https://dc.engconfintl.org/cgi/viewcontent.cgi?article=1032&context=superalloys_ii

A critical review of high entropy alloys and related concepts

Microstructures and properties of high-entropy alloys

High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m(1/2). Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening.

/pdf/a-fracture-resistant-high-entropy-alloy-for-cryogenic-4v6ic2t1x4.pdf

A fracture-resistant high-entropy alloy for cryogenic applications

https://zenodo.org/record/1258680/files/article.pdf

The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

High-entropy alloys (HEAs) are alloys with five or more principal elements. Due to the distinct design concept, these alloys often exhibit unusual properties. Thus, there has been significant interest in these materials, leading to an emerging yet exciting new field. This paper briefly reviews some critical aspects of HEAs, including core effects, phases and crystal structures, mechanical properties, high-temperature properties, structural stabilities, and corrosion behaviors. Current challenges and important future directions are also pointed out.

https://www.tandfonline.com/doi/pdf/10.1080/21663831.2014.912690

High-Entropy Alloys: A Critical Review

https://apps.dtic.mil/dtic/tr/fulltext/u2/a618582.pdf

Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys

Refractory high-entropy alloys

Equimolar FeCoCrNi alloys have been the topic of recent research as “high-entropy alloys,” where the name is derived from the high configurational entropy of mixing for a random solid solution. Despite their name, no systematic study of ordering in this alloy system has been performed to date. Here, we present results from anomalous x-ray scattering and neutron scattering on quenched and annealed samples. An alloy of FeNi3 was prepared in the same manner to act as a control. Evidence of long-range chemical ordering is clearly observed in the annealed FeNi3 sample from both experimental techniques. The FeCoCrNi sample given the same heat treatment lacks long-range chemical order.

/pdf/absence-of-long-range-chemical-ordering-in-equimolar-4sp7h0y5xx.pdf

Absence of long-range chemical ordering in equimolar FeCoCrNi

Computer-generated 2D microstructures of varying second phase area fraction (5–30%), aspect ratio (1–16) and degree of alignment (where the reinforcement major-axis orientation is random, perfectly aligned or semi-aligned) are analyzed via the isotropic and directional forms of the computationally efficient multi-scale analysis of area fractions (MSAAF) technique. The impact of these microstructure parameters on the representative volume element (RVE) necessary to characterize a microstructure is ascertained with variations in isotropic and directional homogeneous length scales, derivative quantities of the MSAAF technique. Analysis of these results produces empirical expressions for the directional homogeneous length scale as a function of area fraction and aspect ratio for the limiting cases of random and 'perfect' second phase alignment. Generally, particle alignment is observed to increase the aspect ratio of a microstructure's RVE—a trend amplified by higher reinforcement aspect ratios and lower area fractions. Particle alignment also decreases the absolute size of such an element by reducing the directional homogeneous length scales transverse to the axis of alignment. Periodic boundary conditions on the perimeter of the synthetic microstructures are used to characterize the error in the MSAAF technique via multiple instantiations of the same microstructure, which further indicates that the statistical variation in the directional homogeneous length scale (measured by the directional MSAAF technique) can be an order of magnitude less than the variation in the isotropic homogeneous length scale (measured by the isotropic MSAAF technique).

https://iopscience.iop.org/article/10.1088/0965-0393/16/6/065009/pdf

Multi-Scale Characterization of Orthotropic Microstructures

Considerable work has been done to engineer materials with high efficiencies of thermoelectric heat-to-electricity conversion and the mechanical strength necessary to withstand the demands of practical applications. In particular, in the bismuth telluride system, extrusion pressing has been found to be effective for improving the mechanical strength of alloys via grain refinement. We review some of the literature relating to processing approaches for the bismuth telluride system. We also present preliminary data for a series of samples obtained by incorporating C60 via ball milling and spark plasma sintering into a matrix consisting of a (Bi,Sb)2Te3 alloy, with a focus on the texture of the composites and its relation to thermoelectric transport properties, in comparison to the parent material. The viability of improving the thermoelectric performance of bismuth telluride alloys by the insertion of nanoparticles into a composite is also considered.

G. B. Wilks

Papers

Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys

Refractory high-entropy alloys

Absence of long-range chemical ordering in equimolar FeCoCrNi

Multi-Scale Characterization of Orthotropic Microstructures

Effect of Processing Route on the Microstructure and Thermoelectric Properties of Bismuth Telluride-Based Alloys