Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiC p composite

The effect of SiC volume fraction and particle size on the fatigue behavior of 2080 Al was investigated. Matrix microstructure in the composite and the unreinforced alloy was held relatively constant by the introduction of a deformation stage prior to aging. It was found that increasing volume fraction and decreasing particle size resulted in an increase in fatigue resistance. Mechanisms responsible for this behavior are described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion in the form of S’ precipitates, and the decrease in strain localization with decreasing reinforcement interparticle spacing as a result of reduced particle size. Microplasticity was also observed in the composite, in the form of stress-strain hysteresis loops, and is related to stress concentrations at the poles of the reinforcement. Finally, intermetallic inclusions in the matrix acted as fatigue crack initiation sites. The effect of inclusion size and location on fatigue life of the composites is discussed.     

Influence of reinforcement volume fraction and size on the microstructure and abrasion wear resistance of hot Isostatic pressed white iron matrix composites

The changes in the microstructure and wear resistance of a powder metallurgical high-Cr white iron after the incorporation of TiC particles were studied in the present work. Various reinforcement volume fractions and sizes were used in order to examine their influence on the three-body abrasion wear resistance. The experiments were carried out at three different austenitizing temperatures. The most important observation after a microstructural examination was the increased amount of martensite in the composites subjected to identical heat treatment procedures with the unreinforced alloy. The austenite-to-martensite transformation in the composites increased with the TiC volume fraction and with the austenitizing temperature. This indicates that the two parameters have a key role in the transformation mechanism, which seems to be mechanically induced. The increasing of martensitic transformation with the TiC content in the composites enhanced continuously the supporting ability of the iron alloy matrix to the TiC particles, which in turn increased the wear resistance of the composites. The abrasion wear resistance increased with the TiC volume fraction until the onset of spalling. However, in composites containing coarse reinforcements, spalling occurred earlier in the wear process. This decreased wear resistance significantly because spalled TiC particles contributed additionally to wear.     

The effect of matrix microstructure on the tensile and fatigue behavior of SiC particle-reinforced 2080 Al matrix composites

The effect of matrix microstructure on the stress-controlled fatigue behavior of a 2080 Al alloy reinforced with 30 pct SiC particles was investigated. A thermomechanical heat treatment (T8) produced a fine and homogeneous distribution of S′ precipitates, while a thermal heat treatment (T6) resulted in coarser and inhomogeneously distributed S′ precipitates. The cyclic and monotonic strength, as well as the cyclic stress-strain response, were found to be significantly affected by the microstructure of the matrix. Because of the finer and more-closely spaced precipitates, the composite given the T8 treatment exhibited higher yield strengths than the T6 materials. Despite its lower yield strength, the T6 matrix composite exhibited higher fatigue resistance than the T8 matrix composite. The cyclic deformation behavior of the composites is compared to monotonic deformation behavior and is explained in terms of microstructural instabilities that cause cyclic hardening or softening. The effect of precipitate spacing and size has a significant effect on fatigue behavior and is discussed. The interactive role of matrix strength and SiC reinforcement on stress within “rogue” inclusions was quantified using a finite-element analysis (FEA) unit-cell model.     

The influence of buoyant forces and volume fraction of particles on the particle pushing/entrapment transition during directional solidification of Al/SiC and Al/graphite composites

Directional solidification experiments in a Bridgman-type furnace were used to study particle behavior at the liquid/solid interface in aluminum metal matrix composites. Graphite or siliconcarbide particles were first dispersed in aluminum-base alloysvia a mechanically stirred vortex. Then, 100-mm-diameter and 120-mm-long samples were cast in steel dies and used for directional solidification. The processing variables controlled were the direction and velocity of solidification and the temperature gradient at the interface. The material variables monitored were the interface energy, the liquid/particle density difference, the particle/liquid thermal conductivity ratio, and the volume fraction of particles. These properties were changed by selecting combinations of particles (graphite or silicon carbide) and alloys (Al-Cu, Al-Mg, Al-Ni). A model which considers process thermodynamics, process kinetics (including the role of buoyant forces), and thermophysical properties was developed. Based on solidification direction and velocity, and on materials properties, four types of behavior were predicted. Sessile drop experiments were also used to determine some of the interface energies required in calculation with the proposed model. Experimental results compared favorably with model predictions. BRU K. DHINDAW Visiting Scholar with the Solidification Laboratory at the time this work was performed.     

