液态金属浸渗动力学模型及其应用

从流体动力学出发导出了液态金属浸渗纤维预制件的动力学模型.实验研究了碳纤维增强铝基复合材料的浸渗过程,测定了浸渗速度和浸渗系数.结果表明:实验数据与浸渗模型吻合;还发现了两种不同的浸渗方式,均匀浸渗和非均匀浸渗;并提出一种复合材料制造新工艺.用此工艺在不润湿的情况下,在低压力下制出高质量的C/Al复合材料。     

MECHANISM AND KINETICS OF TRANSFORMATION OF ALUMINA INCLUSIONS BY CALCIUM TREATMENT

The basic mechanism and kinetics of the transformation process of alumina inclusions in steel was reported when calcium introduced into the steel by wire feeding orpowder injection. To clarify the mechanisms model, experiments were performed by studying reactions between AltOs and CaO in a laboratory furnace and by performing calcium treatments in an 8kg induction furnace for Al deoxidised melt. The phases formed during the reaction between Al2O3 and CaO were examined by SEM-EDS （scanning electron microscope-energy dispersive spectrometer）, and the reaction sequence of Al2O3=〉 CA6=〉 CA2 =〉 CA =〉 CAx（l） was discussed in term of the experimental observations. The kinetics of the reaction of calcium with alumina inclusions were simulated by immersing alumina plates in a Ca treated steel melt in the induction furnace. Results were compared with observations of real inclusion transformation. A kinetic model was proposed based on the results.     

Infiltration kinetics of pressureless infiltration in SiCp／Al composites

The pressureless infiltration kinetics was investigated by plotting the infiltration distance as function of the infiltration time. The effects of key process parameters such as time, temperature, Mg content on the pressureless infiltration of silicon carbide particle compacts were studied and quantified. The preform with high volume fraction SiC was obtained by mixing SiC particles with bimodal size distribution, whose diameters are 5 and 50 btm, respectively. The results show that an incubation period exists before infiltration, the influence of temperature on the incubation time exceeds that of Mg content, infiltration rate increases with the increasing temperature and Mg content, infiltration rate decreases as Mg consumes. A model of macroscopical infiltration and microscopical infiltration of liquid alloy in porous SiC preform was proposed.     

Liquid infiltrated powder interlayer bonding: a process for large gap joining

Abstract

The liquid infiltrated powder interlayer bonding (LIPIB) process is proposed to make large gap, transient liquid phase joints. Some basic aspects of the LIPIB process are discussed. In the LIPIB process, a joint with high mechanical properties requires a low volume fraction of porosity in the powder interlayer and low concentrations of melting point depressants in the joint. To achieve the fully infiltrated large gap joint, it is essential to maintain an open pore structure at the initial stage of infiltration, while allowing complete elimination of pores behind the infiltration front. Approximate analytical solutions are applied to describe dissolution of capillaries and isothermal diffusional solidification during infiltration. Calculations based on the Cu–Ag binary system show that infiltration kinetics are strongly affected by the geometry of the capillaries. When the effective capillary size is smaller than the critical effective capillary radius at a certain infiltration temperature, the isothermal solidification time will be less than the time required for full infiltration, leading to an incompletely infiltrated powder interlayer. Full infiltration of powder compacts requires tight control of the rearrangement rate of the particles such that a relatively large capillary radius can be maintained to prevent diffusional solidification before full infiltration. Meanwhile, the rearrangement should be able to close the pores behind the infiltration front. Small amounts of chromium or nickel powder addition into spherical copper powders were found to affect the rearrangement significantly, whereas little effect was observed for irregular copper powders. Copper–copper square joints with clearance up to 22 mm have been successfully made.     

Thermoelectric semiconductor iron disilicides produced by sintering elemental powders

Development of a sintering process to fabricate iron disilicides with a fine grain structure is pursued using elemental powders as starting materials with additions of Al. The Al additions are expected to involve liquid Al-rich phase during sintering to accelerate the reaction kinetics. At the same time, additions are made of Co and Cu, the former being an n-type dopant while the latter might promote metal-to-semiconductor transition upon annealing after sintering. The effects of such additions on the sintering kinetics, constituent phases in the products, and their thermoelectric properties are examined. It is shown that fabrication of sintered iron disilicides using elemental powders, heretofore believed to be difficult, becomes possible with Al additions. Then a mechanism of sintering in the Fe–Si–Al ternary system is proposed, and finally a series of demonstrations is given for the changes in thermoelectric properties depending upon the doping element used. It becomes evident that the thermoelectric figure of merit of the present materials is equivalent to that of the conventionally fabricated iron disilicides.     

