Publications by authors named "David E J Armstrong"

4 Publications

  • Page 1 of 1

Amorphization in extreme deformation of the CrMnFeCoNi high-entropy alloy.

Sci Adv 2021 Jan 29;7(5). Epub 2021 Jan 29.

University of California, San Diego, La Jolla, CA 92093, USA.

Ever-harsher service conditions in the future will call for materials with increasing ability to undergo deformation without sustaining damage while retaining high strength. Prime candidates for these conditions are certain high-entropy alloys (HEAs), which have extraordinary work-hardening ability and toughness. By subjecting the equiatomic CrMnFeCoNi HEA to severe plastic deformation through swaging followed by either quasi-static compression or dynamic deformation in shear, we observe a dense structure comprising stacking faults, twins, transformation from the face-centered cubic to the hexagonal close-packed structure, and, of particular note, amorphization. The coordinated propagation of stacking faults and twins along {111} planes generates high-deformation regions, which can reorganize into hexagonal packets; when the defect density in these regions reaches a critical level, they generate islands of amorphous material. These regions can have outstanding mechanical properties, which provide additional strengthening and/or toughening mechanisms to enhance the capability of these alloys to withstand extreme loading conditions.
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http://dx.doi.org/10.1126/sciadv.abb3108DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7846165PMC
January 2021

High-Entropy Alloys for Advanced Nuclear Applications.

Entropy (Basel) 2021 Jan 11;23(1). Epub 2021 Jan 11.

Department of Materials Science and Engineering, University of Sheffield, Sheffield S1 3JD, UK.

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.
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http://dx.doi.org/10.3390/e23010098DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7827623PMC
January 2021

Sodium/Na β″ Alumina Interface: Effect of Pressure on Voids.

ACS Appl Mater Interfaces 2020 Jan 20;12(1):678-685. Epub 2019 Dec 20.

Department of Materials , University of Oxford , Parks Road , Oxford OX1 3PH , U.K.

Three-electrode studies coupled with tomographic imaging of the Na/Na-β″-alumina interface reveal that voids form in the Na metal at the interface on stripping and they accumulate on cycling, leading to increasing interfacial current density, dendrite formation on plating, short circuit, and cell failure. The process occurs above a critical current for stripping (CCS) for a given stack pressure, which sets the upper limit on current density that avoids cell failure, in line with results for the Li/solid-electrolyte interface. The pressure required to avoid cell failure varies linearly with current density, indicating that Na creep rather than diffusion per se dominates Na transport to the interface and that significant pressures are required to prevent cell death, >9 MPa at 2.5 mA·cm.
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http://dx.doi.org/10.1021/acsami.9b17786DOI Listing
January 2020

Size effects resolve discrepancies in 40 years of work on low-temperature plasticity in olivine.

Sci Adv 2017 09 13;3(9):e1701338. Epub 2017 Sep 13.

Department of Materials, University of Oxford, Oxford, UK.

The strength of olivine at low temperatures and high stresses in Earth's lithospheric mantle exerts a critical control on many geodynamic processes, including lithospheric flexure and the formation of plate boundaries. Unfortunately, laboratory-derived values of the strength of olivine at lithospheric conditions are highly variable and significantly disagree with those inferred from geophysical observations. We demonstrate via nanoindentation that the strength of olivine depends on the length scale of deformation, with experiments on smaller volumes of material exhibiting larger yield stresses. This "size effect" resolves discrepancies among previous measurements of olivine strength using other techniques. It also corroborates the most recent flow law for olivine, which proposes a much weaker lithospheric mantle than previously estimated, thus bringing experimental measurements into closer alignment with geophysical constraints. Further implications include an increased difficulty of activating plasticity in cold, fine-grained shear zones and an impact on the evolution of fault surface roughness due to the size-dependent deformation of nanometer- to micrometer-sized asperities.
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http://dx.doi.org/10.1126/sciadv.1701338DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5597306PMC
September 2017