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High entropy alloys: PVD has a major role to play

Over the last five millennia, alloys have aided and shaped the advancement of society. A remarkable progression from native metals and native alloys to the inadvertent discovery of arsenical bronzes. Numerous combinations of alloying elements, typically based on a single primary element, were tested. In alloys which including high-tin bronzes and ultrahigh-carbon steels, greater quantities of alloying were utilized. From such concentrated binary alloys to multicomponent alloys signified a significant advancement in the previous century, resulting in alloy steels and superalloys. Nonetheless, all these alloys had one metal component in a disproportionately large amount. In 2004, Jien-Wei Yeh and Brian Cantor separately revealed multicomponent equiatomic or near-equiatomic alloys, marking a new milestone in alloying. Surprisingly, several of these alloys were solid solutions comparable to bronzes from the third millennium BCE. They have revitalized the material world, offering an incredibly diverse family of alloys.

Figure 1: Historical evolution of engineering materials marked with the birth of HEAs Figure Curtesy: Yeh et al., Advanced Engineering Materials 2004
Figure 1: Historical evolution of engineering materials marked with the birth of HEAs Figure Curtesy: Yeh et al., Advanced Engineering Materials 2004

Alloying is the most useful discovery in the history of metallurgy. Although in metallurgy, where pure metals are of little utility, but many alloys have many applications, the English literatures insist on unalloyed joys, meaning that the sense of pleasure must be pure and not admixed with other emotions. This concept of alloying is not just useful for metals, though. Alloying is a technique that may be used to both polymers and ceramics. Further development is possible by combining different kinds of materials into composites.

The society's civilizational journey began with the discovery of pure metals such as gold and copper. We now have access to an enormous amount and variety of materials. The Ashby map (Ashby, 2011) depicted in the following figure provides a panoramic picture of the evolution of material consumption over 10 millennia. A visual representation of the many groups of materials, ranging from ceramics through metals, polymers, and, more recently, composites, is depicted. It is possible to trace the path from discovery through development to material design. Ashby's (2011) chart of strength against density, illustrated in the following figure, vividly depicts the filling of material-property space from 50,000 BCE to the present scenario. In terms of time, the most filling has happened in the last 50 years, during which metal, ceramic, and composite envelopes have expanded significantly, and new envelopes of synthetic polymers and foam materials have taken up substantial space. However, the filled region appears to be approaching some basic boundaries beyond which it is difficult to progress any farther.