Synergistic effects of wear and corrosion for Al2O3 particulate-reinforced 6061 aluminum matrix composites

Wear corrosion of alumina particulate-reinforced 6061 aluminum matrix composites in a 3.5 wt pct NaCl solution with a revised block-on-ring wear tester has been investigated. The studies involved the effects of applied load, rotational speed, and environments (dry air and 3.5 pct NaCl solution) on the wear rates of materials. Also various specimens with Al₂O₃ volume fractions of 0, 10, 15, and 20 pct were employed in this work. Electrochemical measurements and electron micrographic observations were conducted to clarify the micromechanisms of wear corrosion in such metal matrix composites. Experimental results indicated that the wear rate of monolithic 6061 Al in either dry wear or wear corrosion was reduced by adding alumina reinforcements. However, the effect of volume fraction on wear rate is only minor in dry wear, while it is significant in the case of wear corrosion. Wear-corrosion tests also showed that the corrosion potential shifted to the active side and the current density for an applied potential increased with the decrease of Al₂O₃ volume fraction in the materials and the increase in applied load and rotational speed. Although the incorporation of reinforcement in these aluminum matrix composites was deterimental to their corrosion resistance, the influence on wear corrosion was favorable.     

Corrosive wear of SiC Whisker-and particulate-reinforced 6061 aluminum alloy composites

Wear tests on SiC whisker- and SiC particulate-reinforced 6061-T6 aluminum matrix composites (SiCw/Al and SiCp/Al), fabricated using a high pressure infiltration method, were performed in laboratory air, ion-exchanged water and a 3 pct NaCl aqueous solution using a block-on-ring type apparatus. The effects of environment, applied load, and rotational (sliding) speed on the wear prop-erties against a sintered alumina block were evaluated. Electrochemical measurements in ion-ex-changed water and a 3 pct NaCl aqueous solution were also made under the same conditions as the wear tests. A comparison was made with the properties of the matrix aluminum alloy 6061-T6. The SiC-reinforced composites exhibited better wear resistance compared with the monolithic 6061 Al alloy even in a 3 pct NaCl aqueous solution. Increase in the wear resistance depended on the shape, size, and volume fraction of the SiC reinforcement. Good correlation was obtained between corrosion resistance and corrosion wear. The ratios of wear volume due to the corrosive effect to noncorrosive wear were 23 to 83 pct, depending on the wear conditions.     

Damping behavior of 6061Al/Gr metal matrix composites

The damping behavior of graphite particulate-reinforced 6061A1 alloy metal matrix composites (MMCs) processed by spray atomization and codeposition is studied. Four spray deposition experiments are made, yielding materials with graphite volume fractions of 0, 0.05, 0.07, and 0.10. A dynamic mechanical thermal analyzer is used to measure the damping capacity and elastic modulus at 0.1, 1, and 10 Hz over the temperature range of 30 °C to 250 °C. The damping capacity of the materials is shown to increase with increasing volume fraction of graphite. Hot extrusion of the spray-deposited MMCs is shown to further increase the damping capacity. The elastic moduli of the spray-deposited MMCs are reduced with the addition of graphite but are improved by hot extrusion. At low temperatures (below 150 °C), the high damping capacity of the MMCs is attributed primarily to thermal expansion mismatch-induced dislocations and the high intrinsic damping of graphite. At high temperatures (above approximately 200 °C), the damping capacity is attributed to Al/graphite interface viscosity, preferred orientation of the graphite, and the presence of dislocations.     

Analysis of stress-strain,fracture, and ductility behavior of aluminum matrix composites containing discontinuous silicon carbide reinforcement

Mechanical properties and stress-strain behavior were evaluated for several types of commercially fabricated aluminum matrix composites, containing up to 40 vol pct discontinuous SiC whisker, nodule, or particulate reinforcement. The elastic modulus of the composites was found to be isotropic to be independent of type of reinforcement, and to be controlled solely by the volume percentage of SiC reinforcement present. The yield/tensile strengths and ductility were controlled primarily by the matrix alloy and temper condition. Type and orientation of reinforcement had some effect on the strengths of composites, but only for those in which the whisker reinforcement was highly oriented. Ductility decreased with increasing reinforcement content; however, the fracture strains observed were higher than those reported in the literature for this type of composite. This increase in fracture strain was probably attributable to cleaner matrix powder, better mixing, and increased mechanical working during fabrication. Comparison of properties with conventional aluminum and titanium structural alloys showed that the properties of these low-cost, lightweight composites demonstrated very good potential for application to aerospace structures.     

