Multi-phase-field simulation of austenite peritectic solidification based on a ferrite grain

A multi-phase-field model is implemented to investigate the peritectic solidification of Fe-C alloy. The nucleation mode of austenite is based on the local driving force, and two different thicknesses of the primary austenite on the surface of the ferrite equiaxed crystal grain are used as the initial conditions. The simulation shows the multiple interactions of ferrite, austenite, and liquid phases, and the effects of carbon diffusion, which presents the non-equilibrium dynamic process during Fe-C peritectic solidification at the mesoscopic scale. This work not only reveals the influence of the austenite nucleation position, but also clarifies the formation mechanism of liquid phase channels and molten pools. Therefore, the present study contributes to the understanding of the micro-morphology and micro-segregation evolution mechanisms of Fe-C alloy during peritectic solidification.     

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对Nb3Sn超导线低温下残余应力进行了分析。通过分析复合材料产生热应力的机理,给出弹性假设下计算残余应变的公式。用有限元分析计算了超导线制造到降温过程中各组分的应力变化情况,得到Nb3Sn超导丝在低温下的残余应变,模拟了不锈钢管对超导线残余应力的影响。     

Mg-Sn-Si系合金的热力学基础及合金相演变过程分析

研究了Mg-Sn-Si系合金的热力学基础及合金相的演变过程. 结果表明: 对于Mg-Sn-Si系合金, 合金相的比热容随着温度增加而增加, 在低温下变化迅速, 而在高温下变化平缓, 其热膨胀系数在低温范围内随温度升高呈指数形式增加, 而在高温范围内呈线性增大. 在Mg₂ (Si_x, Sn_1-x)、Mg₂ (Sn_x, Si_1-x)相结构中, Sn(Si)原子的取代位置不固定, 可以是面心, 也可以是顶点. 常规凝固过程中, 由于处于非平衡状态, x的取值范围有所波动, 对于Mg₂ (Si_x, Sn_1-x)和Mg₂ (Sn_x, Si_1-x) 两种结构, x的取值范围在0.25或0.75附近. Mg₂ (Si, Sn)的生成温度较高, 可从液相中直接析出, 也可由Mg₂Si转化而来, 而Mg₂ (Sn, Si)的生成温度较低, 只能从基体中析出, 随着Sn含量的增加, 开始析出Mg₂ (Sn, Si)相的温度升高.     

熔体初始温度对液态金属Na凝固过程中微观结构影响的模拟研究

采用分子动力学方法对熔体初始温度热历史条件对液态金属Na凝固过程中微观结构的影响，进行了模拟研究，并采用双体分布函数g(r)曲线、键型指数法和原子团类型指数法对凝固过程中的微观结构进行了分析.结果表明：液态金属Na在不同熔体初始温度条件下以1×10¹¹K/s冷速凝固时，均形成晶化结构，其中1661和1441键型或体心立方基本原子团(14 6 0 8)在凝固过程中对微观结构的转变起决定性作用.同时发现：熔体初始温度对凝固微结构有显著影响，而对液态和过冷态的微观结构影响并不明显，只有在晶化起始温度T_c附近才充分地展现出来.不同熔体初始温度对凝固结构的晶化程度有不同的影响，虽其影响程度是随着熔体初始温度的下降呈非线性变化关系的，但仍表明是可以通过改变熔体初始温度来加以控制的.原子团类型指数法(比键型指数法)更进一步表征了晶化体系中原子团的结构特征，将有利于对液态金属凝固过程中微观结构的转变机理进行更为深入的研究.     

Influence of initial solid–liquid interface morphology on further microstructure evolution during directional solidification

Preparation of the initial solid–liquid interface on which growth is started is a very critical step in directional solidification experiments. Dedicated experiments concerning preparation of the initial solid–liquid interface morphology and its influence on further directionally solidified microstructure were performed on Cu-20 wt% Sn peritectic alloy in a Bridgman-type furnace. To verify the morphology of the initial solid–liquid interface, steady-state directional dendritic growth was interrupted by thermal stabilization ranging from 0 to 1 h prior to quenching. With thermal stabilization duration increase, the solid–liquid interface morphology degenerated from dendritic to cellular and finally to planar. To verify the influence of the initial state on further solidification microstructure, directional solidification experiments were performed at a low pulling rate of 1 μm/s with different initial solid–liquid interface morphologies. The initial state affects solute redistribution and formation of peritectic coupled growth structure in the subsequent directional solidification process.     

