History of High-Entropy Materials

Abstract

Materials are usually categorized by chemical compositions and the phase structure of the materials, e.g., Ti-based alloys, Cu-based alloys, and NdFeB magnets, Ferrite stainless steels, ZrBe-based bulk metallic glasses. It might be JW Yeh in Taiwan, who suggested to use configurational entropy to categorize materials, especially he defined the high-entropy alloys, with at least five elements and each content above 5 at.%, and the configurational entropy is above 1.62R (sometime 1.5R), here R is the gas constant. The configurational entropy was defined by Boltzmann equation, \(S = k\ln W\), here k is Boltzmann constant, W is the numbers of microstates corresponding to the macrostate. Currently, entropic materials may contain high- and medium-entropy alloys and ceramics. Moreover, high entropic films, composite materials are also proposed. The most challenge problem for the entropic materials is the compositional design because there are many principal elements included, and nonlinear behaviors between the properties and the entropy. We have proposed a parameter to evaluate the entropic effect over the enthalpy at the liquid state, which have been verified as effective to predict the solid solution and amorphous phases, both disordered phases, in the multicomponent materials. The most extensive studied high-entropy alloys are [1] CoFeMnNiCr, which is a FCC structured single-phase alloy discovered by Professor Cantor, also called Cantor alloys, and [2] another is a BCC-structured alloy, AlCoCrFeNi, which was discovered by Y. Zhang’s group in the USTB. The high-entropy alloys design conception is very effective to break the trade-off of the materials strength-ductility. Some specified high-entropy alloys can have very high fracture and impact toughness at low temperature, e.g., CoFeCrNi, which has very high strength and is ductile at liquid helium temperature, and some can be very heat softening-resistant at higher temperature, e.g., WTaCrFeV. Recently, new technology such as high-throughput screening and characterization, and additive manufacturing technologies have been applied in the entropic materials.