SiCp/Al Composites Fabricated by Modified Squeeze Casting Technique

A cost effective method was introduced to fabricate pure aluminum matrix composites reinforced with 20% volume fraction of 3.5 μm SiC particles by squeeze casting followed by hot extrusion. In order to lower volume fraction of the composites, a mixed preform containing pure aluminum powder and the SiC particles was used. The suitable processing parameters for the infiltration of pure aluminum melt into the mixed preform are： melt temperature 800℃, preform temperature 500℃, infiltration pressure 5 MPa, and solidification pressure 50 MPa. Microstructure and properties of the composites in both as-cast and hot extruded states were investigated. The results indicate that hot extrusion can obviously improve the mechanical properties of the composite.     

Evolution of Thermoplastic Shear Localization and Related Microstructures in Al／SiCp Composites Under Dynamic Compression

The localized shear deformation in the 2024 and 2124 Al matrix composites reinforced with SiC particles was investigated with a split Hopkinson pressure bar (SHPB) at a strain rate of about 2.0×103 s-1. The results showed that the occurrence of localized shear deformation is sensitive to the size of SiC particles. It was found that the critical strain, at which the shear localization occurs, strongly depends on the size and volume fraction of SiC particles. The smaller the particle size, the lower the critical strain required for the shear localization. TEM examinations revealed that Al/SiCp interfaces are the main sources of dislocations. The dislocation density near the interface was found to be high and it decreases with the distance from the particles. The Al matrix in shear bands was highly deformed and severely elongated at low angle boundaries. The AI/SiCp interfaces, particularly the sharp corners of SiC particles, provide the sites for microcrack initiation. Eventual fracture is caused by the g     

Dependence of corrosion resistance on grain boundary characteristics in a high nitrogen CrMn austenitic stainless steel

Processing schedules for grain boundary engineering involving different types of cold deformation(tension, compression, and rolling) and annealing were designed and carried out for 18Mn18Cr0.6N high nitrogen austenitic stainless steel. The grain boundary characteristic distribution was obtained and characterized by electron backscatter diffraction(EBSD) analysis. The corrosion resistance of the specimens with different grain boundary characteristic distribution was examined by using potentiodynamic polarization test. The corrosion behavior of different types of boundaries after sensitization was also studied.The fraction of low-∑ boundaries decreased with increasing strain, and it was insensitive to the type of cold deformation when the engineering strain was lower than 20%. At the strain of 30%, the largest and smallest fractions of low-∑ boundaries were achieved in cold-tensioned and rolled specimens, respectively. The fraction of low-∑ boundaries increased exponentially with the increase of grain size. The proportion of low-∑ angle grain boundaries increased with decreasing grain size. Increasing the fraction of low-∑ boundaries could improve the pitting corrosion resistance for the steels with the same grain size.After sensitization, the relative corrosion resistances of low-∑ angle grain boundaries, ∑3 boundaries, and ∑9 boundaries were 100%, 95%, and 25%, respectively, while ∑27 boundaries, other low-∑ boundaries and random high-angle grain boundaries had no resistance to corrosion.     

Study on the Thermal Expansion and Thermal Cycling of AlNp／Al Composites

The AIN particle reinforced aluminum matrix composites with 50% volume fraction were fabricated by squeeze-casting technology.The thermal expansion behavior and its response to thermal cycling were studied between 20℃ and 400℃.Compared with four theoretical models,the measured CTEs of the composite lie within the elastic bounds derived by Schapery′s analysis .Schapery′s model and Kerner′s model agree well with the CTEs of the composites at lower temperature and elevated temperature,respectively.Strain hysteresis was observed between heating and cooling curves during cycling.This was attributed primarily to the anelastic behavior of the matrix induced by matrix residual stresses.     

Study on Interface between Sub—micron Particles and Matrix in Al2O3p／Al Composites

The microstructural characteristic of 1070Al matrix composites reinforced by 0.15μm Al2O3 particles whose volume fraction was 40% was investigated by TEM and HREM.The results showed that the interface between the matrix and reinforcements was clean and bonded well,without any interfacial reaction products.There were some preferential crystallographic orientation relationships between Al matrix and Al2O3 particle because of the lattice imperfection on the surface of Al2O3 particles.     

