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Cold spray (CS) has become an appealing technique for both printing and repairing components. Understanding the formation of a powder track along with the deposition porosity is crucial for producing high-quality structures by CS, unfortunately, very limited resources have been addressed to this direction, especially for the pore formation process. We address this gap by conducting experiments to measure particle properties, print and cut tracks, capture cross-section views, and calculate the deposition porosity. To overcome experimental limitations in observing the coating process, we develop and perform high-fidelity process simulations at the powder-scale using a massively parallel smoothed particle hydrodynamics (SPH) method that incorporates over 10 million discretization points to simulate around 200,000 powder particles for the first time. The high-fidelity SPH simulations of CS exhibit excellent agreement with the experimentally measured data in capturing track profiles, surface roughness and deposition porosity, outperforming the numerical results by current part-scale simulation tools. The present work’s key novelties lie in integrating direct powder measurement data into a large-scale CS process simulation, achieving unprecedented computational performance via high accuracy SPH version and massive MPI parallelization, and explaining the mechanisms governing the formation of pores. Three distinct types of pores with varying sizes are characterized, which are attributed to the spatial distribution intensity of powders, differences in powder size, and the phenomena of slipping and rebounding. Through control of feed rate, nozzle speed, feedstock quality, and distribution intensity, the porosity of the deposited track could be significantly reduced.

The application of high-strength steels can improve the safety and lightweight of automobiles without significantly increasing costs. However, with the increase of the strength, the springback after forming is also more serious, which seriously restricts the wide-range promotion of high-strength steels. The springback prediction using numerical simulation is an effective way for the obtainment of the high-precision forming results. In this paper, the 980 MPa-grade complex-phase (CP) steel is investigated, and the springback behavior of the CP980 sheet is studied by combining experiment and simulation. The basic mechanical properties and anisotropic yield model parameters of the CP980 steel are obtained by uniaxial and biaxial tensile tests. The elastic modulus attenuation model of the CP980 steel is established by the cyclic loading-unloading test. The true stress-plastic strain data are used to fit the parameters of the hardening model before necking, and the post-necking stress-strain relation is obtained by the extrapolation and optimization of the hardening model. Four constitutive models are established by combining different elastic, yield and hardening functions and further programmed by UMAT subroutine in ABAQUS. The V-shaped forming/bending tests with the bending angle of 60° and 90° are adopted to verify the effectiveness of the constitutive models. The results show that the constitutive model including the attenuated elastic modulus, anisotropic yield and optimized hardening functions can contribute to the highest springback prediction accuracy. With the decrease of the bending angle, the springback phenomenon becomes more significant. The research of this paper is helpful to evaluate and predict the springback results, and further enhance the forming quality of the CP steel with complex microstructure.

• New ternary carbides, in the formula V 8 P 6 C with a unique layered structure was found. • Thermal expansion coefficients are α a =8.55(5) μK−1 and α c =10.23(5) μK−1. • The out-plane V-P bonds play a key role in the dilation and compressibility along c -axis. • The experimental bulk modulus is b =185(9) GPa with strong anisotropy (B a / B c = 1.64). • Well damage tolerance due to its layered structure was predicted. Due to the laminated structure, MAX phases and MAX-phase-like compounds possess outstanding damage tolerance capability with good thermal/electrical conductivity and other charming functional properties, showing a bright future for structural-functional uses. Here, a new MAX-phase-like ternary carbide V 8 P 6 C with layered characteristics was discovered. Contrary to the alternate stacking of hexagonal-arranged M/A/X atom layers in MAX phases, V 8 P 6 C has a layered structure formed by a repeated stacking of the slabs consisting of V 6 C-octahedrons and VP 6 -distorted-octahedrons in three-atomic layers thick. The thermal expansion anisotropy was revealed, with the thermal expansion coefficients (TECs) along a- axis , c- axis, and the average value, equal to α a =8.55(5) μK−1, α c =10.23(5) μK−1, and α L =9.14(5) μK−1, respectively. The dilation along the c -axis was mainly dominated by out-plane V-P bonds between layered slabs. According to the pressure-dependent lattice parameters, the polycrystalline bulk modulus (B 0 = 185(9) GPa) and high compressibility anisotropy factor (B a / B c = 1.64) was found. Meanwhile, the second-order elastic constants were calculated, and the elastic moduli, E= 303 GPa, B= 198 GPa, G= 122 GPa, and ν = 0.246, were obtained. And the weak connection between layer slabs by out-plane V1-P bond was revealed by the bond stiffness model, with a bond stiffness close to the M-A bond in MAX phases. Furthermore, good damage tolerance capability was predicted according to Paugh's ratio (G / B =0.616) and Cauchy pressure (P a = 0 GPa, P c = -7.5 GPa), even better than those of typical MAX phases. The discovery of V 8 P 6 C provides a new member of MAX-phase-like damage-tolerant ceramic with relatively low density, moderate thermal expansion, and anisotropic compressibility, and the unique layered structure is inspiring for exploring new layered ternary ceramics. [Display omitted] [ABSTRACT FROM AUTHOR]

