Kovaliuk, T.Vondrackova, B.Valenta, J.Colman, R. H....
8页
查看更多>>摘要：Structural and magnetic properties of the prominent magnetic shape memory alloy Ni2MnGa have been investigated in detail, as well as their evolution with tuning parameters - most frequently stoichiometry changes in the Ni-Mn-Ga system or substitution of one or more elements in the parent alloy by other d and/or p-electron elements. Surprisingly, Ga-Ge substitution, although one of the most natural substitutions, has largely been neglected. We present our study on the preparation, phase and structural characterization, and magnetization properties of alloys within the Ni2MnGa1-xGex series. The isostructural substitution of Ga by Ge results in (i) a significant decrease of the melting point of the alloy, (ii) a contraction of the L2(1) crystal lattice following the Vegard's law, (iii) a suppression of martensitic transformation of the lattice for x >= 0.25, and (iv) a substantial decrease of ferromagnetic Curie temperature with Ge content increasing above x = 0.4. Tentative signs of a possible martensitic transition are observed also for the other parent compound Ni2MnGe. The crucial role of sample annealing on its overall properties is demonstrated. The presented data are discussed in the frame of previous results on the parent alloy and other related doped systems. (C) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：The kinetics of transformation of 5 w% TiB2 nano-particles (50 nm diameter) dispersed and pressed (1.5 GPa) in the of Ti matrix (150 mu m diameter) into TiB nano-whiskers was studied as a function of temperature (800-900-1000 degrees C) and time (5-720 min) in 25 experiments in total, by vacuum sintering. The classical one-step Avrami kinetics does not properly describe this transformation due to the formation of oxide inhibitors from contaminants. However, the same results could be perfectly described by a complex Avrami kinetics supposing two parallel steps of transformations: i). fast transformation along the initially metallic Ti/TiB2 interface and ii). delayed transformation along contaminated Ti/TiB2 interface. Data for kinetic calculation was obtained by X-ray diffraction and Rietveld refinement on the sintered samples. Extrapolating the experimental results by the complex Avrami model beyond the measured temperature interval it was found that at and above about 1050 degrees C, the second step is missing due to the fast dissolution of oxide films in the beta-Ti matrix, leading to very fast kinetics. (C) 2021 The Authors. Published by Elsevier B.V.
查看更多>>摘要：Studies of high entropy or multi-principal element alloys are focused mainly on equimolar or near-equimolar compositions for which configurational entropy is maximized. Negligence of interactions between elements forming such alloys, namely enthalpy of mixing, may detract these studies from a desirable target, which is a stabilization of single-phases instead of intermetallic compounds. In this work, both entropy and enthalpy terms are taken into account for the determination of the high stability compositions, for which the Gibbs free energy of mixing is minimized. For the ten 3d metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Al) all possible combinations forming 3-(120 compositions), 4-(210), 5-(252), 6-(210) and 7-component systems (120) are analyzed. Two numerical methods: a genetic algorithm (GA) and a hybrid one (a combination of GA with the gradient method) give convergent results regarding the high stability compositions at 1273 K (a typical temperature used on HEAs synthesis). It is found surprisingly that such compositions are dominated by the Ti and Al or Ni contents with minor ones of the seven remaining elements, especially for 6 and 7-component systems. These numbers can be related to average parameters of interaction (of a given element with the remaining ones) giving almost monotonic dependence; the lower the average value of the interaction parameter of a given element - the higher its content in the alloys of the high stability. Such selective preferences in compositions are reflected by high deviations between the high stability compositions and equimolar ones, with an average reaching 48% for ternary systems to 105% for 7 -component alloys (expressed as normalized root mean square errors). (c) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：Microstructure and morphology of particles play key roles in optimizing the properties of shape-selected ZnO particles, which are essential factors for flexible and reliable applications. In particular, chemical understanding and physical measurement with scientific theory must be further integrated for the realization of finely tuned ZnO nano microstructures with desired sizes and shapes. Herein, we deliver a detailed description of the mechanism that mimics the formation of finely-tuned, spherical ZnO nanoparticles (NPs) at the computational level. We tackled issues that significantly affect the favorable structural motifs of the spherical ZnO NPs grown hydrothermally from ethanolic solution leading to their advancing chemical and physical properties. The excellent photocatalytic activity of the spherical ZnO was addressed by an apparent-rate constant of 9.7(2)x10(-2) min(-1) efficiently degrading the Rhodamine B solution by similar to 99% in 50 min. The apparent-rate constant for tubular ZnO particles is almost six times lower than that of spherical ZnO NPs. Comparative results revealed that the diversity of size and shape of ZnO particles distinguishes the wurtzite-to-rocksalt transformation reversibility phenomena by dictating the microstructure-dependent deformation behavior and ultimately leading to different transition-induced elastic strain responses to hydrostatic pressure up to 30 GPa. (C) 2021 Elsevier B.V. All rights reserved.
