High-Entropy Materials, Ultra-Strong Molecules, and Nanoelectronics Emerging Capabilities and Research Objectives: Proceedings of a Workshop (2019) / Chapter Skim
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2 High-Entropy Materials
2 High-Entropy Materials
Pages 21-42
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; and (2) to favor solid solutions over intermetallic phases by regulating the configurational entropy in complex alloys (Yeh et al., 2004)
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High-entropy effect. High configurational entropy in complex alloys fa vors simple (face-centered cubic [FCC]
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The configurational entropy of an intermetallic in an MPEA can be as much as 60 percent of the configurational entropy of the disordered solid solution. He reiterated that the Gibbs energy of the competing phases needs to be considered, and he cautioned that the Gibbs phase rule2 is often misinterpreted in the available literature.
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. Although these data support the view that HEAs provide single-phase solid solutions with simple structures more often than expected for complex alloys, Miracle explained that it is a far more complicated issue.
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A new area of exploration within the community includes high-throughput calculations of phase diagrams, Miracle explained. The Air Force Research Labo ratory has calculated phase diagrams for more than 135,000 complex alloys; this data set of calculations can be used in comparison to the actual experiments (see Figure 2.2)
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establish basic scientific concepts. Peter Liaw asked Miracle about the future of multicomponent alloys.
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ab initio calculations of thermodynamic potential (Gibbs free energy) to sample the configuration space and understand entropies of mixing.
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Despite the local disorder, the BCC structure is still well ordered on long length scales. Widom concluded his talk by reiterating the importance of applying first principles, total energies, and Monte Carlo and molecular dynamics methods to simulate HEAs; the benefit of using the cluster variation method to evaluate configurational entropy; and the presence of local structure collapse without long range structure collapse in HCP/BCC combinations.
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Once the vibrational free energy is included, the magnitudes of the free energies are approximately one order of magnitude greater. The vibrational free energy is much larger here, but the crossing point has hardly moved.
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DEVIATION FROM HIGH-ENTROPY CONFIGURATIONS IN THE ATOMIC DISTRIBUTIONS OF A MULTI-PRINCIPAL-ELEMENT ALLOY Peter Liaw, Ivan Racheff Chair and Professor, Department of Materials Science and Engineering, University of Tennessee Peter Liaw's current research objective is to understand and relate the advantages of the configurational entropy mixing in single-phase HEAs and MPEAs, with the hope of making new materials appropriate for applications where high strength, fatigue, toughness, and corrosion resistance at higher temperatures are essential (Santodonato et al., 2015; Zhang et al., 2014; Yeh et al., 2004; Senkov et al., 2010; Feuerbacher, Heidelmann, and Thomas, 2015; Youssef et al., 2015; Hemphill et al.,
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The phase with the lowest Gibbs free energy is stable at high temperatures, and a random solid solution phase of elements may be stabilized by the large configurational entropy of mixing, though other factors should be considered, Liaw explained. Simple crystal structures in HEAs are formed in one of the following ways (or combination of the following)
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In order to fully specify the B2 ordered structure, additional information is needed, including the calculation of the configurational entropy. Regarding diffraction studies, the crystal lattice parameters must be determined from the peak positions; the order parameters must be obtained from the relative peak intensities; and a twofold strategy must be used to cross-check the order parameters (e.g., conducting complementary neutron and X-ray scattering studies and introducing a single-parameter fitting technique)
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configurational entropy of mixing during cooling is quantifiable; and (3) the local structure of the liquid state is a precursor to the long-range-ordered structure that forms in solid state.
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Research also demonstrates that the electronic entropy is negligible. Gao defined excess entropy, which could be positive, zero, or negative, as the total entropy of mixing minus the ideal configurational entropy.
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FIGURE 2.5 Select disordered HEA formation rules. SOURCES: Michael Gao, National Energy Technology Laboratory, presentation to the workshop.
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As the number of principal components in the alloy increases, Gao explained, the contribution to the Gibbs free energy associated with the entropy also increases. While CALPHAD does not always indicate why the excess entropy is positive, DFT simulations reveal that positive excess entropy in the FCC Co-Cr-Fe-Mn-Ni system is a result of the positive vibrational entropy of mixing.
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Daniel Miracle asked if the magnitude of the negative excess vibrational entropy for BCC materials was sufficient to overcome the positive term associated with the configurational entropy. Gao said no because although it is small, it is not too small.
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He observed that high or low entropy is determined by the balance of enthalpic and entropic contributions. The amount of thermal energy is the entropy multiplied by the temperature, and the Gibbs free energy is really the amount of enthalpy; the system cannot be maintained without this free energy.
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Miracle noted that the answer to this question depends on what the researcher wants to achieve: single-phase alloys can be processed at low cost with corrosion resistance, while two-phase alloys have both high strength and a good balance of fracture properties. Because of this, Miracle believes that it is a mistake to focus only on the single-phase solid solutions.
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Considering the water vapor example, the mixture can be modeled as pressure and temperature are steadily increased, and DFT can be used to calculate a large number of states and mix them together. For some HEAs, entropy can actually be made higher than the ideal configurational entropy; this is the impetus for more modeling with multicomponent systems, Liu concluded.
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Miracle responded that the mechanical properties of HEAs are not that different from those of conventional alloys. One exception to this, Miracle noted, is the remarkable mechanical fracture toughness of Cantor alloys due to nanotwinning at and below room temperature.
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Liaw responded that Alcoa is already excited about HEAs; they have purchased high-temperature alloy companies, plan to switch their aluminum research to this area, and view HEAs as a great opportunity. Samsung and some of the automobile companies have also expressed interest in HEAs, Liaw noted.
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