大块非晶临界冷却速率的非等温转变计算模型

在非等温转变理论以及非稳态形核理论基础上提出了计算大块非晶合金连续冷却转变曲线和 临界冷却速率的新模型,以用于评估合金的非晶形成能力. 依据此模型对Zr基和Pd基8种合金 进行了计算，计算结果与实验值符合较好. 计算结果表明，影响临界冷却速率的主要因素为 黏度、临界形核功和临界结晶分数. 随着黏度增大，临界冷却速率降低；随临界形核功增大 ，临界冷却速率急剧降低；随临界结晶分数增大，临界冷却速率起初降低较快，达到一定程 度后下降速率趋于缓慢. 关键词： 临界冷却速率 非晶形成能力 大块非晶 连续冷却转变曲线     

落管中Ni-Fe-Ti合金的快速凝固机理及其磁学性能

采用落管自由落体方法实现了Ni_(45)Fe_(40)Ti_(15)合金在微重力无容器条件下的快速凝固,获得了直径介于160—1050μm的合金液滴.理论计算表明冷却速率及过冷度随液滴直径减小而增大,并呈指数函数关系,实验获得的最大过冷度为210 K(0.14 T_L).随着过冷度增大,凝固组织中粗大的γ-(Fe,Ni)枝晶逐渐细化,二次枝晶间距减小,溶质Ti在γ-(Fe,Ni)相中的固溶度显著扩展.对不同直径合金液滴的凝固样品进行磁学性能分析,结果表明随着凝固合金液滴直径减小,其饱和磁化强度增大,矫顽力减小,矩形比下降,软磁性能明显提高.     

磁感应强度和冷却速率对Tb_(0.27)Dy_(0.73)Fe_(1.95)合金凝固过程中取向行为的影响

实验研究了磁感应强度和冷却速率对Tb_(0.27)Dy_(0.73)Fe_(1.95)合金凝固过程中(Tb,Dy)Fe_2相取向行为及合金磁性能的影响.结果表明,将强磁场作用于Tb_(0.27)Dy_(0.73)Fe_(1.95)合金的凝固过程可以制备出(Tb,Dy)Fe_2相沿111取向的组织,同时显著提高了合金的磁致伸缩性能;通过提高磁感应强度可以在更快的冷却速率下得到111取向的组织;在4-10 T范围内,随着冷却速率的增加,(Tb,Dy)Fe_2相沿111取向所需的磁感应强度增加,而发生(110)取向的磁感应强度减小.随着冷却速率的增加,合金的饱和磁化强度增加,而强磁场的施加对合金饱和磁化强度的变化没有明显影响.(Tb,Dy)Fe_2相的取向行为受*Tb,Dy)Fe_3相取向行为的影响,且由磁晶各向异性能与磁场作用时间共同控制.     

耐蚀合金Au3Cu高温冷却过程中能量及结构转变的分子动力学模拟

用分子动学模拟方法对液态Au3Cu冷却过程进行了研究,考察了不同冷却速度下Au3Cu结构变化特点,原子间相互作用势采用F-S多体势,结构分析采用键取向序和对分析技术。计算结果表明,冷却速度对液态Au3Cu能量及结构转变有重要影响,给出了不同冷却速度下液态Au3Cu结构转变的微观信息。     

氩蒸汽异质核化的分子动力学模拟研究

建立了一个分子动力学研究蒸汽异质核化的模型,对氩蒸汽异质核化进行MD模拟研究。氩蒸气的初始温度为300 K,冷却终温为80 K,冷却速率为0.0002 m/s。异质核化成核现象表明核化均以固体颗粒为核化中心,核化温度高于均质核化。同时统计了不同氩蒸汽粒子数异质核化的核化团簇的密度、法向压力、切向压力分布,计算团簇的表面...     

