Entropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy

Special Issue “Advanced Refractory Alloys”: Metals, MDPI

Metallic materials with extreme and often unusual combinations of properties are always in high demand in the competitive world market [...

About the reliability of CALPHAD predictions in multicomponent systems

This study examines one of the limitations of CALPHAD databases when applied to high entropy alloys and complex concentrated alloys. We estimate the level of the thermodynamic description, which is still sufficient to correctly predict thermodynamic properties of quaternary alloy systems, by comparing the results of CALPHAD calculations where quaternary phase space is extrapolated from binary descriptions to those resulting from complete binary and ternary interaction descriptions. Our analysis has shown that the thermodynamic properties of a quaternary alloy can be correctly predicted by direct extrapolation from the respective fully assessed binary systems (i.e., without ternary descriptions) only when (i) the binary miscibility gaps are not present, (ii) binary intermetallic phases are not present or present in a few quantities (i.e., when the system has low density of phase boundaries), and (iii) ternary intermetallic phases are not present. Because the locations of the phase boundaries and possibility of formation of ternary phases are not known when evaluating novel composition space, a higher credibility database is still preferable, while the calculations using lower credibility databases may be questionable and require additional experimental verification. We estimate the level of the thermodynamic description which would be still sufficient to correctly predict thermodynamic properties of quaternary alloy systems. The main factors affecting the accuracy of the thermodynamic predictions in quaternary alloys are identified by comparing the results of CALPHAD calculations where quaternary phase space is extrapolated from binary descriptions to those resulting from ternary system descriptions

Phase Composition of a CrMo0.5NbTa0.5TiZr High Entropy Alloy: Comparison of Experimental and Simulated Data

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Mapping the world of complex concentrated alloys

This work explores the mechanical properties of high entropy alloys (HEAs) and complex concentrated alloys (CCAs) by comparing them with commercially available engineering alloys including industry-standard aerospace alloys. To reach this goal we have developed a materials database covering the main mechanical properties of HEAs and CCAs from the published literature. The database is used to represent various property spaces enabling an assessment of their performance for light weight structures and high-temperature structural applications. In addition, we illustrate the effects of alloying and of specific elements on the room temperature mechanical properties of HEAs and CCAs. With densities between titanium alloys and steels or nickel alloys, the best CCAs exceed commercial alloys in uniaxial loading and beam bending at room temperature. Where use temperature or cost excludes commercial alloys based on Mg, Al or Ti, the best CCAs also offer attractive specific yield strength in panel bending and specific stiffness for all loading conditions at room temperature. Many CCAs have superior structural properties at elevated temperatures

Development of a Refractory High Entropy Superalloy

Microstructure, phase composition and mechanical properties of a refractory high entropy superalloy, AlMo0.5NbTa0.5TiZr, are reported in this work. The alloy consists of a nano-scale mixture of two phases produced by the decomposition from a high temperature body-centered cubic (BCC) phase. The first phase is present in the form of cuboidal-shaped nano-precipitates aligned in rows along <100>-type directions, has a disordered BCC crystal structure with the lattice parameter a1 = 326.9 ± 0.5 pm and is rich in Mo, Nb and Ta. The second phase is present in the form of channels between the cuboidal nano-precipitates, has an ordered B2 crystal structure with the lattice parameter a2 = 330.4 ± 0.5 pm and is rich in Al, Ti and Zr. Both phases are coherent and have the same crystallographic orientation within the former grains. The formation of this modulated nano-phase structure is discussed in the framework of nucleation-and-growth and spinodal decomposition mechanisms. The yield strength of this refractory high entropy superalloy is superior to the yield strength of Ni-based superalloys in the temperature range of 20 °C to 1200 °C

Database on the mechanical properties of high entropy alloys and complex concentrated alloys

This data article presents the compilation of mechanical properties for 370 high entropy alloys (HEAs) and complex concentrated alloys (CCAs) reported in the period from 2004 to 2016. The data sheet includes alloy composition, type of microstructures, density, hardness, type of tests to measure the room temperature mechanical properties, yield strength, elongation, ultimate strength and Young׳s modulus. For 27 refractory HEAs (RHEAs), the yield stress and elongation are given as a function of the testing temperature. The data are stored in a database provided in Supplementary materials, and for practical use they are tabulated in the present paper. The database was used in recent publications by Miracle and Senkov [1], Gorsse et al. [2] and Senkov et al. [3]

Elastic properties of Ca-based bulk metallic glasses studied by resonant ultrasound spectroscopy

We report measurements of the elastic properties and internal friction of Ca-based bulk metallic glasses (BMGs), Ca50Mg20Cu30, Ca55Mg18Zn11Cu16 and Ca65Mg15Zn20 (numbers indicate at.%), as a function of temperature between 5 and 400 K. Below the glass transition temperature, both Young’s modulus and shear modulus decrease with increasing temperature, and the Poisson ratio increases with increasing temperature. Above the glass transition temperature, these trends reverse due to crystallization. At temperatures close to the glass transition temperature, we observe the onset of a peak in the internal friction Q−1, attributed to crystallization