Carter, Carly C.Klein, Ryan A.Zhou, WeiUdovic, Terrence J....
7页
查看更多>>摘要：Dihydrogen ligation to metal centers results in a wide range of metal-hydrogen interactions. Compounds containing non-classical dihydrogen ligands in which the H-H contacts are significantly longer than the 0.74 angstrom distance found in solid H-2 are classified as Kubas-like compounds, stretched dihydrogen ligands, or compressed dihydrides, depending on the H-H contact length. Compounds containing stretched dihydrogen and compressed dihydride ligands are somewhat rare. Developing a more complete understanding of the metal-hydrogen and hydrogen-hydrogen interactions in these species may provide insights into the mechanisms for the oxidative addition reaction and hydrogen storage. Here, we use diffraction and inelastic neutron scattering (INS) measurements, paired with both molecular and solid-state density functional theory (DFT), to fully characterize the nature of the metal-hydrogen and hydrogen-hydrogen interactions in a previously reported compound, OsClH3(PPh3)(3). In that report, the H-H distance was measured as 1.48(2) angstrom by single-crystal neutron diffraction and was described as a "stretched" dihydrogen. The INS-generated phonon density of states is well described by the DFT calculations, including those normal modes dominated by the vibrational motions of the three Os-ligated hydrogen atoms. The resulting calculated electron charge density landscape indicates that there is no remnant bonding character between the Os-ligated hydrogen atoms in the compound. Based on these findings, we update the understanding of the metal-hydrogen bonding in this complex and reclassify it as a compressed dihydride. As such, OsClH3(PPh3)(3) represents a rare example of a non-classical compressed dihydride species. (C) 2021 Elsevier B.V. All rights reserved.
Babanova, O. A.Soloninin, A., VGrinderslev, J. B.Skripov, A., V...
6页
查看更多>>摘要：The lithium borohydride - ammine composite LiBH4 center dot NH3 exhibits a high hydrogen density and combines [BH4]- anions and neutral NH3 molecules within the common crystal structure. To study the dynamical properties of this compound, we have measured the 1H, 11B, and 7Li nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rates over the temperature range of 18 - 293 K. Our measurements have revealed a coexistence of four types of BH4 reorientational jump processes with different activation energies. One of these processes corresponds to extremely fast BH4 reorientations with the jump rate reaching -108 s-1 already at 95 K; this fast process is characterized by the activation energy of 78(2) meV. The local coordination of [BH4]- anions in LiBH4 center dot NH3 suggests that the fastest process is represented by rotations around a single 3-fold symmetry axis of the BH4 tetrahedron. The slower processes characterized by the activation energies of 152(5) meV, 216(7) meV, and 297(10) meV may be attributed to reorientations around other symmetry axes of the BH4 tetrahedron. In the studied temperature range up to 293 K, we have not found any signs of diffusive Li+ jumps in LiBH4 center dot NH3 at the frequency scale of -104 s-1 or higher. (c) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：Magnetocaloric effect (MCE) shows great potential in refrigeration field and especially fundamental scientific research with cooling requirement. In this work, we studied the anisotropic magnetic and magnetocaloric properties of TbScO3 single crystal, a popular substrate used for depositing epitaxial ferroelectric films. Our results indicate a strong antiferromagnetic ordering below T-N similar to 3.1 K in orthorhombic TbScO3 single crystal. Large magnetocrystalline anisotropy exists between the magnetic easy ab plane and the hard c axis, leading to a giant anisotropic MCE. In field changes of 20 and 50 kOe along the a axis, large magnetic entropy changes reach 15.25 and 23.71 J/kg K at 3 K and 7 K, respectively, while the magnetic entropy changes along the c axis are negligible. As a result, large rotating magnetic entropy changes of 15.27 and 23.63 J/kg K are obtained by rotating TbScO3 single crystal with constant fields of 20 and 50 kOe lying in the ac plane. The large anisotropic MCE of TbScO3 single crystal indicates this material as a promise refrigerant candidate for