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1. Superconductivity in a uranium containing high entropy alloy

   https://doi.org/10.1038/s41598-020-61666-z  

   Nelson, W. L. ; Chemey, A. T. ; Hertz, M. ; Choi, E. ; Graf, D. E. ; Latturner, S. ; Albrecht-Schmitt, T. E. ; Wei, K. ; Baumbach, R. E. ( March 2020 , Scientific Reports)

   High entropy alloys (HEA) are an unusual class of materials where mixtures of elements are stochastically arrayed on a simple crystalline lattice. These systems exhibit remarkable functionality, often along several distinct axes: e.g., the examples [TaNb]_1-x(TiZrHf)_xare high strength and damage resistant refractory metals that also exhibit superconductivity with large upper critical fields. Here we report the discovery of anf-electron containing HEA, [TaNb]_0.31(TiUHf)_0.69, which is the first to include an actinide ion. Similar to the Zr-analogue, this material crystallizes in a body-centered cubic lattice with the lattice constanta = 3.41(1) Å and exhibits phonon mediated superconductivity with a transition temperaturesT_c ≈ 3.2 K and uppermore »critical fieldsH_c2 ≈ 6.4 T. These results expand this class of materials to include actinide elements, shows that superconductivity is robust in this sub-group, and opens the path towards leveraging HEAs as functional waste forms for a variety of radioisotopes.

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2. Strong pairing in two dimensions: pseudogaps, domes, and other implications

   https://doi.org/10.1088/1367-2630/ab890b  

   Wang, Xiaoyu ; Chen, Qijin ; Levin, K. ( June 2020 , New Journal of Physics)

   This paper addresses the transition from the normal to the superfluid state in strongly correlated two dimensional fermionic superconductors and Fermi gases. We arrive at the Berezinskii–Kosterlitz–Thouless (BKT) temperatureT_BKTas a function ofattractivepairing strength by associating it with the onset of ‘quasi-condensation’ in the normal phase. Our approach builds on a criterion for determining the BKT transition temperature for atomic gases which is based on a well established quantum Monte Carlo analysis of the phase space density. This latter quantity, when derived from BCS–BEC crossover theory for fermions, leads to non-monotonic behavior forT_BKTas a function of the attractive interaction ormore »inverse scattering length. In Fermi gases, this implies a robust superconducting dome followed by a long tail from the flat BEC asymptote, rather similar to what is observed experimentally. For lattice systems we find thatT_BKThas an absolute maximum of the order of 0.1E_F. We discuss how our results compare with those derived from the Nelson–Kosterlitz criterion based on the mean field superfluid density and the approach to the transition from below. While there is agreement in the strict mean-field BCS regime at weak coupling, we find that at moderate pairing strength bosonic excitations cause a substantial increase inT_BKTfollowed by an often dramatic decrease before the system enters the BEC regime.

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3. Effect of Post Processing Heat Treatment Routes on Microstructure and Mechanical Property Evolution of Haynes 282 Ni-Based Superalloy Fabricated with Selective Laser Melting (SLM)

   https://doi.org/10.3390/met10050629  

   Deshpande, Anagh ; Deb Nath, Subrata ; Atre, Sundar ; Hsu, Keng ( May 2020 , Metals)

   Selective laser melting (SLM) is one of the most widely used additive manufacturing technologies. Fabricating nickel-based superalloys with SLM has garnered significant interest from the industry and the research community alike due to the excellent high temperature properties and thermal stability exhibited by the alloys. Haynes-282 alloy, a γ′-phase strengthened Ni-based superalloy, has shown good high temperature mechanical properties comparable to alloys like R-41, Waspaloy, and 263 alloy but with better fabricability. A study and comparison of the effect of different heat-treatment routes on microstructure and mechanical property evolution of Haynes-282 fabricated with SLM is lacking in the literature. Hence,more »in this manuscript, a thorough investigation of microstructure and mechanical properties after a three-step heat treatment and hot isostatic pressing (HIP) has been conducted. In-situ heat-treatment experiments were conducted in a transmission electron microscopy (TEM) to study γ′ precipitate evolution. γ′ precipitation was found to start at 950 °C during in-situ heat-treatment. Insights from the in-situ heat-treatment were used to decide the aging heat-treatment for the alloy. The three-step heat-treatment was found to increase yield strength (YS) and ultimate tensile strength (UTS). HIP process enabled γ′ precipitation and recrystallization of grains of the as-printed samples in one single step.« less
4. Observation of an apparent first-order glass transition in ultrafragile Pt–Cu–P bulk metallic glasses

   https://doi.org/10.1073/pnas.1916371117  

   Na, Jong H. ; Corona, Sydney L. ; Hoff, Andrew ; Johnson, William L. ( January 2020 , Proceedings of the National Academy of Sciences)

   An experimental study of the configurational thermodynamics for a series of near-eutectic Pt_80-xCu_xP₂₀bulk metallic glass-forming alloys is reported where 14 <x< 27. The undercooled liquid alloys exhibit very high fragility that increases asxdecreases, resulting in an increasingly sharp glass transition. With decreasingx, the extrapolated Kauzmann temperature of the liquid,T_K, becomes indistinguishable from the conventionally defined glass transition temperature,T_g. Forx< 17, the observed liquid configurational enthalpy vs.Tdisplays a marked discontinuous drop or latent heat at a well-defined freezing temperature,T_gm. The entropy drop for this first-order liquid/glass transition is approximately two-thirds of the entropy of fusion of the crystallized eutectic alloy. BelowT_gm,more »the configurational entropy of the frozen glass continues to fall rapidly, approaching that of the crystallized eutectic solid in the low T limit. The so-called Kauzmann paradox, with negative liquid entropy (vs. the crystalline state), is averted and the liquid configurational entropy appears to comply with the third law of thermodynamics. Despite their ultrafragile character, the liquids atx= 14 and 16 are bulk glass formers, yielding fully glassy rods up to 2- and 3-mm diameter on water quenching in thin-wall silica tubes. The low Cu content alloys are definitive examples of glasses that exhibit first-order melting.

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5. A magnetic levitation based low-gravity simulator with an unprecedented large functional volume

   https://doi.org/10.1038/s41526-021-00174-4  

   Sanavandi, Hamid ; Guo, Wei ( October 2021 , npj Microgravity)

   Low-gravity environment can have a profound impact on the behaviors of biological systems, the dynamics of fluids, and the growth of materials. Systematic research on the effects of gravity is crucial for advancing our knowledge and for the success of space missions. Due to the high cost and the limitations in the payload size and mass in typical spaceflight missions, ground-based low-gravity simulators have become indispensable for preparing spaceflight experiments and for serving as stand-alone research platforms. Among various simulator systems, the magnetic levitation-based simulator (MLS) has received long-lasting interest due to its easily adjustable gravity and practically unlimitedmore »operation time. However, a recognized issue with MLSs is their highly non-uniform force field. For a solenoid MLS, the functional volumeV_1%, where the net force results in an acceleration <1% of the Earth’s gravityg, is typically a few microliters (μL) or less. In this work, we report an innovative MLS design that integrates a superconducting magnet with a gradient-field Maxwell coil. Through an optimization analysis, we show that an unprecedentedV_1%of over 4000 μL can be achieved in a compact coil with a diameter of 8 cm. We also discuss how such an MLS can be made using existing high-T_c-superconducting materials. When the current in this MLS is reduced to emulate the gravity on Mars (g_M = 0.38g), a functional volume where the gravity varies within a few percent ofg_Mcan exceed 20,000 μL. Our design may break new ground for future low-gravity research.

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