Search

The effects of precipitates at {10 1‾2} twin boundaries on twin stability and yield strength of a Mg-Nd-Ag-Zr alloy are investigated by transmission electron microscopy and electron back-scattered diffraction. Twin-boundary precipitates are observed in the pre-deformed alloys after subsequent ageing treatments over a temperature range of 200–450 °C. Such twin-boundary precipitates significantly enhance the thermal stability of {10 1‾2} twins, which remain relatively stable even at 480 °C. The stress stability of {10 1‾2} twins is also improved due to the pinning effect of the twin-boundary precipitates, leading to a strengthening effect on the alloy. More twin boundaries can be introduced via pre-deformation at a much higher strain rate of 1000 s−1, and such dynamic pre-compression with subsequent ageing treatments is more effective in improving the yield strength due to the formation of more stable {10 1‾2} twin boundaries.

• The interaction of co-zone {11 2 ¯ 1} twin variants can result in formation of crossing-like twin-twin junctions, different from those formed by interaction of co-zone {10 1 ¯ 2} twin variants. • Four typical twin-twin junctions are observed for the interaction of the {11 2 ¯ 1} twins, including two types of crossing-like junctions. One type of the crossing-like junction is formed by nucleation of a new twin on the back side of the impeded twin, and the other type may be formed by the pre-existing twin simultaneously impeding two closely aligned incoming twins on its two sides. • Twin nucleation is more likely to occur at {11 2 ¯ 1} twin boundary rather than at {10 1 ¯ 2} twin boundary, due to the relatively large strain concentration at the {11 2 ¯ 1} twin boundary. Deformation twinning in a WE43 alloy subjected to uniaxial compression at room temperature is investigated. Active {11 2 ¯ 1} and {10 1 ¯ 2} extension twinning is observed. The activation of {11 2 ¯ 1} twinning in a certain grain is influenced not only by its own Schmid factor, but also by that of the basal slip. Twin-twin interactions of the co-zone {11 2 ¯ 1} twin variants are commonly observed and systematically studied, including the crystallographic characteristics of the four typically observed twin-twin junctions (TTJs) and the associated twin-twin boundaries. Unlike the quilted-looking TTJs formed by the interaction of co-zone {10 1 ¯ 2} twin variants, crossing-like TTJs of co-zone {11 2 ¯ 1} twin variants are observed and their formation mechanisms are discussed. Crossing-like TTJs are also formed by the interaction of {11 2 ¯ 1} and {10 1 ¯ 2} twins. The morphology and thickening rate of {11 2 ¯ 1} and {10 1 ¯ 2} twins vary markedly due to the difference in twinning shear. The needle-like {11 2 ¯ 1} twins thicken more slowly than the lenticular {10 1 ¯ 2} twins. Twin nucleation is observed on the previously formed {11 2 ¯ 1} twin boundary due to the relatively large strain concentration on such interface, while twin nucleation on {10 1 ¯ 2} twin boundary is rarely observed. [ABSTRACT FROM AUTHOR]

• Both L1 2 and B 2 phases remain ordered at 12 GPa. • The accurate hugoniot equation of state of AlCoCrFeNi 2.1 is obtained. • Nanotwins are observed in L1 2 phase. • Both ductile and brittle fracture modes are observed. Shock compression and spallation of a eutectic high-entropy alloy (HEA) AlCoCrFeNi 2. 1 with lamellar structure are investigated via plate impact loading with free-surface velocity measurements. The as-cast and postmortem samples are characterized with transmission electron microscopy, electron back-scatter diffraction and scanning electron microscopy. An accurate Hugoniot equation of state is determined. After shock compression to ∼ 12 GPa, both the L1 2 and B 2 phases retain their ordered structures. Dense dislocations in the {111} slip planes, stacking faults and deformation twins are found in the L1 2 phase, along with fewer dislocations in the {110} slip bands in the B 2 phase. Shock-induced deformation twinning within the L1 2 phase of this HEA is observed as a new deformation mechanism under various loading conditions. For spallation, both ductile and brittle damage modes are observed. The micro voids and cracks prefer to nucleate at the phase boundaries chiefly, then in the B 2 phase. Under similar shock stress, the spall strength of AlCoCrFeNi 2. 1 HEA is about 40% higher than those of other reported dual-phase HEAs due to the high stability of its semi-coherent phase boundaries. [Display omitted] [ABSTRACT FROM AUTHOR]

