![]() ![]() Here, we offer a critical perspective of the field with the intent of broadening the scope to re-engage the concepts of compositional and microstructural complexity. ![]() 5, 6 The compound forming elements Al and Ti are often added to both families. In spite of these advantages, current HEAs cover a surprisingly narrow range of systems that are typically based on the transition metals Cr, Co, Fe, Ni, Mn and Cu or on refractory metals such as V, Cr, Ti, Mo, Nb, Ta, W, Zr and Hf. Supporting this expectation, HEAs have already been produced with a wide range of properties and microstructures, including single phase, multiphase, nanocrystalline and amorphous. This suggests a strong potential for new discoveries in both scientific knowledge and practical usefulness. Each HEA base can be modified by adjusting proportions of the base elements around the equimolar composition, and by relatively minor additions of other elements as is carried out for conventional alloys, so that the number of unexplored compositions is truly astronomical. By contrast, an HEA base alloy has significant concentrations of five or more elements, so that the same palette of 12 elements gives 3302 different HEA bases. With this approach, each element represents an alloy base, so that a palette of a dozen elements (for example) gives only 12 alloy bases (Al base, Ti base, etc.). As many as a dozen alloy elements can be added, but the base element still almost always accounts for >50% of the alloy. 4 Conventional alloy development starts with a base element and then modifies properties by adding relatively minor amounts of other elements. This motivation has led to a strong emphasis on the search for single phase alloys and on development of simple, phenomenological rules to guide their selection.Īnother major benefit is that HEAs provide a vast number of new alloys. 1– 3 The rationale often given to support this strategy is that intermetallic phases can embrittle structural alloys, while solid solution alloys with simple crystal structures can have good strength and may retain ductility. 1 Increasing configurational entropy Δ S conf is a key motivation for high entropy alloys (HEAs), with the intent of stabilising solid solution phases over competing intermetallic compounds. High entropy alloys have five or more elements with atomic concentrations between 5 and 35%. ![]()
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