等静压和温度诱导的PbLa(Zr,Sn,Ti)O3反铁电陶瓷相变和介电性能研究

研究了等静压和温度诱导掺镧La的Pb(Zr,Sn,Ti)O₃(PLZST)陶瓷材料的铁电反铁电相变、介电压力谱和介电温度谱,研究了温度对压致相变和介电压力谱的影响,结果发现温度使铁电反铁电相变压力降低,介电压力谱具有明显的扩散相变和频率弥散的特点;研究了等静压对介电温度谱的影响,结果表明等静压使铁电反铁电相变温度降低,反铁电顺电相变温度升高.这些现象有利于丰富和拓宽人们对温度和压力诱导的多组元弛豫型铁电体和弛豫型反铁电体扩散相变和弛豫行为的认识和理解. 关键词： 等静压和压致相变 铁电反铁电相变 介电压力谱 介电温度谱     

Effect of rotating magnetic field on microstructure formation of directionally solidified Sn–1.6Cd peritectic alloy

In order to investigate the effect of rotating magnetic field on the microstructure formation of peritectic alloys, directional solidification experiments of Sn–1.6Cd peritectic alloy have been conducted under different rotating magnetic field conditions. The directional solidification microstructure of Sn–1.6Cd peritectic alloy changes from banded structure to axisymmetric isolated banded structure to axisymmetric oscillatory tree-like banded structure and to single primary phase structure when the magnetic Taylor number of forced-melt flow generated by a rotating magnetic field increases from 0 to 91 to 364 and to 1456. The second and third banded structures are observed in a peritectic alloy for the first time. The results indicate that it is possible to control solidification microstructure and prepare a single primary phase structure by using a rotating magnetic field during directional solidification of peritectic alloys. The experiments show that the effect of forced-melt flow on solute distribution transforms from solute buildup to homogenization with an increase in the magnetic Taylor number. The formation mechanisms of those structures are also discussed.     

Structural Feature and Solute Trapping of Rapidly Grown Ni3Sn Intermetallic Compound

The rapid dendritic growth of primary Ni3Sn phase in undercooled Ni-30.9%Sn-5%Ge alloy is investigated by using the glass fluxing technique. The dendritic growth velocity of Ni3Sn compound is measured as a function of undercooling, and a velocity of 2.47m/s is achieved at the maximum undercooling of 251 K （0.17TL）. The addition of the Ge element reduces its growth velocity as compared with the binary Ni75Sn25 alloy. During rapid solidification, the Ni3Sn compound behaves like a normal solid solution and it displays a morphological transition of ＂coarse dendrite-equiaxed grain-vermicular structure＂ with the increase of undereooling. Significant solute trapping of Ge atoms occurs in the whole undercooling range.     

Combined bulk and laser heat treatment for optimizing the structure and phase composition of high-speed steels

The influence of laser radiation on the formation of an austenitic grain and on the alloying ability of a solid solution of high-speed cutting steels is investigated using as an example the steel R18 subjected beforehand to various bulk heat treatments. Notice is taken of the role of laser tempering during the initial stage of heating to the temperature at which an austenitic transformation with release of secondary carbides takes place, accompanied by laser action of plastic de formation with formation of a corresponding structurally stressed state. The content, in the solid solution, of the main alloying element tungsten in the R18 high-speed steel is determined from Mössbauer measurements and from x-ray phase analysis data on the degree of the dissolution of the carbide phase in laser-action zone. Determination of the carbon content in the solid solution of the zones of laser action, yields data on the degree of the carbide-phase solution and on the displacement of the diffraction -lines; these data can be attributed to anomalous diffusion of the carbon as the laser heating is accelerated.Quantum Physics Division, Kuibyshev Branch. Translation of Preprint No. 19, Lebedev Institute of Physics, Academy of Sciences of the USSR, Moscow, 1991.     

Exothermic effects during sintering of a mixture of nickel and aluminum powders. II

When a mixture of nickel and aluminum powders is sintered there is an exothermic effect which involves a rise of several hundred degrees in the temperature of the compact in a few seconds. The magnitude and nature of the exothermic effect depend on the aluminum content of the mixture, the degree of dispersion of the powders, and the initial porosity of the specimens. The temperature at which the exothermic effect begins moves towards lower temperatures with increase in the degree of dispersion, with increase in the concentration of aluminum in the mixture, and with decrease in the porosity. The magnitude of the heat effect increases with increase in the aluminum content.It is shown that the exothermic effect in the sintering process arises mainly from decomposition of the liquid phase and crystallization of chemical compounds with large heats of formation. A definite role is played by the metallothermic reduction of nickel oxide by aluminum present in the liquid phase and by the formation of intermetallic compounds at the boundaries of dissimilar particles of powder during sintering in the solid phase.     

