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.
