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Future Directions of High Entropy Alloys
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Future Directions of High Entropy Alloys

A special issue of Entropy (ISSN 1099-4300). This special issue belongs to the section "Multidisciplinary Applications".

Deadline for manuscript submissions: closed (15 December 2020) | Viewed by 29050

Special Issue Editor

Dr. Nicholas G. Jones

E-Mail Website
Guest Editor
Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, UK
Interests: High Temperature Materials; Micromechanics and Functional Fatigue of Transforming Alloys; Themomechanical Processing of Engineering Alloys

Special Issue Information

Dear Colleagues,

Since their first publication in 2004, the concepts, behaviour and properties of high entropy alloys (HEA) have generated huge scientific interest around the world. The nature of their compositions, having multiple principal elements rather than being based on a single element, has led to the exploration of poorly characterised or previously unexplored areas of compositional space and the discovery of some extraordinary properties. During the subsequent period, the field has moved quickly, with the number of published manuscripts increasing annually. As a result, numerous different HEA systems have been studied and reported, some of which have shown promise for commercial applications.

To fulfil their hypothesised potential, these more promising HEA systems require further development before they can be adopted into service. Consequently, efforts need to focus on systems that can either compete with existing classes of materials, which themselves have decades of research behind them, or provide unique capabilities. Examples of such systems include but are not limited to refractory metal high entropy superalloys for high temperature service; alloys with high resistance to radiation damage for nuclear applications; narrow freezing range alloys for joining; and corrosion resistant alloys for environmental resistance. As such, this Special Issue will focus on the potential of high entropy alloys, identify key opportunities for these materials and provide critical opinions as to their future direction.

Dr. Nicholas G. Jones
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Entropy is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • High entropy alloys
  • High entropy materials
  • Applications
  • Alloy design
  • Microstructure
  • Properties
  • Characterisation

Published Papers (5 papers)

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Research

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14 pages, 8703 KiB  
Article
Microstructural Degradation of the AlMo0.5NbTa0.5TiZr Refractory Metal High-Entropy Superalloy at Elevated Temperatures
by Tamsin E. Whitfield, Howard J. Stone, C. Neil Jones and Nicholas G. Jones
Entropy 2021, 23(1), 80; https://doi.org/10.3390/e23010080 - 08 Jan 2021
Cited by 26 | Viewed by 3217
Abstract
Refractory metal high-entropy superalloys (RSA), which possess a nanoscale microstructure of B2 and bcc phases, have been developed to offer high temperature capabilities beyond conventional Ni-based alloys. Despite showing a number of excellent attributes, to date there has been little consideration of their [...] Read more.
Refractory metal high-entropy superalloys (RSA), which possess a nanoscale microstructure of B2 and bcc phases, have been developed to offer high temperature capabilities beyond conventional Ni-based alloys. Despite showing a number of excellent attributes, to date there has been little consideration of their microstructural stability, which is an essential feature of any material employed in high temperature service. Here, the stability of the exemplar RSA AlMo0.5NbTa0.5TiZr is studied following 1000 h exposures at 1200, 1000 and 800 °C. Crucially, the initial nanoscale cuboidal B2 + bcc microstructure was found to be unstable following the thermal exposures. Extensive intragranular precipitation of a hexagonal Al-Zr-rich intermetallic occurred at all temperatures and, where present, the bcc and B2 phases had coarsened and changed morphology. This microstructural evolution will concomitantly change both the mechanical and environmental properties and is likely to be detrimental to the in-service performance of the alloy. Full article
(This article belongs to the Special Issue Future Directions of High Entropy Alloys)
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19 pages, 7960 KiB  
Article
Dynamic Shear Deformation of a Precipitation Hardened Al0.7CoCrFeNi Eutectic High-Entropy Alloy Using Hat-Shaped Specimen Geometry
by Bharat Gwalani, Tianhao Wang, Abhinav Jagetia, Sindhura Gangireddy, Saideep Muskeri, Sundeep Mukherjee, Jeffrey T. Lloyd, Rajarshi Banerjee and Rajiv S. Mishra
Entropy 2020, 22(4), 431; https://doi.org/10.3390/e22040431 - 10 Apr 2020
Cited by 19 | Viewed by 3694
Abstract
Lamellar eutectic structure in Al0.7CoCrFeNi high-entropy alloy (HEA) is emerging as a promising candidate for structural applications because of its high strength-ductility combination. The alloy consists of a fine-scale lamellar fcc + B2 microstructure with high flow stresses > 1300 MPa [...] Read more.
Lamellar eutectic structure in Al0.7CoCrFeNi high-entropy alloy (HEA) is emerging as a promising candidate for structural applications because of its high strength-ductility combination. The alloy consists of a fine-scale lamellar fcc + B2 microstructure with high flow stresses > 1300 MPa under quasi-static tensile deformation and >10% ductility. The response to shear loading was not investigated so far. This is the first report on the shear deformation of a eutectic structured HEA and effect of precipitation on shear deformation. A split-Hopkinson pressure bar (SHPB) was used to compress the hat-shaped specimens to study the local dynamic shear response of the alloy. The change in the width of shear bands with respect to precipitation and deformation rates was studied. The precipitation of L12 phase did not delay the formation of adiabatic shear bands (ASB) or affect the ASB width significantly, however, the deformed region around ASB, consisting of high density of twins in fcc phase, was reduced from 80 µm to 20 µm in the stronger precipitation strengthened condition. We observe dynamic recrystallization of grains within ASBs and local mechanical response of individual eutectic lamellae before and after shear deformation and within the shear bands was examined using nano-indentation. Full article
(This article belongs to the Special Issue Future Directions of High Entropy Alloys)
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12 pages, 3259 KiB  
Article
High-Temperature Nano-Indentation Creep of Reduced Activity High Entropy Alloys Based on 4-5-6 Elemental Palette
by Maryam Sadeghilaridjani, Saideep Muskeri, Mayur Pole and Sundeep Mukherjee
Entropy 2020, 22(2), 230; https://doi.org/10.3390/e22020230 - 18 Feb 2020
Cited by 21 | Viewed by 3627
Abstract
There is a strong demand for materials with inherently high creep resistance in the harsh environment of next-generation nuclear reactors. High entropy alloys have drawn intense attention in this regard due to their excellent elevated temperature properties and irradiation resistance. Here, the time-dependent [...] Read more.
There is a strong demand for materials with inherently high creep resistance in the harsh environment of next-generation nuclear reactors. High entropy alloys have drawn intense attention in this regard due to their excellent elevated temperature properties and irradiation resistance. Here, the time-dependent plastic deformation behavior of two refractory high entropy alloys was investigated, namely HfTaTiVZr and TaTiVWZr. These alloys are based on reduced activity metals from the 4-5-6 elemental palette that would allow easy post-service recycling after use in nuclear reactors. The creep behavior was investigated using nano-indentation over the temperature range of 298 K to 573 K under static and dynamic loads up to 5 N. Creep stress exponent for HfTaTiVZr and TaTiVWZr was found to be in the range of 20–140 and the activation volume was ~16–20b3, indicating dislocation dominated mechanism. The stress exponent increased with increasing indentation depth due to a higher density of dislocations and their entanglement at larger depth and the exponent decreased with increasing temperature due to thermally activated dislocations. Smaller creep displacement and higher activation energy for the two high entropy alloys indicate superior creep resistance compared to refractory pure metals like tungsten. Full article
(This article belongs to the Special Issue Future Directions of High Entropy Alloys)
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Review