We use the Insight-HXMT data collected during the 2019 outburst from X-ray pulsar 4U 1901+03 to complement the orbital parameters reported by Fermi/GBM. Using the Insight-HXMT, we examine the correlation between the derivative of the intrinsic spin frequency and bolometric flux based on accretion torque models. It was found that the pulse profiles significantly evolve during the outburst. The existence of two types of the profile's pattern discovered in the Insight-HXMT data indicates that this source experienced transition between a super-critical and a sub-critical accretion regimes during its 2019 outburst. Based on the evolution of the pulse profiles and the torque model, we derive the distance to 4U 1901+03 as 12.4±0.2kpc.

We study the 2018 outburst of Aql X-1 via the monitor of all sky X-ray image (MAXI) data. We show that the outburst starting in February 2018 is a member of short-lowclass in the frame of outburst duration and the peak count rate although the outburst morphology is slightly different from the other fast-rise-exponential-decay (FRED) type outbursts with a milder rising stage. We study the partial accretion in the weak propeller stage of Aql X-1 via the MAXI data of the 2018 outburst. We report on the spectral analysis of 3 observations of Aquila X-1 obtained by Insight – hard X-ray modulation telescope (Insight-HXMT) during the late decay stage of the 2018 outburst. We discuss that the data taken by Insight-HXMT is just after the transition to the weak propeller stage. Our analysis shows the necessity of a comptonization component to take into account the existence of an electron cloud resulting photons partly up-scattered.

We present a detailed timing study of the brightest persistent X-ray source Sco X-1 using the data collected by the Hard X-ray Modulation Telescope (Insight-HXMT) from July 2017 to August 2018. A complete Z-track hardness-intensity diagram (HID) is obtained. The normal branch oscillations (NBOs) at ∼ 6 Hz in the lower part of the normal branch (NB) and the flare branch oscillations (FBOs) at ∼ 16 Hz in the beginning part of the flaring branch (FB) are found in observations with the Low Energy X-ray Telescope (LE) and the Medium Energy X-ray Telescope (ME) of Insight-HXMT, while the horizontal branch oscillations (HBOs) at ∼ 40 Hz and the kilohertz quasi-periodic oscillations (kHz QPOs) at ∼ 800 Hz are found simultaneously above 20 keV for the first time on the horizontal branch (HB) by the High Energy X-ray Telescope (HE) and ME. We find that for all types of the observed QPOs, the centroid frequencies are independent of energy, while the root mean square (rms) increases with energy; the centroid frequencies of both the HBOs and kHz QPOs increase along the Z-track from the top to the bottom of the HB; and the NBOs show soft phase lags increasing with energy. A continuous QPO transition from the FB to NB in ∼ 200 s are also detected. Our results indicate that the non-thermal emission is the origin of all types of QPOs, the innermost region of the accretion disk is non-thermal in nature, and the corona is nonhomogeneous geometrically.

A new black hole X-ray binary (BHXRB) MAXI J1535-571 was discovered by MAXI during its outburst in 2017. Using observations taken by the first Chinese X-ray satellite, the Hard X-ray Modulation Telescope (dubbed as Insight-HXMT), we perform a joint spectral analysis (2-150 keV) in both energy and time domains. The energy spectra provide the essential input for probing the intrinsic Quasi-Periodic Oscillation (QPO) fractional rms spectra (FRS). Our results show that during the intermediate state, the energy spectra are in general consistent with those reported by Swift/XRT and NuSTAR. However, the QPO FRS become harder and the FRS residuals may suggest the presence of either an additional power-law component in the energy spectrum or a turn-over in the intrinsic QPO FRS at high energies.

