Mechanical behavior of cast particulate SiC/AI (A356) metal matrix composites

Mechanical tests were carried out to study the deformation behavior of particulate SiC-reinforced Al (A356) matrix composites produced through direct casting using the molten metal mixing method. The matrix alloy-Al (A356) was also tested as a control material for comparison. The elastic constant and yield strength of the composite material were found higher than those of the control alloy, but the ultimate tensile strength (UTS) and the ductility were lower. The Tsai-Halpin equation was found applicable for calculating the elastic constant if an average particle aspect ratio could be determined. The strain-hardening behavior of the tested composite material appeared very different from that of the control alloy. The high strain-hardening rate in the early stage of plastic deformation of the composite was rationalized by the interaction between the hard particles and the ductile metal matrix; on the other hand, the low hardening rate recorded from intermediate strain amplitude to fracture was attributed to the early coalescence of voids and other microdamages. Particle-matrix interface debonding, particle cracking, and void for-mation in the metal matrix were considered to be responsible for the low ductility. Deformation asymmetry of the composites was noticed, not only through the Bauschinger effect, but also through the difference in virgin specimens’ yield stresses in tension and compression.     

Determination of fracture toughness and bridging tractions from crack-opening displacement measurements in particulate composites of diamond in zinc sulfide

The fracture toughness of zinc sulfide ceramic and a series of ZnS diamond particle composites was studied through measurements of crack opening displacement profiles. Five composites were fabricated using weight fractions of 10, 20 and 30% of 0–1 μm diamond particles, and 10 and 20% of 1–3 μm particles in a ZnS matrix. The cracks were grown using a novel specimen geometry and a loading technique that permitted stable crack growth even in small specimens. The fracture toughness of each material was calculated on the basis of the displacement profiles and the material properties, as opposed to the applied loads and the specimen geometry. The pure zinc sulfide material exhibited nearly ideal brittle behavior, and the toughness measurements agreed closely with other methods. The greatest toughening occurred in the 1–3 μm particle size composites, in which weak bridging tractions (⩽ 100 MPa) over a short distance along the crack (50–100 μm) could explain the reduced displacements near the crack tip. Even smaller size bridging zones (5–10 μm) may have been present in the 0–1 μm particle size composites, but elastic shielding alone can explain the observed toughening. The specimen geometry used here permitted toughness measurements using small specimens (< 5 mm) but is limited to materials having bridging zones that are less than about 250 μm.     

SiC particle cracking in powder metallurgy processed aluminum matrix composite materials

Particle cracking is one of the key elements in the fracture process of particulate-reinforced metal-matrix composite (MMC) materials. The present study quantitatively examined the amount of new surface area created by particle cracking and the number fraction of cracked particles in a series of SiC-reinforced aluminum-matrix composite materials. These composite materials were fabricated by liquid-phase sintering and contained 9 vol pct of 23, 63, or 142 μm SiC. The matrix properties were varied by heat treating to either an underaged or peak-aged condition. In general, the new surface area created by particle cracking (S _v ) and the number fraction of cracked particles (F_no) were linearly dependent on the local strain along the tensile specimen. Multiple cracks were frequently observed in the composites containing large particles. It was found that the new surface area created by particle cracking per unit strain was higher for the case of high-strength matrices and was not systematically affected by particle size within the range studied. The number fraction of cracked particles was affected by both particle size and matrix strength. A higher number fraction of particles cracked in the composites reinforced with large particles and with high matrix yield strengths. These results are interpreted in terms of the size of the particle defects, which is a function of particle size, and the critical flaw size necessary to crack a given particle, which is a function of the stress on the particle. The new surface area created by cracking and the fraction of cracked particles were related and are in good agreement for the large and medium sized particles.     

