Phase-field modeling of bimodal particle size distributions during continuous cooling

Microstructures in Nickel-base alloys typically contain a two-phase mixture of γ/γ′. The microstructure having a bimodal size distribution of γ′ is of particular interest because it has important property consequences [1]. In this paper, the phase-field method with an explicit nucleation algorithm is employed to investigate the microstructural development during a continuous cooling with various cooling rates. It is demonstrated that bimodal particle size distributions can be achieved at an intermediate cooling rate due to a coupling between diffusion and undercooling, in which the system experiences two peaks of well-isolated nucleation events. It is suggested that this is caused by soft impingement, followed by a renewal of driving force for nucleation, followed by a subsequent soft impingement. Under very high cooling rates, the microstructure becomes unimodal, because undercooling always outruns diffusion and the microstructure never reaches soft impingement.     

Computer simulation of the two-body abrasion process modeling the particle as a paraboloid of revolution

Two-body abrasive wear is a process with strong stochastic characteristic. Abrasive particle geometry, distribution and worn surface morphology can only be statistically determined and analytical models therefore always cause large inaccuracy. In this research, the earlier model of a particle as pyramid with a hemispherical tip has been replaced by a paraboloid model of revolution. In the pyramid model the normal load cannot be large enough to penetrate beyond the height of the hemisphere. Generally, in practice the hemisphere tip is quite small, and it readily penetrates into the surface. A new particle model has, therefore, been devised to extend the normal load range. New contact equations are proposed for the particle geometry used in the present model. The Monte Carlo method and finite element methods (FEMs) have also been combined to calculate the wear rate of the material during simulation. It is found that the linear wear rate increases continuously during the running-in process and reaches a constant value after some travel distance. Computed roughness and worn surface morphologies are in agreement with the experimental data. Finally, a comparison between simulated and experimented wear rates has also made. Both data matched very well.     

非晶合金的晶化动力学与初生相的内在联系

通过差示扫描量热法(DSC)研究了Zr60Al15Ni25非晶合金的晶化动力学。X射线衍射(XRD)和能谱(EDS)分析结果表明:Zr60Al15Ni25非晶合金晶化过程中的初生相为复杂三元化合物Al2NiZr6和AlNi4Zr5。非晶合金的有效晶化激活能反映了晶化初生相与非晶相间的结构差异,二者结构差异越大,有效晶化激活能就越高,Zr60Al15Ni25非晶合金的有效晶化激活能高达345 kJ/mol;当合金高温熔体快冷过程中的初生相与非晶合金的晶化初生相一致时,晶化动力学参数能够实际反映合金的玻璃形成能力,相反则不能。     

热处理条件对FeZrB基非晶合金微观结构及磁性能的影响

采用单辊快淬法制备一系列不同名义成分的FeZrB合金样品,并在第一个晶化峰值温度进行退火。利用同步热分析仪(STA)、X射线衍射仪(XRD)、透射电镜(TEM)和振动样品磁强计(VSM)测试分析合金的热曲线、微观结构和磁性能。初始晶化相随着FeZrB系列合金成分比例的改变而不同。在不同成分比例的合金中观察到四组不同的初始晶化相,例如α-Fe, α-Fe+Fe23B6型, α-Fe+α-Mn型和α-Fe+Fe2B+ZrB。通过TEM观察发现具有不同初始晶化产物的合金具有不同的形貌。具有不同初始晶化产物合金的饱和磁化强度(Ms)和矫顽力(Hc)存在以下关系：Ms(α-Fe)>Ms(α-Fe+α-Mn type)>Ms(α-Fe+Fe2B+ZrB)>Ms(α-Fe+Fe23B6-type), Hc (α-Fe+α-Mn type)>Hc(α-Fe+Fe2B+ZrB)>Hc(α-Fe+Fe23B6-type)>Hc(α-Fe).     

