![]() ![]() ![]() ![]() For the past decade and a half, however, a new alloying strategy that involves the combination of multiple principal elements in high concentrations to create new materials called high-entropy alloys has been in vogue. Typically, it involves the addition of relatively small amounts of secondary elements to a primary element. ![]() They coined a catchy new name, high-entropy alloys (HEAs), for this Abstract | Alloying has long been used to confer desirable properties to materials. But Yeh and colleagues reasoned that, as the number of elements in an alloy increased, the entropic contribution to the total free energy would overcome the enthalpic contribution and, thereby, stabilize solid solutions (Box 1 Fig. This was a counter-intuitive notion because the conventional view-likely based on binary phase diagrams in which solid solutions are typically found at the ends and compounds near the centres-was that the greater the number of elements in concentrated alloys, the higher the probability that some of the elements would react to form compounds. Jien-Wei Yeh and co-workers 1 provided an additional intriguing rationale for investigating these alloys: they hypothesized that the presence of multiple (five or more) elements in near-equiatomic proportions would increase the configurational entropy of mixing by an amount sufficient to overcome the enthalpies of compound formation, thereby deterring the formation of potentially harmful intermetallics. Owing to their sheer numbers, little is known about concentrated, multi-component alloys but, by the same token, because there are so many possible combinations, the concept offers promise to discover interesting new alloys with useful properties in their midst. It was subsequently pointed out that conventional alloys tend to cluster around the corners or edges of phase diagrams, where the number of possible element combinations is limited, and that vastly more numerous combinations are available near the centres of phase diagrams, especially in quaternary, quinary and higher-order systems 3. Two groups independently proposed the study of a new class of alloys containing multiple elements in near-equiatomic concentrations. The related surge in research activity, especially during the past 5 years, can be traced back to the publication of two seminal papers 1,2 in 2004. This approach stands in sharp contrast to the traditional practice and has, therefore, attracted much attention. One such approach is based on mixing together multiple principal elements in relatively high (often equi-atomic) concentrations. New approaches are needed if the compositional space to explore is to be significantly enlarged. However, such a primary-element approach drastically limits the total number of possible element combinations and, therefore, alloys, most of which have been identified and exploited. It is even reflected in the way alloys are named after their principal constituent: ferrous alloys, aluminium alloys, titanium alloys, nickel alloys and so on. With few exceptions, the basic alloying strategy of adding relatively small amounts of secondary elements to a primary element has remained unchanged over millennia. Examples from the modern era include steels that consist primarily of iron, to which elements such as carbon and chromium are added for strength and corrosion resistance, respectively, and copper alloyed with beryllium to make it strong and non-sparking for use in explosive environments. For example, a few percent by weight of copper was added to silver to produce sterling silver for coinage a thousand years ago, because pure silver was too soft. Since the Bronze Age, humans have been altering the properties of materials by adding alloying elements. ![]()
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