Tailoring interfacial shear strength of SiC fiber-reinforced brittle matrix composites

The interfacial shear strength of Nicalon SiC fiber-reinforced glass-ceramic matrix composites was aimed to be tailored via two methods: (1) varying of the thickness of the carbon-rich interfacial layer between the fiber and the matrix by controlling hot pressing period and (2) formation of the secondary interfacial layer, TaC, at the carbon/matrix boundary by doping the Ta₂O₅ matrix addition. In the series of composites with varying carbon-rich layer thickness, fiber/matrix debonding mostly occurred at the carbon/matrix boundary and hence the increase in the carbon-rich layer thickness did not cause any apparent changes in the interfacial shear strength. In the TaC formed series of composites, the interfacial shear strength was affected considerably by the presence of the TaC phase at carbon/matrix boundary. The Ta₂O₅ addition to control the quantity of the TaC phase has shown to be a useful method to tailor the interfacial shear strength of SiC fiber/glass-ceramic composites.     

Thermomechanical behavior of TiNi shape memory alloy fiber reinforced 6061 aluminum matrix composite

The processing and thermomechanical behaviors of TiNi shape memory alloy (SMA) fiber-reinforced 6061 Al matrix smart composites are investigated experimentally and analytically. Optimum processing conditions of hot pressing temperature and pressure are identified. Composite yield stresses are observed to increase with an increase in the volume fraction of TiNi fiber and prestrain given to the composites. An analytical model for thermomechanical behavior of the composites is developed by utilizing an exponential type of SMA constitutive model. The model predicts an increase in the composite yield stress with an increase in prestrain. It is found that the key parameters affecting the composite yield stress are the fiber volume fraction, prestrain, and matrix heat treatment. The predictions are in a reasonably good agreement with the experimental results.     

Thermomechanical behavior of TiNi shape memory alloy fiber reinforced 6061 aluminum matrix composite

The processing and thermomechanical behaviors of TiNi shape memory alloy (SMA) fiber-reinforced 6061 Al matrix smart composites are investigated experimentally and analytically. Optimum processing conditions of hot pressing temperature and pressure are identified. Composite yield stresses are observed to increase with an increase in the volume fraction of TiNi fiber and prestrain given to the composites. An analytical model for thermomechanical behavior of the composites is developed by utilizing an exponential type of SMA constitutive model. The model predicts an increase in the composite yield stress with an increase in prestrain. It is found that the key parameters affecting the composite yield stress are the fiber volume fraction, prestrain, and matrix heat treatment. The predictions are in a reasonably good agreement with the experimental results.     

Analysis of shear stress distribution in pushout process of fiber-reinforced ceramics

The interfacial shear stress distribution of a thin specimen of SiC fiber-reinforced glass matrix composite (fiber volume fraction of 0.1, 0.5 and 0.7) during a fiber pushout process was subjected to finite element analysis using a three concentric axisymmetrical model which consisted of fiber, matrix, and composite. A stress criterion was used to determine interface debonding. Effects of thermally-induced stress and a post debond sliding process at the interface were also included in the analysis. The analytical result showed that shear stress near the specimen surface was introduced during the specimen preparation process. Before the interfacial debonding, the distribution of shear stress during the pushout test was affected by the existence of thermally-induced stress in the specimen. The interfacial shear debonding initiated ≈ 30 μm below the pushing surface and the sliding at the debonded interface proceeded in the direction of both the pushing surface and back surface from the peak shear position; the debonding from the back surface initiated just before the complete debonding of the interface. The pushout load-displacement curve near the origin was straight, however, after the existence of interface sliding at the debonded interface, the curve exhibited non-linearity with the increase in applied load up to the complete debonding at the interface. This debonding process was essentially independent of the fiber volume fraction. The results indicate that the total of thermally-induced stress in the specimen and shear stress distribution generated by applied load are important for the initiation of debonding and the frictional sliding process of the thin specimen pushout test.     

