Phase Diagrams Research Papers - Academia.edu (original) (raw)

Since the Bronze Age, humans have been altering the properties of materials by adding alloying elements. 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. 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. With few exceptions, the basic alloying strategy of adding relatively small amounts of secondary elements to a primary element has remained unchanged over millennia. 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. 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. New approaches are needed if the compositional space to explore is to be significantly enlarged. One such approach is based on mixing together multiple principal elements in relatively high (often equi-atomic) concentrations. This approach stands in sharp contrast to the traditional practice and has, therefore, attracted much attention. 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. Two groups independently proposed the study of a new class of alloys containing multiple elements in near-equiatomic concentrations. 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. 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. 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. 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. 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. 1). They coined a catchy new name, high-entropy alloys (HEAs), for this Abstract | Alloying has long been used to confer desirable properties to materials. Typically , it involves the addition of relatively small amounts of secondary elements to a primary element. 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. The multi-dimensional compositional space that can be tackled with this approach is practically limitless, and only tiny regions have been investigated so far. Nevertheless, a few high-entropy alloys have already been shown to possess exceptional properties, exceeding those of conventional alloys, and other outstanding high-entropy alloys are likely to be discovered in the future. Here, we review recent progress in understanding the salient features of high-entropy alloys. Model alloys whose behaviour has been carefully investigated are highlighted and their fundamental properties and underlying elementary mechanisms discussed. We also address the vast compositional space that remains to be explored and outline fruitful ways to identify regions within this space where high-entropy alloys with potentially interesting properties may be lurking.