Aging Behavior of High Volume Fraction SiC Particles Reinforced 2024Al Composite

2024 Al matrix composite reinforced by SiC particles with 45% volume fraction and 1μm diameter was successfully fabricated by squeeze-exhaust casting method. The aging behavior of SiCp/2024AI composite at four temperatures was investigated and compared to 2024 alloy. It was found that the addition of high volume fraction SiC particles does not alter the aging sequence, but it significantly accelerates the kinetics of precipitation in the composite matrices. Therefore, the aging peak of the composite appears earlier than that of 2024AI alloy. This is attributed to the decrease in the activation energy for the precipitate formation and the increase in the precipitate growth rate due to the high density dislocations in the composite with high volume fraction particles. The high density dislocations, as preferential nucleation sites for precipitates, bring about the tiny and dense precipitates in the composite.     

Effect of Microstructures on Corrosion Behavior of Nickel Coatings:(Ⅱ) Competitive Effect of Grain Size and Twins Density on Corrosion Behavior

Nanocrystalline nickel coatings with grain size of 50 nm were annealed in vacuum at 200°C and 400°C for 10 min.Their microstructures were investigated by transmission electron microscopy(TEM).And their corrosion behaviors were studied by means of polarization and electrochemical impedance spectroscopy(EIS).The results showed that their grain size grew up to about 60 nm(200°C)and 500 nm(400°C),respectively.The specimen annealed at 200°C possessed higher density of twins in compared with the counterparts of as-deposited and annealed at 400°C.The normal grain size effect on the corrosion behavior was not observed.However,it was found that the corrosion resistance of the coating linearly changed with the density of twins.     

Optimization of Machining Characteristics for Al／SiCp Composites using ANN／GA

The present work is focused on optimization of machining characteristics of Al/SiCp composites.The machining characteristics such as specific energy,tool wear and surface roughness were studied.The parameters such as volume fraction of SiC,cutting speed and feed rate were considered.Artificial neural networks(NN) was used to train and simulate the experimental data.Genetic algorithms(GA) was interfaced with ANN to optimize the machining conditions for the desired machining characteristics .Validation of optimized results was also performed by confirmation experiments.     

Fabrication of barium-strontium aluminosilicate coatings on C/SiC composites via laser cladding

Barium-strontium aluminosilicate(BSAS) and Si/BSAS coatings were fabricated on the surface of C/SiC composites via a two-step laser cladding process. The microstructure, mechanical properties, and the water vapor corrosion behavior of the samples were investigated. The BSAS coating was found to be tightly bonded to the substrate and only a few pores and microcracks were observed. The introduction of a silicon middle layer was revealed to reduce thermal stress and promote the healing of defects formed during the laser cladding process. To evaluate the corrosion resistance, the BSAS and Si/BSAS-coated C/SiC composites were exposed to an atmosphere of 50% H_2O and 50% O_2 at 1250 °C. The resulting weight change and flexural strength were measured as a function of the corrosion time. The addition of the silicon middle layer below the BSAS top layer resulted in a better resistance to water vapor corrosion. Furthermore, the Si/BSAS-coated samples showed a lower weight loss and a smaller reduction in flexural strength than the BSAS-coated and the uncoated samples during water vapor corrosion. Thus, laser cladding is demonstrated to be an effective and feasible method to fabricate high-quality ceramic coatings on C/SiC composites. The introduction of a silicon middle layer can inhibit defect formation during the laser cladding process and protect the composite from water vapor corrosion.     

Effect of cooling rate on microstructure,microsegregation and mechanical properties of cast Ni-based superalloy K417G

The effects of different solidification rates after pouring on the microstructures,microsegregation and mechanical properties of cast superalloy K417 G were investigated.Scheil-model was applied to calculate the temperature range of solidification.The casting mould with different casting runners was designed to obtain three different cooling rates.The microstructures were observed and the microsegregation was investigated.Also,high temperature tensile test was performed at 900?C and stress rupture test was performed at 950?C with the stress of 235 MPa.The results showed that the secondary dendrite arm spacing,microsegregation,the size and volume fraction of γ'phase and the size of γ/γ'eutectic increased with decreasing cooling rate,but the volume fraction of γ/γ' eutectic decreased.In the cooling rate range of 1.42?C s~(-1)–0.84?C s~(-1),the cast micro-porosities and carbides varied little,while the volume fraction and size of phase and γ/γ' eutectic played a decisive role on mechanical properties.The specimen with the slowest cooling rate of 0.84?C s~(-1) showed the best comprehensive mechanical properties.     

