Phase Selection, Lattice Distortions, and Mechanical Properties in High‐Entropy Alloys

Abstract

Increasing demand for improved physical, chemical, and mechanical properties led to the development of conventional alloys from single elements. These conventional alloys are made by selecting one or two major elements with desired properties, and other minor components are added to tune the properties to meet performance specifications. The conventional alloy design strategy is mainly based on the "solutesolvent" principle. However, another alloy design strategy based on multiprincipal elements in the near-equimolar ratio has recently been developed. In this multiprincipal element system, it is difficult to distinguish between the solute and the solvent, and various atoms are supposed to be randomly distributed in the crystal lattices (which has not been confirmed yet). This strategy, proposed by Yeh [1] and Cantor, [2] was based on the Boltzmann hypothesis of the relationship between entropy and system complexity. They proposed that the increased configurational entropy of mixing in alloys with multiple alloying elements in near-equiatomic proportions would overcome the enthalpy of compound formation, stabilizing solid solutions (SS) at the expense of intermetallics (IMs) during solidification. These multiprincipal element alloys are named "high entropy alloys" (HEAs), although the exact values of entropy in HEAs have not been experimentally measured yet. Since then, hundreds of HEAs with different combinations have been developed based on transition metals, refractory metals, and compound forming elements. [2-20] Most of these HEAs have been found to possess simple crystal structures such as the face-centered cubic (fcc) Fe-Co-Cr-Mn-Ni HEAs, [2] bodycentered cubic (bcc) Al-Co-Cr-Fe-Ni HEAs, [18] and the hexagonal close-pack (hcp) Ho-Dy-Y-Gd-Tb HEAs [6] when solidified from the melts. Experimental characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), differential scanning calorimetry (DSC), nanoindentation, tensile, wear-and corrosionresistance tests have been used to study the microstructureproperties correlations of HEAs. [3-50] These studies have revealed that HEAs possess superior properties, such as high hardness and strength, [23-29] superior corrosion and wear resistance, [3,30-35] good magnetic properties, [9,36-39,51] structural stability, [7,40,41] attractive mechanical properties at extreme temperature conditions, [21,42-47] and good electronic properties. [48,49] Yeh [50] proposed four core effects of HEAs, namely: 1) high-entropy effect; 2) sluggish diffusion effect; 3) sever lattice distortion effect; and 4) cocktail effect, in which the lattice distortion might be the key factor affecting the properties of HEAs. Although a complete understanding of lattice distortions in HEAs is still not established yet, significant progress in HEAs has been made recently.

High-entropy-alloy nanoparticles with 21 ultra-mixed elements for efficient photothermal conversion

“…In general, there are four types of high-entropy effect—the high-entropy, the lattice distortion, the sluggish diffusion and the ‘cocktail’ effect [ 13 , 14 , 40–42 ]—which result in excellent performances characterized by, for example, good ductility, corrosion resistance and high strength of the HEAs [ 25 , 43 , 44 ]. Here, the lattice distortion effect of HEAs could result in the random occupation of metallic atoms in a crystalline structure, reducing the electrical and thermal conductivity [ 42 , 45 , 46 ]. The reduction of thermal conductivity can result in the accumulation of heat under solar irradiation, increasing the surface temperature of nanoparticles and thus enhancing the water evaporation rate.…”

Characterization of Lattice Defects in Refractory Metal High‐Entropy Alloy HfNbTaTiZr by Means of Positron Annihilation Spectroscopy

“…[3] A genuine HEA is a metallic material characterized by high distortions of the crystalline lattice due to the random occupation of lattice points by atoms of various sizes. [4] This intriguingly new structure may result in superior mechanical properties, [5,6] good resistance against radiation damage, [7][8][9][10][11] as well as interesting functional properties, for example, high hydrogen storage capacity. [12] Refractory metal HEAs [13] crystallize in the body centered cubic (bcc) structure and are characterized by high strength at elevated temperatures.…”

Influence of Local Lattice Distortion on Elastic Properties of Hexagonal Close‐Packed TiZrHf and TiZrHfSc Refractory Alloys

“…As the effect of LLD is dependent on the crystal structures of MPEAs, it is meaningful to discuss the impact of LLD for FCC‐, BCC‐, and HCP‐structured MPEAs separately. [ 20 ] A series of studies on LLD of cubic MPEAs have been intensively conducted. Zheng et al [ 21 ] reported a 25% reduction of the shear modulus for BCC TiZrHfNbTa by LLD.…”