High-entropy alloy

[2][6][10][11][12][13][14] Although HEAs were considered from a theoretical standpoint as early as 1981[15] and 1996,[16] and throughout the 1980s, in 1995 Taiwanese scientist Jien-Wei Yeh came up with his idea for ways of actually creating high-entropy alloys, while driving through the Hsinchu, Taiwan, countryside.

Significant research interest from other countries did not develop until after 2004 when Yeh and his team of scientists built the world's first high-entropy alloys to withstand extremely high temperatures and pressures.

Yeh was also the first to coin the term "high-entropy alloy" when he attributed the high configurational entropy as the mechanism stabilizing the solid solution phase.

In HEAs, those whole-solute matrices' diffusion vacancies are surrounded by different element atoms, and thus have a specific lattice potential energy (LPE).

Ω should be greater than or equal to 1.0, (or 1.1 in practice), which means entropy dominates over enthalpy at the point of solidification, to promote solid solution development.

Formation of a solid-solution phase requires a δ ≤ 6.6%, which is an empirical number based on experiments on bulk metallic glasses (BMG).

When the atomic size difference (δ) is sufficiently large, the distorted lattice would collapse and a new phase such as an amorphous structure would be formed.

[51] However, DFT modeling of complex, random alloys has its own challenges, as the method requires defining a fixed-size cell, which can introduce non-random periodicity.

This is commonly overcome using the method of "special quasirandom structures", designed to most closely approximate the radial distribution function of a random system,[52] combined with the Vienna Ab initio Simulation Package.

theory for multicomponent alloys,[56][57] which evaluates the two-point correlation function, an atomic short-range order parameter, ab initio.

theory has been used with success to study the Cantor alloy CrMnFeCoNi and its derivatives,[58] the refractory HEAs,[59][60] as well as to examine the influence of a material's magnetic state on atomic ordering tendencies.

This method rapidly produces hundreds of samples, allowing the researcher to explore a region of composition in one step and thus can used to quickly map out the phase diagram of the HEA.

This model uses first principle high throughput density functional theory to calculate the enthalpies, thus requiring no experiment input, and it has shown excellent agreement with reported experimental results.

A refractory alloy, VNbMoTaW maintains a high yield strength (>600 MPa (87 ksi)) even at a temperature of 1,400 °C (2,550 °F), significantly outperforming conventional superalloys such as Inconel 718.

[74] A single-phase nanocrystalline Al20Li20Mg10Sc20Ti30 alloy was developed with a density of 2.67 g cm−3 and microhardness of 4.9 – 5.8 GPa, which would give it an estimated strength-to-weight ratio comparable to ceramic materials such as silicon carbide,[12] though the high cost of scandium limits the possible uses.

Small-scale HEAs combining these properties represent a new class of materials in small-dimension devices potentially for high-stress and high-temperature applications.

With the aim of increasing high temperature strength, Chien-Chuang et al. modified the composition of TiZrNbHfTa and studied the mechanical properties of the refractory high-entropy alloys TiZrMoHfTa and TiZrNbMoHfTa.

But upon adding titanium, it forms a complex microstructure consisting of FCC solid solution, amorphous regions and nanoparticles of Laves phase, resulting in superparamagnetic behavior.

[83] They are being used for high-performance applications like power electronics, heat spreaders, sensors, and inductors, and show potential for efficient conductive materials in advanced components.

Two aspects need to be considered for nano-crystalline HEAs: the stability of phases formed, which is dominated by the thermodynamics mechanism (see alloy design), and the retention of nanocrystallinity.

Compared to the preparation methods of HEA bulk materials, HEAFs are easily achieved by rapid solidification with a faster cooling rate of 109 K/s.

The related contents of the as-deposited films are approximately equal to that of the original target alloy even though each element has a different sputtering yield with the help of the pre-sputtering step.

Based on the published papers, lots of researchers doped different quantities of elements such as Al, Mo, V, Nb, Ti, and Nd into the CrMnFeCoNi system, which can modify the chemical composition and structure of the alloy and improve the mechanical properties.

[98] The hardness increased drastically to 8.61 GPa for Ti0.2 by adding Ti atoms to the CoCrFeMnNi alloy system, suggesting good solid solution strengthening effects.

The increase in hardness was due to both the lattice distortion effect and the presence of the amorphous phase that was attributed to the addition of the larger Ti atoms to the CoCrFeMnNi alloy system.

For nitride-HEAFs, Huang et al. prepared (AlCrNbSiTiV)N films and investigated the effect of nitrogen content on structure and mechanical properties.

Compared to pure metallic HEAFs (Table 1), most nitride-based films have larger hardness and elastic modulus due to the formation of binary compound consisting of nitrogen.

The grain size, phase transformation, structure, densification, residual stress, and the content of nitrogen, carbon, and oxygen also can affect the values of hardness and elastic modulus.

The published papers regarding the pure metallic HEAFs and their phase, hardness and related modulus values via magnetron sputtering method.

For instance, powders may be processed using high energy ball milling (HEBM) which relies on the principle of mechanical alloying.