• The accurate Hugoniot equation of state of Al 0.1 CoCrFeNi. • Nanotwins are only observed at high shock stress. • Damage in the Al 0.1 CoCrFeNi is ductile in nature. • Void nucleation depends on GB misorientation and peak stress. Shock compression and spallation damage of a face-center cubic phase high-entropy alloy (HEA) Al 0.1 CoCrFeNi were investigated via plate impact experiments along with free surface velocity measurements. Postmortem samples were characterized with transmission electron microscopy and electron backscatter diffraction. The Hugoniot equation of state and spall strength at different impact strengths were determined. There exists a power-law relation between spall strength and strain rate. The spall strength of Al 0.1 CoCrFeNi HEA is about 50% higher than those of previously studied HEAs and comparable to those widely applied structural stainless steels at the same shock stress. Dislocation glide and stacking faults are the important deformation mechanisms in the Al 0.1 CoCrFeNi HEA. Nanotwins are only observed at high shock stress. Damage in the Al 0.1 CoCrFeNi HEA is ductile in nature. Voids are nucleated preferentially in grain interiors, and the intragranular voids show a strong dependence on grain boundary misorientation and peak stress. [Display omitted] [ABSTRACT FROM AUTHOR]

This paper is concerned with a gas-liquid two-phase model combining with the k–ε turbulent model for numerical simulation of 3D flood waves due to complete or partial dam-break. The flow equations are solved with the finite volume method and solved by the pressure-correction algorithm of the SIMPLE-type. The free fluid surface is simulated by the the volume of fluid(VOF) method. The comparisons with other numerical results show that the proposed method is accurate, reliable and effective in simulation of dam-break flood waves.

In this paper, a numerical two-fluid flow model combining with the Realizable k–ε turbulent model for compressible viscous fluid is presented for the computation of flow characteristics in an aeration tank; and the equations are solved with the finite volume method. A multigrid technique based on the full approximation storage (FAS) scheme is employed to accelerate the numerical convergence. The numerical results for velocity and turbulent kinetic energy distribution in the aeration tank are obtained. It is shown that the Computational Fluid Dynamics (CFD) is a valuable tool to analyze the interaction of flow field and aeration.

A method for orthogonal grid generation is presented. The generating system is based on solution of a system of partial differential equations with finite difference discretization. The influence of the number of grid points, type of boundary, and intensity of the grid quality control function and grid properties are investigated. Specification of both boundary point distribution on all sides is used. The proposed method is applied to various test problems，which shows this method provides a good balance between controlling grid orthogonality and cell aspect ratio.

Abstract:A computational model is implemented to investigate hydraulic characteristics of turbulent flow through a 90 bending square duct. In the numerical solution the governing equations are first transformed by using boundary-fitted coordinate to map the physical domain to a uniform transformed space, subsequently discretized by finite-volume method and collocated grid system, and solved via SIMPLEC algorithm.The flow parameters at the control surface are obtained by momentum interpolation method. The modified k-ε turbulence model is applied to close the Reynolds stresses. The 3D hydraulic characteristics of flows in a bend square duct are computed, and the comparison between numerical results and experimental data shows a very good agreement.

This paper is concerned with the numerical study of gas–liquid flow in bubble columns by large eddy simulations (LES). The Euler–Euler approach is used to describe the equations of motion of the two-phase flow. The mean velocities and the fluctuating velocities are obtained. It is found that, when the drag, lift and virtual mass forces are used, the computed results in agreement with experimental transient behavior can be captured.

large eddy simulation cooperated with a physical fractional-step method was applied to simulate steady flow around two parallel circular cylinders. The total velocity vectors, pressure contours and vorticity magnitude are obtained. The modeling results conform to physical law, and show that the large eddy simulation theory has powerful capacity in simulation of microstructures of turbulent flows, and can be widely applied to the solution of real engineering problems.

In this paper, a numerical two-phase flow model combining with the Realizable k–ε turbulent model for compressible viscous fluid is presented for the computation of flow characteristics in an aeration tank; and the equations are solved with the finite volume method. The free fluid surface is simulated by the VOF method. A multigrid technique based on the full approximation storage (FAS) scheme is employed to accelerate the numerical convergence. The numerical results for velocity and turbulent kinetic energy distribution in the aeration tank are obtained. It is shown that the Computational Fluid Dynamics (CFD) is a valuable tool to analyze the interaction of flow field and aeration.

This paper was concerned with a vertical two-dimensional (2D) flow model with free surface. The water governing equations were discretized with finite difference method. The function of volume method was employed to track the moving free surface. The model was used to predict the characteristics of hydraulic jump flow in a 2D vertical plane. The surface profile and time averaged velocity were calculated, which shows the proposed model can be capable of capturing sharp water and gas interface configuration as time changes.

