Solute redistribution and Rayleigh number in the mushy zone during directional solidification of Inconel 718

The interdendritic segregation along the mushy zone of directionally solidified superalloy Inconel 718 has been measured by scanning electron microscope (SEM) and energy dispersion analysis spectrometry (EDAX) techniques and the corresponding liquid composition profile was presented. The liquid density and Rayleigh number (Ra) profiles along the mushy zone were calculated as well. It was found that the liquid density difference increased from top to bottom in the mushy zone and there was no density inversion due to the segregation of Nb and Mo. However carbide formation in the freezing range and the preferred angle of the orientated dendrite array could prompt the fluid flow in the mushy zone although there was no liquid density inversion. The largest relative Rayleigh number appeared at 1,326 degrees C for Inconel 718 where the fluid flow most easily occurred.

Effect of Counterface Materials on Friction and Wear Behavior of Double Glow Glasma Discharge Surface Alloying on Ti22Al25Nb Alloy

Samples of surface chromising layer were prepared by the double glow plasma discharge technique. X-ray diffraction and X-ray photoelectron spectroscopy（XPS） analysis of dif-ferent elements confirmed the formation of chrome in the layer. Their tribological properties were investigated by pin-on-disk tribometer. Silicon nitride, GCr15, and nickel-based alloy were selected as counterface materials. Results indicated that the lowest friction coefficients and wear rate were ob-tained when substrate and chromising layer against nickel-based alloy, and tribological properties of chromising layer were better than those of substrate. The highest friction and wear rate were samples against silicon nitride alloys. In the case of three rubbing pairs, the unchangeable materials against different hardness counterfaces leaded to different wear mechanisms. Samples against silicon nitride exhibited abrasive mechanism, and when GCr15 and nickel-based alloy were used as counterface, transfer film and glaze layer formed on the contact surface, which played the main role in decreasing friction and wear.

High-temperature tribological behaviors of Ti_2AlNb-based alloys by plasma surface duplex treatment

Plasma tungstening followed by carburization(W-C duplex treatment)was performed on the Ti2AlNb-based(O phase) alloy by using the double glow plasma process to enhance its wear resistance.The microstructure and high-temperature tribological behaviors of the un-treated and W-C duplex-treated samples were investigated.The results show that the duplex-treated layer is mainly composed of W2C or W6C2.54 phases and the contents of W and C elements in the alloyed layer change gradually along the depth by surface plasma duplex treatment.The diffusion depth of W is about 12μm,while the carbon atoms most exist in the depth more than 12μm.High temperature tribometer tests indicate that the friction coefficient of the W-C duplex-treated layer is approximately 1/6 that of substrate.The wear rate of the duplex-treated layer is about 28%that of the untreated one.So,plasma surface W-C duplex treatment can obviously improve the high-temperature tribological resistance of Ti2AlNb-based alloy.The tribological mechanism of the duplex-treated layer is discussed by dividing the friction process of the duplex-treated layer into three fluctuate stages.The first stage is the formation of oxide film between W-C duplex-treated layer and counterface.The second stage is the detachment of oxide film,acting as "the third body" .The last stage is the period that the friction and wear occur between the compact particle layer and counterface.

Improving Corrosion Resistance of Q235 Steel by Ni-Cr Alloyed Layer

Ni-Cr alloyed layer was formed on surface of Q235 steel by double glow plasma surface metallurgy to improve the corrosion resistance of substrate. The composition and microstructure of alloyed layer was analyzed by SEM and XRD. Potentiodynamic polarization and electrochemical impedance spectroscopy was applied to evaluate the corrosion resistance of the alloyed layer. The results showed working pressure had a great effect on structure of Ni-Cr alloyed layer, and the dense and smooth alloyed layer was prepared at 50 Pa working pressure. Compared with substrate, Ni-Cr alloyed layer exhibited higher corrosion potential, lower corrosion current density and larger charge transfer resistance, which indicated that Ni-Cr alloyed layer significantly modified the corrosion resistance of Q235 steel.

