Liquid Phase Separation in High-Entropy Alloys—A Review

Abstract

It has been 14 years since the discovery of the high-entropy alloys (HEAs), an idea of alloying which has reinvigorated materials scientists to explore unconventional alloy compositions and multicomponent alloy systems. Many authors have referred to these alloys as multi-principal element alloys (MPEAs) or complex concentrated alloys (CCAs) in order to place less restrictions on what constitutes an HEA. Regardless of classification, the research is rooted in the exploration of structure-properties and processing relations in these multicomponent alloys with the aim to surpass the physical properties of conventional materials. More recent studies show that some of these alloys undergo liquid phase separation, a phenomenon largely dictated by low entropy of mixing and positive mixing enthalpy. Studies posit that positive mixing enthalpy of the binary and ternary components contribute substantially to the formation of liquid miscibility gaps. The objective of this review is to bring forth and summarize the findings of the experiments which detail liquid phase separation (LPS) in HEAs, MPEAs, and CCAs and to draw parallels between HEAs and the conventional alloy systems which undergo liquid-liquid separation. Positive mixing enthalpy if not compensated by the entropy of mixing will lead to liquid phase separation. It appears that Co, Ni, and Ti promote miscibility in HEAs/CCAs/MPEAs while Cr, V, and Nb will raise the miscibility gap temperature and increase LPS. Moreover, addition of appropriate amounts of Ni to CoCrCu eliminates immiscibility, such as in cases of dendritically solidifying CoCrCuNi, CoCrCuFeNi, and CoCrCuMnNi.

VISCOSITY AND ELECTRICAL RESISTIVITY OF LIQUID CuNiAl, CuNiAlCo, CuNiAlCoFe ALLOYS OF EQUIATOMIC COMPOSITIONS

“…The fields of MPEA, CCA, HEAs have expanded and now include microstructures with any number of phases and phases of any type.The scope of their application includes both structural and functional materials [1][2][3][4][5][6]. Regardless of classification, the researchers are based on the study of structure-properties and processing relations in these multicomponent alloys with the aim to surpass the physical properties of conventional materials [7]. Currently, two concepts HEA are being formulated: the 'entropy effect' and the 'sluggish diffusion' [1,2].…”

High Entropy Alloys Mined From Binary Phase Diagrams

“…Many mixed phase HEAs are found to form because of interatomic repulsions 30,31 . Specific element pairs, such as Cr and Cu, separate because of the large positive mixing enthalpy, causing multiphase formations in HEAs 30 . This effect is included in the model with the phase separation parameter (PSP).…”

Evaluation of microstructural and corrosion resistance of as-cast Cu45Mn25Al15Fe5Cr5Ni5 high entropy alloy

“…For example, restricting the content Ni in the proposed alloy minimizes the possibility of formation of Al-Ni intermetallic. It should be taken into account that the Al-Ni (B 2 ) compounds are thermodynamically stable at elevated temperature [10], while the Cu-Mn rich phase has a lower melting point nearly in the range of 1050°C [12]. As a result, there will be two competing phases during solidification; however, controlling the Ni content in conjunction with increasing the entropy of mixing will promote the formation of a solid solution instead of intermetallic compounds.…”

“…Cu containing high entropy alloys attracted attention due to its unique solidification path in terms of presence of liquid phase separation phenomenon [10][11][12]. The addition of Cu to FeMnCrNiCo alloy shows the formation of two fcc phases fcc1/fcc2 dual structure [10][11][12][13][14][15][16][17]. In addition, Tong et al [18], investigated the effect of Al on the mechanical properties of CoCrCuFeNi system.…”