钯在一级相变和玻璃转化过程的拓扑结构

用分子动力学法模拟了（ＮＰＴ）系综下金属Ｐｄ的一级相变和玻璃转化的整个过程，考察了不同冷却速度条件下Ｐｄ在相转变过程中的结构变化特征，用Ｖｏｒｏｎｏｉ多面体分析的方法对以上各个过程进行了拓扑分析，得出对应于晶态、液态和非晶态物质的各种特征结构，从原子尺度上解释了一级相变和玻璃转化过程中系统微观结构的变化。     

分子动力学法对Cu-Ag合金熔化及凝固过程的模拟

借助镶嵌原子势(EAM)利用等压等温的分子动力学方法(NPT-MD)模拟共晶Cu40Ag60合金的熔化,凝固过程.利用径向分布函数和对分析技术研究Cu40Ag60台金在不同的冷却速率(1×1011,1×1012,5×1012,1×1013。及1×1014 K/s)下的非晶形成能力和微观结构演化.研究发现Cu40Ag60合金形成非晶态需要极大的冷却速率.模拟中的结构分析揭示了合金在冷却过程中微观结构的演化规律     

Deformation and fracture behaviors of Co-based metallic glass and its composite with dendrites

Microstructures and mechanical properties of Co-based metallic glass with nominal composition of Co₄₃Fe₂₀Ta_5.5B_31.5 (at.%) cast at the different cooling rates were investigated. When cooling rate is low enough, some dendritic crystalline phases were in situ precipitated from the glass matrix, forming the Co-based metallic glass composite with dendrites. Macroscopically, the fully amorphous samples often split apart or were broken into some particles, displaying a fragmentation failure mode. The size of particles became larger with the decrease in cooling rate. But, strength reduces slightly. Besides, the composites with dendrites show a small compressive plasticity, plus local melting behaviors with vein-like structure on the fracture surfaces. Furthermore, the effects of cooling rate on microstructures, deformation and fracture behaviors were discussed systemically. It is proposed that the fragmentation mechanism can be attributed to the inherent brittle character and high stress concentrations around the free volume. And the local melting behavior is due to the more receiving elastic energy and local shearing.     

Formation and crystallization of bulk Pd_(82)Si_(18) amorphous alloys

1　INTRODUCTIONMetallicglassisregardedasastatethatisofdis orderunlikecrystalalloyswith periodicatomstruc ture.Soitshowsexcellentcapabilitiesofsoftmag netism ,mechanics ,corrosionresistance ,etc .How ever ,mostofamorphousalloyswereproducedbyus ingrapidsolidificationmethodssuchassplatquench ing ,meltspinning ,andsoon ,withcharacteristiccoolingratesof 10 4 10 6 K/s .Becauseamorphousal loysarepreparedwithsilk ,powderandribbon ,itisgreatlylimitedinengineeringapplication .Recently ,severalbulk…     

Temperature dependence of the crystal–melt interfacial energy of metals

A model to express the dependence of the crystal–melt interfacial energy on the temperature for metals is proposed. The crystal–melt interfacial energies, the homogeneous nucleation undercoolings and the critical cooling rates to form ideal metallic glasses of silver, copper and nickel have been predicted according to the present model and simulated by the molecular dynamics method. The results show that the crystal–melt interfacial energy of metals increases nonlinearly with temperature. Over a wide temperature range from the melting point to the glass transition temperature the predicted results for the crystal–melt interfacial energy, the homogeneous nucleation undercooling and the critical cooling rate to form ideal metallic glasses from the present crystal–melt interfacial energy model are in good agreement with the experimental results reported, as well as the results of molecular dynamics simulations based on different EAM potentials of the metals.     