Effect of particle characteristics on deformation of particle reinforced metal matrix composites

The particle characteristics of 15% SiC particles reinforced metal matrix composites (MMC) made by powder metallurgy route were studied by using a statistical method. In the analysis, the approach for estimation of the characteristics of particles was presented. The study was carried out by using the mathematic software MATLAB to calculate the area and perimeter of each particle, in which the image processing technique was employed. Based on the calculations, the sizes and shape factors of each particle were investigated respectively. Additionally, the finite element model (FEM) was established on the basis of the actual microstructure. The contour plots of von Mises effective stress and strain in matrix and particles were presented in calculations for considering the influence of microstructure on the deformation behavior of MMC. Moreover, the contour maps of the maximum stress of particles and the maximum plastic strain of matrix in the vicinity of particles were introduced respectively.     

Non-conventional machining of particle reinforced metal matrix composite

Particle Reinforced Metal Matrix Composites (PRMMC's) have proved to be extremely difficult to machine using conventional manufacturing processes due to heavy tool wear caused by the presence of the hard reinforcement. This paper presents details and results of an investigation into the machinability of SiC particle reinforced aluminium matrix composites using non-conventional machining processes such as Electro Discharge Machining (EDM), laser cutting and Abrasive Water Jet (AWJ). The surface integrity of the composite material for these different machining processes are examined and compared. The influence of the ceramic particle reinforcement on the machining process was analysed by tests performed on samples of the non-reinforced matrix material.     

TiC颗粒增强钨基复合材料的烧蚀性能

用自制的氧乙炔烧蚀装置对ＴｉＣ颗粒增强钨基复合材料的烧蚀性能进行了测试,同时用多波长比色高温计对烧蚀试样表面温度和用热电偶对试样背面温度进行了在线监测。复合材料的质量烧蚀率和和线烧蚀率由低到高的排列顺序为：Ｗ〈３０％ＴｉＣｐ／Ｗ〈４０％ＴｉＣｐ＝Ｗ。ＴｉＣ颗粒加入到Ｗ中可明显提高材料的抗烧蚀性能,而且ＴｉＣ颗粒含量越高,材料的抗烧性能越好。ＴｉＣｐ／Ｗ复合材料的烧机理是Ｗ和ＴｉＣ的氧化烧和燃气流的     

Computer modeling of chemical vapor deposition processes

A general purpose computer model for describing the transport phenomena and resulting rate of deposition has been described. Partial differential equations describing the conservation of mass, momentum, energy, and chemical species are solved by a computer program employing the finite difference method. The system considered in this paper is deposition of silicon in a vertical stagnation flow reactor by the reaction of silicon tetrachloride and hydrogen. The program allows for multiple chemical species and natural convection effects. Predicted silicon deposition rates along the substrate are in reasonable agreement with experimental values available in the literature. The effect of gas inlet configuration on the uniformity of deposition has been studied. The model can be used as a tool for design optimization of such reactors.     

Structural formation of high-speed steels during crystallization

Conclusions The L+ peritectic transformation in tungsten-molybdenum high-speed steels goes to completion prior to the start of the eutectic reaction (L+K), which may precede segregation of austenite from the liquid (with a high carbon content). In this case, the eutectic is crystallized on the base of the metastable M₂C carbide. Conditions for the formation of thermodynamically more stable MC and M₆C carbides are created during slow cooling characteristic for the central zones and upper horizons of an ingot. Eutectics based on these carbides begin to crystallize at comparatively high temperatures, when the peritectic transformation has yet to go to completion. In a certain temperature interval, these transformations occur simultaneously; in this case, they can be suppressed by the eutectic crystallization owing to the characteristic features of the peritectic transformation. As a result, -ferrite (the products of its decomposition) is present in the steel.The mechanism responsible for the peritectic transformation in tungsten-molybdenum highspeed steels is characterized by the penetration of austenite into the depths of the -crystals during steady contact between the liquid and both the austenite, and the -phase, and by the absence of solid-phase pericrystallization. Diffusion processes are carried out via a network of channels connecting the thin liquid interlayer at the -transformation front with the interdendrite melt. This dictates the high rate of the peritectic transformation, and also the possibility of the suppression of its eutectic crystallization of the liquid in the channels.Decreases in the macro-, micro-, and submicroheterogeneity and increases in the technological plasticity can be attained with this effect on crystallization, which prevents overlapping of the regions of the peritectic and eutectic transformations. The amount of residual -ferrite and skeletal eutectic, and the chemical heterogeneity of the austenite should be reduced in this case.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 11, pp. 23–30, November, 1982.     