采用SECM分析7075铝合金的局部腐蚀行为

使用0.6 mol/LNaCl溶液腐蚀7075铝合金, 采用扫描电化学显微镜进行逼近曲线、面扫描测试, 用X射线衍射仪对合金的形貌进行分析, 用能谱仪分析腐蚀产物成分, 研究7075铝合金局部腐蚀电化学机理.结果显示:逼近曲线呈现正反馈, 探针的电流随着与合金基底距离的减小变大; 合金表面的活性点因为氯离子活化作用不断增加, 形成大范围的点蚀; 氯离子通过吸附在合金表面钝化层并与之反应, 破坏钝化层使得内部裸露, 内部的第2相电极电位更负, 和铝基体构成腐蚀微电池, 第2相的阳极腐蚀溶解降低合金的强度和抗腐蚀能力.      

SiC颗粒增强铝基复合材料的研究进展

简要介绍了SiC颗粒增强铝基复合材料的优点及几种制备方法,包括搅拌法、浸渗法、喷射法、粉末冶金法和固液分离法;并对其后热变形加工参数对复合材料的性能影响进行了论述;最后,展望了粉末冶金法制备铝基复合材料的发展前景。     

Influence of fabrication technique on the fiber pushout behavior in a sapphire-reinforced NiAl matrix composite

Directional solidification (DS) of “powder-cloth” (PC) processed sapphire-NiAl composites was carried out to examine the influence of fabrication technique on the fiber-matrix interfacial shear strength, measured using a fiber-pushout technique. The DS process replaced the fine, equiaxed NiAl grain structure of the PC composites with an oriented grain structure comprised of large columnar NiAl grains aligned parallel to the fiber axis, with fibers either completely engulfed within the NiAl grains or anchored at one to three grain boundaries. The load-displacement behavior during the pushout test exhibited an initial “pseudoelastic” response, followed by an “inelastic” response, and finally a “frictional” sliding response. The fiber-matrix interfacial shear strength and the fracture behavior during fiber pushout were investigated using an interrupted pushout test and fractography, as functions of specimen thickness (240 to 730 μm) and fabrication technique. The composites fabricated using the PC and the DS techniques had different matrix and interface structures and appreciably different interfacial shear strengths. In the DS composites, where the fiber-matrix interfaces were identical for all the fibers, the interfacial debond shear stresses were larger for the fibers embedded completely within the NiAl grains and smaller for the fibers anchored at a few grain boundaries. The matrix grain boundaries coincident on sapphire fibers were observed to be the preferred sites for crack formation and propagation. While the frictional sliding stress appeared to be independent of the fabrication technique, the interfacial debond shear stresses were larger for the DS composites compared to the PC composites. The study highlights the potential of the DS technique to grow single-crystal NiAl matrix composites reinforced with sapphire fibers, with fiber-matrix interfacial shear strength appreciably greater than that attainable by the current solid-state fabrication techniques.     

Strength of Al-Zn-Mg-Cu matrix composite reinforced with SiC particles

The AA7075 alloys reinforced with SiC and without SiC particles were fabricated by a pressureless infiltration method, and then, their tensile properties and microstructures were analyzed. The spontaneous infiltration of molten metal at 800 °C for 1 hour under a nitrogen atmosphere made it possible to fabricate 7075 Al matrix composite reinforced with SiC, as well as a control 7075 Al without SiC. A significant strengthening even in the control alloy occurred due to the formation of in-situ AlN particle even without an addition of SiC particles. Composite reinforced with SiC particles exhibited higher strength values than the control alloy in all aging conditions (underaged (UA), peak-aged (PA), and overaged (OA)), as well as a solution treated condition. Spontaneous infiltration was further prompted owing to the combined effect of both Mg and Zn. This may lead to an enhancement of wetting between the molten alloy and the reinforcement. Consequently, strength improvement in a composite may be attributed to good bond strength via enhancement of wetting. The grain size of the control alloy is greatly decreased to about 2.5 μm compared to 10 μm for the commercial alloy. In addition, the grain size in the composite is further decreased to about 2 μm. These grain refinements contributed to strengthening of the control alloy and the composite.     

Influences of matrix ductility,interfacial bonding strength,and fiber volume fraction on tensile strength of unidirectional metal matrix composite

A trial to predict the influences of ductility of matrix, interfacial bonding strength, and volume fraction of fiber on the tensile strength of unidirectional metal matrix composites was attempted by means of a Monte Carlo computer simulation method. The main results are summarized as follows. (1) The strength of strongly bonded composites increased with increasing ductility of matrix and then remained nearly constant. (2) When the matrix was ductile, the strength of composite increased with increasing interfacial bonding strength and then remained nearly constant. When the matrix was not ductile, the strength increased but then decreased with interfacial bonding strength. In this case, there was an optimum bonding strength, for which the strength of composite was highest. (3) Concerning the strength of composite as a function of volume fraction of fiber, there arose the case where it is approximately described by the rule of mixtures and also the case where it is not described by this rule, depending on the ductility of matrix, interfacial bonding strength, and scatter of strength of fiber.     

