Nuclear Energy: High-Entropy Alloy

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Table of Contents

Introduction
Methods of Formation
Structure and Composition
Mechanical Properties
References

Introduction

The main reason for today’s global trend towards climate change is the changing concentration of greenhouse gases in the atmosphere. Reducing anthropogenic emissions into the atmosphere is an important task for the world community on the way to solving the problem of climate change. One of the tools for reducing the level of greenhouse gas emissions is the development of nuclear energy, which is characterized by a high degree of environmental efficiency and the absence of a significant impact on the carbon balance.

In this regard, nuclear energy is competitive, since it has a positive effect on climate change, and this allows concluding that in the future it may become the most attractive and affordable form of energy for sustainable development. At the same time, the share of nuclear power in the world energy production is small and its further development depends on the growth of investments in new technologies – one of these technologies is the use of high-entropy alloys.

The scientific and practical interest in new coatings based on multicomponent alloys is due to their excellent physicochemical properties, which are manifested due to the deformation effect of solid solution hardening, distortions of the crystal lattice and even the nanocomposite structure. The properties exhibited by high-entropy alloys make them attractive for use in various fields, including the construction of nuclear reactors.

The traditional approach to the creation of new structural materials is to select one main element as a matrix, which is alloyed to obtain the desired combination of mechanical and/or technological properties. As a result, a large number of practically used alloys based on iron, copper, aluminum, titanium, nickel, and so on have been created. In recent years, an approach to the creation of multicomponent alloys with several elements as a matrix in equiatomic proportions seems to be of interest. Special attention is paid to alloys in which disordered solid solutions are formed.

In the last decade, a new concept for the creation of metal alloys, based on the achievement of a high entropy of mixing of alloy components, has been proposed and experimentally confirmed. In accordance with this concept, the high entropy of mixing of different metal elements with a concentration close to equimolar can significantly reduce the Gibbs free energy and stabilize solid solutions with a relatively simple crystal structure and a good combination of properties [1]. Today, special attention is paid to the study of high-entropy alloys based on transition metals.

Methods of Formation

High entropy alloys, with five or more components in their composition, according to the theory of regular solutions, have a high entropy of mixing, which is determined according to the Boltzmann equation: λ Smix λ R lnn, where R is the gas constant, n is the number of chemical elements in the alloy [2, 3]. For example, the entropy of mixing λ Smix for equimolar alloys of 3, 5, 7 and 9 elements is 1.1R, 1.61R, 1.95R and 2.2R J λ mol λ¹ λK λ¹, respectively.

Instead of numerous intermetallic or other complex compounds, HEAs tend to form simple substitutional solid solutions with predominantly cubic crystal structure: bcc, fcc, or fcc λ bcc [4]. This tendency is explained by the influence of high entropy and is based on a simple relationship: λG_mix λ λH_mix λT λS_mix (G – Gibbs energy, H – enthalpy, T – absolute temperature, S – entropy) and the second law of thermodynamics [5]. In this case, the number of phases in HEAs is less than the allowed equilibrium number, which follows from the Gibbs phase rule.

As shown in the literature, for the formation of stable solid solutions in multicomponent alloys, in addition to the high entropy of mixing (λSmix λ 1.61R or λ 12 J λmol λ¹ λK λ¹), which is the dominant factor controlling them formation, it is necessary that the atomic radii of the constituent elements differ slightly (λλ 8.5—12%), whereas the enthalpy of mixing (binding energy) λHmix can vary from λ15 to 5 kJ/mol [6]. The formation of simple solid solutions with increased entropy and a large number of chemical elements of equiatomic composition significantly influence the alloy properties and predetermines the combination of high strength, plasticity, thermal stability, wear and corrosion resistance both at ambient and at high temperatures [7, 8].

High-entropy alloys can be produced by the same methods as traditional alloys. Casting technologies, melt quenching, mechanical alloying, deposition of films and coatings are used to obtain them. Each method has its own advantages and disadvantages. Mechanical alloying provides the formation of a stable microstructure with a homogeneous chemical composition, in comparison with other methods of obtaining these alloys. In addition, mechanical alloying provides the formation of a nanocrystalline structure, which improves the mechanical properties of these alloys.