液滴室内的氢液滴超冷条件下的结晶(英文)

液滴室内超冷现象的存在使得氢微球在自身温度低于其三相点温度的情况下依然处于液态。这可能导致液滴在真空注入过程中发生碎裂。结合液氢的超冷知识, 对液滴室内的氢液滴进行了热力学模拟, 理论上提出了利于氢液滴尽快冷却成固体小丸的最佳实验条件, 即保持液滴室内氦背景气体和氢蒸汽的压强尽可能低, 并将液滴室长度延长到6 cm以上。Because of the existence of supercooling in a droplet formation chamber, the hydrogen micro spheres are still in liquid phase even though their temperature is lower than the triple point temperature. This may cause the droplets to shatter in the vacuum injection capillary. Based on the knowledge about supercooling of liquid hydrogen, we have done a thermodynamic simulation of the droplets in the droplet formation chamber, and theoretically suggested the optimal working conditions under which the droplets will most properly nucleate to solid pellets. The suggested working conditions are that the helium gas pressure and the hydrogen vapor pressure in the droplet formation chamber are kept as low as possible, and the droplet formation chamber should be no less than 6 cm in the length.     

Molecular Weight Dependence of Primary Nucleation Rate of Poly(Ethylene Succinate)

The molecular weight (MW) dependence of the primary nucleation rate (I) was studied for poly(ethylene succinate). The primary nucleation rate was measured in a wide range of temperatures. It has a bell shape with a maximum nucleation rate (I _max ) that showed a remarkable MW dependence. I _max decreased first with MW to about 3000 and then increased. The MW of about 3000 was attributed to the transition from intermolecular nucleation to intramolecular nucleation. The MW dependence of I _max was expressed as I _max ∝MW^α. The value of α was negative unity for intermolecular nucleation and positive unity for intramolecular nucleation.     

镍基合金激光熔覆数值模拟及凝固参数预测

本文采用有限体积法对激光熔覆过程的温度场分布和熔体流动进行了数值模拟.基于CALPHAD相图法计算了基体和粉末的热物理性质,采用三维热源精确预测了凝固过程和温度分布,研究了Marangoni对流对熔池尺寸的影响.在熔池凝固过程中模拟所得出的温度梯度和凝固速度,预测了熔覆层凝固组织的演变趋势,相应的显微组织与实验结果吻合...     

Effect of Planar Interfaces on Nucleation in Melting and Crystallization

The effect of planar interfaces on nucleation (namely, on the work of critical cluster formation and their shape) is studied both for crystallization and melting. Advancing an approach formulated about 150 years ago by J. W. Gibbs for liquid phase formation at planar liquid–liquid interfaces, we show that nucleation of liquids in the crystal at crystal–vapor planar interfaces proceeds as a rule with a much higher rate compared to nucleation in the bulk of the crystal. Provided the surface tensions crystal–liquid (σcl), liquid–vapor (σlv), and crystal–vapor (σcv) obey the condition σcv=σcl+σlv, the work of critical cluster formation tends to zero; in the range σcv<σcl+σlv, it is less than one half of the work of critical cluster formation for bulk nucleation. The existence of a liquid–vapor planar interface modifies the work of critical cluster formation in crystal nucleation in liquids to a much less significant degree. The work of critical crystal cluster formation is larger than one half of the bulk value of the work of critical cluster formation, reaching this limit at σcv=σcl+σlv. The shape of the critical clusters can be described in both cases by spherical caps with a radius, R, and a width parameter, h. This parameter, h, is the distance from the cutting plane (coinciding with the crystal–vapor and liquid–vapor planar interface, respectively) to the top of the spherical cap. It varies for nucleation of a liquid in a crystal in the range (h/R)≤1 and for crystal nucleation in a liquid in the range 2≥(h/R)≥1. At σcv=σcl+σlv, the ratio (h/R) of the critical cluster for nucleation in melting tends to zero ((h/R)→0). At the same condition, the critical crystallite has the shape of a sphere located tangentially to the liquid–vapor interface inside the liquid ((h/R)≅2). We present experimental data which confirm the results of the theoretical analysis, and potential further developments of the theoretical approach developed here are anticipated.     

