**2. High entropy alloys as aero-engine materials**

High entropy alloys (HEAs) are alloys with at least five metallic components and every one of these components has a molar atomic concentration somewhere between 5 and 35% [4]. Reports on most HEAs show the amalgams contain a basic face-centered cubic (FCC) or body-centered cubic (BCC) or hexagonal closed pack (HCP) solid solution phases without intermetallic phases because of their high-entropy impact [5]. These solid solution phases empower high entropy alloys combinations to have remarkable properties, for example, increased hardness, high fracture strength, yield stress, and plastic strain. HEAs exhibit good ductility, they have a superb work hardenability and high-temperature oxidation resistance [6]. They contain particular attractive magnetic properties, have high wear resistance and exhibit good erosion opposition [7]. As a result of these properties, HEAs are remarkable refractory materials, fatigue resistant materials, and have a corrosionresistant surface layers and diffusion obstruction layers for various structural jet engine applications. However, most HEAs were fabricated using conventional techniques and an enhancement in the manufacturing process of the alloys will upgrade the mechanical properties of these alloys.

**139**

*High Entropy Alloys for Aerospace Applications DOI: http://dx.doi.org/10.5772/intechopen.84982*

Metals: ferrous metals, e.g., stainless 15-5PH and nonferrous metals, e.g., aluminum, e.g., aluminum 6062, aluminum 7050, aluminum 7075, Al-Li and

Heat resistant super alloys, e.g., nickel 718, Ti6Al4V, Ti-5553,

magnesium

TiAl-based alloys

**Material Properties Short comings**

Good electric and thermal conductivity, high strength and corrosion resistance, light weight and ductility, hardness, high heat resistance

Elevated temperature strength and corrosion

strength and compression

resistance

resistance

Composite materials Low weight, high tensile

Magnesium is exceedingly combustible; steel is non-receptive to warm treatment. There is a surprising expense of delivering aluminum parts and thin aluminum cannot be compound bent

Titanium and nickel alloys and some ceramic materials are grouped under heat-resistant alloys; in any case, they are hard to machine meaning shorter life expectancy and decreased process

The most common composite material used in the aerospace is fiberglass and it is expensive to fix while resins used as matrix discharges poisonous fumes

while conveying load

security

ing of high entropy alloys [8–10].

∆*Sconf* = −*KInw* = −*RIn*\_\_1

number of elements in the composition [8].

equilibrium.

determined by:

When designing high entropy alloys, the components in the blend need not be equivalent or near equal. The structure does not separate a minor or major element in its composition rather, the major consideration for designing high entropy alloys amalgams is; the elements in the mix ought to be at least 5 in number and additionally, their atomic concentrations should be between 5 and 35%. Higher configurational entropy is achieved by having no less than five elements in the composition because an increment in the number of components increases the mixing entropy and best explains the high entropy effect which is an important factor to the design-

Nmajorelements ≥ 5 at.%≤ 35 at.% (1)

Nminorelements ≥ 0 ≥ 5 at.% (2)

When the Gibbs free energy (∆Gmix) is at its base, the system is said to be at

∆Gmix = ∆Hmix − T∆Smix (3)

where the enthalpy of mixing is the ∆Hmix, ∆Smix is the entropy of mixing and T is the temperature. From the Boltzmann hypothesis on the entropy of mixing, the molar configuration entropy design (∆SConf) gives more prominent outcomes of forming a multi-element solid solution phase through statistical thermodynamics

∆*Sconf* = *RIn*(*n*) (5)

where ∆*Sconf* is the molar concentration, K is the Boltzmann constant, w is the thermodynamic probability, R is the gas constant (8.134 J/kmol) and *n* is the

High entropy alloys form stable solid solutions because of the crystal structure of the elements which is unaltered when different elements are included. Furthermore,

*<sup>n</sup>* = −*RInn*∆*Smix* = *RIn*(*n*) (4)

Other materials used for aerospace applications are:


When designing high entropy alloys, the components in the blend need not be equivalent or near equal. The structure does not separate a minor or major element in its composition rather, the major consideration for designing high entropy alloys amalgams is; the elements in the mix ought to be at least 5 in number and additionally, their atomic concentrations should be between 5 and 35%. Higher configurational entropy is achieved by having no less than five elements in the composition because an increment in the number of components increases the mixing entropy and best explains the high entropy effect which is an important factor to the designing of high entropy alloys [8–10].

$$\mathbf{N}\_{\text{major elements}} \ge \mathbf{5} \text{ at}\\ \mathbf{9} \le \mathbf{35} \text{ at}\\ \mathbf{9} \tag{1}$$

$$\mathbf{N}\_{\text{minor elements}} \ge \mathbf{0} \succeq \mathbf{5} \text{ at} \mathbf{9} \tag{2}$$

When the Gibbs free energy (∆Gmix) is at its base, the system is said to be at equilibrium.

$$
\Delta \mathbf{G}\_{\text{mix}} = \Delta \mathbf{H}\_{\text{mix}} - \mathbf{T} \Delta \mathbf{S}\_{\text{mix}} \tag{3}
$$

where the enthalpy of mixing is the ∆Hmix, ∆Smix is the entropy of mixing and T is the temperature. From the Boltzmann hypothesis on the entropy of mixing, the molar configuration entropy design (∆SConf) gives more prominent outcomes of forming a multi-element solid solution phase through statistical thermodynamics determined by:

$$
\Delta S\_{\text{conf}} = -K \text{Im} \omega = -R \text{Im} \, \frac{1}{\overline{n}} = -R \text{Im} \, n \, \Delta S\_{\text{mix}} = R \, \text{Im}\_{(n)} \tag{4}
$$

$$
\Delta S\_{\text{conf}f} = R \operatorname{Im}\_{\text{(n)}} \tag{5}
$$

where ∆*Sconf* is the molar concentration, K is the Boltzmann constant, w is the thermodynamic probability, R is the gas constant (8.134 J/kmol) and *n* is the number of elements in the composition [8].