rotating magnetic refrigeration technique near the helium temperature. (C) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：TiCx particle is a promising reinforcement in Al-based composites. However, the unstable interfaces of TiCx/Al have seriously affected the mechanical properties due to the chemical reaction throughAl + TiCx -> Al4C3. By utilizing B atoms to modify the interfaces between ex-situ TiCx particles and Al matrix, the (4TiC(x) + 5AlN)/Al composite has been synthesized. It is confirmed that B atoms distribute as coating around the TiCx particles by the electron probe micro-analyzer (EPMA) and high resolution transmission electron microscope (HRTEM) analyses. The interfaces among Al, AlN and B-modified TiCx particles have been identified to be clean and clear. Compared with 5AlN/Al composite, the (4TiC(x) + 5AlN)/Al composite exhibits enhanced hardness and tensile strength. The ultimate tensile strength (UTS) of 5AlN/Al composite at room temperature (25 degrees C) and elevated temperature (350 degrees C) are 325 MPa and 143 MPa, while (4TiC(x) + 5AlN)/Al composite is increased to 350 MPa and 187 MPa, respectively. Except for the synergistic strengthening performance of nano AlN and submicron TiCx particles, it is regarded that the B-modified interfaces of TiCx/Al are beneficial to the load transfer strengthening. This work may provide some new insights for preparing high performance Al-based composites, by promoting the interface modification between ex-situ reinforcement and Al matrix. (C) 2021 Elsevier B.V. All rights reserved.
查看更多>>摘要：The effect of the WCoB addition on the wear resistance of the ultra-coarse WC-8Co cemented carbides was studied by experiments and finite element simulations. The results showed that with the increase of the WCoB content, the wear resistance of the ultra-coarse cemented carbides was greatly improved. The cemented carbide containing 1.0 wt.% WCoB had a friction coefficient of 0.281 and a wear rate of 0.77 x 10-6 mm3/N.m, which was 72% and 46% of those of the WC-8Co counterpart. The reason of the significant reduction of the friction coefficient and wear rate was analyzed. WCoB with high hardness distributed homogeneously at WC/WC and WC/Co interfaces, which led to relatively homogeneous distribution of the stress in WC grains and supported the WC grains after Co removal. Consequently, the probability of brittle fracture and pull-out of the WC grains during wear process of the cemented carbides was decreased. (c) 2021 Elsevier B.V. All rights reserved.
Maurya, A. K.Yeom, Jong-TaekKang, Seung WonPark, Chan Hee...
17页
查看更多>>摘要：Recently, the hybrid method has been developed in which wire and arc additive manufacturing (WAAM) use to produce the near net shape preform for the single-step hot forging process. The hybrid method over-comes the defects and anisotropic properties of WAAM processed preform and produce the net shape of the component with better mechanical properties. This study investigates the hot deformation behavior and mechanical properties of WAAM Ti-6Al-4V alloy containing widmantatten microstructure (0.34 - 0.48 mu m) produced by the hybrid method. Hot deformation tests were conducted in the temperature range 700-1000 degrees C and strain rate range 0.01 s(-1) - 10 s(-1) up to the height reduction of 60%, using the thermal-mechanical simulator gleeble-380 0. The artificial neural network model (ANN) has been developed to correct the non-isothermal flow curve at finer intervals of experimental conditions. The microstructural studies were carried out at various regions using a developed processing map. The microstructures show an instability region at a high strain rate and lower temperature, associated with flow localization and lamellae kinking. At the same time, the high efficiency and stable regions are related to dynamic recrystallization in the temperature range 900-950 degrees C at a strain rate below 1 s-1. The self-deformation activation energy in the alpha + beta and beta phase regions was 308.7 kJ/mol and 493.2 kJ/mol, respectively. The forged sample at 920 degrees C and strain 0.6, 0.8, and 0.9 show high strength, elongation, and weak texture compared to the received and stress relieved sample. (C) 2021 Elsevier B.V. All rights reserved.
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