Creep behavior of fiber reinforced metal matrix composites

A theoretical model of the creep behavior of metal matrix composites having strong fiber-matrix interfaces is described in terms of creep parameters of the matrix and fibers. The available experimental data, obtained from the unidirectionally solidified aluminum-nickel eutectic containing 10 vol pct Al₃Ni fibers, are in good agreement with the theoretical model. The creep activation energy of the composite is described in terms of the creep activation energy of fibers and the matrix. The experimentally de-termined data of (Co, Cr)-(Co, Cr)₇C₃ and Al-Al₃Ni eutectics are in agreement with those values as predicted.     

Creep behavior of fiber reinforced metal matrix composites

A theoretical model of the creep behavior of metal matrix composites having strong fiber-matrix interfaces is described in terms of creep parameters of the matrix and fibers. The available experimental data, obtained from the unidirectionally solidified aluminum-nickel eutectic containing 10 vol pct Al₃Ni fibers, are in good agreement with the theoretical model. The creep activation energy of the composite is described in terms of the creep activation energy of fibers and the matrix. The experimentally de-termined data of (Co, Cr)-(Co, Cr)₇C₃ and Al-Al₃Ni eutectics are in agreement with those values as predicted. Formerly a Visiting Scholar, Materials Department, University of California, Los Angeles.     

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.     

Effect of SiC and graphite particulates on the damping behavior of metal matrix composites

The effect of SiC and graphite (Gr) particulates on the resultant damping behavior of 6061 A1 metal matrix composites (MMCs) was investigated in an effort to develop a high damping material. The MMCs were processed by a spray atomization and deposition technique and the damping characterization was conducted on a dynamic mechanical thermal analyzer. The damping capacity, as well as the dynamic modulus, was measured at frequencies of 0.1, 1, 10 and 30 Hz over a 30 to 250°C temperature range. The microstructural analysis was performed using scanning electron microscopy, optical microscopy and image analysis. The damping capacity of the 6061 Al/SiC and 6061 Al/Gr MMCs, with two different volume fractions of reinforcements, were compared with that of as-received 6061-T6 Al and spray deposited 6061 Al. It was shown that the damping capacity of 6061 Al could be significantly improved by the addition of either SiC or graphite particulates through spray deposition processing. Finally, the operative damping mechanisms were discussed in light of the data obtained from characterization of microstructure and damping capacity.     

Influence of porosity and hBN content on the damping capacity of metal matrix composites

ABSTRACT

Sintered iron samples were produced using different pressing loads, resulting in different residual porosities of approximately 14%, 18%, 22% and 30%. Iron-matrix composites, containing 2.5, 5.0, 7.5 and 10.0 %vol. of dispersed hexagonal boron nitride (hBN) particles, were also produced. The influence of these parameters on the amplitude-dependent damping capacity was assessed using a dynamic-mechanical analyser. The simultaneous effect on mechanical strength was assessed through a tensile test. The microstructure was analysed with optical and electronic microscopy and quantitatively evaluated through a digital image analysis. It was verified that the increase of porosity did not lead to a representative increase in the damping capacity of sintered iron. On the other hand, higher hBN content leads to a higher damping capacity due to the introduction of robust new damping mechanisms. However, hBN reservoirs, which are bigger and more elongated than the pores, are more detrimental to mechanical strength.     

Processing,microstructure, and mechanical behavior of cast magnesium metal matrix composites

Magnesium metal matrix composites (MMCs) have been receiving attention in recent years as an attractive choice for aerospace and automotive applications because of their low density and superior specific properties. This article presents a liquid mixing and casting process that can be used to produce SiC particulate-reinforced magnesium metal matrix composites via conventional foundry processes. Microstructural features, such as SiC particle distribution, grain refinement, and particle/matrix interfacial reactions of the cast magnesium matrix composites, are investigated, and the effects of solidification-process parameters and matrix alloys (pure Mg and Mg-9 pct Al-1 pct Zn alloy AZ91) on the microstructure are established. The results of this work suggest that in the solidification processing of MMCs, it is important to optimize the process parameters both to avoid excessive interfacial reactions and simultaneously achieve wetting, so that a good particle distribution and interfacial bonding are obtained. The tensile properties, strain hardening, and fracture behavior of the AZ91/SiC composites are also studied and the results are compared with those of the unreinforced AZ91 alloy. The strengthening mechanisms for AZ91/SiC composite, based on the proposed SiC particle/matrix interaction during deformation, are used to explain the increased yield strength and elastic modulus of the composite over the magnesium matrix alloy. The low ductility found in the composites is due to the early appearance of localized damages, such as particle cracking, matrix cracking, and occasionally interface debonding, in the fracture process of the composite.     

