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

A critical review of high entropy alloys and related concepts

Oxygen, one of the most abundant elements on Earth, often forms an undesired interstitial impurity or ceramic phase (such as an oxide particle) in metallic materials. Even when it adds strength, oxygen doping renders metals brittle1–3. Here we show that oxygen can take the form of ordered oxygen complexes, a state in between oxide particles and frequently occurring random interstitials. Unlike traditional interstitial strengthening4,5, such ordered interstitial complexes lead to unprecedented enhancement in both strength and ductility in compositionally complex solid solutions, the so-called high-entropy alloys (HEAs)6–10. The tensile strength is enhanced (by 48.5 ± 1.8 per cent) and ductility is substantially improved (by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA with 2.0 atomic per cent oxygen, thus breaking the long-standing strength–ductility trade-off11. The oxygen complexes are ordered nanoscale regions within the HEA characterized by (O, Zr, Ti)-rich atomic complexes whose formation is promoted by the existence of chemical short-range ordering among some of the substitutional matrix elements in the HEAs. Carbon has been reported to improve strength and ductility simultaneously in face-centred cubic HEAs12, by lowering the stacking fault energy and increasing the lattice friction stress. By contrast, the ordered interstitial complexes described here change the dislocation shear mode from planar slip to wavy slip, and promote double cross-slip and thus dislocation multiplication through the formation of Frank–Read sources (a mechanism explaining the generation of multiple dislocations) during deformation. This ordered interstitial complex-mediated strain-hardening mechanism should be particularly useful in Ti-, Zr- and Hf-containing alloys, in which interstitial elements are highly undesirable owing to their embrittlement effects, and in alloys where tuning the stacking fault energy and exploiting athermal transformations13 do not lead to property enhancement. These results provide insight into the role of interstitial solid solutions and associated ordering strengthening mechanisms in metallic materials. Ordered oxygen complexes in high-entropy alloys enhance both strength and ductility in these compositionally complex solid solutions.

Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes

Open literature publications, in the period from 2010 to the end of January 2018, on refractory high entropy alloys (RHEAs) and refractory complex concentrated alloys (RCCAs) are reviewed. While RHEAs, by original definition, are alloys consisting of five or more principal elements with the concentration of each of these elements between 5 and 35 at.%, RCCAs can contain three or more principal elements and the element concentration can be greater than 35%. The 151 reported RHEAs/RCCAs are analyzed based on their composition, processing methods, microstructures, and phases. Mechanical properties, strengthening and deformation mechanisms, oxidation, and corrosion behavior, as well as tribology, of RHEA/RCCAs are summarized. Unique properties of some of these alloys make them promising candidates for high temperature applications beyond Ni-based superalloys and/or conventional refractory alloys. Methods of development and exploration, future directions of research and development, and potential applications of RHEAs are discussed.

Development and exploration of refractory high entropy alloys—A review

High entropy alloys: A focused review of mechanical properties and deformation mechanisms

Mechanical properties of high-entropy alloys with emphasis on face-centered cubic alloys

Phase equilibria, microstructure, and high temperature oxidation resistance of novel refractory high-entropy alloys

https://publikationen.bibliothek.kit.edu/1000052574/7900676

Microstructure and mechanical properties at elevated temperatures of a new Al-containing refractory high-entropy alloy Nb-Mo-Cr-Ti-Al

Topological management of materials at a Micro-scale is one of the fundamental building principles of nature This combination of material and structural properties results in marked changes in the properties of solids Nowadays physicists, chemists, materials scientists and engineers explore those effects by synthesizing, characterizing, and modeling Micro-lattice materials from all material classes Applications have been identified in the fields of ultra-lightweight structures, thermal equipment, electrochemical devices, high absorption capacity and bio-repair materials This article aims to review recent progress in the development of such advanced Micro-lattice materials

Advanced Micro-Lattice Materials

Mechanisms of oxide scale formation on yttrium-alloyed Mo–Si–B containing fine-grained microstructure

Mo-Si-B-alloys are promising materials for high-temperature applications because of their high melting point, excellent phase stability, large alloying capabilities, and reasonable mechanical as well as oxidative properties. A continuing alloy development is, however, required because of the catastrophic oxidation taking place at intermediate temperatures and the rather high density. The addition of Ti stabilizes a new ternary phase field including the Mo5Si3 (T1) phase instead of the Mo3Si (A15) phase. Alloys comprising the phases Moss, T1 and Mo5SiB2 (T2) show very high creep resistance, improved oxidation behavior and significantly reduced density. The new T1 phase seems to play a crucial role in the improved oxidation resistance of these new materials, since this phase exhibits excellent oxidation behavior at intermediate and high temperatures. The 4-component alloys possess superior creep behavior compared to Mo-Si-B alloys with the same microstructural phase arrangement and size or to the single crystal Ni-base superalloy CMSX-4. The main reason was found to be the formation of Ti-rich silicide precipitates during processing.

H.-J. Christ

Papers

Phase equilibria, microstructure, and high temperature oxidation resistance of novel refractory high-entropy alloys

Microstructure and mechanical properties at elevated temperatures of a new Al-containing refractory high-entropy alloy Nb-Mo-Cr-Ti-Al

Advanced Micro-Lattice Materials

Mechanisms of oxide scale formation on yttrium-alloyed Mo–Si–B containing fine-grained microstructure

Creep Resistance and Oxidation Behavior of Novel Mo-Si-B-Ti Alloys