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28 pages, 3278 KiB  
Review
High-Entropy Alloys for Advanced Nuclear Applications
by Ed J. Pickering, Alexander W. Carruthers, Paul J. Barron, Simon C. Middleburgh, David E. J. Armstrong and Amy S. Gandy
Entropy 2021, 23(1), 98; https://doi.org/10.3390/e23010098 - 11 Jan 2021
Cited by 131 | Viewed by 11725
Abstract
The expanded compositional freedom afforded by high-entropy alloys (HEAs) represents a unique opportunity for the design of alloys for advanced nuclear applications, in particular for applications where current engineering alloys fall short. This review assesses the work done to date in the field [...] Read more.
The expanded compositional freedom afforded by high-entropy alloys (HEAs) represents a unique opportunity for the design of alloys for advanced nuclear applications, in particular for applications where current engineering alloys fall short. This review assesses the work done to date in the field of HEAs for nuclear applications, provides critical insight into the conclusions drawn, and highlights possibilities and challenges for future study. It is found that our understanding of the irradiation responses of HEAs remains in its infancy, and much work is needed in order for our knowledge of any single HEA system to match our understanding of conventional alloys such as austenitic steels. A number of studies have suggested that HEAs possess ‘special’ irradiation damage resistance, although some of the proposed mechanisms, such as those based on sluggish diffusion and lattice distortion, remain somewhat unconvincing (certainly in terms of being universally applicable to all HEAs). Nevertheless, there may be some mechanisms and effects that are uniquely different in HEAs when compared to more conventional alloys, such as the effect that their poor thermal conductivities have on the displacement cascade. Furthermore, the opportunity to tune the compositions of HEAs over a large range to optimise particular irradiation responses could be very powerful, even if the design process remains challenging. Full article
(This article belongs to the Special Issue Future Directions of High Entropy Alloys)
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23 pages, 5149 KiB  
Review
High Entropy Alloys as Filler Metals for Joining
by Dan Luo, Yong Xiao, Liam Hardwick, Robert Snell, Matthew Way, Xavier Sanuy Morell, Frances Livera, Nicholas Ludford, Chinnapat Panwisawas, Hongbiao Dong and Russell Goodall
Entropy 2021, 23(1), 78; https://doi.org/10.3390/e23010078 - 07 Jan 2021
Cited by 19 | Viewed by 6073
Abstract
In the search for applications for alloys developed under the philosophy of the High Entropy Alloy (HEA)-type materials, the focus may be placed on applications where current alloys also use multiple components, albeit at lower levels than those found in HEAs. One such [...] Read more.
In the search for applications for alloys developed under the philosophy of the High Entropy Alloy (HEA)-type materials, the focus may be placed on applications where current alloys also use multiple components, albeit at lower levels than those found in HEAs. One such area, where alloys with complex compositions are already found, is in filler metals used for joining. In soldering (<450 °C) and brazing (>450 °C), filler metal alloys are taken above their liquidus temperature and used to form a metallic bond between two components, which remain both unmelted and largely unchanged throughout the process. These joining methods are widely used in applications from electronics to aerospace and energy, and filler metals are highly diverse, to allow compatibility with a broad range of base materials (including the capability to join ceramics to metals) and a large range of processing temperatures. Here, we review recent developments in filler metals relevant to High Entropy materials, and argue that such alloys merit further exploration to help overcome a number of current challenges that need to be solved for filler metal-based joining methods. Full article
(This article belongs to the Special Issue Future Directions of High Entropy Alloys)
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