High Entropy Alloys

High entropy alloys (HEAs) are classes of new multicomponent materials consisting of five or more principal elements of alloy or have an entropy of mixing higher than 1.5 R. HEAs have an increasing interest in many engineering applications because of their outstanding performance and unique properties, namely: high thermal stability, strong mechanical strength, excellent corrosion resistance, fine trade-off effect on performance, and very promising catalyst for hydrogen oxidation reaction (HOR) to enhance the energy conversion efficiency of the hydrogen fuel cell.

Because the alloys consist of various elements with different percentage representations and foremostly wide range of melting temperatures, there are many techniques used for their preparation. Usually, HEAs are prepared by arc melting, vacuum induction melting, physical vapour deposition or mechanical alloying (MA), which is accompanied by subsequential compaction via spark plasma sintering (SPS) or hot isostatic pressing (HIP).

On the other hand, a vast majority of literatures have reported HEAs as multi-phase alloys, while single phase alloys (solid solutions) with broader applicability have attracted more attention in the practice since important metallurgical variables (i.e., the number, types and concentrations of alloying elements) could be systematically varied and directly correlated with physical-mechanical properties (i.e., elastic constants, stacking-fault energy, diffusion coefficient, strength, and ductility) of the material. 

. Previous works have shown more interest in the microstructure examination and the relationship with physical-mechanical properties of HEAs using the combination of different characterization techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). Among these methods, EBSD has been powerful to reveal important information of the material related to microstructure, grain orientations, grain boundary character, and deformation levels, which are fundamental for any (post) treatment proposal and/or modification of processing parameters to improve the respective material’s properties. Thus, several works have used EBSD to demonstrate the strong influence of the (re)solidification rate on the microstructure and properties of HEA system (AlxCoCrFeNi, x = 0 – 1.8), to analyze microstructural evolution and formation of annealing twins of CoCrFeMnNi after thermomechanical processing, strengthening mechanism of the CoCrFeNiCu HEA via friction stir processing, as well as to investigate microstructures and deformation substructure evolution in FeCoNiCrN alloy. 

It is worth mentioning that the application of HAEs as electrocatalyst is a topic of current interest. However, a study dealing with both mechanical and electrochemical behaviors of HAEs is still lacking.

Recently, our lab presents the successful synthesis of single-phased HEA samples, using a simple electric-arc melting method, with not only outstanding mechanical properties (strength, ductility), but also excellent corrosion resistance and electrocatalytic activity in acidic media. The design of the HAEs was performed based on thermodynamic and phase diagram calculations using Thermo-calc software. Mechanical behavior of the studied HAEs was based mainly on Vicker hardness and compression tests. Microstructure and grain orientations determination of the HAEs were studied by SEM, TEM, EBSD measurements. Corrosion behavior and electrocatalytic activity of the HAEs were evaluated using Tafel, cyclic voltammetry, and linear sweep voltammetry techniques.