液态金属的初始状态对凝固微结构影响的模拟研究

采用分子动力学方法对液态金属Ａｌ在不同的初始状态下，以相同急冷速率凝固的过程进行模拟跟踪研究，发现：在玻璃转变温度Ｔｇ以上（即过冷液态）时，系统的微结构组态情况基本一致，相差甚微；但在Ｔｇ以下时，不同的初始液态微结构对其固态微结构有明显的影响；且在Ｔｇ处各种固态微结构之间的差别发生突发性的变化。这一结果对于深入理解液－固微结构之间的转变关系，具有一定的重要意义。     

First principles studies on the elastic,thermodynamic properties and electronic structure of Ti15−xMoxSn compounds

The structural, elastic, thermodynamic and electronic properties of the Ti_15−xMo_xSn compounds were systematically investigated by means of first-principles calculations based on the density functional theory (DFT). The calculated results demonstrate the Ti_15−xMo_xSn compounds still remain the stable β phase structure. The calculation of cohesive energy shows that the structural stability of the Ti_15−xMo_xSn compounds increases apparently with the increase of Mo content. According to Hooke's law, the single crystal elastic constants were obtained and show that all the calculated compounds keep mechanical stability. Then the bulk modulus B, shear modulus G, Young's modulus E and Poisson's ratio ν of polycrystalline aggregates were calculated at zero pressure. The calculated results show that among these Ti_15−xMo_xSn compounds, Ti₄Mo₁₁Sn exhibits the largest stiffness while Ti₁₂Mo₃Sn shows the greatest ductility. The compounds Ti₁₂Mo₃Sn and Ti₁₁Mo₄Sn with the two lowest elastic Young's modulus of 61.01 GPa and 65.59 GPa are expected to be promising metallic biomaterials for implant applications. Besides, the Debye temperature Θ_D and the electronic density of states (DOS) are also investigated and discussed.     

Characteristics and heat treatment of cold-sprayed Al-Sn binary alloy coatings

In this study, Al-Sn binary alloy coatings were prepared with Al-5 wt.% Sn (Al-5Sn) and Al-10 wt.% Sn (Al-10Sn) gas atomized powders by low pressure and high pressure cold spray process. The microstructure and microhardness of the coatings were characterized. To understand the coarsening of tin in the coating, the as-sprayed coatings were annealed at 150, 200, 250 and 300 °C for 1 h, respectively. The effect of annealing on microstructure and the bond strength of the coatings were investigated. The results show that Al-5Sn coating can be deposited by high pressure cold spray with nitrogen while Al-10Sn can only be deposited by low pressure cold spray with helium gas. Both Al-5Sn and Al-10Sn coatings present dense structures. The fraction of Sn in as-sprayed coatings is consistent with that in feed stock powders. The coarsening and/or migration of Sn phase in the coatings were observed when the annealing temperature exceeds 200 °C. Furthermore, the microhardness of the coatings decreased significantly at the annealing temperature of 250 °C. EDXA analysis shows that the heat treatment has no significant effect on fraction of Sn phase in Al-5Sn coatings. Bonding strength of as-sprayed Al-10Sn coating is slightly higher than that of Al-5Sn coating. Annealing at 200 °C can increase the bonding strength of Al-5Sn coatings.     

Microstructure and properties of 6FeNiCoSiCrAlTi high-entropy alloy coating prepared by laser cladding

The content of each constituent element in the newly developed high-entropy alloys (HEAs) is always restricted in equimolar or near-equimolar ratio in order to avoid the formation of complex brittle phases during the solidification process. In this study, a 6FeNiCoSiCrAlTi high-entropy alloy coating with simple BCC solid solution phase has been prepared by laser cladding on a low carbon steel substrate. The microstructure, hardness and magnetic properties have been investigated. The experimental results show that the tendency of component segregation in the conventional solidification microstructure of multi-component alloy is effectively relieved. The microstructure of the coating is mainly composed of equiaxed polygonal grains, discontinuous interdendritic segregation and nano-precipitates. EBSD observation confirms that the polygonal grains and interdendritic segregation have similar BCC structure with lots of low angle grain boundaries at the interface. The microhardness of the coating reaches 780 HV_0.5, which is much higher than most of the HEAs prepared by other methods. In addition, the coating shows excellent soft magnetic properties.     