Micromechanical modeling of reinforcement fracture in particle-reinforced metal-matrix composites

Finite element analyses of the effect of particle fracture on the tensile response of particle-reinforced metal-matrix composites are carried out. The analyses are based on two-dimensional plane strain and axisymmetric unit cell models. The reinforcement is characterized as an isotropic elastic solid and the ductile matrix as an isotropically hardening viscoplastic solid. The reinforcement and matrix properties are taken to be those of an Al-3.5 wt pet Cu alloy reinforced with SiC particles. An initial crack, perpendicular to the tensile axis, is assumed to be present in the particles. Both stationary and quasi-statically growing cracks are analyzed. Resistance to crack growth in its initial plane and along the particle-matrix interface is modeled using a cohesive surface constitutive relation that allows for decohesion. Variations of crack size, shape, spatial distribution, and volume fraction of the particles and of the material and cohesive properties are explored. Conditions governing the onset of cracking within the particle, the evolution of field quantities as the crack advances within the particle to the particle-matrix interface, and the dependence of overall tensile stress-strain response during continued crack advance are analyzed. Formerly Graduate Research Assistants, Brown University     

Effect of loading condition and stress state on damage evolution of silicon particles in an Al-Si-Mg-Base cast alloy

Damage evolution of Si particles in a Sr modified cast A356(T6) Al alloy is quantitatively characterized as a function of strain under tension, compression, and torsion. The fraction of damaged Si particles, their size distributions, and orientation distribution of particle cracks are measured by image analysis and stereological techniques. Silicon particle cracking and debonding are the predominant damage modes. Particle debonding is observed only under externally applied tensile loads, whereas particle cracking is observed under all loading conditions. The relative contributions of Si particle debonding and fracture to the total damage strongly depend on stress state and temperature. For all loading conditions and stress states studied, the average size of damaged Si particles is considerably larger than the bulk average size. The rate of damage accumulation is different for different loading conditions. At a given strain level, Si particle damage is lowest under compression and highest under torsion. The anisotropy of the damage is highly dependent on the deformation path and stress state. Under uniaxial tension, the cracks in the broken Si particles are mostly perpendicular to the loading direction, whereas in the compression test specimens they are parallel to the loading direction. The Si particle cracks in the torsion and notch-tension test specimens do not exhibit preferred orientations. The quantitative microstructural data are used to test damage evolution models.     

陶瓷颗粒增强扩散合金化钢复合材料的微观结构和力学性能

采用传统粉末冶金工艺制备了陶瓷颗粒增强Fe?0.5Mo?1.75Ni?1.5Cu?0.7C扩散合金化钢复合材料,选用的陶瓷颗粒为SiC、TiC和TiB₂。采用光学显微镜和扫描电子显微镜观察了烧结材料微观结构,并对烧结材料的硬度、强度和摩擦磨损性能进行了测试。结果表明,由于SiC和TiB₂与基体的化学相容性好,陶瓷颗粒与基体界面结合良好;由于TiC颗粒具有极高的化学稳定性,TiC颗粒与基体界面结合情况不理想。随着陶瓷相含量（质量分数）的增加,添加SiC和TiC的烧结试样相对密度降低;添加TiB₂的烧结试样相对密度先增加后降低,当添加TiB₂质量分数为0.9%时达到最大值。随着陶瓷含量增加,添加SiC和TiB₂烧结试样的硬度增大,当陶瓷相质量分数超过1.2%时,硬度增加缓慢;添加TiC烧结试样的硬度先增加后降低,当添加TiC质量分数为0.9%时达到最大值。随着陶瓷相含量增加,添加SiC和TiC烧结试样的强度降低,少量添加SiC对强度没有明显损害;添加TiB₂烧结试样的强度先增加后降低,当添加TiB₂质量分数为0.6%时达到最大值（971.7MPa）,比基体提高了14.1%以上。添加陶瓷相对烧结钢性能的积极影响依次是TiB₂、SiC和TiC。     

Microstructure effects on tensile properties of tungsten-Nickel-Iron composites

Controlled processing of heavy alloys containing 88 to 97 pct W resulted in high sintered densities and excellent bonding between the tungsten grains and matrix. For these alloys, deformation and fracture behavior were studiedvia slow strain rate tensile testing at room temperature. The flow stress increased and the fracture strain decreased with increasing tungsten content. The tradeoff between strength and ductility resulted in a maximum in the ultimate tensile strength at 93 pct W. Microstructure variations, notably grain size, explain sintering temperature and time effects on the properties. During tensile testing, cracks formed on the surface of the specimens at tungsten-tungsten grain boundaries. The crack density increased with plastic strain and tungsten content. The surface cracks, though initially blunted by the matrix, eventually increased in density until catastrophic failure occurred. An empirical failure criterion was developed relating fracture to a critical value of the surface crack tip separation distance. Application of the model explains the effects of microstructural variables on tensile properties. Formerly Graduate Research Assistant at Rensselaer Polytechnic Institute.     