低浸渗压力制备纤维增强铝基复合材料

利用液态浸渗技术制备了氧化铝纤维增强铝基复合材料。研究结果表明,在低压下使液态合金浸渗纤维预制件制备铝基复合材料是可行的。在浸渗过程中,液态合金的温度对浸渗压力有较大影响。所制备的复合材料具有均匀的显微组织,基体中的共晶组织可依附在纤维表面形核生长。     

高拉速板坯连铸结晶器液态渣消耗机理分析

基于弯月面渣道压力计算,分析2.0 m/min高拉速下板坯连铸结晶器振动周期内初凝坯壳变形行为,提出新的液态渣消耗机制,并定义了渣耗时间和渣耗强度的概念,阐明了结晶器非正弦振动参数对液态渣渣耗量影响规律.研究表明:渣道宽度变化引起渣道压力改变导致液态渣周期性持续消耗,振动正滑脱末期至负滑脱末期渣道负压抽吸液态渣进入渣道;降低振频延长了液态渣渣耗时间,但使渣耗强度削弱;提高振幅可加大渣耗强度,对渣耗时间影响很小;非正弦振动因子对渣耗强度影响较小,渣耗时间随非正弦振动因子减小而增大.     

Infiltration mechanism and factors influencing carbon/carbon–Zr–Ti–C composites prepared by liquid metal infiltration

The effects of fibre architecture, reaction temperature and holding time on the infiltration performance of carbon/carbon (C/C)–Zr–Ti–C composites prepared by liquid metal infiltration were investigated. The results indicated that samples with a chopped-web needled preform and low initial density had a high final density. Increasing the reaction temperatures resulted in a decrease of the final density of samples. Additionally, increasing the initial holding time appeared to obviously result in a high final density, but its effectiveness was not obvious in later observations. An analysis of the infiltration kinetics and mechanisms indicated that the diffusivity of carbon in the carbide, the open-pore sizes and their distribution in C/C composites were the essential characteristics that controlled the height of infiltrating melts.     

Zr_(55)Al_(10)Ni_5Cu_(30)大块非晶合金在过冷液相区内虚拟应力模型的改进

针对描述非晶合金在过冷液相区流动行为的虚拟应力模型虽然能定性分析应力-应变关系,但定量计算误差很大的问题,以Zr55Al10Ni5Cu30为例,采用MATLAB遗传算法优化弹性模量、最大应力和最大松弛时间,提高了模型对应力峰值及稳态应力值的计算精度;提出时间调整因子概念,可提高对应力变化历史描述的准确度;改进后的虚拟应力模型与Zr55Al10Ni5Cu30大块非晶合金单轴压缩实验的结果吻合较好。     

Phase-field simulation of solidification dendritic segregation in Ti-45Al alloy

The microstructures and mechanical properties of Ti Al alloys are directly linked to micro-segregation which cannot be avoided during solidification. So a thorough understanding of the micro-segregation should be a great help to further enhance the mechanical properties of the cast products. Theoretical analysis and experiments have been used to predict the micro-segregation, but it is very difficult to observe and determine the dendritic segregation in the micro region. Phase-field method has been employed for the simulation of dendritic growth. However, due to the complicated quasi-sub regular solution model for Ti-45Al(at.%) alloy, the classic phase-field models have difficulty to deal with the free energy. In this work, a phase-field model by linking thermodynamic calculation was used to simulate solidification dendritic segregation of Ti-45 Al alloy for Liquid→Liquid+β(Ti). The free energies of solid phase and liquid phase for Ti-45 Al alloy were calculated by Thermo-Calc and then coupled with the phase-field equations. The simulation results show the dendritic morphology and Al content variations between liquid and growing solid phase for Ti-45 Al alloy. With the growth of the β(Ti), dendritic segregation is formed in the liquid and solid phases due to the solute partitioning and rejection into the liquid. As a result, the dendrite arms are depleted of Al element, while the inter-dendrites are enriched. The dendritic tip growth velocity decreases with the progress of solidification, whereas the segregation ratio increases.     