Processes Proceeding in Fe–0.12C–1Cr–1Mo–1V Steel after Pressure Cycling

The evolution of the phase content and morphology and crystalline defects during temperature and pressure cycling is studied by the methods of transmission electron microscopy and x-ray diffraction analysis. Thermostable Fe–0.12C–1Cr–1Mo–1V steel in the initial ferrite-pearlite state is examined. The evolution of the grain structure and dislocation density is quantitatively described. A low stability of large pearlite grains at temperatures above 600°C is established. The problem of oxidizing of steel of this class is also discussed.     

低温容器夹层真空度对内罐压升率的影响研究

针对低温容器夹层真空失效工况,建立了低温容器夹层真空度与通过内罐壁面热流密度的理论模型,结合ANSYS-FLUENT软件对某立式低温容器内的液氮蒸发过程进行了仿真模拟,研究了初始充满率为50%时,低温容器内罐空间温度及压力随夹层真空度变化的规律。结果表明,随着夹层真空度降低,通过低温容器内罐壁面的平均热流密度增大,内罐中液体温度升高速率增大;当夹层真空完全失效时,内罐压升率分别是夹层真空度为10-3 Pa、1 Pa和10 Pa时的10.4倍、5倍和1.2倍。     

低压闪蒸液滴形态和温度变化的研究

将液滴在常压下突然置于低压环境中,液滴由最初的平衡状态变成过热状态,发生闪蒸.本文实验研究了低压闪蒸液滴内部形态和温度的变化,系统描述了液滴闪蒸过程中的各种形态变化,总结了稳态闪蒸和稳态结冰过程中环境压力和初始温度对温度变化的影响.实验结果表明液滴闪蒸分六种形态.稳态闪蒸中环境压力越低,液滴的最终温度也越低;液滴的初始温度越高,降到最低温度的时间越长.稳态结冰过程中,液滴初始温度增加,液滴结冰温度和结冰回升最高温度也随之增加;液滴的结冰温度和回升最高温度随环境压力的增高而减小.     

Ultrasonic-accelerated metallurgical reaction of Sn/Ni composite solder: Principle,kinetics, microstructure,and joint properties

The high-melting-point joints by transient-liquid-phase are increasingly playing a crucial role in the die bonding for the high temperature electronic components. In this study, three kinds of Sn/Ni composite solder pastes composed of different sizes of Ni particles were synthesized to accelerate metallurgical reaction among Sn/Ni interfaces under the ultrasonic-assisted transient liquid phase (U-TLP) soldering. The temperature evolution, microstructure and mechanical property in joints composed by these composite solder pastes with or without ultrasonic energy were systemically investigated. The intermetallic joint consisted of high-melting-point sole Ni₃Sn₄ intermetallic compound with a little residual Ni was obtained under the conditions of no pressure and lower power (200 W) in a high-temperature duration of only 10 s, its shear strength was up to 45.3 MPa. Ultrasonic effects significantly accelerated the reaction among the interfaces of liquid Sn and solid Ni, which attributed to the temperature rise caused by acoustic cavitation because of large number of liquid/solid interfaces during U-TLP, resulting in accelerated solid/liquid interfacial diffusion and growth of intermetallic compounds. This intermetallic joint formed by U-TLP soldering has a promising potential for applications in high-power device packaging.     

快速降压下Tween20 液滴真空闪蒸特性

为探究闪蒸喷雾冷却的微观机理, 设计并搭建了液滴悬挂式真空闪蒸实验装置, 利用可视化窗口探究Tween20 液滴闪蒸过程中的闪蒸特性及气泡生长机理. 液滴在快速降压过程中形态会经历气泡成核、气泡生长、伴随气泡生长、爆裂这四个阶段的变化, 并反复循环这一过程直至液滴稳定蒸发. 对于液滴温度的变化, 闪蒸室的终态压力起到了决定性的作用, 并且其终态温度随压力的升高明显上升. 同时通过液滴闪蒸过程形态图分析发现, 液滴在剧烈爆炸阶段其温度也发生明显下降; 在稳定蒸发阶段, 其温度也将开始稳定不变. 因此可知液滴的剧烈爆炸会带走其自身的大量热量. 而 Tween20 浓度对液滴温度的影响微乎其微, 但其会使液滴内气泡的初始成核时间发生明显滞后, 并抑制液滴内的气泡发生破裂.     

液化石油气蒸汽爆炸的模拟实验研究

液化石油气蒸汽爆炸是对其安全储运构成很大威胁的一种观象。本文设计了一个实验装置用以检测液化石油气容器在突然出现裂口后压力急剧变化的过程。该装置可模拟液化石油气容器发生蒸汽爆炸的现象。实验结果表明,液化石油气的突然泄放过程中确实可能出现压力的急剧反弹,有时甚至超过初始压力,很可能导致剧烈的蒸汽爆炸。