Variations of water's local-structure induced by solvation of NaCl

The researches on the structure of water and its changes induced by solutes are of enduring interests. The changes of the local structure of liquid water induced by NaCl solute under ambient conditions are studied and presented quantitatively with some order parameters and visualized with 2-body and 3-body correlation functions. The results show that, after the NaCl are solvated, the translational order t of water is decreased for the suppression of the second hydration shells around H20 molecules; the tetrahedral order （q） of water is also decreased and its favorite distribution peak moves from 0.76 to 0.5. In addition, the orientational freedom k and the diffusion coefficient D of water molecules are reduced because of new formed hydrogen-bonding structures between water and solvated ions.

A Universal Formula for the Secondary Electron Yield of Metals at an Incident Angle of θ

Based on the main physical processes,we deduce the relationships among the incident energy Wp0 of the primary electron,the number of released secondary electrons(i.e.δ_(PEθ))per primary electron entering the metal at incident angleθand the angleθitself.In addition,the relationship ofδPEθatθ=0°,i.e.δ_(PE0),with Wp0 is determined.From the experimental results,the relationship of the ratio atθ=0°,i.e.β_(0) which is the ratio of the average number of released secondary electrons generated by a single primary electron backscattered at the metal surface to that generated by a single primary electron entering the metal,with Wp0 is determined.Moreover,the relationships among the ratioβθ,Wp0 andθare obtained.Based on the relationships among the secondary electron yield atθ(i.e.δθ),the yield atθ=0°(i.e.δ_(0)),the backscattering coefficient atθ(i.e.η_(θ)),the coefficient atθ=0°(i.e.η0),δ_(PEθ)andδ_(PE0),we deduce the universal formula forδ_(θ),δ_(0),η_(θ),η_(0),and W_(p0) for the primary electrons at an incident energy of 2–10 keV.The secondary electron yields calculated from the universal formula and the experimental yields of some metals are compared,and the results suggest that the proposed formula is universal for estimation of secondary electron yields atθ=0°−80°.

Bidirectional Quantum Secure Direct Communication Network Protocol with Hyperentanglement

We propose a bidirectional quantum secure direct communication(QSDC) network protocol with the hyperentanglment in both the spatial-mode ad the polarization degrees of freedom of photon pairs which can in principle be produced with a beta barium borate crystal.The secret message can be encoded on the photon pairs with unitary operations in these two degrees of freedom independently.Compared with other QSDC network protocols,our QSDC network protocol has a higher capacity as each photon pair can carry 4 bits of information.Also,we discuss the security of our QSDC network protocol and its feasibility with current techniques.

High-capacity three-party quantum secret sharing with superdense coding

This paper presents a scheme for high-capacity three-party quantum secret sharing with quantum superdense coding, following some ideas in the work by Liuet al （2002 Phys. Rev. A 65 022304） and the quantum secret sharing scheme by Deng et al （2008 Phys. Lett. A 372 1957）. Instead of using two sets of nonorthogonal states, the boss Alice needs only to prepare a sequence of Einstei^Podolsky-l^osen pairs in d-dimension. The two agents Bob and Charlie encode their information with dense coding unitary operations, and security is checked by inserting decoy photons. The scheme has a high capacity and intrinsic efficiency as each pair can carry 21bd bits of information, and almost all the pairs can be used for carrying useful information.

Multiparty quantum secret conference based on quantum encryption with pure entangled states

We present a scheme for multiparty quantum remote secret conference （MQRSC） with pure entangled states, not maximally entangled multipartite quantum systems. The conferees first share a private quantum key, a sequence of pure entangled states and then use them to encode and decode the secret messages. The conferees exploit the decoy-photon technique to ensure the security of the transmission of qubits. This MQRSC scheme is more feasible and efficient than others.

Thermal Stability and Transformation-mediated Deformability of Cu-Zr-Al-Ni Bulk Metallic Glass Composite

The glass-forming ability and the thermal stability of Cu45Zr45AI5Ni5 metallic glass were systematically investigated by the differential scanning calorimetry. The activation energies for the glass transition and the onset crystallization and the crystallization peak were calculated to be 255.1, 308.7 and 311.5 kJ/mol, respectively, while the corresponding liquid fragility was determined to be 23.4. By controlling the casting process, Cu45Zr45AI5Ni5 bulk metallic glass composite with the presence of ductile B2 CuZr phase was fabricated. The composite shows a pronounced plastic strain of 6.8 ~ 0.05% with obvious work hardening, which results from the formation of multiple shear bands and the deformation-induced martensitic transformation.

