The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Xiangdong Ding - One of the best experts on this subject based on the ideXlab platform.
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nanostructured high strength molybdenum alloys with unprecedented tensile ductility
Nature Materials, 2013Co-Authors: Guoyu Zhang, F Jiang, Xiangdong DingAbstract:Although molybdenum alloys — often used in turbines and fusion reactors — can be easily hardened, they suffer from low ductility and toughness. Now, a nanostructuring processing route that leads to a microstructure consisting of submicrometre Grains with nanometric oxide particles uniformly distributed in the Grain Interior achieves high-strength molybdenum alloys with large tensile elongation at room temperature.
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nanostructured high strength molybdenum alloys with unprecedented tensile ductility
Nature Materials, 2013Co-Authors: Gang Liu, Xiangdong Ding, F Jiang, Guoyu Zhang, Yuanjun Sun, Jun SunAbstract:The high-temperature stability and mechanical properties of refractory molybdenum alloys are highly desirable for a wide range of critical applications. However, a long-standing problem for these alloys is that they suffer from low ductility and limited formability. Here we report a nanostructuring strategy that achieves Mo alloys with yield strength over 800 MPa and tensile elongation as large as ~ 40% at room temperature. The processing route involves a molecular-level liquid-liquid mixing/doping technique that leads to an optimal microstructure of submicrometre Grains with nanometric oxide particles uniformly distributed in the Grain Interior. Our approach can be readily adapted to large-scale industrial production of ductile Mo alloys that can be extensively processed and shaped at low temperatures. The architecture engineered into such multicomponent alloys offers a general pathway for manufacturing dispersion-strengthened materials with both high strength and ductility.
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origin of 2 stage r phase transformation in low temperature aged ni rich ti ni alloys
Acta Materialia, 2005Co-Authors: Yumei Zhou, Xiangdong Ding, Jian Zhang, Kazuhiro OtsukaAbstract:Abstract After aging at intermediate temperatures (400–500 °C), Ni-rich Ti–Ni alloys undergo an abnormal 3-stage martensitic transformation behavior (1-stage R and 2-stage B19′), which stems from a preferential Ti3Ni4 precipitation around Grain boundary. On the other hand, if aged at low-temperatures (250–300 °C), they undergo 2-stage R-phase transformation, but the origin of this strange phenomenon is unclear. In the present study, we made a systematic study of this phenomenon by considering the Grain boundary effect and composition effect. We found that all single crystals undergo 1-stage R-phase transformation; in contrast, the transformation behavior of polycrystals is dependent on Ni content: low-Ni (50.6Ni, 51Ni) polycrystals undergo 2-stage R-phase transformation while high-Ni (52Ni) polycrystals undergo 1-stage R-phase transformation. The abnormal 2-stage R-phase transformation is attributed to a large-scale compositional heterogeneity in B2 matrix between Grain boundary region and Grain Interior, due to the heterogeneity in precipitate density between the Grain boundary and Grain Interior. But for high-Ni polycrystals, precipitates are essentially homogeneously distributed across the whole Grain and this leads to normal 1-stage R-phase transformation. The different transformation behavior of low-Ni and high-Ni polycrystals stems from a competition between two opposing tendencies: (1) for preferential precipitation in the Grain boundary; (2) for homogeneous precipitation across the whole Grain with high-Ni content. The difference between the effect of intermediate-temperature and low-temperature aging lies in the difference in the ability for long-range diffusion of Ni (from the Grain Interior to the Grain boundary), which results in whether or not Ti3Ni4 precipitates can form in the Grain Interior. Our results lead to a unified explanation for different transformation behaviors of both low-temperature and intermediate-temperature aged alloys in terms of the kinetics of precipitation in supersaturated polycrystals.