Analysis of the literature shows that conventional casting methods are used as methods for producing high-entropy alloys. However, it should be noted that the formation of the structure of a solid solution doped with many elements, apparently, should complicate the casting process, in particular, an inhomogeneous distribution of elements can be assumed, as well as the presence of significant internal stresses in the ingot [9]. There is an obvious need to increase the number of heats to improve the homogeneity of the chemical composition and control the cooling rates during crystallization. Meanwhile, the latter is associated with the possible formation of undesirable phases and the prevention of the formation of a coarse structure in the ingot.

The most common methods include vacuum arc melting; the process of obtaining high-entropy alloys has definite scheme. A mixture of high-purity metals with a purity of more than 99% is alloyed and then remelted several more times to achieve greater chemical homogeneity of the alloy. Melting of a mixture of metals occurs in an inert gas atmosphere [10]. Samples of various compositions were obtained in a similar way.

Gas Formation Methods

Advances in the development of reactive sputtering processes make it possible to deposit coatings of transition metal nitrides or oxide films of fairly high quality using reactive magnetron sputtering. The technique of obtaining coatings, the conditions for their deposition – all this is of decisive importance for the properties of the coatings obtained. Using modern magnetron sputtering methods, it is possible to control the size, orientation (texture) of grains and nanostructures in general. The advantages of the magnetron sputtering method over similar methods are as follows [16]:

High spraying rate at relatively low voltages (about 600-800 V) and at low gas pressures in the working chamber (5λ10-1 – 10 Pa);
No overheating of the substrate;
Low degree of contamination of the resulting films;
The possibility of obtaining films uniform over the entire thickness on a relatively large area of substrates.

Using this method, high-entropy alloys based on nitrides are mainly obtained. The general characteristic features of the use of this method for the production of HEAs based on nitrides is that, first, using the vacuum arc melting method, targets of high-entropy alloys are prepared by fusing a (often equimolar) mixture of the required metals. Basically, this melting process is repeated at least five times in order to achieve the most uniform distribution of elements in the alloy.

The nitride coating is obtained directly in the working chamber of the magnetron. Basically, a mixture of Ar + N2 is used as working gases. The conditions for magnetron sputtering of a target onto a previously prepared substrate can be very different – various pressures of the working gas in the chamber, the distance from the target to the substrate, the presence or absence of potential on the substrate, its value, etc. are used. All these parameters ffect the quality and properties of the resulting coatings. Using the method of magnetron sputtering, one can also obtain high-entropy carbide alloys [17]. In this case, naturally, a mixture of reactive gases of a different composition is used, namely, Ar + CH4. Otherwise, the technological process does not differ from the production of HEAs based on nitrides.

As to vapor-phase deposition, chemical vapor deposition (CVD) is a chemical process that is designed to produce hard coatings, usually of higher purity. The essence of this method is that the final product is formed on the target substrate as a result of the interaction of gaseous precursor substances or thermolysis of the precursor substance vapor. At the same time, Hybrid Physical-Chemical Vapor Deposition (HPCVD) is a process that uses both the chemical decomposition of a precursor and the evaporation of a solid material.

Atomic deposition is based on sequential chemical reactions between vapor and a solid and is self-limiting. Most of such reactions use two chemical compounds commonly referred to as precursors, which alternately react with the surface. As a result of the multiple influence of precursors, a thin film grows. It is a technology that uses the principle of molecular assembly of materials from the gas phase.

Liquid Formation Methods

One of the most widespread methods of obtaining high-entropy alloys is the casting method in combination with various methods of melting: arc induction, electric arc. The method of vacuum-arc deposition is widely used in the production of nitride coatings. The main feature of this method is the presence of highly ionized plasma flows of the evaporated material. A vacuum arc arises between the cathode and the anode, which vaporizes the cathode material with the formation of so-called cathode spots. However, in contrast to cathode sputtering, the erosion product is not a flux of atoms, but a flux of ions of the cathode material. When a high negative potential is applied to the substrate, the high energy of the particles provides cleaning and activation of its surface due to the ion bombardment of the coating material. During the subsequent deposition of the coating, mutual diffusion of the atoms of the coating and the substrate occurs, thereby ensuring the adhesion of the coating to the strength levels of the atomic bond with the substrate [11]. One of the disadvantages of the method is the presence of a stream of micron-sized molten material droplets included in the coating in the form of particulates, which adversely affects the performance characteristics.