碳化硅颗粒增强7A04铝基复合材料的腐蚀行为

碳化硅颗粒增强7A04(SiCp/7A04)铝基复合材料的应用环境及其腐蚀状况都较复杂.过去,对它的研究方法多样,依据不同,因而其腐蚀结论有异,甚至矛盾.为了研究SiCp/Al复合材料的耐蚀性,用电化学法和失重法研究了SiC颗粒含量和粒度对SiCp/7A04铝基复合材料及基体合金耐腐蚀性能的影响.结果表明,与基体合金相比,SiCp/7A04铝基复合材料耐蚀性下降,SiC含量高的复合材料腐蚀较快,SiC颗粒尺寸越大,复合材料耐蚀性越好;采用扫描电镜(SEM)观察腐蚀后的微观形貌表明,SiC颗粒破坏了基体表面氧化膜的完整性,促进了点蚀的形成,但其自身的稳定性又阻碍了蚀孔的长大.     

Pitting behavior of SiCp/2024 Al metal matrix composites

The effects of the volume fraction of SiC particulate reinforcements and the concentration of chloride ions in solution on the localized corrosion characteristics of SiCp/2024 Al metal matrix composites (MMC) were investigated. A scanning micro reference electrode (SMRE) technique was employed to study the dynamic process of pitting initiation and development on the surface of the composites at open-circuit potential. Potentiodynamic polarizations were performed to characterize the electrochemical behavior of the MMCs. The morphology of the localized attack on the MMC sample after corrosion tests were examined by scanning electron microscopy (SEM). The results of electrochemical measurement showed that the composites were less resistant to pit initiation than the corresponding unreinforced metrix alloy. Increase in the volume fraction of SiCp reinforcement in the SiCp/2024 Al composites resulted in a significant decrease of pitting potential. In situ potential mapping of active centers on the surfaces of the composites revealed that local breakdown of passivity and initiation of micro pitting corrosion could take place even at an open-circuit potential which was more negative than the pitting potential, and the number of active centers on the surfaces of the composites increased as the volume fraction of SiC particulates in MMCs increased. Micro-structural analysis indicated that pitting attack on the composites mainly occurred at SiCp-Al interfaces or inclusions-Al interfaces.     

SiC颗粒增强镁基复合材料的制备与组织性能研究

目的 研究SiC体积分数对复合材料显微形貌、物相组成、拉伸性能、磨损性能和腐蚀性能的影响。方法 采用粉末冶金法制备SiC_p/AS81复合材料,采用相关仪器设备对不同SiC体积分数的SiC_p/AS81复合材料的显微形貌进行观察,对物相组成和组织性能进行分析。结果 随着SiC体积分数的增加,SiC增强镁基复合材料的平均晶粒尺寸逐渐减小。0.25%(体积分数,下同)–SiC_p/AS81和0.50%–SiC_p/AS81复合材料中的Si元素分布较为均匀,而在1.0%–SiC_p/AS81复合材料中可见较多的Si元素团聚。随着SiC体积分数的增加,复合材料的抗拉强度、屈服强度和断后伸长率先增大后减小,均在SiC体积分数为0.50%时取得最大值。在5～30 N载荷下,0.50%–SiC_p/AS81复合材料的耐磨性能要优于AS81复合材料的耐磨性能。复合材料按耐腐蚀性能从高至低顺序排列如下：0.50%–SiC_p/AS81>0.25%–SiC     

颗粒尺寸对颗粒增强型金属基复合材料动态特性的影响

利用有限元模型分析了颗粒增强型金属基复合材料 ( PMMCs ) Al/SiC的颗粒尺寸对复合材料在不同应变率下的动态特性的影响。采用有限元三维立方体单胞模型嵌入单个和多个球形增强颗粒,颗粒直径分别为16 μ m和7.5 μ m,多颗粒模型内部颗粒随机分布。基体材料假设为弹塑性,应变强化及应变率强化均符合指数规律。模拟结果表明:颗粒尺寸、颗粒体积含量及应变率对金属基复合材料的动态特性的影响是相互耦合的。颗粒体积含量一定时,颗粒尺寸越小,复合材料流动应力越高;颗粒含量越高,材料流动应力越高;应变率越高,材料流动应力越高。      

增强颗粒含量和尺寸对SiCp/Fe复合材料性能的影响

利用电流直加热动态热压烧结工艺,分别制备了增强颗粒体积含量从5%到15%,尺寸从3 μm到45 μm的SiCp/Fe复合材料,研究了粒子含量与尺寸对复合材料硬度、强度、延伸率和耐磨性能的影响.研究表明:增强颗粒的体积含量从5%提高到10%,可以明显提高材料的性能;随着增强颗粒含量进一步提高,颗粒团聚将导致材料性能降低;...     