Particle weakening in superplastic SiC/2124 Al composites at high temperature

High-strain-rate superplastic behavior of powder-metallurgy-processed 2124 Al matrix alloy and 10%, 20% and 30% SiC particulate reinforced 2124 Al composites were investigated over the temperature range from 370°C to 565°C, and the strain rate range from 10⁻⁴/s to 1/s. The true activation energy for the plastic flow after threshold stress compensation was close to that for lattice diffusion in aluminum for the 2124 Al alloy, while the activation energies for the 2124 composites were considerably higher than those for the unreinforced alloy, increasing with an increase in the volume fraction of SiC. The strength of the 2124 Al composites is lower than the strength of the 2124 Al alloy at high temperatures. The strength differential between the unreinforced and reinforced 2124 Al alloys is a function of temperature and is seen to decrease systematically with decrease in temperature and virtually vanishes at 460°C. Particle weakening is discussed in the light of load transfer effect, interphase diffusion, dissolution of second phase particles into matrix and the presence of liquid phase. It is proposed that interphase weakening, possibly with some liquid formation, is the principal factor contributing to the results obtained. Interphase and boundary sliding is believed to be the rate-controlling process in plastic flow of the SiC/2124 Al composites.     

Impurity effects on the intergranular liquid bismuth penetration in polycrystalline nickel

The effects of impurities on the intergranular penetration of Bi based liquids in polycrystalline Ni at 700 °C were systematically investigated. In comparison with a nominally pure Ni (99.9945%), the presence of a total amount of <0.5 at.% impurities of Mn, Fe and Si in the Ni increased the penetration length by six times when a near-equilibrium Bi-Ni liquid was applied; when an initially pure Bi liquid was applied, this increment further enlarged to ∼20 times in the initial penetration stage. In a second set of controlled experiments, the addition of Mn, Sn and Fe to the liquid Bi-Ni all enhanced the intergranular penetration, but produced different kinetics and morphologies. We extended a concept that was initially proposed in the Rice-Wang model for grain boundary embrittlement to explain our observations of the impurity-enhanced intergranular penetration based on a theory that segregation of an impurity could reduce the grain boundary energy more rapidly than the solid-liquid interfacial energy. Correspondingly, a new analytical model for the effect of adding a third impurity on changing the equilibrium dihedral angle and the associated intergranular penetration kinetics has been derived for the dilute solution limit. Furthermore, we demonstrated that the interplay of bulk phase equilibria, interfacial segregation, transport (dissolution, precipitation and diffusion) processes and stress generation could effectively explain a variety of different intergranular penetration behaviors and morphologies that have been observed in the experiments conducted using 10 different combinations of the solid and liquid metals. The framework for understanding the impurity effects on intergranular liquid penetration developed in this study can be applied to other materials systems. This study has practically importance for understanding and controlling liquid metal corrosion and embrittlement.     

A mathematical model coupled to CALPHAD to predict precipitation kinetics for multicomponent aluminum alloys

A mathematical model was developed to simulate the precipitation kinetics during heat treatment of multicomponent aluminum alloys. The model is based on the general numerical framework proposed by Kampmann and Wagner, and features a full coupling with CALPHAD software for the evaluation of the Gibbs–Thomson effect. It also does not rely on the assumption that precipitate phase composition is stoichiometric or uniform, and is therefore applicable for predicting complex precipitation kinetics encountered in industrial practices. Applications of the model to various aging treatments of binary Al–Sc alloys and a ternary Al–Sc–Zr alloy were conducted. It was found that the model predictions for extended time coarsening kinetics are in good agreement with the analytical Lifshitz–Slyozov–Wagner coarsening theory. Its ability to reproduce the complex precipitation pathways in multicomponent alloys was demonstrated by simulation of the precipitation kinetics for an Al–0.09 at.% Sc–0.03 at.% Zr alloy. Comparison of the simulation results with experimental measurement has also highlighted research directions that require further effort.     

SIMULATION OF LIQUID ALUMINIUM INFILTRATION IN POROUS MEDIUM

1.IntroductionTheinfiltrationofliquidaluminuminporousmediumistheoneofthekeysubjectsinresearchesonthefabricationofMMCsandporousmetalinrecentyears.Theprocessparametersofinfiltrationarecertainlyrelatedwiththequalityofthefinalproducts.Whenfabricatingporousaluminumalloybymeansofpressureinfiltration,thefirststepistomakeaporouspreformwithrounddissolvableormeltableparticles.Thentheliquidaluminumpenetratesintothepreformundercertainconditions.Whentheparticlesareremoved,theporousaluminumalloyisobtained…     

Al2O3sf.SiCp／LY12复合材料液态下浸渗与塑性成形的研究

采用液态浸渗和渗后直接挤压的方法,成功地制备出Al2O3sf·SiCp／LY12非连续混杂增强金属基复合材料。其增强物分布均匀,基体组织致密,与基体LY12相比具有较高的弹性模量、屈服强度、抗拉强度,并且延伸率也保持较高的水平。同液态浸渗制备的复合材料相比,各项力学指标均有提高。     