In situ synthesis of TiC particulate-reinforced aluminum matrix composites

Particulate TiC-reinforced aluminum composite specimens were processed by compacting a mixture of titanium, carbon, and aluminum powders into preforms that were infiltrated with molten aluminum and subsequently heated in a differential thermal analyzer to about 1573 K under argon atmosphere. The onset of formation of TiC particles began at about 1150 K by reaction of TiAl₃ with Al4C₃. Subsequent formation of TiC particles at higher temperatures to approx-imately 1265 K occurred by direct reaction of carbon with TiAl₃. Above this temperature, the TiC particles coarsened with increasing temperature from an initial size of about 0.15μm. TiC particles were also produced in preforms that were not infiltrated; however, the presence of liquid aluminum in infiltrated specimens inhibited particle agglomeration and sintering. Infil-trated preforms could, therefore, serve as excellent "master alloys" for subsequent dilution in an aluminum melt and processing of metal-matrix composites (MMCs) reinforced with sub-micron TiC particulates. Formerly Research Scientist, Massachusetts Institute of Technology, Cambridge, MA 02139     

An analysis of the effects of matrix void growth on deformation and ductility in metal-ceramic composites

Deformation and failure of metal-matrix composites, by the nucleation and growth of voids within the ductile matrix, are studied numerically and experimentally. The matrix material is modelled as an elastic-viscoplastic ductile porous solid to characterize the evolution of damage from void formation. The material systems chosen for parametric analyses and for quantitative comparisons between numerical analyses and experiments are aluminum alloys discontinuously reinforced with SiC. The brittle reinforcement phase, in the form of spheres, particulates with sharp corners, or cylindrical whiskers, is modelled as elastic or rigid, with the interfaces between the ductile matrix and the brittle reinforcement assumed to be perfectly bonded. The overall constitutive response of the composite and the evolution of matrix failure are analyzed using finite element models within the context of axisymmetric and plane strain formulations. Detailed parametric analyses of the effects of (i) reinforcement shape, (ii) reinforcement volume fraction, (iii) mechanical properties of the matrix, (iv) nucleation strain and volume fraction of void-nucleating particles, and (v) reinforcement distribution on the overall deformation and ductility of the composite are discussed. The numerical predictions of yield strength, strain hardening exponent and ductility for the composites with different volume fractions of SiC particulates are also compared with experimental measurements.     

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.     

Mechanical property and fracture behavior of squeeze-cast Mg matrix composites

The present study aims to investigate the microstructure and fracture properties of AZ91 Mg matrix composites fabricated by the squeeze-casting technique, with variations in the reinforcement material and applied pressure. Microstructural and fractographic observations, along with in situ fracture tests, were conducted on three different Mg matrix composites to identify the microfracture process. Two of them are reinforced with two different short fibers and the other is a whisker-reinforced composite. From the in situ fracture observation of Kaowool-reinforced composites, the effect of the applied pressure on mechanical properties is explained using a competing mechanism: the detrimental effects of fiber breakage act to impair the beneficial effects of the grain refinement and improved densification as the applied pressure increases. On the other hand, for the composites reinforced with Saffil short fibers, microcracks were initiated mainly at the fiber/matrix interfaces at considerably higher stress intensity factor levels, while the degradation of fibers was not observed even in the case of the highest applied pressure. This finding indicates that the higher applied pressure yields better mechanical properties, attributable to the Saffil short fibers having relatively high resistance to cracking. Although an improved microstructure was obtained by accommodating the appropriate applied pressure in the short fiber-reinforced composites, their mechanical properties were far below those of conventional A1 matrix composites. In this regard, the Alborex aluminum borate whisker is suggested as a replacement for the short fibers used in the present investigation, to achieve better mechanical properties and fracture toughness.     

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.     

Application of the generalized method of cells principle to particulate-reinforced metal matrix composites

A three-dimensional model based on the generalized method of cells (GMC) principle has been used to predict the effective properties of particulate-reinforced metal matrix composites (PMMCs). The effects of constituent phases on the elastic properties of PMMCs are predicted using GMC. The predictions are compared with an assortment of finite-element predictions and experimental results available in the literature. The accuracy and the computational efficiency of the GMC model are also discussed. Moreover, the effect of particle shape and orientation on the elastic properties of PMMCs has been predicted and analyzed. Cubical and parallelepiped shaped particles having different orientations are considered for this study. Significant variations are noted on the elastic properties of the PMMC systems by altering the shape and orientation of the particles.     