Effect of the SiC particle size on the dry sliding wear behavior of SiC and SiC–Gr-reinforced Al6061 composites

The effect of size of silicon carbide particles on the dry sliding wear properties of composites with three different sized SiC particles (19, 93, and 146 μm) has been studied. Wear behavior of Al6061/10 vol% SiC and Al6061/10 vol% SiC/5 vol% graphite composites processed by in situ powder metallurgy technique has been investigated using a pin-on-disk wear tester. The debris and wear surfaces of samples were identified using SEM. It was found that the porosity content and hardness of Al/10SiC composites decreased by 5 vol% graphite addition. The increased SiC particle size reduced the porosity, hardness, volume loss, and coefficient of friction of both types of composites. Moreover, the hybrid composites exhibited lower coefficient of friction and wear rates. The wear mechanism changed from mostly adhesive and micro-cutting in the Al/10SiC composite containing fine SiC particles to the prominently abrasive and delamination wear by increasing of SiC particle size. While the main wear mechanism for the unreinforced alloy was adhesive wear, all the hybrid composites were worn mainly by abrasion and delamination mechanisms.     

Main and interaction effects of matrix particle size,reinforcement particle size and volume fraction on wear characteristics of Al–SiCp composites using central composite design

The aim of this study was to investigate the effects of matrix particle size, reinforcement particle size, volume fraction, and their interactions on the wear characteristics of Al–SiC_p composites. Central composite design method was used to perform a series of experiments. The statistical analysis of experimental results showed that both main effect and interaction effect of factors investigated were effective on the wear behavior of Al–SiC_p composites. Wear loss decreased as volume fraction increased; however, beyond volume fraction of 17.5%, it increased due to reinforcement particle clustering depending on volume fraction and matrix particle size to reinforcement particle size ratio. With decreasing of matrix particle size and increasing of reinforcement particle size, wear loss also decreased. However, after a certain volume fraction, large sized reinforcement particles had a negative effect on the wear resistance.     

Production and properties of SiCp-reinforced aluminium alloy composites

A vacuum infiltration process was developed to produce aluminium alloy composites containing various volume fractions of ceramic particles. The matrix composites of aluminium with 9.42 wt%Si and 0.36 wt%Mg containing up to 55 vol% SiCp were successfully infiltrated and the effect of infiltration temperature and volume fraction of particle on infiltration behaviour was investigated. In addition to aluminium powder, magnesium was used to improve the wetting of SiC particles by the molten aluminium alloy. The infiltration rate increased with increasing infiltration time, temperature and volume fraction of particle, but full infiltration appeared at the optimum process parameters for the various volumes of fraction composite compacts. In addition, the microstructure, hardness, density, porosity and wear resistance of the composites were also examined. It is observed that the distribution of SiC particles was uniform. The hardness and density of the composite increased with increasing reinforcement volume fraction and porosity decreased with increasing particle content. Moreover, the wear rate of the composite increased with increasing load and decreased with increasing particle content.     

不同界面SiC纤维束复合材料的拉伸力学行为

利用国产三代SiC纤维通过化学气相渗透工艺(CVI)制备不同界面厚度和基体体积分数的SiC纤维束复合材料,并对其拉伸力学行为进行研究;同时,通过有限元方法研究界面厚度和基体体积分数对SiC纤维束复合材料热残余应力的影响。有限元分析结果表明:该纤维束复合材料的界面存在较为明显的径向和环向热残余应力,而且这两种应力均随着界面厚度增加而减小,随着基体体积分数的增加而增加。拉伸实验结果表明:随着界面厚度增加SiC纤维束复合材料的拉伸强度有增大趋势,且纤维拔出长度也相应增加;但在界面厚度相同的情况下,过高的基体体积分数将导致复合材料拉伸强度和韧性下降。     

SiCp(w)/Al复合材料的研究现状

概述了增强相SiC颗粒的尺寸、体积分数、以及热处理工艺等参数对SiCp/Al复合材料力学性能的影响,并系统阐述了SiCp(w)/Al复合材料的界面、腐蚀行为以及强化和断裂机理.