Efficient Polarization Entanglement Purification Using Spatial Entanglement

The protocol using spatial entanglement to purify polarization entanglement by entanglement transformation between different degrees of freedom in a realistic environment is elaborated. Our analyses show that the bit-flip error can be completely purified, but the pure maximally entangled state can not be obtained ultimately if the spatial entanglement is impure. The fidelity of the purified state is decided by the spatial entanglement. Furthermore, this protocol can also be extended to purify the multi-particle Greenberg—Horne—Zeilinger （GHZ） state. It is presented that the spatial entanglement can be served as another source to improve the quality of entanglement.

Global attractors associated with general partly dissipative reaction-diffusion systems in unbounded domains

The asymptotic behaviour of solutions for general partly dissipative reaction-diffusion systems in Rn is studied. The asymptotic compactness of the solutions and then the existence of the global attractor are proved in L2(Rn )× L2(Rn ) .

High thermoelectric performance sheds new light on low-grade waste heat recovery

Recovering waste heat from fuel energy supply is a haunting challenge for greatly reducing the emissions of greenhouse gas CO_(2),and achieving sustainable economy.Importantly,a significant amount of energy is dissipated to environment as low-grade heat(below 250~oC),largely produced in transportation and chemical engineering industry.It accounts for~30%of the total global energy products but is much more difficult to recycle than high-grade heat(Figure 1 A)[1].So the reuse of low-grade heat is of great importance for the future energy demands and the issue of climate change[2,3].So far,several technologies have been developed to recover low-grade waste heat,such as organic Rankine cycle,water desalination,piezoelectric,and thermoelectric conversion[4].

Deterministic secure quantum communication over a collective-noise channel

We present two deterministic secure quantum communication schemes over a collective-noise. One is used to complete the secure quantum communication against a collective-rotation noise and the other is used against a collective-dephasing noise. The two parties of quantum communication can exploit the correlation of their subsystems to check eavesdropping efficiently. Although the sender should prepare a sequence of three-photon entangled states for accomplishing secure communication against a collective noise,the two parties need only single-photon measurements,rather than Bell-state measurements,which will make our schemes convenient in practical application.

Robust quantum secure direct communication with a quantum one-time pad over a collective-noise channel

We present two robust quantum secure direct communication (QSDC) schemes with a quantum one-time pad over a collective-noise channel. Each logical qubit is made up of two physical qubits and it is invariant over a collective-noise channel. The two photons in each logical qubit can be produced with a practically entangled source, i.e., a parametric down-conversion source with a beta barium borate crystal and a pump pulse of ultraviolet light. The information is encoded on each logical qubit with two logical unitary operations, which will not destroy the antinoise feather of the quantum systems. The receiver Bob can read out the sender's message directly with two single-photon measurements on each logical qubit, instead of Bell-state measurements, which will make these protocols more convenient in a practical application. With current technology, our two robust QSDC schemes are feasible and may be optimal ones.

二维拓扑反铁磁材料MnBi_(2)Te_(4)中超快磁化率和磁振子动力学的实时观测

原子级薄的范德华磁性材料不仅为探索二维极限下的基本物理学提供了肥沃的土壤,也为新型超快功能器件创造了巨大的机会.本文系统研究了少层拓扑反铁磁MnBi_(2)Te_(4)晶体在不同层厚、温度和磁场下的超快磁化率和自旋波动力学.作者发现激光诱导的(退)磁化过程可用于精确地跟踪不同磁场区域中的不同磁状态,包括显示出明显的奇偶层数效应.此外,频率为数十千兆赫兹的反铁磁磁振子模式可以被光脉冲激发并在时域中直接被观察到,其频率强烈地依赖于磁场.值得注意的是,磁化率和磁振子动力学不仅可以在时间分辨的磁光克尔效应中观察到,而且在时间分辨的反射率中也可以观察到,这表明磁态和电子结构之间存在很强的相关性.本工作研究了这种新型二维反铁磁体中的超快自旋动力学,为二维反铁磁自旋电子学和磁振子学的潜在应用以及磁态和拓扑量子态的超快调控研究铺平了道路.