High entropy alloys (HEAs) have emerged as promising candidates for high-temperature applications due to their ability to withstand high temperatures with high thermal stability, superior oxidation and corrosion resistance, thermal fatigue, and creep resistance.
These alloys possess distinct characteristics known as core effects, including high entropy effect, sluggish diffusion effect, severe lattice distortion, and cocktail effect. Thanks to these unique core effects, HEAs often exhibit exceptional properties.
(see figure 1 showing tensile properties of different high entropy alloys at different temperatures. The HEA CoFeNi2V0.5Mo0.2 exhibit outstanding properties of ductility and strength).
High entropy alloys (HEAs) have become a topic of great interest in materials due to their unique properties. This review article presents some results of the mechanical and corrosion properties of HEAs, as a function of crystal structure, temperature and alloying elements. Figure 2 shows the effect of FCC percentage on the corrosion potential and corrosion current density. It is seen that there is an optimum at 40%.
Alloy design has evolved with the introduction of High-Entropy Alloys (HEAs), which consist of five or more principal elements mixed in equimolar or near-equimolar ratios. Examples include AlCoCrFeMn and FeCrMnNiAlCo. Although K. F. Achard investigated the first multi-component system with five to seven elements in the 18th century, it wasn't until 2004 that HEA's research gained notable attention. Further studies have since been conducted on non-equiatomic multi-component materials, such as FeMnCoCr, and have been widely reported.
One thing that sets HEA apart from other alloys is that it mostly comprises a simple solid solution rather than complex phases or intermetallic compounds. This is because of its high mixing entropies and slow diffusion, which also help to reduce the brittleness of the material.
High Entropy Alloys (HEAs) are ideal for high-temperature applications because of their exceptional thermal stability, oxidation resistance, and superior mechanical properties. These properties, such as high-temperature strength and thermal fatigue, are achieved through their unique compositional and structural features. Due to their remarkable properties, HEAs have gained much attention in various research areas for their credible applicability in extreme conditions.
Moreover, it is easy to mass-produce HEAs with existing PVD equipment and technologies, as they do not require special processing techniques. Over 320 HEAs have been processed with more than 35 different elements and their combinations.
There have been several significant contributions to the development of High Entropy Alloys (HEAs) that exhibit excellent mechanical properties, as well as strong resistance to oxidation and corrosion. When considering high-temperature applications, it is particularly important to prioritize oxidation resistance. Typically, the oxidation resistance of conventional alloys can be improved by incorporating elements that form a stable and thick oxide layer on the surface of the material at higher temperatures. For example, adding Al, Si, and Cr can significantly enhance oxidation resistance. In the case of HEAs, research into oxidation resistance has been conducted primarily on selected alloys such as CrMnFeCoNi, CoCrFeNiAlx, and CoCrCuFeNiSix, which often exhibit good oxidation resistance due to the presence of Al, Cr, and Si elements. Furthermore, most of the research on HEAs has focused on the effects of alloying addition on oxidation behaviour.
Figure 3 shows the corrosion behavior of an AlTiCrNiTa coating fabricated on Zr-4 substrate using the radiofrequency (RF) MS technique. (Source: High entropy alloy coatings for biomedical applications: A review, Smart Materials in Manufacturing, Volume 1, 2023, 100009). In this reference, the corrosion behavior of AlTiCrNiTa is explained as follows: An Al2O3 outermost layer is formed because of the reaction of Al present in the coating with O2. Spinel NiCr2O4 particles are formed on the Al2O3 layer due to the contact of O2 with Cr. Al2O3 reacts with H2O, resulting in the formation of AlO(OH), leading to the dissolution of the Al2O3 layer, followed by peeling of NiCr2O4 particles. This would suggest that with time, the durability of the coating is reduced. The authors suggest that this problem can be solved by reducing the Al content to reduce the dissolution of Al2O3 and increasing the Cr content to promote the formation of a dense spinel NiCr2O4 coating.