The evidences for the influence of thermonuclear (type-I) X-ray bursts upon the surrounding environments in neutron star low-mass X-ray binaries (LMXB) were detected previously via spectral and timing analyses. Benefitting from a broad energy coverage of Insight-HXMT, we analyze one photospheric radius expansion (PRE) burst, and find an emission excess at soft X-rays. Our spectral analysis shows that, such an excess is not likely relevant to the disk reflection induced by the burst emission and can be attributed to an enhanced pre-burst/persistent emission. We find that the burst and enhanced persistent emissions sum up to exceed Eddington luminosity by ∼ 40 percentages. We speculate that the enhanced emission is from a region beyond the PRE radius, or through the Comptonization of the corona.

The long-term evolution of the centroid energy of the CRSF in Her X-1 is still a mystery. We report a new measurement from a campaign between Insight-HXMT and NuSTARperformed in February 2018. Generally, the two satellites show well consistent results of timing and spectral properties. The joint spectral analysis confirms that the previously observed long decay phase has ended, and that the line energy instead keeps constant around 37.5keV after flux correction.

Explosive welding (EXW) involves processes like the detonation of explosive charge, impact of metal structures and strong fluid-structure interaction with complex features. The EXW has not been well modeled before since in most numerical simulations the EXW is simply treated as the impact of two plates at high velocities. Recently, the authors developed a density adaptive smoothed particle hydrodynamics (SPH) method to successfully model the whole EXW process considering the high explosive explosion and its driving effects on flyer plate [1]. Later, this model is applied to investigate the mechanisms in EXW such as the wave formation mechanism and weldability windows [2]. The explosive TNT used in the previous model can be regarded as an ideal explosive while the complex and non-ideal explosive ANFO is more frequently used in practical welding operations. In this paper, the JWL++ model is incorporated into the density adaptive SPH to simulate the detonation of ANFO. The slab detonation and the explosive welding with TNT or ANFO explosive are simulated using the developed models. Good agreement can be observed between computational results and analytical solutions or experimental observations. Some typical characteristics in TNT and ANFO EXW including the detonation of explosive, wavy interface, jet formation, weldability windows are comparatively studied.

Abstract Hypervelocity impact (HVI) is associated with large deformations of structures, phase transitions of materials and scattered debris cloud. It is a great challenge to accurately model the HVI process and predict the damage produced by HVI for conventional numerical methods. In this paper, a recently developed corrective smoothed particle hydrodynamics (SPH) method, decoupled finite particle method (DFPM), is extended to model HVI problems. Validation examples show that DFPM is as flexible as SPH while having much better performance in improving accuracy and removing tensile instability. DFPM is also very attractive for modeling problems with extremely disordered particle distribution (e.g., HVI) as no matrix inversion is required. DFPM is then applied to model the penetration of a sphere on a target plate at various impact velocities and impact angles. It is found that as the impact velocity increases, the hole size increases accordingly and maintains basically constant in a steady state after the impact velocity reaches a critical value. Based on extensive DFPM simulation results, novel formulae are given for predicting the hole size produced by normal and oblique HVIs, which can reproduce experimental data and shows better performance than existing empirical formulae in a wider range of impact velocity. [ABSTRACT FROM AUTHOR]

•A density adaptive SPH method is used to model the whole process of explosive welding (EXW).•An initial welding angle or a sufficient distance between two plates is necessary for a successful welding.•Density ratio (flyer to base plate) is significant to the composition of jet materials.•The effectiveness of two important wave formation mechanisms is confirmed.•Numerical weldability windows and the concept of effective explosive charge for EXW are given.