The most perfect type of heating is one in which heat is generated directly in the heated body. This heating method is very well carried out by passing an electric current through the body. However, direct inclusion of a heated body in an electrical circuit is not always possible for technical and practical reasons. In these cases, a proper type of heating can be carried out using induction heating, in which heat is also generated in the heated body itself, which eliminates unnecessary, usually large, energy consumption in the heating elements. Therefore, despite the relatively low efficiency of generating currents of increased and high frequency, the total efficiency of induction heating is often higher than with other heating methods. A particularly valuable property of induction heating is the possibility of high concentration of energy in the heated body, easily amenable to precise dosage. Only an electric arc can obtain the same order of energy density, however, this heating method is difficult to control.

The working process of induction melting furnaces is characterized by the electrodynamic and thermal movement of liquid metal in a bath or crucible, which contributes to obtaining a metal that is uniform in composition and uniform temperature throughout the volume, as well as low metal waste (several times less than in arc furnaces).

In later works, the method of laser cladding was used to obtain high-entropy alloys. This method has the following advantages [13, 14]:

High energy density, high heating rate, and little influence of thermal effects on the substrate;
Low solubility is limited by the control of the applied laser energy, to achieve excellent properties of the clad starting material;
The method allows obtaining a pore-free ball between the coating and the substrate;
Due to the fast heating and cooling processes, the coatings obtained by the laser cladding method have a homogeneous structure without a large number of defects.

For HEAs obtained by laser cladding, the formation of three different regions is characteristic, consisting of a cladding zone, limiting the zones, and heat-affected zones. In this case, the alloys have a bcc or fcc structure, and also include Laves phases [15].

Studies of the properties of coatings were carried out on a high-entropy alloy of the Ti20-Zr20-Nb20-Hf 20-V20 system. The vacuum sprayed coating has a high hardness of 8.2 hPa and a very high H/Er ratio of 0.077. Similar values of hardness and H / Er ratio were also observed in cast high-entropy alloys cooled at a high rate, which are characterized by a nanocrystalline structure. High-entropy coatings obtained by spraying in a vacuum are characterized by high values of hardness (8.0-9.0 hPa) and thermal stability in the temperature range up to 1000°C.

Phase composition and physical and mechanical characteristics of metal coatings depending on composition and method pf spurring are given in the table below.

Table. Phase composition and physical and mechanical characteristics of metal coatings depending on composition and method pf spurring.

Phase composition	Spraying method	Crystal lattice type	a, nm	H_IT, hPa	Er, hPa	H_IT/Er
TiZrHfNbTaV	VAR	bcc	0,3323	8±0,3	106±3	0,077
TiZrHfNbV	The same	bcc	0,3389	8±0,3	101±3	0,080
TiZrHfNbTaV	Compressed discharge plasma	bcc	0,3356	7±0,3	103±3	0,059
TiZrNbTaV	The same	bcc	0,3264	10±0,3	125±3	0,08
TiZrHfNbTaY	The same	bcc	0,3427	7±0,3	100±3	0,068
TiZrVNbMo	The same	bcc	0,3230	12±0,3	130±5	0,092
AlCrFeCoNiCuV	The same	Bcc (29%) fcc (71%)	0,3894 0,3327	8±0,3	160±5	0,051
TiZrHfNbTaCr	The same	C14 (75,4%) bcc (24.6%)	0,3284	19±0,3	195±8	0,097
AlCrFeCoNiCuV	Magnetron	Bcc (38%) fcc (62%)	0,2977 0,3663	15±0,3	170±5	0,091

As can be seen, the phase composition does not change qualitatively, only for two-phase HEA the phase ratio changes. A characteristic feature of coatings based on HEA is a decrease in the lattice parameter during spraying. Thus, when using a solid solution based on TiZrHfNbTaV with a bcc lattice to obtain coatings, a decrease in the lattice parameter from 0.3348 to 0.3323 nm and a significantly smaller size of the structural component are observed.

Solid Formation Method

The main factors that can affect the properties of the alloys obtained are the number of melts of the finished alloy (this affects the chemical homogeneity of the alloys), the method of cooling (air or water cooling), the cooling rate, etc. Mechanical fusion refers to the method of mechanical processing in a high-energy ball mill of a mixture of elemental powders in a solid state. In this case, repeated processes of destruction of powder particles and cold welding take place, which make it possible to obtain a homogeneous powder material [12]. Therefore, this method makes it possible to obtain high-entropy alloys with excellent chemical homogeneity and with some features of the structure and properties. Generally, one can conclude that high-entropy alloys obtained using the method of mechanical alloying are not inferior in their properties to HEAs obtained by other methods, especially if after mechanical alloying the alloys are subjected to HIP [84].