Thermodynamic evaluation of the Al-H system

The thermodynamic properties of the Al-H system were assessed using models for the Gibbs energy of the individual phases, including the metastable hydride AlH₃ phase. The model parameters were obtained through optimization by best fitting to selected experimental data. Particular attention was paid to hydrogen solubility in liquid and face-centered-cubic (fcc) Al. It was shown that the hydrogen can be treated as an ideal gas under normal conditions. The hydrogen solubility in liquid and fcc Al can be described very well with a regular solution model for liquid and fcc. The present calculations show satisfactory agreement with most experimental data for hydrogen solubility in fcc Al and selected data for hydrogen solubility in liquid Al, qualifying the extension of this binary model to higher-order Al-H-bearing systems.     

Zr_(55)Cu_(30)Ni_5Al_(10)大块金属玻璃的等时和等温晶化(英文)

采用差示扫描量热仪(DSC)和X射线衍射仪(XRD)研究Zr55Cu30Ni5Al10大块金属玻璃的非等温晶化转变动力学和在过冷液相区的等温晶化动力学行为。在非等温过程中,采用不同方法(Kissinger,Flynn-Wall-Ozawa和Augis-Bennett)得到的Zr55Cu30Ni5Al10大块金属玻璃平均激活能彼此之间吻合很好。此外,采用Johnson-Mehl-Avrami(JMA)模型描述Zr55Cu30Ni5Al10大块金属玻璃的等温转变动力学。研究结果表明:Zr55Cu30Ni5Al10大块金属玻璃的Avrami指数n介于2.2和2.9之间,表明其晶化机制主要是扩散控制过程。在等温晶化的过程中,晶核长大主要是三维的长程有序扩散控制的过程,平均激活能为469kJ/mol。     

B4C/Al复合材料力学性能及其断裂机理的研究

对无压浸渗法制备的B4C/Al复合材料进行了力学性能测试。结果表明,B4C/Al复合材料的抗弯强度和断裂韧性与单一B4C材料相比有显著提高。B4C/Al复合材料的抗弯强度及断裂韧性分别比单一B4C提高了18.39%和75.27%,但其硬度降低。B4C/Al复合材料经扫描电镜和背散射仪分析后发现,无压浸渗法制备的B4C/Al复合材料中没有大尺寸的显微缺陷,组织分布比较均匀、致密;B4C以连续的骨架结构存在,而渗入的铝相也以连续基体的形式存在;单一B4C存在较多的穿晶断裂,而B4C/Al复合材料的断裂方式主要以沿晶断裂为主,这是B4C/Al复合材料断裂韧性提高的主要原因。     

DETERMINATION OF THRESHOLD PRESSURE FOR INFILTRATION OF LIQUID ALUMINIUM INTO SHORT ALUMINA FIBER PREFORM

ＤＥＴＥＲＭＩＮＡＴＩＯＮＯＦＴＨＲＥＳＨＯＬＤＰＲＥＳＳＵＲＥＦＯＲＩＮＦＩＬＴＲＡＴＩＯＮＯＦＬＩＱＵＩＤＡＬＵＭＩＮＩＵＭＩＮＴＯＳＨＯＲＴＡＬＵＭＩＮＡＦＩＢＥＲＰＲＥＦＯＲＭ￥ＨｕＬｉａｎｘｉ；ＬｕｏＳｈｏｕｊｉｎｇ；ＨｕｏＷｅｎｃａｎ（Ｓ...     

Solidification structures of Ti–Al–Cr alloys

Phase transformations in the ternary Ti–Al–Cr alloy system have been studied by combining preliminary phase equilibria calculations and microstructural studies of a number of model alloys. The results have contributed to a better understanding of phase equilibria in the Ti–Al–Cr alloy system above 1273 K. A liquid surface projection has been tentatively proposed. Micro-twins have been observed in the monolithic γ phase within a B2 matrix. This supports a previously proposed mechanism for the formation of such a structure in a B2 matrix. The results also suggest that there is no representative orientation relationship between γ and the Ti(Cr,Al)₂ Laves phase. The L1₂ τ phase can be in direct equilibrium with the liquid phase. The ω phase stability has also been studied. The stability of the ω phase is attributed to the electron density of the prior B2 phase. This leads to changes in the effective heat of formation of the ω structure, as concluded from total energy LMTO calculations.