A study was conducted to examine the isothermal oxidation behavior of a new Ni-Cr-W-AI alloy in air at 1250°C with different time. Oxidation kinetics was determined from weight-change measurements. The microstructure and composition of the oxide scale were investigated by means of scanning electron microcopy and X-ray diffraction. The results showed that the oxide scales of the alloy were a compact and continuous outer Cr2O3 and NiCr2O4 layer and an inner AI2O3 layer that was in dendrite shape. Oxides scales with good adherence were formed on the surface of the alloy, which made the alloy perform excellent high-temperature oxidation resistance. [Copyright &y& Elsevier]

Not Available

Not Available

Reduction of nitric oxide (NO) to nitrous oxide (N2O) is catalyzed by bovine heart cytochrome coxidase (CcO) in anaerobic solutions at pH 7.2 and 20°C. Cyanide inhibits and forms Fe3+a3CN. The mononitrosyl (Fe2+a3NO), but not the dinitrosyl (Fe2+a3NO; Cu+BNO), is a likely intermediate in N2O formation. One-electron reduction of NO at Fe2+a3could yield N2O via HNO. However, a two-electron reduction of the NO ligand to give an intermediate that reacts with a second NO to give N2O and H2O appears more likely. Conversion of NO to N2O is favored by low levels of both NO and O2; higher NO levels can inhibit both cytochrome coxidase and NO reductase activities. Raising the O2level will favor catalysis of NO oxidation to NO2by CcO. The reactions of NO and the specific CcO activity that occur in tissue will be critically dependent on NO, O2, and CcO levels.

Infrared spectra permit direct measurements of cysteine thiols as well as nitric oxide bound to heme iron in human hemoglobin A nitrosyl. A single symmetric N-O stretch band of nitric oxide hound to Fe2+ is detected amid strong water and protein hands in the Hb14N16O minus Hb15N16O difference spectrum. Nitric oxide accepts electron density from metal in bent-end-on Fe2+-14N -16O (ν_NO=1616.5 cm−1) and donates electron density to metal in linear Fe3+-14N -16O (ν_NO= 1925 cm−1). S-H stretch hands reveal that changes in protein conformation occur at α-104, β-93, and β-112 cysteines upon conversion of deoxyHb to HbNO but that no reactions of thiols with NO occur. Furthermore, no infrared hand for S-nitrosothiol is detected. Changes in amide I spectra reflect NO binding induced changes in protein secondary structure.Copyright 1994, 1999 Academic Press

The stable binding of nitric oxide to both Cu+Band Fe2+a3in cytochrome coxidase is shown in infrared spectra. The N-O stretch band for Fe2+a3-NO at 1610 cm−1is similar to the band for MbNO at 1612 cm−1. The Cu+B-NO band is at 1700 cm−1. Thus, electron donation from metal to NO is greater with Fe2+a3than Cu+B. However, the affinity for NO is only slightly greater at Fe2+a3than at Cu+B. In contrast CO binds stably only to Fe2+a3. Infrared spectra of oxidase, myoglobin and hemoglobin NO and CO complexes are consistent with only one stable protein structure at the ligand binding site of nitrosyls and with dynamic multiple protein conformers at carbonyl sites. The binding of NO to both Cu+Band Fe2+a3of cytochrome coxidase can contribute to NO cytotoxicity and to the catalysis of NO reduction to N2O and makes possible the effective use of infrared spectroscopy in investigations of Cu as well as Fe at the binuclear O2reduction sites of oxidases.

The structures of nickel deposits on sulfided CoMo/Al₂O₃ hydrodemetallation catalyst surface were characterized by using scanning transmission electron microscopy and high-resolution transmission electron microscopy techniques to study the interaction between the deposits and catalytic components. The deposits were found in crystallite form of nickel sulfide (Ni₇S₆) on the catalyst surface. Within the crystallites cobalt is uniformly distributed. Molybdenum is only partially associated with the nickel deposits as a segregated surface layer of molybdenum sulfide (MoS₂). For crystallites smaller than about 15 nm, the extent of segregation decreases. About 75% of the molybdenum is not directly associated with nickel deposits. Extensive cover-up of molybdenum by nickel deposits was not observed, and is not the main reason of catalyst deactivation. It was found that nickel deposits migrated towards cobalt sites even though molybdenum sites were the active sites for the hydrodemetallation reaction. Structure affinity favored the formation of solid solution between cobalt and nickel sulfides, which was the driving force for the association between nickel and cobalt on the catalyst surface. Microscopic characterization showed that deposition of nickel sulfide on the catalyst surface enhanced the mobility of the catalytic components MoS₂. Coalescence of MoS₂ leads to significant reduction of accessible molybdenum sulfide sites, and is the main reason for the deactivation of hydrodemetallation catalyst.Copyright 1994, 1999 Academic Press, Inc.