Design of the microstructural scale for optimum toughening in metallic composites

The role of the microstructural scale in toughening of composites containing a metallic constituent is described by invoking a dislocation pile-up mechanism for debonding at metal ceramic (intermetallic) interfaces. Below a certain size of the metal phase, the pile-up length becomes subcritical precluding debonding. The toughening effect below and above this critical size separates into two kinds of behavior leading to a maximum toughness when plotted against the characteristic length of the metal phase. The position of this maximum defines the optimum microstructural scale. The concept is applied successfully to metal-ceramic and metal-intermetallic composites with a microstructural length scale of less than about 10 μm. Metal matrix composites where the metal forms a continuous network through the microstructure are shown to have much higher toughness than the case where the same volume of the metal phase is distributed as discrete particles in the composite.     

Fatigue crack growth and stress redistribution at interfaces

The role of the interface in redistributing stress around cracks in multilayered ceramic/metal composites is investigated. The emphasis is on the different effects of interfacial debonding or of plastic slip in the metal phase adjacent to strongly bonded interfaces. The experiments are conducted on alumina/aluminum multilayered composites. Monotonic loading precracked test pieces causes plastic shear deformation within the aluminum layer at the tip of the notch without debonding. However, interfacial debonding can be induced by cyclic loading, in accordance with a classical fatigue mechanism. Measurements of the stress around the crack demonstrate that debonding is much more effective than slip at reducing the stress ahead of the crack.     

Microscopic characteristics of fatigue crack propagation in aluminum alloy based particulate reinforced metal matrix composites

Microscopic characteristics of fatigue crack propagation in two aluminum alloy (A356 and 6061) based particulate reinforced metal matrix composites (MMCs) were investigated by carrying out three point bending fatigue tests. The impedance offered by the reinforcing particles against fatigue crack propagation has been studied by plotting the nominal and actual crack lengths vs number of cycles. Surface observation shows that fatigue cracks tend to develop along the particle-matrix interface. In the case of Al (A356) MMCs, stronger interaction of fatigue crack with Si particles, as compared to SiC particles, was evident. In both MMC materials, particle debonding was more prominent as compared to particle cracking. The attempted application of Davidson's model to calculate ΔK_th indicated that for cast MMCs the matrix grain including the surrounding reinforcing particles has to be considered as a large “hard particle”, and the grain boundary particles themselves behave like an hard “egg-shell” to strengthen the material.     

High-Temperature Fatigue of a Hybrid Aluminum Metal Matrix Composite

An aluminum metal matrix composite (MMC) brake drum was tested in fatigue at room temperature and extreme service temperatures. At room temperature, the hybrid composite did not fail and exceeded estimated vehicle service times. At higher temperatures (62 and 73 pct of the matrix eutectic), fatigue of a hybrid particle/fiber MMC exhibited failure consistent with matrix overloading. Overaging of the A356 matrix coupled with progressive fracture of the SiC particles combined to create the matrix overload condition. No evidence of macro-fatigue crack initiation or growth was observed, and the matrix–particle interface appeared strong with no debonding, visible matrix phases, or porosity. An effective medium model was constructed to test the hypothesis that matrix overloading was the probable failure mode. The measured particle fracture rate was fit using realistic values of the SiC Weibull strength and modulus, which in turn predicted cycles to failure within the range observed in fatigue testing.     