高温钛合金和颗粒增强钛基复合材料的研究和发展

简要回顾了高温钛合金的研究和发展历程,指出现代高温钛合金进一步发展需要解决的主要难题.综述了颗粒增强钛基复合材料的研究现状,从基体的选择、增强相的选择和制备工艺等3个方面,较详细地阐述了颗粒增强钛基复合材料设计中的基本任务.最后对今后的发展趋势进行了展望.     

Effects of misfit strain and reverse loading on the flow strength of particulate-reinforced A1 matrix composites

Experimental studies conducted on particulate-reinforced A1 alloys are used to critically assess flow models. For this purpose, the influence of thermal expansion misfit and of reverse loading provide a particularly critical assessment. Comparison with models indicates that continuum cell calculations provide good predictions of trends, subject to an in situ matrix strength that may differ from that for the unreinforced alloy.     

The effects of superimposed hydrostatic pressure on deformation and fracture: Part II. Particulate-reinforced 6061 composites

The effects of various levels of superimposed hydrostatic pressure on the tensile ductility and fracture micromechanisms were determined for 6061 + 15 pct Al₂0₃ composites heat-treated to underaged (UA) and overaged (OA) conditions of equivalent yield strength. Superimposed pressures of 0.1, 150, and 300 MPa were selected, while the ductility significantly increased with each increment in pressure. At 300 MPa pressure, the monolithic 6061 and 6061 composite exhibited nearly identical ductility. It is shown that the levels of pressure chosen significantly inhibit void growth and coalescence in the composite. Void nucleation in the composites occurredvia the fracture of the reinforcement, followed by void growth and coalescence in the matrix. Tests conducted with 500 MPa pressure additionally provided evidence for suppression of void nucleation. Neither the ductility nor the pressure response was significantly affected by the heat treatments chosen. This article is based on a presentation made in the symposium “Quasi-Brittle Fracture” presented during the TMS fall meeting, Cincinnati, OH, October 21–24, 1991, under the auspices of the TMS Mechanical Metallurgy Committee and the ASM/MSD Flow and Fracture Committee.     

Failure of SiC particulate-reinforced metal matrix composites induced by laser thermal shock

Thermal failure of SiC particulate-reinforced 6061 aluminum alloy composites induced by both laser thermal shock and mechanical load has been investigated. The specimens with a single-edge notch were mechanically polished to 0.25 mm in thickness. The notched-tip region of the specimen is subjected to laser beam rapid heating. In the test, a pulsed Nd:glass laser beam is used with duration 1.0 ms or 250 μs, intensity 15 or 70 kW/cm², and spot size 5.0 mm in diameter. Threshold intensity was tested and fracture behavior was studied. The crack-tip process zone development and the microcrack formation were macroscopically and microscopically observed. It was found that in these materials, the initial crack occurred in the notched-tip region, wherein the initial crack was induced by either void nucleation, growth, and subsequent coalescence of the matrix materials or separation of the SiC particulate-matrix interface. It was further found that the process of the crack propagation occurred by the fracture of the SiC particulates.     

Trace element effects on ductility and fracture of Ni-Cr-Ce alloys

The effect of trace additions of Ce, ranging from Oto 180 at. ppm, on the tensile behavior of a Ni-20Cr alloy is presented. For alloys without Ce a transition from ductile transgranular to brittle intergranular fracture mode is observed at high temperatures and for low strain-rate tests. Additions of Ce suppress this transition with a resulting increase in ductility. Maximum effects are observed for temperature and strain rate values where fracture in Ce-free alloys occursvia grain boundary cavitation. The reduced cavitation rate of Ce-containing alloys is suggested to be the result of an increase in both interfacial energy and grain boundary mobility. Formerly Graduate Assistant, Department of Mechanics and Materials Science, Rutgers     

