A critical review of high entropy alloys (HEAs) and related concepts

The field of high entropy alloys (HEAs) is barely 10 years old. It has stimulated new ideas and has inspired the exploration of the vast composition space. Here we give a critical review of this body of work, with the intent of summarizing key findings, uncovering major trends and providing guidance for future efforts. Based on detailed analysis, the following major results emerge. An intense focus is apparent on alloys based on first row transition metal elements. These efforts have discovered an extended, single-phase solid solution phase field and are exploring the extent and properties of this phase field. As a result of this focus, the vast range of complex composition space remains relatively unexplored and still offers a compelling motivation for future studies. Seven distinct alloy families have been reported, and the design of alloy families by selecting elements to achieve a specific, intended purpose is becoming established. A major hypothesis is that configurational entropy in HEAs may favor solid solution phases with simple crystal structures. Detailed analysis of current experiments and calculations does not support this hypothesis. Increasing the number of alloy constituents increases configurational entropy slowly, but increases much more rapidly the probability of including a pair of atoms with interaction energies that are sufficiently negative to overcome configurational entropy. The hypothesis that diffusion may be unusually slow in HEAs is also not supported by the current data. Data for mechanical properties of HEAs will be reviewed and suggestions for new research directions will be offered

Layering in the Ising model

We consider the three-dimensional Ising model in a half-space with a boundary field (no bulk field). We compute the low-temperature expansion of layering transition lines

Oxygen impurities in NiAl: Relaxation effects

We have used a full-potential linear muffin-tin orbital method to calculate the effects of oxygen impurities on the electronic structure of NiAl. Using the supercell method with a 16-atom supercell we have investigated the cases where an oxygen atom is substitutionally placed at either a nickel or an aluminum site. Full relaxation of the atoms within the supercell was allowed. We found that oxygen prefers to occupy a nickel site over an aluminum site with a site selection energy of 138 mRy (21,370 K). An oxygen atom placed at an aluminum site is found to cause a substantial relaxation of its nickel neighbors away from it. In contrast, this steric repulsion is hardly present when the oxygen atom occupies the nickel site and is surrounded by aluminum neighbors. We comment on the possible relation of this effect to the pesting degradation phenomenon (essentially spontaneous disintegration in air) in nickel aluminides.Comment: To appear in Phys. Rev. B (Aug. 15, 2001

Processing of ultrafine-size particulate metal matrix composites by advanced shear technology

Copyright @ 2009 ASM International. This paper was published in Metallurgical & Materials Transactions A 40A(3) and is made available as an electronic reprint with the permission of ASM International. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplications of any material in this paper for a fee or for commercial purposes, or modification of the content of this paper are prohibited.Lack of efficient mixing technology to achieve a uniform distribution of fine-size reinforcement within the matrix and the high cost of producing components have hindered the widespread adaptation of particulate metal matrix composites (PMMCs) for engineering applications. A new rheo-processing method, the melt-conditioning high-pressure die-cast (MC-HPDC) process, has been developed for manufacturing near-net-shape components of high integrity. The MC-HPDC process adapts the well-established high shear dispersive mixing action of a twin-screw mechanism to the task of overcoming the cohesive force of the agglomerates under a high shear rate and high intensity of turbulence. This is followed by direct shaping of the slurry into near-net-shape components using an existing cold-chamber die-casting process. The results indicate that the MC-HPDC samples have a uniform distribution of ultrafine-sized SiC particles throughout the entire sample in the as-cast condition. Compared to those produced by conventional high-pressure die casting (HPDC), MC-HPDC samples have a much improved tensile strength and ductility.EP-SR

Comprehensive data compilation on the mechanical properties of refractory high-entropy alloys

This data article presents the compilation of mechanical properties for 122 refractory high entropy alloys (RHEAs) and refractory complex concentrated alloys (RCCAs) reported in the period from 2010 to the end of January 2018. The data sheet gives alloy composition, type of microstructures and the metallurgical states in which the properties are measured. Data such as the computed alloy mass density, the type of mechanical loadings to which they are subjected and the corresponding macroscopic mechanical properties, such as the yield stress, are made available as a function of the testing temperature. For practical use, the data are tabulated and some are also graphically presented, allowing at a glance to access relevant information for this attractive category of RHEAs and RCCAs

Localized Einstein modes in Ca-based bulk metallic glasses

Low-temperature specific heat and elastic moduli measurements are reported for Ca-based bulk metallic glasses. The deviation from the Debye behaviour observed in the specific heat is modelled with a simple Einstein oscillator with characteristic temperature 80 K. The presence of this local mode can also account for the deviation from normal ‘Varshni behaviour’ observed in the temperature dependence of the elastic moduli

Shear bands in metallic glasses: Size effects on thermal profiles

International audienceThe characteristic dimensions of the hot, liquid zone behind a moving shear-band front in a metallic glass are analyzed. In addition to the expected dependence on material constants, the thickness of the zone is proportional to the shear offset, while its width is proportional to the square of the offset. Considering the bending of a plate, the size and shape of the hot zone are found to be strongly dependent on plate thickness. For shear offsets << 1 mu m, typical of plate thickness << 100 mu m, local temperature rises are insignificant. For larger dimensions, local temperature rises give a liquid zone centered on the shear plane with width comparable to the sample dimensions. The scaling of characteristic lengths and times with plate thickness facilitates the interpretation of the transitions observed in mechanical behavior, and the variation in behavior from glass to glass. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserve