镍基高温合金SLM过程中熔池内的微观结构演变数值模拟

采用有限元耦合相场的方法研究IN718高温合金在激光选区熔化过程中熔池内的竞争性生长和晶粒/枝晶结构的演变。通过有限元解决热演化问题,然后将结果输入相场模型,相场模拟涉及晶粒成核、晶粒外延生长和晶粒竞争的微观结构演变。基于对熔池横向和纵向截面微观结构演变的分析,讨论SLM过程中枝晶竞争性生长的机制,并量化计算熔池中溶质元素Nb的浓度分布。结果表明,枝晶竞争性生长受温度梯度和晶体学取向的影响,具有高错配角(>22.5°)的不利取向晶粒将很快被有利取向晶粒淘汰。

重力驱动的自然对流下hcp金属枝晶生长的相场-格子玻尔兹曼方法研究

开发了一种耦合并行-自适应网格划分算法的相场-格子玻尔兹曼方法来模拟hcp金属的等轴和柱状枝晶生长,重点讨论了重力驱动的自然对流以及自然对流与强制对流耦合对枝晶生长的影响。模拟结果表明,二维情况下的hcp金属枝晶在重力驱动的自然对流下呈现非对称生长,同时揭示了溶质偏析和溶质羽流的演化过程。研究得出,溶质羽流的形成是由枝晶的溶质阻塞和熔体流动时溶质传输之间的竞争决定的。引入适当的强制对流可以消除溶质羽流的形成并抑制枝晶尖端溶质浓度的局部波动。研究还发现,重力驱动的自然对流丰富了hcp金属3D枝晶形貌的多样性。

Progress and prospects in Mg-alloy super-sized high pressure die casting for automotive structural components

Since the introduction of Tesla's Giga-Casting process, the automotive industry has widely accepted the concept of super-sized structural components due to their significant potential for enhancing the light-weighting of both electric and internal combustion engine vehicles.These super-sized components can be further lightened by using Mg alloys because of their exceptional lightweight characteristics, with a density only two-thirds that of aluminium alloys and one-fourth that of steel. This outstanding attribute offers the attractive prospect of achieving significant weight reduction without compromising structural integrity. This review examines studies on the Mg-alloy HighPressure Die Casting(HPDC) process, providing insights into the future prospects of incorporating Mg alloys into super-sized automotive HPDC components.

Precise measurement of 171Yb magnetic constants for 1S_(0)–3P_(0) clock transition

We present a precise measurement of171Yb magnetic constants for 1S_(0)-3P_(0) clock transition. The background magnetic field is firstly compensated to < 1 m Gs(1 Gs = 10^(-4)T) through measuring the splitting of two π transitins of171Yb clock transition at different compensation coils currents. Then, the splitting ratios of the π and σ components of171Yb clock transition at different bias magnetic fields are measured, and the first-order Zeeman coefficient is determined to beα = 199.49(5) Hz/Gs. The second-order Zeeman shifts at various bias magnetic fields are also measured through interleaved self-comparison in which the bias magnetic fields are modulated between high and low values. The second-order Zeeman coefficient is fitted to be β =-6.09(3) Hz/m T^(2), which is consistent with the result of NIST group.

Recent advances in micro-alloyed wrought magnesium alloys:Theory and design

Micro-alloying design of wrought magnesium(Mg) alloys is an important strategy to achieve high mechanical properties at a low cost. In the last two decades, significant progress has been made from both theory and experiment. In the present review, we try to summarize recent advances in micro-alloying design of wrought Mg alloys from both theoretical and pragmatic perspectives, and provide fundamental data required for establishing the relationship between chemical composition and mechanical properties of Mg alloys. We start with theoretical attempts for understanding the mechanical properties of Mg alloys at different scales, by involving first principle calculations,molecular dynamics, cellular automata, and crystal plasticity. Then, the role of alloying elements is discussed for a series of promising Mg alloys such as Mg-Al, Mg-Zn, Mg-RE(rare-earth element), Mg-Sn, and Mg-Ca families.Potential challenges in the micro-alloying design of Mg alloys are highlighted at the end. The review is expected to provide helpful guidance for the intelligent design of novel wrought Mg alloys and inspire more innovative ideas in this field.

CHES: A Space-borne Astrometric Mission for the Detection of Habitable Planets of the Nearby Solar-type Stars

The Closeby Habitable Exoplanet Survey(CHES) mission is proposed to discover habitable-zone Earth-like planets of nearby solar-type stars(~10 pc away from our solar system) via microarcsecond relative astrometry.The major scientific objectives of CHES are:to search for Earth Twins or terrestrial planets in habitable zones orbiting100 FGK nearby stars;further to conduct a comprehensive survey and extensively characterize nearby planetary systems.The primary payload is a high-quality,low-distortion,high-stability telescope.The optical subsystem is a coaxial three-mirror anastigmat(TMA) with a 1.2 m-aperture,0°.44 × 0°.44 field of view and 500 nm-900 nm working wave band.The camera focal plane is composed of a mosaic of 81 scientific CMOS detectors each with4 k × 4 k pixels.The heterodyne laser interferometric calibration technology is employed to ensure microarcsecond level(1 μas) relative astrometry precision to meet the requirements for detection of Earth-like planets.The CHES satellite operates at the Sun-Earth L2 point and observes all the target stars for 5 yr.CHES will offer the first direct measurements of true masses and inclinations of Earth Twins and super-Earths orbiting our neighbor stars based on microarcsecond astrometry from space.This will definitely enhance our understanding of the formation of diverse nearby planetary systems and the emergence of other worlds for solar-type stars,and finally provide insights to the evolution of our own solar system.

Phase-field-lattice Boltzmann study for lamellar eutectic growth in a natural convection melt

In the present study, the influence of natural convection on the lamellar eutectic growth is determined by a phase-field-lattice Boltzmann study for Al-Cu eutectic alloy. The mass difference resulting from concentration difference led to the fluid flow, and a robust parallel and adaptive mesh refinement algorithm was employed to improve the computational efficiency without any compromising accuracy. Results show that the existence of natural convection would affect the growth undercooling and thus control the interface shape by adjusting the lamellar width. In particular, by alternating the magnitude of the solute expansion coefficient, the strength of the natural convection is changed. Corresponding microstructure patterns are discussed and compared with those under no-convection conditions.

A study of metal/die interfacial heat transfer behavior of vacuum die cast pure copper

High pressure die casting copper is used to produce rotors for induction motors to improve efficiency.Experiments were carried out for a special"step-shape"casting with different step thicknesses.Based on the measured temperature inside the die,the interfacial heat transfer coefficient(IHTC)at the metal/die interface during vacuum die casting was evaluated by solving the inverse problem.The IHTC peak value was 4.5×10^3-11×10^3 W·m^-2·K^-1 under the basic operation condition.The influences of casting pressure,fast shot speed,pouring temperature and initial die surface temperature on the IHTC peak values were investigated.Results show that a greater casting pressure and faster shot speed could only increase the IHTC peak values at the location close to the ingate.An increase of pouring temperature and/or initial die surface temperature significantly increases the IHTC peak values.

Phase-field modeling of dendritic growth of magnesium alloys with a parallel-adaptive mesh refinement algorithm

A two-dimensional phase field(PF)model was developed to simulate the dendritic solidification in magnesium alloy with hcp crystal structure.By applying a parallel-adaptive mesh refinement(Para-AMR)algorithm,the computational efficiency of the numerical model was greatly improved.Based on the PF model,a series of simulation cases were conducted and the results showed that the anisotropy coefficient and coupling coefficient had a great influence on the dendritic morphology of magnesium alloy.The dendritic growth kinetics was determined by the undercooling and equilibrium solute partition coefficient.A significant finding is acquired that with a large undercooling,the maximum solute concentration is located on both sides of the dendrite tip in the liquid,whereas the maximum solute concentration gradient is located right ahead of the dendrite tip in the liquid.The dendrite tip growth velocity decreases with the increase of the equilibrium solute partition coefficient,while the variation trend of the dendrite tip radius is the opposite.Quantitative analysis was carried out relating to the dendritic morphology and growth kinetics,and the simulated results are consistent with the theoretical models proposed in the previously published works.

Effective sideband cooling in an ytterbium optical lattice clock

Sideband cooling is a key technique for improving the performance of optical atomic clocks by preparing cold atoms and single ions into the ground vibrational state.In this work,we demonstrate detailed experimental research on pulsed Raman sideband cooling in a 171 Yb optical lattice clock.A sequence comprised of interleaved 578 nm cooling pulses resonant on the 1st-order red sideband and 1388 nm repumping pulses is carried out to transfer atoms into the motional ground state.We successfully decrease the axial temperature of atoms in the lattice from 6.5μK to less than 0.8μK in the trap depth of 24μK,corresponding to an average axial motional quantum number<nz><0.03.Rabi oscillation spectroscopy is measured to evaluate the effect of sideband cooling on inhomogeneous excitation.The maximum excitation fraction is increased from 0.8 to 0.86,indicating an enhancement in the quantum coherence of the ensemble.Our work will contribute to improving the instability and uncertainty of Yb lattice clocks.

Segregation of Mg-6Gd alloy under natural convection:From macro solute distribution to micro dendrite growth

Segregation is a serious defect in alloy ingots which severely deteriorates materials performance.The segregation defect in Mg-6Gd alloy is studied by coupling macro thermal-solutal-convection transport and micro dendrite growth.The macroscopic fluid dynamics and mass transfer equations are resolved to forecast the segregation behavior under conditions of continuous temperature variation during the solidification process.The numerical model is validated by testing double-diffusive natural convection in a closed square cavity.A phase field model is then applied to simulate the micro dendrite growth,using macro undercooling and liquid flow velocity as boundary conditions.Results show that the multiscale segregation behavior,including macro solute distribution and micro dendritic morphology,is strongly dependent on the temperature condition and the liquid convection,which provides guidance for reducing and eliminating the segregation defect.

Pump scheme optimization of an incoherently pumped high-power random fiber laser

Optical signal-to-noise ratio(OSNR) is one of the most significant parameters for the performance characterization of random fiber lasers(RFLs) and their application potentiality in sensing and telecommunication. An effective way to improve the OSNR of RFLs is pump scheme optimization, for example, employing a temporally stable source as the pump. In this paper, the output performance of an incoherently pumped RFL dependence on the pump bandwidth has been investigated both in experiment and theory. It is found that a high-OSNR RFL can be achieved with broadband amplified spontaneous emission(ASE) source pumping, and a relatively broad pump bandwidth can also help suppress the spectral broadening while maintaining an ultra-high spectral purity.By optimizing the pump bandwidth to ~10 nm, maximum OSNR of ~39 dB(corresponding to a spectral purity of ~99.96%) with more than 99 W output power can be obtained. Moreover, for the pump bandwidth of 0.6–40 nm, the spectral purity can reach as high as >99% with the pump power ranging from ~85 to ~117 W.In addition, with the aid of theoretical simulation based on a modified power balance model, we find that the increment of pump bandwidth can decrease the effective Raman gain coefficient, further influencing the gain characteristics, nonlinear effects, and eventually the output performance. This work provides new insight into the influence of the pump characteristics on the output performance of incoherently pumped RFLs.

Telescope alignment based on the sharpness function of under-sampled images

The precise alignment of a high-performance telescope is a key factor to ensure the imaging quality.However,for telescopes with a wide field of view,the images are sometimes under-sampled.To study the effects of undersampled images on the precision of telescope alignment,numerical simulations are implemented with the stochastic parallel gradient descent algorithm.The results show that the alignment program can converge stably and quickly.However,with the reduction of the full width at half-maximum of images,the relative residual errors increase from 9.5%to 19.5%,and the wavefront errors raise from 0.0972λto 0.1074λ,indicating that the accuracy of the alignment decreases.