TiC颗粒增强钛基复合材料的静动态力学性能

利用伺服式疲劳实验机和杆一杆型冲击拉伸实验机对TiC颗粒增强钛基复合材料TP650和基体钛合金的静动态力学性能进行研究,得到不同应变率下复合材料的应力一应变曲线.结果表明,复合材料和基体材料的屈服应力均随应变率的增加而提高,属于应变率敏感材料;TP650的破坏形式以颗粒附近基体的撕裂以及颗粒与基体合金的脱粘为主,几乎没有发生颗粒破碎现象.假设复合材料的微观结构为非均质单胞在空间的周期性重复排列,利用有限元软件对钛基复合材料的静动态力学性能进行数值模拟研究,计算结果与实验结果吻合良好.进一步通过数值模拟预测了颗粒形状和颗粒体积分数的变化对TiC颗粒增强钛基复合材料静动态力学性能的影响.     

外加颗粒增强表层复合材料制备方法

按照采用热源和陶瓷增强颗粒的添加方式,对外加颗粒增强表层复合材料的制备方法分类.详细介绍了堆焊、激光熔覆、激光熔射、等离子熔化-注射等颗粒增强表层复合材料的制备方法,并分析了各种制备技术的优缺点.堆焊特点是基体与表层为冶金结合,效率高.激光熔覆可以实现输入的准确控制,冷却速度快,热畸变小.但是堆焊和激光熔覆过程都存在裂纹问题.激光熔射不受基体可焊性限制,可制备颗粒增强相连续分布的表层,避免裂纹的形成.等离子熔化-注射技术与激光熔射技术类似,可以制备出增强相体积分数从0～100%连续变化的梯度复合材料.避免由于增强颗粒分布不均引起的裂纹,实现低投入、低成本运行.     

Microstructure and hardness of WC-Co particle reinforced iron matrix surface composite简

In this study, a high Cr cast iron surface composite material reinforced with WC-Co particles 2-6 mm in size was prepared using a pressureless sand mold infiltration casting technique. The composition, microstructure and hardness were determined by means of energy dispersive spectrometry （EDS）, electron probe microanalysis （EPMA）, scanning electron microscope （SEM） and Rockwell hardness measurements. It is determined that the obtained composite layer is about 15 mm thick with a WC-Co particle volumetric fraction of -38%. During solidification, interface reaction takes place between WC-Co particles and high chromium cast iron. Melting and dissolving of prefabricated particles are also found, suggesting that local Co melting and diffusion play an important role in promoting interface metallurgical bonding. The composite layer is composed of ferrite and a series of carbides, such as （Cr, W, Fe）23C6, WC, W2C, M6C and M12C. The inhomogeneous hardness in the obtained composite material shows a gradient decrease from the particle reinforced metal matrix composite layer to the matrix layer. The maximum hardness of 86.3 HRA （69.5 HRC） is obtained on the particle reinforced surface, strongly indicating that the composite can be used as wear resistant material.     

颗粒几何特征对SiC颗粒增强Al基复合材料力学行为的影响

利用有限元方法建立二维模型分析了不同外力下SiC颗粒形状及其尺寸对Al基复合材料力学行为的影响.结果表明,颗粒形状对材料的应力和应变分布的影响很大,颗粒尖角附近的应力和外力方向上基体相应位置的应变均有严重的集中现象.随颗粒角度的减小和外力的增大,颗粒的应力和基体的应变均很快增大.颗粒尺寸较小时,对颗粒的应力的影响小.     