Simulation study on non-linear effects of initial melt temperatures on microstructures during solidification process of liquid Mg7Zn3 alloy

The non-linear effects of different initial melt temperatures on the microstructure evolution during the solidification process of liquid Mg₇Zn₃ alloys were investigated by molecular dynamics simulation. The microstructure transformation mechanisms were analyzed by several methods. The system was found to be solidified into amorphous structures from different initial melt temperatures at the same cooling rate of 10 × 10¹² K/s, and the 1551 bond-type and the icosahedron basic cluster (12 0 12 0) played a key role in the microstructure transition. Different initial melt temperatures had significant effects on the final microstructures. These effects only can be clearly observed below the glass transition temperature T_g; and these effects are non-linearly related to the initial melt temperatures, and fluctuated in a certain range. However, the changes of the average atomic energy of the systems are still linearly related with the initial melt temperatures, namely, the higher the initial melt temperature is, the more stable the amorphous structure is and the stronger the glass forming ability will be.     

Cu64Zr36合金非晶剪切带形成的分子动力学模拟

用分子动力学(MD)模拟方法研究在轴向压缩下,冷却速率、应变率、环境温度、裂缝对Cu64Zr36二元非晶合金力学性能的影响。在模拟中,采用EAM势函数表述原子间的相互作用。计算结果表明:非晶的弹性模量和抗压强度都比晶体试样大一倍多,而当应变≥15%时两种试样的流动应力几乎相等;冷却速率缓慢得到的非晶试样由于原子发生重组变疏松,产生剪切带,而冷却速率较快得到的试样则没有发生重组;试样的弹性模量、抗压强度和流动应力对应变率变化很不敏感;随着环境温度的升高,流动应力、抗压强度和弹性模量降低;有初始裂纹的试样剪切带集中,从裂纹尖端开始,与加载方向呈45o方向扩展。     

Bulk amorphous metal—An emerging engineering material

During the last two decades, researchers have developed families of metal alloys that exhibit exceptional resistance to crystallization in the undercooled liquid state. Upon cooling, these alloys readily form glass or vitrify to form bulk amorphous alloys or bulk metallic glasses. The stability of the undercooled molten alloys with respect to crystallization has enabled studies of liquid thermodynamics, rheology, atomic diffusion, and the glass transition previously not possible in metallic systems. Bulk amorphous alloys exhibit very high strength, specific strength, and elastic strain limit, along with unusual combinations of other engineering properties. These factors, taken together, suggest that bulk amorphous metals will become widely used engineering materials during the next decade.     

Influence of the molten quenching temperature on the thermal physical behavior of quenched Zr-based metallic glasses

Thermo-physical behavior of some Zr-based metallic glasses prepared by different molten quenching temperatures was studied by Differential scanning calorimetry (DSC) measurements. The characteristic thermo-physical properties are normally used for evaluating the glass-forming ability (GFA) of metallic glasses. Our results show that the glass transition temperature, crystallization temperature and supercooled liquid region of these metallic glasses increased with increasing the molten quenching temperature. Their glass-forming abilities were discussed in terms of the GFA criterion γ and the reduced glass transition temperature, T_rg, using these thermo-physical properties.     

保存环境对不同结构水淬渣活性的影响

采用将矿渣在不同温度重新熔融并水淬的方法制备了不同水淬渣样品，应用XRD、DTA等分析手段对不同样品的结构特征进行了分析，同时结合胶凝材料抗压强度实验，研究了不同水淬样品在潮湿环境保存7d后的胶凝活性变化特点及原因。研究表明，形成过程对水淬渣在不同保存环境下胶凝活性的影响不同，较低水淬温度下形成水淬渣的胶凝活性在潮湿环境中下降最大，样品玻璃相中存在不同程度的分相结构是产生这一影响的主要原因。     

Ti–Cu–Ni shape memory bulk metallic glass composites

New Ti–Cu–Ni shape memory bulk metallic glass composites were obtained by carefully controlling the cooling rate upon quenching. This allows for the formation of a metastable microstructure consisting mainly of ductile, spherical martensitic Ti(Ni,Cu) precipitates embedded in an amorphous matrix also containing a small volume fraction of TiCu and Ti₂Cu precipitates. These composites exhibit large ductility and high strength combined with a strong work-hardening behaviour. A deformation mechanism is proposed with the help of experimental observations and finite-element simulation. The simulation results demonstrate that stress concentrations occur around the precipitates, which promotes a heterogeneous stress distribution and the formation of multiple shear bands. Additionally, different transformation temperatures were observed for martensitic precipitates depending on whether they are completely or partially embedded in the amorphous matrix.     

