrate was higher than that on the copper substrate, and its composition was more uniform than that on the copper substrate. The microhardness of the CoCrFeMnNi high-entropy alloys prepared on copper and glass substrates were 192.90 and 186.63 HV, respectively.

Brian Cantor from Bradford University, UK, and Junwei Ye from National Tsing Hua University, Taiwan, China, broke the concept of conventional alloys and proposed the concept of high-entropy alloys. High entropy alloys are generally defined as alloys with five or more major elements, with atomic percentages of each element ranging from 5% to 35% and elements alloyed at or near isoatomic ratios. Each atom in a high entropy alloy can be a solute atom, and each atom has the same chance to occupy any position in the lattice and form a simple solid solution. When the size difference between the atoms is large enough, the lattice deformation increases, so the complete lattice configuration cannot be maintained. The distorted lattice will form an amorphous organization. The phase composition of the alloy can be changed by adding some elements; for example, Cr can promote the generation of BCC body-centered cubic, while Co and Ni are beneficial to the generation of FCC face-centered cubic phase. From the literature, it can be seen that appropriate alloy composition and proper synthesis process can lead to excellent properties of high entropy alloys, such as high hardness, high work hardening, high temperature softening resistance, high-temperature oxidation resistance, corrosion resistance, and high resistivity, which are better than those of conventional alloys.

The CrMnFeCoNi alloy Bernd Gludovatz et al. studied consists of single-phase FCC with excellent damage tolerance at tensile strengths above 1 GPa. Its excellent damage tolerance exceeds that of most pure metals and metal-based alloys. PPBhattacharjee et al. rolled and annealed the CrMnFeCoNi alloy. The weaving behavior was studied. However, these alloys were prepared by the conventional electric arc furnace melting method, which requires several meetings to make the alloy composition homogeneous. The metals with low boiling points are prone to evaporation during the melting process, making it difficult to control their composition. In this paper, we tried to prepare CoCrFeMnNi high-entropy alloy by aluminothermal reaction method, and argon was used as the protective gas during the experiment. The effect of copper and glass substrates on the phase composition and hardness of CoCrFeMnNi high-entropy alloy prepared by the aluminothermal reaction method was investigated in this paper because of the difference in thermal conductivity between copper and glass substrates, resulting in different cooling rates of the alloy during crystallization.

1. Experimental materials and methods

The raw materials used for the experiments were Cr₂O₃, MnO₂, Fe₂O₃, Co, Ni, and Al powders with purity higher than 99.9%. In this experiment, the isoatomic ratio high entropy alloy CoCrFeMnNi was prepared on copper and glass substrates with a thickness of 10 mm by the aluminothermal reaction method.

The mixed materials were mechanically dry-milled in a QM-BP planetary ball mill for 8 h. The ball milling medium was quasi-5 mm Al₂O₃ balls with a ratio of 1:2 and a speed of 150 r/min. The mixture was dried in an electric thermostatic blast dryer DHG-9067A at 120°C for 10 h. 300 g of the mixed reaction materials were pressed into quasi-atomic ratios by a press at 25 MPa. The mixture was pressed into round cakes of about 80mm under 25MPa pressure and placed in molds with copper and glass substrates, respectively.

The flake igniting agent was placed on top of the reaction material, and the mold with the reaction material was put into the reaction vessel. Argon gas was to remove the air in the reaction vessel, the temperature of the vessel to 200 ℃ to remove the water vapor, and then pass argon gas continued to increase the temperature of the vessel; when the temperature of the vessel reached about 280 ℃, the initiator began to react and trigger the reaction between the reaction materials. The material reaction mainly manifests as a sharp increase in pressure and temperature in the reaction vessel. The oxides are reduced to single atoms and then solidified in the mold. The resulting products are cooled to room temperature in the furnace under the protection of an argon atmosphere. When the reaction vessel is opened at room temperature, and the product is removed, there is a layer of gray Al₂O₃ on the surface of the alloy, which can be removed manually.

The alloy was cut into 10mm×10mm block specimens and then analyzed by D/MAX-2400 X-ray diffractometer for physical phase analysis, JSM-6700F scanning electron microscope for microscopic morphology and composition analysis; EPMA-1600 electronic probe analyzer for chemical composition analysis, and MH-5-VM micro vickers hardness tester for the hardness test.

2. Experimental results

Figure 1 shows the XRD patterns of the CoCrFeMnNi high-entropy alloy prepared on copper and glass substrates. It can be seen that the organization of the alloy is a simple face-centered cubic solid solution without the formation of intermetallic compounds. From the XRD comparison between the copper substrate and the glass substrate, it can be seen that the position of the main peak does not change. In contrast, the diffraction peak at 51° of the alloys prepared on the glass substrate does not exist in the alloy prepared on the copper substrate. Moreover, the crystallinity of the alloy prepared on the glass substrate is better than that on the copper substrate.

Figure.1 XRD patterns of CoCrFeMnNi high-entropy alloys prepared on different substrates

Table 1 shows the atomic percentages of the elements of the CoCrFeMnNi high-entropy alloys prepared on different substrates. It can be seen that the alloy prepared on the copper substrate has less Cr content and contains 10.82at% of Al element; the alloy prepared on glass substrate has less Mn element and contains 3.63at% of Al element, and the alloy prepared on glass substrate has more uniform composition.