镀层对金刚石/玻璃复合材料性能的影响

为满足现代电子工业日益增长的散热需求,急需研究和开发新型高导热陶瓷（玻璃）基复合材料,而改善复合材料中增强相与基体的界面结合状况是提高复合材料热导率的重要途径.本文在对金刚石和镀Cr金刚石进行镀Cu和控制氧化的基础上,利用放电等离子烧结方法制备了不同的金刚石增强玻璃基复合材料,并观察了其微观形貌和界面结合状况,测定了复合材料的热导率.实验结果表明:复合材料中金刚石颗粒均匀分布于玻璃基体中,Cu/金刚石界面和Cr/Cu界面分别是两种复合材料中结合最弱的界面;复合材料的热导率随着金刚石体积分数的增加而增加;金刚石/玻璃复合材料的热导率随着镀Cu层厚度的增加而降低,由于镀Cr层实现了与金刚石的化学结合以及Cr在Cu层中的扩散,镀Cr金刚石/玻璃复合材料的热导率随着镀Cu层厚度的增加而增加.当金刚石粒径为100μm、体积分数为70%及镀Cu层厚度为约1.59μm时,复合材料的热导率最高达到约91.0 W·m-1·K-1.      

Processing and Mechanical Properties of Cast Al (Mg,Mn)-Al2O3 (MnO2) Composites Containing Nanoparticles and Larger Particles

The composites reinforced with nanoparticles result in improved strength and ductility while those containing coarser particles of micron size have limited ductility. The present study investigates the outcome of mechanical properties in a composite reinforced simultaneously with coarse and fine particles. High energy milling of manganese dioxide particles with excess of aluminum powder ensures that nanoparticles generated, either of MnO₂ or alumina, are mostly separate and surrounded by aluminum particles. The milled powder when added to aluminum alloy melt, the excess aluminum particles will melt leaving behind separate oxide nanoparticles without significant agglomeration. Different amounts of milled powder mix have been stirred into molten aluminum alloy where nanoparticles of MnO₂ react with melt to form alumina. The resulting slurry is cast into composites, which also contains coarser (nearly micron size) alumina particles formed by internal oxidation of the melt during processing. The microstructure of the composites shows good distribution of both the size categories of particles without significant clustering. The oxide particles are primarily γ-alumina in a matrix of aluminum-magnesium-manganese alloy containing some iron picked up from the stirrer. These composites fail during tensile test by ductile fracture due to debonding of coarser particles. The presence of nanoparticles along with coarser particles in a composite improves both strength and ductility considerably, presumably due to delay in debonding of coarser particles to higher stress because of reduced mismatch in extension caused by increased strain hardening in presence of nanoparticles in the matrix. The composites containing only coarser oxide particles show limited strength and ductility attributed to early debonding of particles at a relatively lower stress due to larger mismatch in extension between matrix and larger particles. Higher addition of powder mix beyond a limit, however, results in deterioration of mechanical properties, possibly due to clustering of nanoparticles. The present work, however, did not optimize the relative amounts of the different sized particles for achieving maximum ductility.     

Influence of neodymium addition on microstructure,tensile properties and fracture behavior of cast Al-Mg2Si metal matrix composite

The influence of Nd on the microstructures,tensile properties and fracture behavior of cast Al-18 wt.%Mg2Si in situ metal matrix composite was investigated.Experimental results showed that,after introducing a proper amount of Nd,both primary and eutectic Mg2Si in the Al-18 wt.%Mg2Si composite were well modified.The morphology of primary Mg2Si was changed from irregular or dendritic to polyhedral shape,and its average particle size was significantly decreased from 47.5 to 13.0 μm.Moreover,the morphology of the eutectic Mg2Si phase was altered from flake-like to a thin laminar,short fibrous or dot-like structure.Tensile tests revealed that Nd addition improved the tensile strength and ductility of the material.Compared with those of unmodified composite,the ultimate tensile strength and percentage elongation with 0.5% Nd were increased by 32.4% and 200%,respectively.At the same time,Nd addition changed the fracture behavior from brittle to ductile.     

On the particle-size dependence of fatigue-crack propagation thresholds in SiC-particulate-reinforced aluminum-alloy composites: Role of crack closure and crack trapping

A study has been made of ambient-temperature fatigue-crack propagation behavior in P/M Al-Zn-Mg-Cu metal-matrix composites reinforced with either 15 or 20 vol.% silicon-carbide particulate, with specific emphasis on the role of SiC-particle size on the fatigue-crack growth threshold condition. It is found that measured threshold stress-intensity levels ΔK_TH, are a function of both SiC-particle size and volume fraction; however, whereas coarse-particle distribution results in higher ΔK_TH values at low load ratios, fine particles give higher threshold at high load ratios. Such behavior is analyzed in terms of the interaction of SiC particles with the crack path, both in terms of the promotion of (roughness-induced) crack closure at low load ratios and by crack trapping by particles. Consideration of the latter mechanism yields a limiting requirement for the intrinsic threshold condition in these materials that the maximum plastic-zone size must exceed the effective mean particle size; this implies that for near-threshold crack advance, the tensile stress in the matrix must exceed the yield strength of the material beyond the particle.     