液态Ca70Mg30合金快速凝固过程中纳米团簇结构形成演变特性模拟

采用分子动力学方法对液态Ca70Mg30合金快速凝固过程中纳米团簇结构的形成和演变特性进行模拟。采用原子团类型指数法(CTIM)对凝固过程中纳米团簇结构的演变进行分析。结果表明：系统在5×1011 K/s的冷速条件下形成以1551、1541和1431为主的非晶态结构,非晶转变温度约为530 K;(120120)二十面体基本原子团对系统非晶结构的形成起到决定性的作用,并且原子半径较小的Mg原子更容易占据二十面体基本原子团中心原子的位置;同时,纳米团簇主要是通过中等尺寸团簇的合并而形成,纳米级大团簇的形成演变过程呈现出类似于非晶晶化过程的3个阶段式的变化。     

Atomic simulation of amorphization and crystallization of Ag50Au50 alloy during rapid solidification

By means of constant temperature and constant pressure molecular dynamic simulation technique,a series of simulations of the glass transition and crystallization processes of Ag50 Au50 were performd.The atoms interact via EAM potential function.Pair correlation functions of liquid Ag50 Au50 during different cooling rates and temperatures were simulated to reveal the structural features of liquid,super-cooled liquid,glass state and crystal.The thermodynamics and kinetics of structure transition of Ag50Au50 during cooling processes were performed.     

冲击波淬火条件下凝固的锆基合金

利用二级轻气炮驱动飞片高速撞击产生的高温高压对ZrTiCuNiBe合金试样进行冲击波淬火,制备了Zr41Ti14Cu12.5Ni10Be22.5大块金属玻璃。由于冲击波对合金熔体的复杂效应,冲击波淬火制备的试样具有与水淬试样不同的结构、热稳定性和物性参数。与水淬试样相比,冲击波淬火的试样具有较高的玻璃转变温度和晶化温度,较低的密度和较高的声速。冲击波淬火是一种极具前途的大块金属玻璃制备方法。     

Molecular dynamics simulation of amorphous segregation in Ag-Rh alloys

Molecular dynamics simulation was carried out to investigate the liquid and amorphous microstructures of binary Agx-Rh(100-x) (x = 25, 50, 75 in atom fraction) alloys. Segregation feature of homogeneous interatomic binding of Ag-Rh liquid was found and probed, which can be retained into amorphous solids upon rapid cooling. Homogeneous binding may occur when the difference in the elemental atomic sizes is less than 10%. The icosahedra in liquid before the formation of amorphous state exist in a stable state and the network formed by 1551-clusters in molten alloys would inhibit the crystallization and diffusion of atoms. A higher degree of 1551-clusters will be favorable to form metallic glasses.     

TiZrNiCuBe块体非晶合金激光焊接行为及温度场数值模拟研究

采用数值模拟研究了Ti40Zr25Ni3Cu12Be20块体非晶合金激光焊接过程中的温度场变化和冷却速度变化规律,分析了Ti基非晶合金在激光焊接过程中保持非晶态结构的机理。以高斯表面热源和圆柱体热源相结合的复合热源模型作为焊接热源,考察了不同焊接参数下Ti Zr Ni Cu Be非晶合金在激光焊接过程中的热历史,结合实验结果分别对接头焊缝区和热影响区显微组织变化进行了分析,分别从加热和冷却两阶段给出非晶合金激光焊接接头非晶相得以保持的理论解释。完全非晶态接头的显微硬度测试表明:热影响区体现出较母材以及焊缝区更高的显微硬度,从玻璃转变温度、冷却速度以及屈服强度三者之间关系的角度对该现象进行理论分析,并通过实验验证分析的准确性。     