SiC颗粒增强Al基复合材料及其性能研究

SiC颗粒的加入使SiC增强铝基复合材料拥有了优异的综合性能,从而成为具有广泛使用价值的先进复合材料。本文综述了SiC颗粒增强铝基复合材料的第二相特征及其对使用性能的影响规律。特别是对近年来倍受关注的SiC颗粒形状、尺寸、体积分数、颗粒分布和界面特征等对复合材料宏、微观性能的影响进行了详细论述。     

颗粒增强铝基梯度复合材料的摩擦磨损性能

摘要采用新型喷射沉积技术制备SiC体积分数呈连续分布(0～30%)的Al-Si基梯度复合材料,利用MG-2000型销-盘磨擦磨损试验机,研究不同滑动转速和载荷对该梯度复合材料摩擦磨损性能的影响.采用SEM和MHV-2000型维氏硬度计研究该梯度复合材料的显微组织、硬度及其耐磨性的梯度分布规律.结果表明随着滑动转速和载荷的增大,梯度材料的摩擦因数逐渐降低;材料的磨损率随载荷的增加而增大,随滑动转速的提高先增大后减小,在转速500 r/min时达到最大;对比研究沉积态与热压态材料的摩擦磨损行为,喷射沉积态由于孔隙等缺陷的存在,其磨损形式主要是磨粒磨损和剥层磨损;热压后,梯度材料的磨损形式以磨粒磨损和粘着磨损为主;随基体中SiC含量的逐渐增加,锭坯各部分硬度和耐磨性也随之提高.     

Multiscale modeling of failure in metal matrix composites

A multiscale approach to composite failure, in which detailed information on small-scale micromechanics is incorporated approximately yet accurately into larger-scale models capable of simulating extensive damage evolution and ultimate failure, is applied to the deformation and failure of a Ti–matrix composite. The composite is reinforced with SiC fibers under conditions of matrix yielding and interfacial sliding via Coulomb friction. Specifically, a fully three-dimensional finite element model is employed to investigate the load transfer from broken to unbroken fibers as a function of applied stress and interface friction coefficient. With a von Mises matrix yield criterion, constraint effects permit the matrix to carry some of the transferred load near the fiber break, a feature not captured in previous composite models. The single-break results for stress concentrations are then used as the discrete Green's functions for load transfer in the full composite, and the predicted load transfer around a seven-fiber-break cluster is shown in good agreement with finite element results. The Green's function model is then used to predict overall damage evolution and composite failure for an IMI-834 Ti/SCS-6 SiC system for various interface friction coefficients. The composite tensile strength is rather insensitive to the friction coefficient and, for values of μ comparable to those measured experimentally, the predicted tensile strength is in excellent agreement with the measured value. Analytic models for scaling of the tensile strength to very large sizes are then shown to agree well with strengths obtained from simulations. These results suggest that the hierarchical coupling approach used here may be useful for approaching a wide variety of damage and failure problems in fiber composites.     

Computer simulation of particle rearrangement in the presence of liquid

A model was developed to describe the motion of particles which are connected by liquid bridges. The model takes into account capillary force, particles inertia, viscous drag of liquid, and elastic collision. Below a critical thickness of the liquid film between particles, the liquid is assumed to be in a visco-elastic state and follows the Maxwell model. For a single pair of particles, the model predicts that, as viscosity increases, the first particle contact occurs later, and subsequent particle bounce is weaker. In the case of planar arrays of 5000 particles, the particles agglomerate, generating pores between particle clusters during the rearrangement. The final density obtainable by particle rearrangement increases as viscosity increases. This result is attributed to the suppression of fast local densification in the case of high viscosity. The implications and limitations of the present analysis are also presented and discussed.