G. J. Weng - One of the best experts on this subject based on the ideXlab platform.
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mechanics of very fine Grained nanocrystalline materials with contributions from Grain Interior gb zone and Grain boundary sliding
International Journal of Plasticity, 2009Co-Authors: Pallab Barai, G. J. WengAbstract:Abstract In this paper, we formulated an atomically-equivalent continuum model to study the viscoplastic behavior of nanocrystalline materials with special reference to the low end of Grain size that is typically examined by molecular dynamic (MD) simulations. Based on the morphology disclosed in MD simulations, a two-phase composite model is construed, in which three distinct inelastic deformation mechanisms disclosed from MD simulations are incorporated to build a general micromechanics-based homogenization scheme. These three mechanisms include the dislocation-related plastic flow inside the Grain Interior, the uncorrelated atomic motions inside the Grain-boundary region (the GB zone), and the Grain-boundary sliding at the interface between the Grain and GB zone. The viscoplastic behavior of the Grain Interior is modeled by a Grain-size dependent unified constitutive equation whereas the GB zone is modeled by a size-independent unified law. The GB sliding at the interface is represented by the Newtonian flow. The development of the rate-dependent, work-hardening homogenization scheme is based on a unified approach starting from elasticity to viscoelasticity through the correspondence principle, and then from viscoelasticity to viscoplasticity through replacement of the Maxwell viscosity of the constituent phases by their respective secant viscosity. The developed theory is then applied to examine the Grain size- and strain rate-dependent behavior of nanocrystalline Cu over a wide range of Grain size. Within the Grain-size range from 5.21 to 3.28 nm, and the strain rate range from 2.5 × 108 to 1.0 × 109/s, the calculated results show significant Grain-size softening as well as strain-rate hardening that are in quantitative accord with MD simulations [Schiotz, J., Vegge, T., Di Tolla, F.D., Jacobsen, K.W., 1999. Atomic-scale simulations of the mechanical deformation of nanocrystalline metals. Phys. Rev. B 60, 11971–11983]. We have also applied the theory to investigate the flow stress, strain-rate sensitivity, and activation volume over the wider Grain size range from 40 nm to as low as 2 nm under these high strain rate loading, and found that the flow stress initially displays a positive slope and then a negative one in the Hall–Petch plot, that the strain-rate sensitivity first increases and then decreases, and that the activation volume first decreases and then increases. This suggests that the maximum strain rate sensitivity and the lowest activation volume do not occur at the smallest Grain size but, like the maximum yield strength (or hardness), they occur at a finite Grain size. These calculated results also confirm the theoretical prediction of Rodriguez and Armstrong [Rodriguez, P., Armstrong, R.W., 2006. Strength and strain rate sensitivity for hcp and fcc nanopolycrystal metals. Bull. Mater. Sci. 29, 717–720] on the basis of Grain boundary weakening and the report of Trelewicz and Schuh [Trelewicz, J.R., Schuh, C.A., 2007. The Hall–Petch breakdown in nanocrystalline metals: a crossover to glass-like deformation. Acta Mater. 55, 5948–5958] on the basis of hardness tests. In general the higher yield strength, higher strain rate sensitivity, and lower activation volume on the positive side of the Hall–Petch plot are associated with the improved yield strength of the Grain Interior, but the opposite trends displayed on the negative side of the plot are associated with the characteristics of the GB zone which is close to the amorphous state.
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a secant viscosity composite model for the strain rate sensitivity of nanocrystalline materials
International Journal of Plasticity, 2007Co-Authors: Jackie Li, G. J. WengAbstract:Abstract In order to address the strain-rate sensitivity of nanocrystalline solids, a secant-viscosity composite model is developed in this article. The microgeometry of the composite is taken to consist of the Grain-Interior phase and the Grain-boundary affected zone (GBAZ) as suggested by Schwaiger et al. [Schwaiger, R., Moser, B., Dao, M., Chollacoop, N., Suresh, S., 2003. Some critical experiments on the strain-rate sensitivity of nanocrystalline nickel. Acta Mater. 51, 5159–5172], while the constituent properties are modeled by a unified viscoplastic constitutive law. The drag stress of the Grain Interior is assumed to follow the Hall–Petch relation, but that of the GBAZ is independent of Grain size, d . Then in terms of the secant viscosity of the constituent phases, the strain-rate sensitivity of the nanocrystalline solid is determined with the help of a linear viscous comparison composite and a field-fluctuation approach. To test the applicability of the developed model, it is applied to predict the strain-rate effect of a nanocrystalline Ni, and the Grain-size dependence of its stress–strain relations. Our theoretical calculations indicate that the tensile strength of a nanocrystalline Ni with d = 40 nm is about five times that of a microcrystalline one with d = 10 μm under the same strain rate of e ˙ = 3 × 10 - 4 s - 1 , and that the nanocrystalline Ni exhibits a much stronger strain-rate effect. These predictions are found to be consistent with the experimental data of Schwaiger et al. Possible Grain-size softening with further Grain-size reduction such as reported in molecular dynamic simulations is also demonstrated.
Guoyu Zhang - One of the best experts on this subject based on the ideXlab platform.
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nanostructured high strength molybdenum alloys with unprecedented tensile ductility
Nature Materials, 2013Co-Authors: Guoyu Zhang, F Jiang, Xiangdong DingAbstract:Although molybdenum alloys — often used in turbines and fusion reactors — can be easily hardened, they suffer from low ductility and toughness. Now, a nanostructuring processing route that leads to a microstructure consisting of submicrometre Grains with nanometric oxide particles uniformly distributed in the Grain Interior achieves high-strength molybdenum alloys with large tensile elongation at room temperature.
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nanostructured high strength molybdenum alloys with unprecedented tensile ductility
Nature Materials, 2013Co-Authors: Gang Liu, Xiangdong Ding, F Jiang, Guoyu Zhang, Yuanjun Sun, Jun SunAbstract:The high-temperature stability and mechanical properties of refractory molybdenum alloys are highly desirable for a wide range of critical applications. However, a long-standing problem for these alloys is that they suffer from low ductility and limited formability. Here we report a nanostructuring strategy that achieves Mo alloys with yield strength over 800 MPa and tensile elongation as large as ~ 40% at room temperature. The processing route involves a molecular-level liquid-liquid mixing/doping technique that leads to an optimal microstructure of submicrometre Grains with nanometric oxide particles uniformly distributed in the Grain Interior. Our approach can be readily adapted to large-scale industrial production of ductile Mo alloys that can be extensively processed and shaped at low temperatures. The architecture engineered into such multicomponent alloys offers a general pathway for manufacturing dispersion-strengthened materials with both high strength and ductility.