Another widespread method for obtaining HEA with a more uniform and stable nanocrystalline structure is mechanical alloying followed by spheroidization. This technology for producing alloys consists in the use of components in the rolling of powders and their subsequent processing in a high-energy ball mill. There, repeated processes of powder particle destruction and cold welding take place, which makes it possible to obtain a homogeneous powder.

The basis of mechanical alloying is pulsed mechanical processing of powders or their mixtures in mills. This method makes it possible to obtain powder particles of uniform composition and structure with the simultaneous formation of a fine-grained structure, including a nanocrystalline one. Powders obtained by grinding are currently not used for additive technologies, since the powder particles have a fragmented, irregular shape. To impart a spherical shape to powders, various methods of spheroidization are used – the process of transforming the initial powder material of an unequal shape in order to obtain particles with a shape close to spherical.

Mechanical alloying advantages in comparison with other methods include the formation of a stable microstructure with a homogeneous chemical composition. However, an increase in the homogeneity of solid solutions under processing conditions at ambient temperature is the main advantage of mechanical alloying. Moreover, the mechanical alloying method allows obtaining alloys in a nanostructured state to improve their mechanical properties.

Structure and Composition

The study of the physical and mechanical characteristics of high-entropy alloys showed that in most cases, with an electron concentration in the range of 5.4–7.0, such alloys have reduced ductility at room temperatures; however, for all high-entropy alloys, high heat resistance characteristics are noted. For the formation of stable solid solutions in multicomponent alloys, in addition to the high entropy of mixing (λSmix λ 1.61R or λ 12 Jλmolλ1 λKλ1), which is the dominant factor controlling their formation, it is necessary that the atomic radii of the constituent elements differ insignificantly (λ8.5—12%), while the enthalpy of mixing (binding energy) λHmix can vary from λ15 to 5 kJ / mol [18]. The formation of simple solid solutions with increased entropy and a large number of chemical elements of equiatomic composition significantly infliences the alloy properties and predetermines the combination of high strength, plasticity, thermal stability, wear and corrosion resistance both at room and at high temperatures. The formation of a simple structure of solid solutions facilitates structural studies and helps to eliminate the brittleness inherent in ordered compounds.

It is possible to establish the relationship between the phase composition, as well as electron concentration, lattice parameter. Moreover, properties of solid solutions based on bcc and fcc lattices also are included in this link. In such alloys, the phase composition is easy to predict on the basis of double or triple phase diagrams, and the introduction of alloying additions leads either to solid solution strengthening of the initial lattice or to the precipitation of dispersed phases in it. As a rule, when developing HEAs, one strives to obtain a single-phase structure – usually a disordered solid solution based on an fcc or bcc structure. This can be achieved due to the high contribution of the configurational entropy to free energy through the presence of several elements in equiatomic or commensurate quantities. However, the addition of Cr in an equiatomic amount with respect to the rest of the elements in the NbTiVZr alloy leads to the appearance the Laves phase, as a result of which the plasticity of this alloy at room temperature significantly decreases [20].

The fundamental difference between the development of HEAs from the traditional strategy of developing alloys is the formation of a disordered solid solution, in which the atoms of the constituent elements have an equal probability of occupying one or another site of the crystal lattice. Due to the heterogeneity of atoms, the potential energy between the sites of the crystal lattice changes. Fluctuations in the potential energy of an interatomic bond significantly affect the kinetics of diffusion and the energy of its activation [18].

These cast materials, along with the characteristics typical of metal alloys, have unique and unusual properties inherent, for example: high hardness and resistance to softening at high temperatures, precipitation hardening, positive temperature coefficient of hardening and a high level of strength characteristics at increased temperatures, attractive wear resistance, corrosion resistance and a number of other properties. Moreover, according to the data of scanning electron microscopy, they differ in a specific dendritic multiphase microstructure [19]. It is worth paying attention to the fact that even at high temperatures the alloys did not lose their sufficiently high mechanical properties, significantly exceeding the properties of traditional widely used alloys and mainly consist of simple bcc and fcc phases. After annealing, the alloys retained high hardness, corrosion resistance, oxidation resistance, and a number of other properties [19].