High velocity impact (HVI) is associated with large deformations of structures, phase transition of materials, big craters and possible flying debris. It is thus challenging for conventional numerical methods to well capture the major physics of HVI problems. This paper presents the development of an improved smoothed particle hydrodynamics (SPH) method with kernel gradient correction (KGC) technique and the original application of the improved SPH method to modeling high velocity impact problems in two-dimensional and three-dimensional spaces. The SPH method with KGC is first validated by the 3D Taylor-Bar-Impact (TBI) test with Armco iron and OFHC copper. It is demonstrated that the obtained results agree well with experimental observations and with existing numerical results. The improved SPH method is then used to model a hypervelocity impact problem with an aluminum sphere impacting onto an aluminum plate in two- and three-dimensional spaces. It is shown that the presented SPH method can qualitatively and quantitatively model the HVI problem with main physics well captured. It is also demonstrated that the presented SPH method can reproduce experimental observations better than other numerical approaches, including the characteristic shape and evolution of the debris cloud as well the particle distribution. [ABSTRACT FROM AUTHOR]

AbstractThe combination of H2O2and hydrodynamic cavitation has been used to degrade phenol in water. The effects of parameters including inlet pressure (P1), orifice geometries of hydrodynamic reactor, initial concentration of H2O2, the presence of dissolved gases and catalysts (CuO, Fe, and TiO2) have been discussed. It revealed that increased P1, more number of holes on plates, optimum initial H2O2concentration, the higher flow rate of oxygen, and the presence of Fe or CuO are more favorable in phenol degradation. Nitrogen has different effect on cavitation from oxygen. Furthermore, the identification of primary intermediates of the reaction (hydroquinone, catechol, benzoquinone, and resorcin) indicated that hydroxyl radicals are involved in phenol degradation mechanisms.

As wind turbines increase in magnitude without a proportional increase in stiffness, the risk of dynamic instability is believed to increase. Wind turbines are time dependent systems due to the coupling between degrees of freedom defined in the fixed and moving frames of reference, which may trigger off internal resonances. Further, the rotational speed of the rotor is not constant due to the stochastic turbulence, which may also influence the stability. In this paper, a robust measure of the dynamic stability of wind turbines is suggested, which takes the collective blade pitch control and non-linear aero-elasticity into consideration. The stability of the wind turbine is determined by the maximum Lyapunov exponent of the system, which is operated directly on the non-linear state vector differential equations. Numerical examples show that this approach is robust for stability identification of the wind turbine system.

Taking 1.4%C and 1.6%C ultrahigh carbon steels (UHCSs) with ultra-fine structures developed through thermo-mechanical processing as research objects, the influences of superplastic deformation conditions and electric field on compression superplasticity were conducted without electric field and with electric field. The experimental results indicate that 1.4%C and 1.6%C UHCSs have respectively superplasticity or electro-superplasticity under the conditions of deformation temperature of 800°C, the initial strain rate of 0.75-3.75×10-4 s-1 and deformation temperature of 780 °C, the initial strain rate of 0.5-5.0×10-4 s-1 and the suitable external electric field. Its strain rate sensitivity are respectively 0.32 and 0.46.When the electric field intensity of 3 kV/cm in the manner of the specimen connected to positive terminal, the steady flow stress of 1.6%C UHCS decreases more than 10.5% as compared with electric field intensity of 0 kV/cm.

Impurity segregation to grain boundaries in ultra low carbon steel was investigated by Auger electron spectroscopy and SEM during isothermal annealing at 900°C and continuous cooling. The results of isothermal annealing at 900°C show that a concentration peak appears at different times for phosphorus, sulphur, and boron, which is contrary to the equilibrium segregation theory of McLean. The phenomena could be satisfactorily explained by the non-equilibrium segregation theory based on the impurity–vacancy complex mechanism. Under continuous cooling, the segregation concentration at the grain boundary largely depends on the cooling rate. At a low cooling rate the concentration of phosphorus and boron at the grain boundary is higher than that of sulphur, while at the higher cooling rate the concentration of sulphur is higher.

The uniaxial true stress logarithmic strain curve for a thick section can be determined from the load–diameter reduction record of a round tensile specimen. The correction of the true stress for necking can be performed by using the well-known Bridgman equation. For thin sections, it is more practical to use specimens with rectangular cross-section. However, there is no established method to determine the complete true stress–logarithmic strain relation from a rectangular specimen. In this paper, an extensive three-dimensional numerical study has been carried out on the diffuse necking behaviour of tensile specimens made of isotropic materials with rectangular cross-section, and an approximate relation is established between the area reduction of the minimum cross-section and the measured thickness reduction. It is found that the area reduction can be normalized by the uniaxial strain at maximum load which represents the material hardening and also the section aspect ratio. Furthermore, for the same material, specimens with different aspect ratio give exactly the same true average stress–logarithmic strain curve. This finding implies that Bridgmans correction can still be used for necking correction of the true average stress obtained from rectangular specimens. Based on this finding, a method for determining the true stress–logarithmic strain relation from the load–thickness reduction curve of specimens with rectangular cross-section is proposed.