Yuichi Ikuhara - One of the best experts on this subject based on the ideXlab platform.
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crystalline Grain Interior configuration affects lithium migration kinetics in li rich layered oxide
Nano Letters, 2016Co-Authors: Akihide Kuwabara, Eita Tochigi, Naoya Shibata, Tetsuichi Kudo, Haoshen Zhou, Yuichi IkuharaAbstract:The electrode kinetics of Li-ion batteries, which are important for battery utilization in electric vehicles, are affected by the Grain size, crystal orientation, and surface structure of electrode materials. However, the kinetic influences of the Grain Interior structure and element segregation are poorly understood, especially for Li-rich layered oxides with complex crystalline structures and unclear electrochemical phenomena. In this work, cross-sectional thin transmission electron microscopy specimens are “anatomized” from pristine Li1.2Mn0.567Ni0.167Co0.067O2 powders using a new argon ion slicer technique. Utilizing advanced microscopy techniques, the Interior configuration of a single Grain, multiple monocrystal-like domains, and nickel-segregated domain boundaries are clearly revealed; furthermore, a randomly distributed atomic-resolution Li2MnO3-like with an intergrown LiTMO2 (TM = transitional metals) “twin domain” is demonstrated to exist in each domain. Further theoretical calculations based on...
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yttrium doping effect on oxygen Grain boundary diffusion in α al2o3
Acta Materialia, 2007Co-Authors: Tsubasa Nakagawa, Naoya Shibata, Isao Sakaguchi, Katsuyuki Matsunaga, Teruyasu Mizoguchi, Takahisa Yamamoto, Hajime Haneda, Yuichi IkuharaAbstract:Abstract The yttrium doping effect on Grain boundary diffusion was directly estimated using bicrystal experiments. For this purpose, pristine and yttrium-doped α-Al2O3 bicrystals with the same geometrical configuration were fabricated. The Grain boundary oxygen diffusion coefficients were measured by the isotopic tracer profiling technique using secondary ion mass spectrometry. The Grain boundary diffusion coefficients of the pristine and yttrium-doped boundary were best described as δ D gb = 8.4 × 10 - 6 exp - 627 [ kJ/mol ] / RT and δ D gb = 6.5 × 10 - 4 exp - 729 [ kJ/mol ] / RT , respectively. It was thus found that yttrium doping retards Grain boundary diffusivity by approximately 10 times compared to the pristine crystals, while their activation energies were not greatly different. On the other hand, the simultaneously obtained volume diffusion coefficient showed similar values to previously reported results, indicating that extrinsic diffusion occurred in the Grain Interior. Taking these facts into account, the yttrium effect can be explained by a “site blocking” mechanism or a “swamp out” mechanism, or by both of these.
T Schober - One of the best experts on this subject based on the ideXlab platform.
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electrical conductivity of the high temperature proton conductor bazr0 9y0 1o2 95
Journal of the American Ceramic Society, 2004Co-Authors: Hans G Bohn, T SchoberAbstract:The impedance of the cubic perovskite BaZr0.9Y0.1O3-δ has been systematically investigated in dry and wet atmospheres at high and low oxygen partial pressures. In the Grain Interior, conductivity contributions from oxygen ions, electron holes, and protons can be identified. Below 300°C, proton conduction dominates and increases linearly with the frozen-in proton concentration. The proton mobility, with an activation energy of 0.44 ± 0.01 eV is among the highest ever reported for a perovskite-type oxide proton conductor. For dry oxygen atmos-pheres, electron hole conduction dominates with an activation energy of ∼0.9 eV. At temperatures <500°C, the Grain-boundary conductivity can be separated and increases upon incorporation of protons. The high electrical conductivity and chemical stability make acceptor-doped barium zirconate a good choice for application as a high-temperature proton conductor.
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the high temperature proton conductor ba3ca1 18nb1 82o9 δ i electrical conductivity
Solid State Ionics, 1999Co-Authors: Hans G Bohn, T Schober, T Mono, W SchillingAbstract:Abstract The electrical conductivity of the complex perovskite Ba3Ca1.18Nb1.82O9−δ (BCN18) was systematically investigated by repeatedly charging identical samples with increasing amounts of water. Below 300°C the bulk conductivity increases linearly with the frozen-in proton concentration. The proton mobility derived from these data shows an activation energy of (0.53±0.01) eV and agrees with results from quasi-elastic neutron scattering. For these samples a significant Grain boundary impedance is found. At temperatures below 500°C the Grain boundary conductivity is much smaller than the proton conduction in the Grain Interior and dominated by electron holes even in the wet samples. Under equilibrium conditions, i.e. for temperatures above 700°C ionic transport numbers are calculated from the results of the impedance measurements. They agree with those from EMF measurements. In summary, a consistent picture of the electrical conductivity of ceramic BCN18 is derived. With respect to the Grain Interior excellent protonic conduction is found while the low Grain boundary conductivity in our samples requires further investigations.
