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Influence of hydrogen concentration and defects introduced by neutron irradiation on hydrogen diffusion in tungsten has been investigated by molecular dynamics simulation at elevated temperatures. Hydrogen diffusion is shown to be significantly restrained at high concentrations due to spontaneous formation of platelet-like hydrogen clusters. For neutron irradiation defects, self-interstitials, mono-vacancies and vacancy clusters are considered. By clustering and acting as dislocation loops, self-interstitials show considerable trapping effects on hydrogen, leading to the suppression of hydrogen effective diffusion and the change of diffusion model in which hydrogen mainly diffuses along dislocation lines instead of hopping between tetrahedral interstitial sites. Moreover, an equation connecting hydrogen diffusion parameters and the total length of dislocation loops is empirically established. Different influences of mono-vacancies and vacancy clusters on hydrogen diffusion have been carefully identified. With the same vacancy concentration, hydrogen diffusivity is lower with mono-vacancies than that with vacancy clusters because more isolated trapping sites are provided by mono-vacancies. This work is not only helpful for understanding the synergistic effects of neutron irradiation and plasma interaction, but also potentially applicable for larger scale simulations as input data.

This paper reports the modeling and simulation of cyclic behavior of single crystal nickel-based superalloy by using the crystal plasticity finite element method. Material constitutive model based on the crystal plasticity theory is developed and is implemented in a parallel way as user subroutine modules embedded in the commercial Abaqus[Formula: see text] software. For simplicity in calibration and without loss of generality, the crystal plasticity constitutive relationship used in this work takes the form that only contains a few parameters. The parameters are optimized by using the Powell algorithm. We employ the calibrated constitutive model with the finite element solver on a cuboid and a blade to simulate cyclic and anisotropic properties of single crystal superalloy. Results show that the predicted stress–strain curves are in good agreement with the experimental measurements, and anisotropic results are presented in both elastic and plastic regions.

High-entropy oxides (HEOs) have attracted great interest in diverse fields because of their inherent opportunities to tailor and combine materials functionalities. The control of local order/disorder in the class is by extension a grand challenge toward realizing their vast potential. Here we report the first examples of pyrochlore HEOs with five M-site cations, for Nd2M2O7, in which the local structure has been investigated by neutron diffraction and pair distribution function (PDF) analysis. The average structure of the pyrochlores is found to be orthorhombic Imma, in agreement with radius-ratio rules governing the structural archetype. The computed PDFs from density functional theory relaxed special quasirandom structure models are compared with real space PDFs in this work to evaluate M-site order/disorder. Reverse Monte Carlo combined with ab initio molecular dynamics and Metropolis Monte Carlo simulations demonstrates that Nd2(Ta0.2Sc0.2Sn0.2Hf0.2Zr0.2)2O7 is synthesized with its M-site local to nanoscale order highly randomized/disordered, while Nd2(Ti0.2Nb0.2Sn0.2Hf0.2Zr0.2)2O7+x exhibits a strong distortion of the TiO6 octahedron and small degree of Ti chemical short-range order (SRO) on the subnanometer scale. Calculations suggest that this may be intrinsic, energetically favored SRO rather than due to sample processing. These results offer an important demonstration that the engineered variation of participating ions in HEOs, even among those with very similar radii, provides richly diverse opportunities to control local order/disorder motifs-and therefore materials properties for future designs. This work also hints at the exquisite level of detail that may be needed in computational and experimental data analysis to guide structure-property tuning in the emerging HEO materials class.