Table.1 Elemental content of CoCrFeMnNi high-entropy alloys prepared on different substrates (atomic fraction, %)

	Co	Cr	Fe	Mn	Ni	Al
Copper substrate	22.84	13.76	17.5	14.24	20.84	10.82
Glass substrate	19.74	19.17	23.98	9.7	23.78	3.63

Figure 2 shows the EPMA diagram of the CoCrFeMnNi high-entropy alloy prepared on different substrates. It can be seen that the elements are uniformly distributed in the substrate.

Figure 3 shows the SEM and EDS spectra of the CoCrFeMnNi high-entropy alloy prepared on different substrates. It can be seen that the materials prepared on both substrates consist of an off-white matrix.

The hardness of the alloy prepared on the copper substrate is 192.90 HV, and that of the alloy prepared on the glass substrate is 186.63 HV. It can be seen that the hardness of the alloy prepared on the copper substrate is greater than that on the glass substrate. Still, its hardness value is lower and not much different from the conventional alloy’s, mainly related to its phase structure.

Fig.2 EPMA diagram of CoCrFeMnNi high-entropy alloys prepared on different substrates

Figure.3 SEM and EDS spectra of CoCrFeMnNi high-entropy alloys prepared on different substrates

3. Discussion

In the aluminothermic reaction process, due to the large heat release, the reaction products are melted to form the melt. The oxides are reduced to monomers and diffused to form the alloy because the density of Al₂O₃ is different from that of metal monomers, the less dense Al₂O₃ floats to the surface of the liquid phase, and the denser metal monomers are deposited on the substrate so that the alloy is purified. Finally, due to the heat dissipation of the substrate, the alloy melt crystallizes and gradually solidifies. As a high thermal conductivity material, the thermal conductivity of copper substrate is 401 W-m-1-K-1, and the heat dissipation during the reaction process leads to rapid cooling of the melt. In contrast, the experimental reaction is extremely complex, and the atomic migration diffusion is slow, which makes the crystallization time short. The thermal conductivity of quartz glass is 2.4W-m-1-K-1, much smaller than that of the copper substrate. Therefore, the choice of quartz glass as the substrate to extend the time of the melt at high temperature makes the thermal reaction of aluminum more adequate and the atomic diffusion more uniform, and prolongs the crystallization time of the alloy. As a result, the crystallinity of the alloy prepared on the glass substrate is higher than that on the copper substrate, as seen in Figure 1. Most liquid metals are less dense than solids, so volume shrinkage occurs during crystallization. The volume shrinkage of the first crystallized part can be supplemented by the liquid metal that has not yet crystallized, while the volume shrinkage of the later crystallized part cannot be supplemented. After the reduction reaction, Al₂O₃ floats to the surface of the liquid phase due to its low density and separates from the metal monomers. In the process of floating Al₂O₃, the alloy liquid starts to crystallize due to the heat dissipation of the substrate, which makes the alloy liquid reach a certain crystallization temperature. Therefore, there are some holes in the solidified alloy.

The characterization of the alloy composition shows that the alloy prepared on the glass substrate contains a small amount of Al elements, while the copper substrate contains more. This indicates that the small thermal conductivity of the glass substrate makes the heat dissipation during the reaction process slow and the aluminothermic reaction more adequate, and prolongs the time for Al₂O₃ to float, which results in the low content of Al elements in the alloy. Moreover, the content of Mn elements on the glass is low because the glass substrate keeps the melt at a high temperature for a longer time, which makes the Mn elements with low boiling points volatilize. The reaction material is mixed by mechanical ball milling before the reaction, which, together with the diffusion in the reaction, results in a uniform distribution of the elements in the alloy.

For a multi-group solid solution, the number of group elements is at least 5, and each group element can be considered a solute atom. The atomic radii of various group elements are different, and the crystal structure of group elements is also different. This leads to the formation of solid solutions in which the atomic arrangement deviates from the equilibrium position, and the phenomenon of atomic misalignment is very prominent. As a result, the lattice distortion phenomenon is particularly serious in this type of alloy, which will lead to an increase in the solid solution strengthening effect of the alloy and an increase in the hardness of the alloy. The hardness of the alloys prepared on copper and glass substrates was 192.90 and 186.63 HV, respectively, because the atomic radii of the elements in the CoCrFeMnNi alloy differed very little, so the degree of lattice distortion caused was small. Alloy has FCC structure because the slip is usually carried out along the dense row direction of the dense row surface, a slip surface, and a slip direction forms a slip system, the FCC phase has 12 slip systems, and the slip of the FCC dense row surface is easier. Therefore, the hardness of the CoCrFeMnNi alloy with FCC structure is low, and the crystal structure factor dominates here, overshadowing the effect of solid solution strengthening on hardness.

4. Conclusion

• (1) The organization of the CoCrFeMnNi high-entropy alloys prepared on copper and glass substrates is a single-phase FCC solid solution. The crystallinity of the alloys prepared on glass substrates is higher than that on copper substrates, and their compositions are more uniform than those on copper substrates.
• (2) The hardness of the CoCrFeMnNi high-entropy alloys prepared on copper and glass substrates were 192.90 HV and 186.63 HV, respectively.

Author: Xiao Haibo