The effects of carrier and metal particle size on the catalytic performance (initial intrinsic activity and deactivation characteristics) of Rh in the reaction of reforming of methane with carbon dioxide were investigated. The specific activity of Rh catalysts was found to strongly depend on the carrier employed to disperse the metal, decreasing in the order yttria-stabilized zirconia (YSZ) > Al2O3> TiO2> SiO2> La2O3> MgO, a result which correlates directly with the acidity characteristics of the carrier. The initial intrinsic activity and rate of deactivation of Rh were also found to be sensitive to the metal particle size, in the range 1 ∼ 6 nm. Both activity and rate of deactivation were found to decrease with increasing metal particle size. However, the degree of these dependences was found to be largely affected by the nature of the carrier, suggesting that the dependence of activity and rate of deactivation on metal particle size is likely to be related to metal–support interactions. While relatively high deactivation rates were observed over Rh supported on TiO2and MgO, lower deactivation rates were observed over Rh supported on YSZ, Al2O3, La2O3, and SiO2. Evidence was found that at least three kinds of factors contribute to catalyst deactivation, namely, carbon deposition, metal sintering, and poisoning of surface Rh sites by species originating from the carrier. The importance of each of these factors was found to be determined by the nature of the carrier

Structures subjected to severe cyclic loading may fail due to low cycle fatigue. During the latter part of the fatigue life the crack growth rate may increase due to the occurence of crack growth from static failure modes, e.g. void growth. The present investigation attempts to predict the combined crack growth by means of nonlinear FE methods. The case studied is an axially loaded flat plate with embedded, nearly circular cracks growing under nominal cyclic plasticity, as test data for this case has been obtained previously. The fatigue part of the crack growth is determined by using the computed cyclic J-integral and the static mode crack growth from ductile tearing is determined from computations accounting for void nucleation/growth/coalescence by means of a modified Gurson-Tvergaard model. Comparison with the test results shows acceptable correspondence.

The kinetic behavior of Rh supported on SiO2, Al2O3, and TiO2in the CO oxidation reaction has been investigated. It was found that the maximum turnover frequency (TOF) is approximately the same over the three catalysts, although the maximum is shifted to lower CO pressures for Rh/Al2O3and Rh/TiO2than for Rh/SiO2catalyst. At high CO pressures, a change from −1. to −0.4 in the reaction order with respect to CO was observed over Rh/Al2O3, while in the case of Rh/TiO2a similar change from −1 to −0.2 was observed. These changes are attributed to the contribution of the CO dissociation step to the total oxidation rate. Oscillatory behavior and hysteresis were observed on all catalysts at 393 K. However, the window of oscillation with respect to the CO pressure was much wider on Rh/SiO2than on the other catalysts. On-line mass spectrometry was used to follow the precise oscillatory patterns as a function of reaction temperature over the Rh/SiO2catalyst. The oxidation/reduction of the rhodium surface is proposed to be the dominant mechanism responsible for the observed oscillatory behavior.

Plasma source ion nitriding is a new low temperature, low pressure nitriding approach for the surface modification of iron and steel. A nitriding apparatus based on an electron cyclotron resonance (ECR) microwave plasma source has been developed. Nitrogen ions are accelerated from the ECR microwave plasma by a pulsed negative bias (typically − 2 kV) applied directly to the sample, are implanted, and are finally diffused to a depth below the surface at elevated temperatures which are regulated up to 450 °C by an auxiliary heater. An experimental investigation of plasma source ion nitriding into pure iron is described. The nitrided samples were characterized using optical metallography, microhardness measurement, glancing angle X-ray diffraction and transmission electron microscopy. The effect of the process temperatures on the surface hardness, the hardness-depth profile and the microstructure of nitrided pure iron has been investigated. On the basis of these findings, it has been established that plasma source ion nitriding of pure iron can produce an iron nitride layer and a hardened diffusion zone at process temperatures from 150 to 450 °C.