UO2/BeO interfacial thermal resistance (ITR) is calculated by diffuse mismatch model (DMM) and the effects of ITR on UO2-BeO thermal conductivity are investigated. ITR predicted by DMM is on the order of 10-9 m2K/W. Using this ITR, UO2-BeO thermal conductivities are calculated by theoretical models and compared with experimental data. The results indicate that DMM prediction is applicable to the interface between UO2 and dispersed BeO, while not applicable to the interface between UO2 and continuous BeO. If the thermal conductivity of UO2 containing continuous BeO was to be in agreement with experimental data, its ITR should be on the order of 10-6 - 10-5 m2K/W. Therefore, the vibrational mismatch between UO2 and BeO considered by DMM is the major mechanism for attenuating the heat flux through UO2/dispersed-BeO interface, but not for UO2/continuous-BeO interface. Furthermore, it is found that the presence of ITR leads to the dependence of the thermal conductivity of UO2 containing dispersed BeO on BeO size. With the decrease in BeO size, UO2-BeO thermal conductivity decreases. When BeO size is smaller than a critical value, UO2-BeO thermal conductivity becomes even smaller than UO2 thermal conductivity. For UO2 containing continuous BeO, the thermal conductivity decreases with the decrease in the size of UO2 granule surrounded by BeO, but not necessarily smaller than UO2 thermal conductivity. Under a critical temperature, UO2-BeO thermal conductivity is always larger than UO2 thermal conductivity. Above the critical temperature, UO2-BeO thermal conductivity is larger than UO2 thermal conductivity only when UO2 granule size is large enough. The conditions for achieving the targeted enhancement of UO2 thermal conductivity by doping with BeO are derived. These conditions can be used to design and optimize the distribution, content, size of BeO, and the size of UO2 granule.

The L21 (NiCo)2TiAl phase plays a key role in the excellent mechanics of 3d high-entropy alloys. In this work, we investigate the phase stability and intrinsic mechanical properties of L21 (NixM1-x)2TiAl (M = Co, Fe, x = 0–1) alloys by using ab initio methods. The phonon dispersion relations suggest that the thermodynamic stability of L21 Ni2TiAl, Co2TiAl and Fe2TiAl as well as Y-type NiTiCoAl, NiTiFeAl and CoTiFeAl intermetallic compounds. The formation enthalpy and the energy contribution from vibrational entropy indicated that (NiCo)2TiAl is easier to form with respect to (NiFe)2TiAl and (CoFe)2TiAl. With the increase of the Co content, both the ductility and Young's modulus of L21 (Ni1-xCox)2TiAl (x = 0–1) alloy decrease, while anisotropy become strong. The Co content has no almost effect on the ductility, Young's modulus and anisotropy of the Y-type (Ni1-yCoy)Ti(NiyCo1-y)Al (y = 0–0.5) solid-solution alloys, whereas the (Ni1-yFey)Ti(NiyFe1-y)Al (y = 0–0.5) solid solutions show different ductility and anisotropy, compared with NiTiFeAl intermetallic compound.

Relaxor ferroelectrics exhibit superior properties for converting mechanical energy into electrical energy, and vice versa, but the structural disorder hampers an understanding of structure–property relationships and impedes rational design of new, lead-free materials. Bi0.5K0.5TiO3 (BKT) is a prototypical lead-free relaxor ferroelectric, but the microscopic origins of polarization, nature of the ferroelectric transition (TC), and structural changes across the tetragonal to pseudocubic transition (T2) are poorly understood. Here the local and intermediate structure of BKT is studied from room temperature to above TC by pair distribution functions (PDFs) from synchrotron X-ray total scattering experiments and complemented by ab initio molecular dynamics (AIMD) simulations. The local structure varies smoothly across T2 as well as TC, in contrast to the abrupt changes at TC inferred from conventional diffraction. Ferroelectric distortions are larger on the local scale than in the average structure, with polar Ti4+ displacements prevailing above TC. We find that local polar regions partly cancel each other below TC, while completely averaging out above, implying that BKT goes through a transition from partial to complete disorder across TC. © American Chemical Society 2018. This is the authors accepted and refereed manuscript to the article.

The local and average structure of (1-x)Bi0.5K0.5TiO3-xBiFeO3 (BKT-xBFO, x = 0.25 and 0.5) solid solutions are studied by synchrotron X-ray total scattering from ambient to 773 K. Pair distribution functions (PDFs) demonstrate that disordered BKT-0.25BFO and BKT-0.5BFO show the same structural coherence length of ∼16 A as pure Bi0.5K0.5TiO3, while their average structures are pseudocubic at all temperatures. Complementary density functional theory (DFT) calculations suggest distinctly different local structural distortions in BKT-0.25BFO and BKT-0.5BFO with random cations distribution on both A- and B-lattice. Based on the experimental and theoretical analysis, we propose that the optimal piezoelectric properties found at the structural phase boundary composition of x = 0.25 originate from tetragonal and rhombohedral polar nanoregions (PNRs) in an on average pseudocubic matrix. In contrast, for x = 0.5 there are only pseudorhombohedral polar distortions in a pseudocubic matrix phase.