Acoustic emission during fatigue crack propagation in SiC particle reinforced Al matrix composites

The acoustic emission (AE) behavior during fatigue propagation in aluminum 6061 and aluminum 6061 matrix composites containing 5, 10, and 20 wt pct SiC particle reinforcement was investigated under tension-tension fatigue loading. The purpose of this investigation was to monitor fatigue crack propagation by the AE technique and to identify the source(s) of AE. Most of the AEs detected were observed at the top of the load cycles. The cumulative number of AE events was found to correspond closely to the fatigue crack growth and to increase with increasing SiC content. Fractographic studies revealed an increasing number of fractured particles and to a lesser extent decohered particles on the fatigue fracture surface as the crack propagation rate(e.g., †K) or the SiC content was increased. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ ASM-MSD Composite Materials Committee.     

Effect of particle-size distribution on the properties of high-volume-fraction SiC p -Al-based composites

High-volume-fraction SiC-Al-based composites have been fabricated by squeeze casting. The effect of particle-size distribution and squeeze-cast parameters on the metal-matrix composites (MMCs) was investigated. The results showed that bulk density of the composites was 2.855 to 3.067 g/cm³ with the various component mixtures of SiC particulates, i.e., the SiC volume fraction was 51.6 to 74.4 pct. The young’s modulus of the composites was between 220 and 226 GPa. The maximum four-point bending strength and fracture toughness reached 478 MPa and 9.42 MPa(m)^−1/2, respectively. The coefficient of thermal expansion (CTE) of the composites was from 5 to 8 × 10⁻⁶/K, depending on the volume fraction of SiC.     

不同粒径的SiC体积配比对SiC_p/Al基复合材料显微组织和拉伸性能的影响

为了研究不同粒径的Si C体积配比对SiC_p/Al基复合材料显微组织及拉伸性能的影响,采用高压扭转法(High-pressure torsion,HPT)将3.5μm(小)、7.0μm(大)SiC颗粒体积比分别为4∶1、1∶1、1∶4的SiC颗粒和纯Al粉末混合物制备成10%SiC_p/Al基复合材料(体积分数)。用金相显微镜、万能试验机、扫描电镜等分析2种粒径的Si C体积比对SiC_p/Al基复合材料显微组织和拉伸性能的影响。结果表明,随扭转半径增大,各试样的SiC颗粒分布更加均匀,颗粒团聚、偏聚现象减少,其中小、大SiC颗粒体积比为1∶1的试样性能最优,伸长率、相对密度最高,分别达到14.3%和99.1%,拉伸断裂形式为塑性断裂。     

超细SiC增强纯铝基复合材料显微组织与力学性能的研究

采用平均粒径为800nm的超细SiC颗粒作为增强体,制备含SiC体积分数为15%的铝基复合材料,研究烧结温度和强压处理对复合材料微观组织和力学性能的影响。研究表明,提高烧结温度可有效加速复合材料的致密化,与520℃下烧结制备的复合材料相比,610℃下烧结制备的复合材料具有更高的密度和较低的孔隙度,从而具有更高的硬度。610℃下烧结制备的复合材料的硬度为83.9HBS,远高于520℃烧结制备的复合材料的硬度(53.7HBS)。这主要是由于烧结温度的提高可加速原子扩散,有利于Al粉之间以及Al粉与SiC颗粒之间的结合,并改善界面结合情况。研究还表明,强压处理可以有效提高复合材料的致密度和降低孔隙的体积分数,610℃下烧结制备的复合材料经强压处理以后的密度为2.68g/cm3,接近于理论密度(2.78g/cm3),且硬度可达121HBS,抗拉强度、屈服强度和伸长率分别可达177.6MPa、168.6MPa和3.97%。     

造孔剂含量对SiC/Al复合材料抗弯强度的影响

采用无压熔浸法制备SiC/Al复合材料,并利用颗粒堆积和毛细管力的静力学理论研究造孔剂含量对SiC/Al复合材料抗弯强度的影响.通过扫描电镜对试样的断口形貌进行分析,发现造孔剂含量为20%(质量分数)时,残余孔隙较小,而造孔剂含量为10%和15%时,残余孔隙较大.造孔剂含量对抗弯强度产生影响,随造孔剂含量增加,抗弯强度先增大后减小,造孔剂为20%时,抗弯强度出现最大值343.63 MPa.     