Surface species formed over Rh/TiO₂ (W6+) catalysts during CO and CO₂ hydrogenation were investigated by in situ FTIR spectroscopy and transient techniques. It was found that at least four carbon-containing species, namely, linear CO, bridged CO, active carbon (C_α) and less-active carbon (C_β), exist on Rh crystallites, while formate and/or carbonate and/or hydrocarbonate species are on the TiO₂ support. A reduction of CO coverage by 25 ~ 40% and a shift of the linear CO band to higher wavenumber values by 10 ~ 14 cm−1 were observed under CO and CO₂ hydrogenation upon doping the TiO₂ carrier with small amounts of W6+ cations (< 1 at.%). This implies that the Rh-CO bond is significantly weakened by doping, presumably due to an alteration of the electronic structure of the TiO₂ carrier, which modifies the electronic state of surface Rh via electronic interactions at the metal-support interface. The concentrations of the C_α and C_β species on Rh crystallites and carbon-containing species on the support were also found to be influenced by doping. While the concentration of the C_α species (including CH_x species) was reduced and that of the C_β species was enhanced with increasing W6+ dopant content, the concentration of the carbonate and/or hydrocarbonate species on the carrier exhibited a maximum at a W6+ content of 0.11 ~ 0.22 at.%. It is reasoned that hydrogen chemisorption is favored on the doped catalysts, as the CO coverage and the strength of the Rh-CO bond are significantly reduced. This has twofold consequences which result in a large increase of the CO and CO₂ hydrogenation activity, as previously observed over the Rh/TiO₂ (W6+) catalysts: there is an increase in the concentration of surface hydrogen participating in the rate-determining step and an enhancement of the formation of Rh carbonyl hydride species, through which the reaction proceeds via a route of lower activation energy (H-assisted CO dissociation).Copyright 1995, 1999 Academic Press, Inc.

A study of CO2hydrogenation on Rh/TiO2catalysts and of the influence of doping the TiO2carrier with W6+cations on kinetic parameters has been conducted. It is found that while the reaction order with respect to H2and CO2partial pressures does not significantly change, the specific activity of CO2methanation is enhanced by up to 24 times upon doping the TiO2support with less than 1 at.% W6+cations. A reduction of the apparent activation energy by ca. 25-34 kJ/mol and improvement of the stability of the catalytic performance are also observed by carrier doping. The greatly increased activity caused by doping is found not to be related to the SMSI phenomenon but to enhanced hydrogen adsorption capacity and weakening of the Rh-CO bond, as revealed by H2chemisorption measurements and infrared spectroscopic studies. The rate of gaseous CO formation is also found to be affected by carrier doping to a significant extent, via the alteration of surface CO coverage and of the strength of the MCO bond. The intrinsic reason for the dependence of catalytic parameters upon doping of the carrier is discussed with respect to alterations of the electronic structure of the support upon doping, and electronic interactions between rhodium crystallites and the doped carrier.

Abstract: The photovoltaic industry relies heavily on solar-grade silicon multicrystals. Understanding their mechanical behavior requires the development of adequate constitutive models accounting for the effects of both high dislocation densities and complex loading situations in a wide range of temperature, strain rate, and impurity contents. The traditional model of Alexander and Haasen poses several limitations. We introduce in this work a novel constitutive model for covalent single crystals and its implementation into a rate-dependent crystal plasticity framework. It is entirely physically based on the dislocation generation, storage and annihilation processes taking place during plastic flow. The total dislocation density is segmented according to the dislocation mobility potential and their character. A dislocation multiplication law for the yield region more accurate than the one of Alexander and Haasen is proposed. The influence of additional dislocation sources created on forest trees, usually disregarded in models for semiconductors, is assessed. The dislocation velocity law combines three potentially rate-limiting mechanisms: the standard double kink mechanism, jog dragging and the influence of localized obstacles. The model is valid at finite strains, in multiple slip conditions and captures accurately the high temperature- and strain rate sensitivity of semiconductors. The experimental stress overshoot is qualitatively reproduced only when jog dragging is accounted for. Localized obstacles are shown not to have any significant effect on dislocation motion in silicon. The broader case of extrinsic semiconductors is discussed and the influence of dissolved oxygen on the upper yield stress of silicon monocrystals is successfully reproduced. [Copyright &y& Elsevier]