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.

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A fracture-resistant high-entropy alloy for cryogenic applications

Amorphous alloys were first developed over 40 years ago and found applications as magnetic core or reinforcement added to other materials. The scope of applications is limited due to the small thickness in the region of only tens of microns. The research effort in the past two decades, mainly pioneered by a Japanese- and a US-group of scientists, has substantially relaxed this size constrain. Some bulk metallic glasses can have tensile strength up to 3000 MPa with good corrosion resistance, reasonable toughness, low internal friction and good processability. Bulk metallic glasses are now being used in consumer electronic industries, sporting goods industries, etc. In this paper, the authors reviewed the recent development of new alloy systems of bulk metallic glasses. The properties and processing technologies relevant to the industrial applications of these alloys are also discussed here. The behaviors of bulk metallic glasses under extreme conditions such as high pressure and low temperature are especially addressed in this review. In order that the scope of applications can be broadened, the understanding of the glass-forming criteria is important for the design of new alloy systems and also the processing techniques.

https://www.tcd.ie/Physics/people/Graham.Cross/SF%20Poster%202011/Shek-MaterSciEng04-Bulk%20Metallic%20Glasses.pdf

Bulk metallic glasses

Herein we cover the key concepts in the field of thermoelectric materials research, present the current understanding, and show the latest developments. Current research is aimed at increasing the thermoelectric figure of merit (ZT) by maximizing the power factor and/or minimizing the thermal conductivity. Attempts at maximizing the power factor include the development of new materials, optimization of existing materials by doping, and the exploration of nanoscale materials. The minimization of the thermal conductivity can come through solid-solution alloying, use of materials with intrinsically low thermal conductivity, and nanostructuring. Herein we describe the most promising bulk materials with emphasis on results from the last decade. Single-phase bulk materials are discussed in terms of chemistry, crystal structure, physical properties, and optimization of thermoelectric performance. The new opportunities for enhanced performance bulk nanostructured composite materials are examined and a look into the not so distant future is attempted.

New and Old Concepts in Thermoelectric Materials

Simple rules for the understanding of Heusler compounds

Plasticity in small-sized metallic systems: Intrinsic versus extrinsic size effect

Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T≈650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.

Effect of Sb doping on the thermoelectric properties of Ti-based half-Heusler compounds, TiNiSn1−xSbx

Bulk icosahedral-quasicrystalline aluminum-palladium-rhenium alloys of high structural quality and thermal stability are found to exhibit low-temperature electrical resistivities that are four orders of magnitude larger than those found in disordered metals and metallic glasses. Experiments suggest that these quasiperiodic alloys, which have a semimetallic electron density, are insulators at low temperature. The findings are discussed in light of theories on electron localization and band-gap formation in ordered metallic systems.

https://science.sciencemag.org/content/sci/261/5122/737.full.pdf

Electron Localization in Metallic Quasicrystals

We observe for the first time the transformation of glassy alloys (${\mathrm{Pd}}_{100\ensuremath{-}x\ensuremath{-}y}{\mathrm{U}}_{x}{\mathrm{Si}}_{y}$, $x\ensuremath{\simeq}y\ensuremath{\simeq}20$ at.%) into a metastable icosahedral phase with quasicrystalline order upon annealing. The icosahedral symmetry of the latter phase is confirmed by x-ray and electron diffraction. This phase can be formed only within a very narrow compositional range ( 2 at.%) of U and Si, suggestive of a strong chemical ordering in it. The icosahedral and glassy phases have similar electrical transport properties. However, magnetic-susceptibility results indicate that the electronic structure of the icosahedral phase is very different from its glassy and crystalline (U${\mathrm{Pd}}_{3}$) counterparts.

Glassy to icosahedral phase transformation in Pd-U-Si alloys.

Unlike semiconducting TiCoSb, ZrCoSb and HfCoSb half-Heusler phases are semimetallic below room temperature and exhibit small Seebeck coefficients of ∼−10 μV/K at 300 K. However, upon substituting (doping) the Co and Sb sites with Pt and Sn, respectively, much larger thermopowers (S) are obtained. For ZrCoSb, S reaches −110 and +130 μV/K while resistivity ρ decreases from ∼5×104 μΩ cm in the undoped phase to 1–2×103 μΩ cm in the substituted phases at 300 K. The lowest thermal conductivity obtained in the substituted alloys is ∼3.0 W/m K at 300 K, which is among the lowest reported for this class of structural phases. There are indications that the thermoelectric properties have not been optimized in these multinary alloys.

Thermoelectric properties of semimetallic (Zr, Hf)CoSb half-Heusler phases

We report measurement of the oxygen isotope effect ${\ensuremath{\alpha}}_{\mathrm{O}}$ in ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Sr}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ as a function of x For values of xl015, ${\ensuremath{\alpha}}_{\mathrm{O}}$ in the is large (04--06) but decreases by about a factor of 5 for x\ensuremath{\ge}015, where we obtain ${\ensuremath{\alpha}}_{\mathrm{O}\mathrm{\ensuremath{\approxeq}}01}$, in good agreement with earlier results The large values of ${\ensuremath{\alpha}}_{\mathrm{O}}$ obtained for small x are reproducible when the $^{18}\mathrm{O}$${,}^{16}$O sample pairs are cross exchanged The maximum measured values of ${\ensuremath{\alpha}}_{\mathrm{O}}$ fall outside the range of standard Bardeen-Cooper-Schrieffer theory and may imply anharmonic phonon-mediated superconductivity The variability of ${\ensuremath{\alpha}}_{\mathrm{O}}$ may be related to a nearby lattice instability, similar to that observed in (La,Ba${)}_{2}$${\mathrm{CuO}}_{4}$ Isotope frequency shifts of the far-infrared transverse-optic phonons of ${A}_{2u}$ symmetry at 240 and 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ also display an anomaly between x=011 and 015 and, in addition, demonstrate $^{18}\mathrm{O}$ exchange for both oxygen sites

S. J. Poon

Papers

Effect of Sb doping on the thermoelectric properties of Ti-based half-Heusler compounds, TiNiSn1−xSbx

Electron Localization in Metallic Quasicrystals

Glassy to icosahedral phase transformation in Pd-U-Si alloys.

Thermoelectric properties of semimetallic (Zr, Hf)CoSb half-Heusler phases

Anomalous oxygen isotope effect in La2-xSrxCuO4.