The fracture characteristics of Al-9Ti/SiC p metal matrix composites

A fracture mechanics approach was used to determine the plane strain fracture toughness (K _IC) of a mechanically alloyed Al-9Ti 20 vol pct cobalt sol-gel-coated SiC particle-reinforced composite. Processing defects consisting of clumped SiC particulate, bonded by the sol-gel, initiated failure in tensile tests. The defects were measured and the fracture toughness was calculated using the Irwin relation. The value ofK _IC for the as-received material was determined to be equal to 4.7 MPa·m^1/2 at room temperature. Annealing the material for 120 hours and 400 hours at 500 °C increased the fracture toughness. This can be attributed to coarsening of an Al₃Ti strengthening phase. Tensile tests conducted at 200 °C show thatK _IC decreases at that temperature for each annealing condition. The sensitivity to the presence of the defects is greatest for samples annealed at 500 °C for 120 hours. The effect of the defects on the failure mechanism of the composite material as a function of temperature was determined. At room temperature, the Co/SiC processing defects provide low-energy paths for crack propagation; at 500 °C, the defects serve as void nucleation sites.     

The interactive role of inclusions and SiC reinforcement on the high-cycle fatigue resistance of particle reinforced metal matrix composites

The effect of intermetallic inclusions on the fatigue crack initiation and growth in 2080 Al alloy and 2080/SiC _p composites was investigated. Using surface replication, it was determined that, in the high-cycle fatigue region, life is dominated by the initiation process. It was also determined that the majority of initiation sites were associated with intermetallic inclusions. While 2080/SiC/20 _p showed a definitive relationship between inclusion size and fatigue life, i.e., a higher inclusion size resulted in lower fatigue life, there was no correlation in 2080/SiC/30 _p . This was attributed to more of the load being shared by the higher volume fraction of SiC particles and smaller average inclusion sizes in the latter composite. A conceptual model is proposed that accounts for these observations and qualitatively shows the effect of reinforcement on stress enhancement in near-surface inclusions. N. CHAWLA, formerly Research Fellow, Department of Materials Science and Engineering, University of Michigan C. ANDES is former Research Fellow, Department of Materials Science and Engineering, University of Michigan. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

The interactive role of inclusions and SiC reinforcement on the high-cycle fatigue resistance of particle reinforced metal matrix composites

The effect of intermetallic inclusions on the fatigue crack initiation and growth in 2080 Al alloy and 2080/SiC _p composites was investigated. Using surface replication, it was determined that, in the highcycle fatigue region, life is dominated by the initiation process. It was also determined that the majority of initiation sites were associated with intermetallic inclusions. While 2080/SiC/20 _p showed a definitive relationship between inclusion size and fatigue life, i.e., a higher inclusion size resulted in lower fatigue life, there was no correlation in 2080/SiC/30 _p . This was attributed to more of the load being shared by the higher volume fraction of SiC particles and smaller average inclusion sizes in the latter composite. A conceptual model is proposed that accounts for these observations and qualitatively shows the effect of reinforcement on stress enhancement in near-surface inclusions. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee.     

Modification of the interface in SiC/Al composites

Methodologies both to avoid the formation of Al₄C₃ and to tailor the interfacial structures in a SiC/2014 Al composite were demonstrated. Modification of the interfacial structures in the SiC/2014 Al composite was made by forming SiO₂ layers on the surfaces of SiC via passive oxidation at elevated temperatures. In the 2014 Al composite reinforced with the oxidized SiC, MgAl₂O₄ and Si crystals were observed to be present at the interfacial region as a result of the reaction between the SiO₂ layer and the matrix. On the other hand, in the case of the 2014 Al composite reinforced with unoxidized SiC, SiC was found to react with the Al matrix to form both Al₄C₃ and Si. Qualitative measurements of the interfacial bonding strength were carried out on composites having various types of interfaces and thicknesses. Detailed interfacial structures and phase identifications, which were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were presented.     