平面射流场中纳米颗粒的成核与凝并

采用大涡模拟和直接积分矩方法,数值模拟了在Reynolds数为8300的平面射流中,水蒸气（相对湿度φ=70%）和硫酸蒸气（质量分数为5×10^-6）二元体系中纳米颗粒的成核与凝并,详细分析了颗粒数密度、体积密度和平均粒径的分布.计算结果表明.射流场混合动量厚度的增长和实验结果一致;射流场的拟序结构导致了涡核中心处硫酸蒸气浓度的明显减小,而纳米颗粒数密度则明显增加;拟序结构的出现导致颗粒碰撞概率增大,提高了颗粒凝并效率;在颗粒数密度较大的涡核中心,颗粒成核作用增强,从而加 关键词： 纳米颗粒 成核 凝并 平面射流     

铝锂合金材料状态方程的研究

 多数材料是应变率正敏感的,其屈服应力随着应变率的增加而提高;但是在应变率相关的动力学实验中,发现铝锂合金是应变率负敏感的,具有明显的冲击韧性特征。为了全面认识该种材料的动态力学行为,有必要进一步研究其在较高压力下的冲击压缩性能。通过轻气炮实验,测量了靶板自由面质点速度时程曲线等相关参量,得到了材料的D-u型Hugoniot线。实验揭示了在突加载荷作用之下,在应变率负敏感的铝锂合金材料中可出现类似于弹塑性材料中的双波结构,从而证明了所提出的“极限应变率负敏感粘塑性材料”这一概念的合理性。最后,在弱激波假设的前提下,推导了材料Hugoniot方程与Murnagham状态方程之间的关系,并以实验数据为基础得到了材料的状态方程参数。     

单分散性CdSe纳米晶的合成——成核及其生长过程

 在氩气保护下,用三辛基亚磷酸和油酸分别作为Se和Cd的配位体,在260～300 ℃的十八烯溶液中合成了尺寸可控的CdSe纳米晶。该纳米晶不用进行进一步的提纯和尺寸分离就具有单分散性,其量子尺寸限域效应非常明显——吸收光谱的第一吸收峰峰位随着纳米晶的长大而发生红移;该纳米晶质量很高,具有很强的发光能力,其发射光谱对称的形状和合理的Stokes频移（13 nm）显示,该纳米晶的发光完全出自带隙,没有来自缺陷的发光。此外,实验结果表明,通过改变三辛基亚磷酸的浓度可以调节成核和生长的快慢,从而可以在不同的反应时间得到不同尺寸的单分散性CdSe纳米晶。     

Density and related thermophysical properties of metastable liquid Ni-Cu-Fe ternary alloys

The densities of liquid Ni-Cu-Fe ternary alloy system were investigated by molecular dynamics method in combination with a MEAM (Modified Embedded Atom Method) potential model. The temperature range is from 1000 to 2200 K, including both a broad superheating range and a large undercooled regime. The densities of six Ni100−2xCuxFex alloys (x=0,10,20,30,40,50) decrease linearly with the rise of temperature at the superheated and undercooled states, and increase with the enhancement of Ni content. Among the simulated alloys, the densities of only liquid pure Ni and Ni₆₀Cu₂₀Fe₂₀ alloy are available in literatures, which are in good agreement with the calculated values. According to the relationship between the excessive volume and the alloy composition, it can be deduced that Ni100−2xCuxFex alloys deviate from ideal solution. Moreover, a comparison was also performed between the calculated results and the approximated values by Neumann-Kopp's rule. Based on the obtained density data, the thermal expansion coefficients are also derived. It firstly decreases to a minimum value, and then displays a rise with the increase of Ni content.     

钽铌合金材料的冲击压缩特性研究

 对一种新型的钽铌合金材料进行了冲击压缩特性实验研究，通过超声测量得到了其常态下的横波和纵波声速，进而计算得到了其相关的一些静态力学参量。通过二级轻气炮加载技术，得到其在60~196 GPa压力范围内的冲击Hugoniot线。这一测量结果与混合物叠加法估算的结果符合较好，并将这种新型合金材料的力学特性与两种钨合金材料进行了简单对比。比较结果显示，Ta-Nb合金材料是一种具有较好工程应用前景的材料。