Liquid Phase Separation in High-Entropy Alloys-A Review.

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

Solidification, Thermodynamics, and Mechanical Properties of Multi-Principal Element Alloys

The structure and solidification of CoCrCu-X MPEAs with X = Fe, Mn, Ni, Ti, V, FeMn, FeNi, FeTi, FeV, MnNi, MnTi, MnV, NiTi, NiV, and TiV were studied via arc-melting and electromagnetic levitation melting. The ternary mixture of CoCrCu was found to form no single phase liquid, however by systematically introducing the remaining 3d transition metals, it was found that Ni and Ti promote single phase liquid formation, eventually leading to dendritic microstructures as opposed to the highly phase separated microstructures found in ten of the alloys. The thermodynamics of the liquid phase separation in these alloys is largely dictated by the positive mixing enthalpy contributions of Cu in these systems. Of the six dendritically solidifying alloys, CoCrCuNi, CoCrCuFeNi, and CoCrCuMnNi solidified with a face-centered cubic (FCC) crystal structure for both the dendritic and interdendritic phases while CoCrCuTi and CoCrCuTiV solidified with FCC and body-centered cubic (BCC) phases. In contrast CoCrCuMnTi solidified with a hexagonal closed packed Laves C14 dendritic phase and FCC interdendritic matrix. The three FCC alloys were then prepared via powder metallurgy and processed via spark plasma sintering (SPS) to compare microstructure, crystal structure, and mechanical properties with the solidification processed alloys. It was found that the powder metallurgical processing and SPS led to a doubling of the hardness in these alloys due to the nanocrystallinity of the powder being preserved. Liquid phase separation was further investigated by neutron imaging techniques. It was found that the technique not only allows for the direct observations of molten metals, but also shows mixing and de-mixing in the liquid for these alloys. The neutron imaging technique was applied to the CoCrCuNi high entropy alloy to study the remixing of the de-mixed liquid with the addition of Ni to CoCrCu. The CoCrCuMnTi alloys were systematically studied by varying the amount of Mn in order to find the critical Mn concentration for the formation of the Laves C14 phase in these alloys. The particular composition of Co22Cr18Cu20Mn16Ti24 has high hardness of 996.6 HV 0.01 for the dendritic phase, while the hardness of the interdendritic phase is 457.3 HV 0.01. In this alloy, there is also a small dispersed Ti-rich phase. The effects of varied cooling rates on this alloy were studied, and it was found that higher cooling rates led to the suppression of the tertiary Ti-rich phase

Processing Pathway Effects in CoCrCuNi + X (Fe, Mn) High-Entropy Alloys

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Liquid Phase Separation in High-Entropy Alloys—A Review

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

In-Situ Imaging of Liquid Phase Separation in Molten Alloys Using Cold Neutrons

Understanding the liquid phases and solidification behaviors of multicomponent alloy systems becomes difficult as modern engineering alloys grow more complex, especially with the discovery of high-entropy alloys (HEAs) in 2004. Information about their liquid state behavior is scarce, and potentially quite complex due to the presence of perhaps five or more elements in equimolar ratios. These alloys are showing promise as high strength materials, many composed of solid-solution phases containing equiatomic CoCrCu, which itself does not form a ternary solid solution. Instead, this compound solidifies into highly phase separated regions, and the liquid phase separation that occurs in the alloy also leads to phase separation in systems in which Co, Cr, and Cu are present. The present study demonstrates that in-situ neutron imaging of the liquid phase separation in CoCrCu can be observed. The neutron imaging of the solidification process may resolve questions about phase separation that occurs in these alloys and those that contain Cu. These results show that neutron imaging can be utilized as a characterization technique for solidification research with the potential for imaging the liquid phases of more complex alloys, such as the HEAs which have very little published data about their liquid phases. This imaging technique could potentially allow for observation of immiscible liquid phases becoming miscible at specific temperatures, which cannot be observed with ex-situ analysis of solidified structures