Effect of Cu infiltration on static and dynamic properties of PM steels

Abstract

Performance improvement that can be achieved by Cu infiltration is quantified in this paper. Tensile and fatigue properties of a Fe–2·0Cu–0·7C powder metallurgy steel were compared to the same alloy infiltrated with 8 wt-%Cu. Microstructural characterisation, using optical and electron microscopies, was carried out to understand the effect of Cu infiltration on mechanical properties. Cu infiltration improves the ultimate tensile strength by 40% by increasing the load bearing structure, decreasing the stress concentrations associated to open porosity and increasing the hardness of the steel matrix. Fractographic observations show the evidence of stress transmission from the sinternecks to the steel particles due to infiltrated Cu. The beneficial effect of Cu infiltration is less pronounced for the fatigue properties as the endurance limit is increased by 10%. This lower improvement is explained by crack initiating at the Cu/steel matrix interface.     

冶金炉窑微孔陶瓷管收集粉尘的特性及润湿性

为了研究有色冶金烟气中的高温熔融金属对微孔陶瓷管过滤性能及寿命的影响,采用静滴法进行熔融金属铋与微孔陶瓷基板之间的润湿性试验。采用金相显微镜、扫描电镜和EDS等手段分析了接触面的结构和成分,并对实际生产中微孔陶瓷管膜除尘系统收集的烟气粉尘进行了元素和粒径分析。结果表明,熔融金属铋与陶瓷基板之间黏结性较差,结合力较弱,容易进行反吹清洗,回收有价金属元素;实际生产收集到的烟气粉尘中主要含铅、砷、锌、铜、铁、钙等元素,其中铅含量最高,烟气粉尘粒径较小,生产稳定运行后粒度范围缩小至1～30 μm,小于PM2.5的颗粒占8%～12%。     

The mechanical behavior of a hybrid metal matrix composite

The mechanical behavior of a unidirectionally reinforced hybrid metal matrix composite in two different states has been studied: one with a “weakly bonded” interface and the other with a “strong” interface. Similarities and contrasts in mechanical behavior have been related to the properties of the interface. The longitudinal tensile strength and the crack growth initiation resistance are found to be insensitive to the condition of the interface. However, the material with the “weak” interface exhibits extensive debonding, resulting in a steeply increasing resistance curve and a large work of rupture. Furthermore, the weak interface reduces the transverse and torsional strength of the composite. This study illustrates how the tailoring of interfacial properties can improve the mechanical performance of composites for certain structural applications.     

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.     

生物医用Ti6Al4V合金粉末注射成形工艺研究

钛及钛合金具有高比强度、低弹性模量、优良的耐蚀性和绝佳的生物相容性,但较差的加工性能大大限制了其应用范围。钛及钛合金金属粉末注射成形工艺克服了机加工、模压等传统加工工艺的缺点,结合传统粉末冶金和注塑成型的优势,实现了结构复杂的钛及钛合金产品低成本、大批量近净成形,提高了材料利用率。本文利用水溶性黏结剂和粉末粒度为16 μm和22 μm的商用球形Ti6Al4V合金粉制备了注射料和相应的试样,通过实验确定了气氛热脱黏结合真空烧结的最佳工艺,基于该工艺制备得到了两种注射料的烧结试样。结果表明:粉末粒度为16 μm注射料烧结件杂质含量未能满足外科植入用金属注射成形Ti6Al4V组件标准;粉末粒度为22 μm注射料烧结件物理化学性能如下,极限拉伸强度880 MPa,屈服强度830 MPa,延伸率13.2%,相对密度96.8%,氧质量分数为0.195%,氮质量分数为0.020%,碳质量分数为0.022%,该试样整体性能满足外科植入用金属注射成形Ti6Al4V组件标准。