N. A. Krapivka

Effect of Electron Density on Phase Composition of High-Entropy Equiatomic Alloys

A series of high-entropy equiatomic alloys have been analyzed to determine the main factors that influence the formation of various solid solutions and chemical compounds. The key factor leading to the formation of phases in high-entropy equiatomic alloys is mean electron density (e/a). The necessary condition for the high-entropy σ-phase to emerge is the presence of elements forming it in two-component alloys in various ratios, the electron density of the alloy is to be between 6.7 and 7.3 e/a. The Laves phase shows up in the high-entropy equiatomic alloys at a mean electron density of 6–7 e/a in the presence of atoms differing by more than 12% in size and having mixing enthalpy lower than −30 kJ/mol. It is revealed that the lattice parameter in bcc high-entropy equiatomic alloys influences their elastic modulus and hardness.

Superhard Vacuum Coatings Based on High-Entropy Alloys

High-entropy TiZrVNbTa, AlCrFeCoNiCuV, and TiZrHfNbTaCr alloy coatings with a thickness of 2.5–6 μm and with various phase compositions were deposited by dc magnetron sputtering. The chemical and phase composition of the TiZrVNbTa and TiZrHfNbTaCr coatings do not change substantially during deposition. Only when the substrate bias is higher than –180 V, the deposited AlCrFeCoNiCuV alloy coatings are depleted of Al and Cu. All the coatings are nanostructured, and their microhardness varies between 10 and 19 GPa, and reduced elastic modulus changes between 106 and 192 GPa depending on the phase composition.

Structural Features and Solid-Solution Hardening of High-Entropy CrMnFeCoNi Alloy

The phase composition, microstructure, and mechanical properties of the single-phase CrMnFeCoNi high-entropy alloy with fcc lattice produced by argon-arc vacuum melting are studied. The alloy solid-solution hardening mechanism is analyzed. The abnormally high athermic solid-solution hardening of the alloy is due to variation in the Burgers vector along the dislocation line and a vector component perpendicular to the slip plane. The activation energy of dislocation movement and activation volume are calculated. The activation energy of dislocation movement is close to the activation energy of pure metals with fcc lattices, and the activation volume is significantly lower compared to that of pure metals because picosized distortions grow in the crystal lattice of the multicomponent alloy. The ratio of hardness and yield stress for this alloy is examined.

Secondary Ion Emission of High-Entropy Cr14.3Mn14.3Fe14.3Ni28.6Co14.3Cu14.3 Alloy

To understand the unique mechanical properties of high-entropy alloys, it is important to know the nature and strength of interatomic interactions between similar and dissimilar atoms. In this regard, the objective of this study is to use the phenomenon of secondary ion emission for Cr14.3Mn14.3Fe14.3Ni28.6Co14.3Cu14.3 alloy with fcc structure. The yield of secondary ions for all alloy components and corresponding pure metals is quantitatively compared for the first time and an equation is proposed to calculate the atomic bond energy based on the existing models of secondary ion emission mechanism. Compared to pure metals, the bond energy increases in the alloy for Cr and Fe atoms. The greatest decrease in the bond energy is observed for Mn atoms. Reduction in the bond energy for Co and Ni is insignificant. It is suggested that the atomic interaction energy is influenced by changes in the local electron density in fusion as compared with pure metals.

Role of Various Parameters in the Formation of the Physicomechanical Properties of High-Entropy Alloys with BCC Lattices

An analysis of simple structures of the solid-solution non-ordered high-entropy alloys (HEAs) with a bcc crystal lattice has allowed us to determine the effect of various parameters on their physicomechanical properties. It was found that, as the hardness increases, the size mismatch results in a decrease in the modulus of elasticity; however, the normalized hardness characteristic increases. It has been found that, when the enthalpy of mixing of the bcc high-entropy alloys shifts to negative values, its effect on the hardness and modulus of elasticity is nonmonotonic. A formula for calculating the modulus of elasticity of high-entropy alloys with a bcc structure has been suggested that is based on the alloy composition and role of the most refractory metallic component.

Wear Resistance of High-Entropy Alloys

The tribotechnical properties of high-entropy alloys in pair with 65G steel in air under dry sliding friction conditions are investigated in comparison with wear-resistant steel and powder materials. The sliding friction rate was 6, 8, and 12 m/sec and the pressure was 0.5 and 1.0 MPa. It is determined that the wear intensity of high-entropy alloys at the sliding friction rate 5–10 m/sec under 0.5 and 1.0 MPa loads ranges from 6.1 · 10–10 g/km to 1.6 · 10–9 g/km for the samples and from 5.5 · 10–8 g/km to 1.1 · 10–8 g/km for the counterface. It is established that, when friction, the shear deformations promote the formation of thermally stable nanostructures with grains 30–70 nm in size in the surface layer of the secondary structures. It is shown that the formation of nanostructures is accompanied with 20–30% increase in hardness for both high-entropy alloys and counterface material. It is established that, when friction, high temperatures at the contact points promote the formation of ordered β-phase with BCC lattice on the friction surface of the Fe25Cr20Ni20 Mn15Co10Al10 high-entropy alloy.

High-entropy alloys: Interrelations between electron concentration, phase composition, lattice parameter, and properties

An analysis of more than 200 high-entropy alloys (HEA) allowed us to find interrelations between the electron concentration, phase composition, lattice parameter, and properties of solid solutions with bcc and fcc crystal lattices. Main conditions for the appearance of high-entropy chemical compounds, such as Laves, σ, and μ phases were determined. The necessary condition for the formation of 100% high-entropy σ phase is the formation of σ phase in two-component alloys for different combinations of elements, which are components of the HEA, and the electron concentration should be 6.7–7.3 electrons per atom. To form a 100% high-entropy Laves phase, the following conditions should be fulfilled: the total negative enthalpy of mixing of alloy is about –7 kJ/mol and less; the difference between the atom sizes in a pair is more than 12%; the enthalpy of the mixing of two present elements is less than –30 kJ/mol; and the average electron concentration is 6–7 electrons per atom. It was shown that the ratios of lattice parameters of solid-solution HEA, which were experimentally determined, to the lattice parameter of the most refractory metal in the HEA determine the value of the modulus of elasticity.

Effect of aluminum, chromium, and iron doping on the heat resistance of zirconium

The influence of aluminum, chromium, and iron doping on the heat resistance of zirconium is studied. It is shown that a dense oxide film starts forming in the oxidation of the alloy containing 8 wt.% aluminum at 700°C regardless of its phase composition and ensures high heat resistance. At lower temperatures, to achieve the heat resistance comparable with that of the É110 commercial alloy, the content of the Zr–Al solid solution based on α-Zr, which should not contain more than 4 wt.% Al, is to be minimum. Additions of 1% Cr and 1% Fe can be used to strengthen the Zr–8Al alloy without a significant decrease in its heat resistance.