碳纤维增强铝基复合材料的研究现状

碳纤维增强铝基复合材料同时具备了增强材料和金属材料的优良特性,具有高强度、高模量、高耐磨性等特征,并且可以在导热、导电和高温下提供高强度、高弹性系数和高尺寸强度,在航空航天、汽车等行业的应用方面表现出巨大的发展空间.介绍了几种制备碳纤维增强铝基复合材料方法,从制备工艺、微观组织、力学性能等方面评述了制备的关键问题和研究...     

钢纤维对摩擦材料性能的影响

树脂基复合材料性能优异,被大量用做汽车制动材料.以酚醛树脂为基体,钢纤维为增强纤维,添加填料,采用热压法制成刹车片,并进行摩擦试验.通过试验研究了酚醛树脂基复合材料中钢纤维不同添加量(质量分数)对复合材料冲击强度、摩擦性能的影响.结果表明:该配方类型中,在100～300℃温度范围内,材料冲击强度先增大后减小,钢纤维质量分数为24%时为转折点;材料的摩擦系数先增大后减小,钢纤维质量分数为26%时为转折点;磨损率随钢纤维添加量的增加而增大,钢纤维最佳添加量为24%～26%.     

Creep rupture of a silicon carbide reinforced aluminum composite

The microstructure, texture, and whisker orientations in 6061 Al-20 wt pct SiC whisker composites have been examined using transmission electron microscopy and X-ray diffraction. Tension creep tests of the composite material have also been conducted in the temperature range 505 to 644 K (450 to 700 F). The steady state creep rate of the composite depends strongly on the temperature and applied stress. The stress exponent for the steady state creep rate of the composite is approximately 20.5 and remains essentially constant within the range of test temperatures. The activation energy is calculated to be 390 kJ/mol, nearly three times as high as the activation energy for self-diffusion of aluminum. No threshold stress was observed. Fracture surface examination using scanning electron microscopy shows that the composite fails by coalescence of voids in the aluminum matrix which originate at the aluminum-SiC interface. It is demonstrated that SiC paniculate composites are less creep resistant than SiC whisker composites.     

Measurement of interfacial debonding and sliding resistance in fiber reinforced intermetallics

A technique is described for measuring fiber debonding and sliding characteristics in intermetallic matrix composites. The method involves pulling a single fiber that protrudes from the composite, while measuring the applied force and relative displacements of the fiber and matrix. High resolution displacement mapping methods are used to obtain data during initial loading, where sliding progresses only partly along the fiber. Measurements from a Ti₃Al/SiC composite are interpreted using the analysis of the companion paper. The results indicate a strong effect of constraint on the sliding response and provide quantitative measurements of sliding resistance, residual stresses, and debond energy. Preliminary measurements of changes in sliding resistance with cyclic loading are presented.     

Degradation mechanism of SiC/Super α 2 composite due to interfacial reaction

The degradation mechanism of SiC(SCS-6)/Super α ₂ composite due to the interfacial reaction was studied using single-fiber composite specimens fabricated by the sputtering method, heat treated at 1273 K for various times, and tensile tested at room temperature. The main results are summarized as follows. (1) The tensile strength was reduced with progress of interfacial reaction by the formed defects on the fiber surface, while the formation of the reaction layers in the matrix side was not the direct reason for the reduction. (2) From the fracture mechanical analysis of the experimentally observed relation of the size and shape of the surface defects to the fiber strength, the fracture toughness of the fiber employed in the present work was estimated to be 2 to 4 MPa ρm. (3) The change in distribution of strength of the reacted fiber with progressing reaction was simulated successfully by combining the Monte Carlo method with the Weibull distribution function for the strength of the unreacted fiber, the Gumbell distribution function for the maximum effective size of the surface defect of the reacted fiber, and the fracture mechanics.