A characteristic feature of equiatomic high-entropy alloys is a fairly close coincidence of the calculated averaged data in modulus, lattice parameter, specific gravity, and CTE. A high-entropy alloy acquires average values of most of its physical characteristics, with the exception of strength properties. For the characteristics of hardness and yield point, significantly higher values are observed, which is associated with anomalously high athermal solid solution hardening [20]. Studies have shown that an addition of the number of atoms of any basic element to a high-entropy alloy will affect the lattice parameter and, accordingly, such characteristics as the modulus of elasticity and hardness [20]. A decrease in the calculated atomic radius is accompanied by an increase in the contact modulus of elasticity and hardness, regardless of the modulus of elasticity of atoms of the additionally introduced element.

High-entropy alloys in the cast state have strength characteristics inherent in individual elements that are part of equiatomic alloys in a nanostructured state. Such high values of hardness in the cast state are the guarantee of obtaining significantly higher hardness values in the coatings. Studies of various types of coatings have confirmed that coatings obtained on the basis of HEAs retain increased characteristics of hardness and heat resistance [21]. A feature of high-entropy alloys is the ability to order the lattice parameter during annealing, which is accompanied by a decrease in that parameter and a slight increase in the elastic modulus and hardness.

Mechanical Properties

High-entropy nitride systems such as (Ti, Hf, Zr, V, Nb) N are of great interest due to their unique properties. As shown in the literature, with an extreme increase in the entropy of the system, relaxation processes do not have time to occur, and the system remains in a nonequilibrium state. In the case of coatings, this contributes to the improvement of such important performance properties as hardness, wear resistance, corrosion resistance, and heat resistance.

Atoms of multielement high-entropy alloys have different electronic structures, sizes, and thermodynamic properties in the crystal lattice of a substitutional solid solution. With a disordered arrangement, a significant distortion of the crystal lattice occurs. The resulting microdistortions of the crystal lattice contribute to solid solution hardening [21]. The reduced free energy of high-entropy alloys ensures the stability of the solid solution at high temperatures.

Large lattice distortions caused by the substitution of several metal elements with different atomic sizes lead to a decrease in the diffusion rate of atoms and enhance the effect of the formation and stabilization of a solid solution. Moreover, large distortions contribute to a decrease in the crystallite growth rate. In this case, the formation of a nanoscale or amorphous structure occurs. As a result of the high entropy of mixing of such alloys, lattice deformation, and a decrease in diffusion in high-entropy alloys, solid-solution phases with a simple crystal lattice are formed. Such crystal lattices are face-centered cubic or body-centered, but double or ternary intermetallic compounds are not formed [22]. The importance of the difference in the atomic radii of the constituent components for the formed structural state was revealed [22]. In this case, the more constituent elements with strongly different atomic radii, the greater the intracrystalline deformation and the higher the probability of the formation of an amorphous-like state.

The main advantage of high-entropy alloys is a significant increase in hardness characteristics in comparison with the hardness of the elements included in the alloy. The selected initial components are close to each other in terms of hardness characteristics and the hardness value for pure components does not exceed 1.5 GPa, while for any high-entropy alloy presented in table 1, the value of hardness exceeds 4.0 GPa. Attention is drawn to the fact that the calculated lattice period is always less than that determined by X-ray studies, and the calculated elastic modulus is always greater than that established by the instrumental indentation method.

According to some authors, HEAs can be used as radiation-resistant structural materials. It is indicated that, in terms of the level of exposure to ion irradiation, vacuum-arc multi-element coatings (Ti-Zr-V-Hf-Nb-Ta) N are the most resistant in comparison with mononitrides [20]. The change in the entropy of the system is directly proportional to the change in free energy and is thermodynamically unfavorable in a metastable state. Consequently, it can be assumed that external influences, such as thermal heating, the action of aggressive media (acids, alkalis), ionizing radiation will have a rather weak effect on the change in the structure and properties of high-entropy alloys, if the energy barrier is large enough. It was found that ion irradiation has little effect on the strength characteristics of the coatings. Changes in the value of microhardness are observed within 9–17% upon irradiation with helium ions with an energy of 500 keV in the fluence range from 5 × 1016 to 3 × 1017 ions/cm2 [20]. The established high radiation resistance of high-entropy coatings is due to the efficient mechanisms of recombination of radiation-induced point defects in nanostructured coatings, primarily at crystallite boundaries. It can be predicted that these coatings are promising as radiation-resistant nuclear reactor fuel claddings.

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