Study of 6061-Al2O3p composites produced by reciprocating extrusion

A reciprocating extrusion process was developed to consolidate 6061-Al₂O_3p composites from mixed powders. The 6061 alloy powder was first dehydrated in a vacuum chamber at 450 °C and then mixed with 12.5 μm Al₂O₃ powder in various volume fractions: 0, 5, 10, 20, and 30 pct. The mixed powders were hot pressed at 300 °C under a pressure of 300 MPa and finally extruded reciprocatingly 14 times at 460 °C. The results show that the composites were fully densified, with no sign of pores or oxide layers observable in the optical microscope. The Al₂O₃ particles were distributed uniformly in the matrix. As compared with 6061 alloys, the composites demonstrated a smaller precipitation hardening and elongation, but exhibited a higher Young’s modulus and a larger work hardening capacity. The degradation of precipitation hardening was due to the loss of Mg, which reacts with Al₂O₃ to form MgAl₂O₄. The large work-hardening capacity is attributable to the incompatibility between Al₂O₃ and the matrix, which possibly generates more dislocations to harden the matrix. The composites had much higher friction coefficients and greater wear resistances than the 6061 alloy against steel disc surface. The friction coefficient of the 6061-30 vol pct Al₂O_3p composite was double that of the 6061 alloy and the wear resistance was 100-fold. As compared with similar composites reported previously, these composites possessed much higher elongation at the same strength level. A 30 vol pct Al₂O_3p still displayed an elongation of 9.8 pct in the T6 condition. All of these improvements are attributed to the merits, including full densification of the bulk, uniform dispersion of the Al₂O₃ particles in the matrix, and strong binding between the Al₂O₃ particles and the matrix resulting from reciprocating extrusion.     

Investigation of wear behaviour and microstructure of hot-pressed TiB2 particulate-reinforced magnesium matrix composites

In this study, magnesium and magnesium metal matrix composites (Mg-MMCs) reinforced with 10, 20 and 30 weight (wt.)% TiB₂ particulates were produced by powder metallurgy using the hot pressing technique. The hardness, density, wear behaviour and microstructure of samples were investigated. Uniform distributions of TiB₂ particulates were observed except with some partial agglomeration for 30 wt-% TiB₂. When compared to pure Mg, the hardness increment of Mg-MMCs reinforced with 10, 20 and 30 wt-%TiB₂ particulates was 10.7, 31.9 and 65.3%, respectively. As compared to pure Mg, under load of 20 N, the decrease in wear rate in 10, 20 and 30 wt-% reinforcement was 28.71, 34.98 and 42.92%, respectively. It is believed that the reason of decrease in wear rate was the presence of harder TiB₂ particulates, which resisted to wear and plastic deformation. For pure Mg, oxidative wear changed to oxidative and mild abrasive wear transition from 10 to 20 N. Mg/TiB₂ composites exhibited abrasive wear mechanism under load of 10 and 20 N except 30 wt-% TiB₂ composite indicated oxidative and adhesive wear. However, a transition from mild abrasive wear to severe abrasive wear was observed with increasing load in composites.     

Tensile ductility and fracture of superplastic Aluminum-SiC composites under thermal cycling conditions

The tensile and fracture creep behavior of aluminum-SiC composites has been evaluated under iso-thermal and cyclic heating conditions. The true strain to fracture under thermal cycling conditions is shown to decrease linearly with the logarithm of the applied stress. This trend is unexpected since the strain rate sensitivity exponent,m, is high (typical of superplastic materials) and constant over the range of stresses studied. The results obtained are explained by a fracture mechanics model in which the crack size increases by a strain-induced mechanism. The results are compared with similar behavior obtained in superplastic ceramics.     

Optimization of the strength-fracture toughness relation in particulate-reinforced aluminum composites via control of the matrix microstructure

The evolution of the microstructure and mechanical properties of a 17.5 vol. pct SiC particulate-reinforced aluminum alloy 6092-matrix composite has been studied as a function of postfabrication processing and heat treatment. It is demonstrated that, by the control of particulate distribution, matrix grain, and substructure and of the matrix precipitate state, the strength-toughness combination in the composite can be optimized over a wide range of properties, without resorting to unstable, underaged (UA) matrix microstructures, which are usually deemed necessary to produce a higher fracture toughness than that displayed in the peak-aged condition. Further, it is demonstrated that, following an appropriate combination of thermomechanical processing and unconventional heat treatment, the composite may possess better stiffness, strength, and fracture toughness than a similar unreinforced alloy. In the high- and low-strength matrix microstructural conditions, the matrix grain and substructure were found to play a substantial role in determining fracture properties. However, in the intermediate-strength regime, properties appeared to be optimizable by the utilization of heat treatments only. These observations are rationalized on the basis of current understanding of the grain size dependence of fracture toughness and the detailed microstructural features resulting from thermomechanical treatments.