Thermodynamic approach to quantitative assessment of propensity of metallic melts to amorphization

On thermodynamic grounds, it was found that key properties that control the capacity of molten metallic alloys for transition into an amorphous state are the excess (configurational) entropy and heat capacity of the liquid. Chemical short-range order in liquids exhibiting various ten-dencies to glass formation was analyzed on the basis of the associated solution theory and the results of detailed thermodynamic research on a wide set of alloys. An interrelation was established between the association, characteristics of molten alloys (viscosity η, activation energies of viscous flow, and crystallization) that determine the possibility of amorphization and characteristics of glassy state stability (glass transition point, Gibbs energy, and enthalpy of crystallization). It was demonstrated that the magnitude of the key functions is completely determined by the covalent constituent of chemical interaction between components and depends mainly on the entropy terms of association reactions. The prospects for developing the quantitative criteria of amorphization on the basis of the entropy of association was discussed. It was also shown that the suggested approach based on taking into accoun the specificity of chemical interaction between components can be useful for prediction of physical, chemical, and mechanical properties of solid amorphous metallic materials.     

液淬工艺和微量元素对非晶态Fe—B—Si合金热稳定性的影响

研究了液淬工艺参数和添加微量稀土元素Ｃｅ对非昌态Ｆｅ－Ｂ－Ｓｉ合金热稳定性的影响,结果表明,制备非晶条带时的液淬速率愈高,在低温退火时合金的结构弛豫过程愈难进行。而非晶晶化与液淬条件之间不存在直接的联系。Ｃｅ的适量添加,在氏温阶延迟了结构弛豫过程中非晶条带延－脆转变的发生;在高温阶段使合金开始晶化的温度降低,且民条带中Ｃｅ的含量成正比。     

A molecular dynamics study on the liquid-amorphous-crystalline transition in a Lennard-Jonesian FCC system: I. bulk crystal

Constant-pressure molecular dynamics simulations are carried out to study the liquid-amorphous-crystalline transition behavior in a model system composed of 500 Lennard-Jones particles under three-dimensional periodic boundary conditions. The critical quenching rate (CQR) for amorphization, i.e., the minimum rate above which no crystallization occurs on quenching, is found to be about 5.52 x 10¹² K/s for a one-component, unary system. For lower quenching rates, the amorphous phase transforms to a crystalline phase with 0.3~0.5 of the melting point as the transition temperature. A binary system with a misfit in atomic size shows a smaller CQR, while a system with a higher bond strength for solute atoms requires a greater CQR. A crystallization behavior of an amorphous phase on reheating is also studied. Its crystallization temperature depends on the history: the higher the quenching rate for amorphization, the higher the crystallization temperature.     

A molecular dynamics study on the liquid-amorphous-crystalline transition in a Lennard-Jonesian FCC system: II. Nanocrystalline plates

Constant-pressure molecular dynamics simulations are carried out to study the liquid-amorphous-crystalline transition behavior in a model system composed of 500 Lennard-Jones particles under three-dimensional periodic boundary conditions. The critical quenching rate (CQR) for amorphization, i.e., the minimum rate above which no crystallization occurs on quenching, is found to be about 5.52 x 10¹² K/s for a one-component, unary system. For lower quenching rates, the amorphous phase transforms to a crystalline phase with 0.3~0.5 of the melting point as the transition temperature. A binary system with a misfit in atomic size shows a smaller CQR, while a system with a higher bond strength for solute atoms requires a greater CQR. A crystallization behavior of an amorphous phase on reheating is also studied. Its crystallization temperature depends on the history: the higher the quenching rate for amorphization, the higher the crystallization temperature.