粉体粒径级配比对粉末冶金SiC_p/6061Al复合材料组织与性能的影响

选择不同粒径的6061Al粉末和SiC颗粒,采用真空热压法制备含35%SiC体积分数的SiCp/6061Al复合材料,研究不同级配比对复合材料显微组织和抗拉强度的影响。结果表明:复合粉末的粒径级配比可影响复合材料的微观组织和力学性能;当增强体颗粒粒径为15μm时,随基体6061粉末与SiC颗粒粒径比降低,SiC颗粒在复合材料中的分布越来越均匀,抗拉强度提高;当基体6061Al粒径为10μm时,随SiC颗粒粒径减小,复合材料微观组织的均匀性降低,但抗拉强度提高。并建立了理想的复合粉末颗粒分布模型,模型的理论计算结果与Slipenyuk公式计算结果接近。     

混料时间和挤压对SiC增强纯Al基复合材料显微组织和力学性能的影响

采用粉末冶金法制备SiC颗粒增强工业纯Al基复合材料,研究混料时间和挤压对复合材料显微组织和力学性能的影响。研究表明:机械混粉过程存在最佳的混料时间,混料时间为16 h时SiC颗粒分布均匀,复合材料的密度高、力学性能好。挤压可以改善复合材料的界面结合强度、减少孔洞的数量,从而提高材料的致密度和力学性能。烧结态复合材料的断裂机制以基体的脆性断裂以及增强相与基体的界面脱粘为主。挤压态复合材料的断裂以基体的韧性断裂以及SiC颗粒的脆性断裂为主,伴随着少量的基体与SiC颗粒的界面脱粘。     

Anin situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites

ANNEALED aluminum/silicon carbide (Al/SiC) composites exhibit a relatively high density of dislocations, which are frequently decorated with fine precipitates, in the Al matrix. This high dislocation density is the major reason for the unexpected strength of these composite materials. The large difference (10:1) between the coefficients of thermal expansion (CTE) of Al and SiC results in sufficient stress to generate dislocations at the Al/SiC interface during cooling. In thisin situ investigation, we observed this dislocation generation process during cooling from annealing temperatures using a High Voltage Electron Microscope (HVEM) equipped with a double tilt heating stage. Two types of bulk annealed composites were examined: one with SiC of discontinuous whisker morphology and one of platelet morphology. In addition, control samples with zero volume percent were examined. Both types of composites showed the generation of dislocations at the Al/SiC interface resulting in densities of at least 10¹³ m^-2. One sample viewed end-on to the whiskers showed only a rearrangement of dislocations, whereas, the same material when sectioned so that the lengths of whiskers were in the plane of the foil, showed the generation of dislocations at the ends of the whiskers on cooling. The control samples did not show the generation of dislocations on cooling except at a few large precipitate particles. The results support the hypothesis that the high dislocation density observed in annealed composite materials is a result of differential thermal contraction of Al and SiC. The SiC particles act as dislocation sources during cooling from annealing temperatures resulting in high dislocation densities which strengthen the material.     

Effect of thermal residual stresses on fatigue crack opening and propagation behavior in an Al/SiC p metal matrix composite

The effects of a thermal residual stress field on fatigue crack growth in a silicon carbide particle-reinforced aluminum alloy have been measured. Stress fields were introduced into plates of material by means of a quench from a solution heat-treatment temperature. Measurements using neutron diffraction have shown that this introduces an approximately parabolic stress field into the plates, varying from compressive at the surfaces to tensile in the center. Long fatigue cracks were grown in specimens cut from as-quenched plates and in specimens which were given a stress-relieving overaging heat treatment prior to testing. Crack closure levels for these cracks were determined as a function of the position of the crack tip in the residual stress field, and these are shown to differ between as-quenched and stress-relieved samples. By monitoring the compliance of the specimens during fatigue cycling, the degree to which the residual stresses close the crack has been evaluated. formerly Research Student, Department of Materials Science and Metallurgy, University of Cambridge formerly Lecturer, Department of Materials Science and Metallurgy, University of Cambridge This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

Mechanical properties of 2014Al/SiC composites with oxidized SiC particles

Metal-matrix composites (MMCs) are known to have wide applications in parts of transportation devices such as automobiles and aircraft. Al-matrix composites using SiC particles as reinforcements are especially spotlighted because of their low cost, superior specific modulus, specific strength, wear resistance, and high-temperature stability. However, Al₄C₃ formed by the interfacial reaction between Al and SiC weakens the interfacial bonding strength. It is also known to be unstable in the water-soluble atmosphere. In this study, the passive oxidation of SiC powder is used as a protective layer against the reaction between the Al matrix and the SiC particles. We investigated the changes in interfacial product of the composites, and mechanical properties such as interfacial bonding strength and tensile strength, in terms of the oxidized-layer thickness of the reinforcement.