High entropy alloys form stable solid solutions because of the crystal structure of the elements which is unaltered when different elements are included. Furthermore, they are stable when the chemical components remain in a single homogeneous phase. This happens when the elements in the composition are firmly packed together on the periodic table.

There is a relationship between the entropy of fusion and the phase transition metals used in the compositional design of high entropy alloys. At the point when the number of elements in a composition is increased, the framework will be progressively stable.

For instance, from Eq. (5),

$$
\Delta S\_{conf} = R \operatorname{Im}\_{(n)}.
$$

When the number of elements in a system *n* = 5, ∆*Sconf* <sup>=</sup> 1.61*R* where *R* is melting point value, when the number increases, the value of *R* increases. Thus, increasing the number of elements in a composition increases the mixing entropy which increases the stability of the alloy system.

Cantor et al. [11] demonstrated the number of alloys that have been studied including unitary, binary, ternary and high entropy alloy in a system with the total number of different possible alloys *N* as;

$$N = \left(\frac{100}{\mathcal{X}}\right)^{\varepsilon - 1} \tag{6}$$

With 60 elements in the alloying range of the periodic table, at a material specification of 1, he gave the conservative number of possible alloys design as ≈10177.

#### **2.1 Properties and production techniques of high entropy alloys**

The property, microstructure and design of high entropy alloys are dependent on some core effects, the phase composition and technique of fabrication respectively [12].

#### *2.1.1 Core effects*

#### *2.1.1.1 High configurational entropy*

The high configurational entropy impact hinders the phase transformation influencing the thermodynamics of the system yet builds the formation of solid solutions. Despite having numerous meta-stable states, the equilibrium state has the most reduced free energy of mixing in solids from the second law of thermodynamics. This suggests that combining five or more components in equimolar or near equimolar concentrations may bolster two phases; the solid solution phase and the intermetallic phase. Intermetallic phase is a stoichiometric compound with super-lattices that give the high entropy alloy amalgams ominous properties while single-phase solid solution phases show a system with a total blend of elements forming simple (BCC, FCC, HCP) crystal structures over intermetallic compounds. The presence of a prolonged range order isolates the intermetallic phase for the solid solution phases. At elevated temperatures the higher the number of elements in a disordered state, the less the possibility of forming intermetallic phases.

This uses the Boltzmann equation:

$$\text{(S = kIn(N))}\tag{7}$$

**141**

*High Entropy Alloys for Aerospace Applications DOI: http://dx.doi.org/10.5772/intechopen.84982*

Sluggish diffusion involves the kinetics of the system; low diffusion rate, increases thermal stability, an increase in recrystallization temperature, slows down grain growth, slows down phase separation and improves creep resistance which might benefit the microstructure. The presence of nanocrystals in as-cast material and amorphous materials in sputter-deposited thin films and high recrystallization temperatures enlivened the sluggish diffusion theory. Cross-diffusion happens when the focus angle of one element prompts or changes the motion of another element. This occurs when one component changes the chemical attributes of other elements in the alloy system. Increasing the number of the composition of the elements in an alloy may make the diffusion become sluggish and reduce the

Lattice distortion manages the structure of the high entropy alloy system. A noteworthy contrast in the atomic radii; the movement of large and small atoms causes' lattice strain making the entire framework to have a distorted lattice. Lattice distortion prompts solid solution strengthening by restricting dislocation, the displacement occurring at each lattice spot relies on the atom dwelling at that spot.

Cocktail effect affects the properties of the system designed. It includes the compositional mixtures of elements where the consequence of the blend is both unpredictable and unexpected because of the distinctive properties the individual element provides. Adding an element to the mixture with properties realized will enhance the combination through the cocktail effect. High entropy alloys may exhibit properties dependent on the reaction between elements in the system. On the off chance that a high-temperature high entropy alloy is desired, elements with

In recent times, a few techniques for anticipating the phase(s) high entropy alloys will possess have emerged. The phases have been predicted most often, however not continually, using the calculation of phase diagram [13], the valence electron concentration or utilizing the thermodynamic and geometry effect.

P = C + 2 − *F* (8)

where C is the number of elements in the system, F is the degree of freedom and P is the maximum number of phases at equilibrium. This standard proposes that high entropy alloys can exhibit multiple phases, nonetheless, high entropy alloys (HEAs) are usually a single phase or double phase system but rarely having multiple phases regardless of containing multiple elements. Solid solution high entropy alloys form FCC, BCC or HCP phases due to their mixing entropies. BCC structured HEAs have high yield strengths, low ductility, limited plasticity and are brittle while the FCC structured HEAs have a low yield strength, inferior cast ability, compositional segregation high plasticity and ductility [14]. The ductility of HEAs decreases as the yield stress and comprehensive strength increases and the blend of

*2.1.1.2 Sluggish diffusion*

temperature of the system.

*2.1.1.3 Lattice distortion*

*2.1.1.4 Cocktail effect*

*2.1.2 Phase formation*

According to Gibbs rule:

elevated temperature strength ought to be used.

To show the configurational entropy S, of an ideal solution with N, number of elements as it only regards configurational entropy each at an equimolar concentration and *k* is the Boltzmann constant however, this hypothesis presumes that the HEA studied constitute random sampling of all HEA systems which is not hypothetically valid.