Damage initiation in metal matrix composites

The process of damage by particle cracking has been followed in a composite of A356 Al containing 20% by volume SiC. The probability of particle cracking is influenced by both particle size and aspect ratio and the results indicate that the relative importance of these factors depends on the Weibull modulus of the SiC particles. A simple model is developed to describe the process of load transfer and crack initiation in the particles. This initial model does not take into account particle interactions and the role of clustering but does provide an initial fragment to describe the damage initiation process.     

Fabrication of metal matrix composites of

Fine fibrous titanium carbide (TiC) was processed through the self-propagating high-temperature synthesis (SHS) method and employed to fabricate aluminum matrix composites. Two consol-idation methods were investigated: (1) combustion synthesis of TiC fiber/Al composites directly using titanium powders and carbon fibers ignited simultaneously with varying amounts of the matrix metal powder and (2) combustion synthesis of TiC using titanium powders and carbon fibers followed by consolidation into different amounts of the metal matrix powder, Al,via hot isostatic pressing (HIP). In the former method, when the amount of the Al in the matrix was increased, the maximum temperature obtained by the combustion reaction decreased and the propagation of the synthesis reactions became difficult to maintain. Preheating was required for the mixture of reactants with more than approximately 5 mole pct aluminum matrix powders in order to ignite and maintain the propagation rate. Microstructural analysis of the products from the Al/C/Ti reaction without preheating shows that small amounts of an aluminum carbide phase (AI4C3) are present. In the second method, following separation of the individual fibers in the TiC product, dense composites containing the SHS products were obtained by HIP of a mixture of the TiC fibers and Al powders. No ternary phase was formed during this procedure. Formerly Graduate Research Assistant, Department of Chemical Engineering, Michigan Technological University, is with Particle Technology, Inc., Hanover, MD 21076. This paper is based on a presentation made in the symposium “Reaction Synthesis of Materials” presented during the TMS Annual Meeting, New Orleans, LA, February 17–21, 1991, under the auspices of the TMS Powder Metallurgy Committee.     

The strength of metal matrix composites

There is intensive interest in metal matrix composites (MMCs) for automotive components, and the first production applications in Japan use discontinuous fibers as the reinforcements. These fibers are randomly oriented, resulting in an MMC with isotropic properties. However, there are conflicting reports on the tensile strengths attainable. In some cases, the strength increases with increasing volume fraction(V _f) of fibers, while in other cases, there is little or no benefit. A simple method is proposed to calculate the strength of this type of MMC. It is shown that the fibers oriented perpendicular to the stress direction play a key role, and the strength depends upon the strength of the interfacial bond. Upper and lower limits of the composite strength are calculated. If the bond strength is larger than the matrix strength, the composite strength has a maximum value which increases withV _f. If the bond strength is weaker than the matrix, the composite strength has a minimum value which is either weakly dependent or even independent ofV _f. These calculations are in good agreement with examples taken from the literature of aluminum composites reinforced with either A1₂O₃, graphite, or SiC. The strength of the matrix alloy is shown to be a very important parameter: weak alloys are easily strengthened, while in certain cases, strong alloys may be weakened.     

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.     

Fundamental aspects of creep in metal matrix composites

The primary characteristics of the creep of metal matrix composites (MMCs) are reviewed, including the shapes of the creep curves, the origin of the threshold stresses, and the nature of the rate-controlling processes. A detailed analysis of two representative MMCs provides no support for the concept of a constant substructure model for creep with a stress exponent (n) of 8. Analysis of the data demonstrates that creep is controlled by deformation in the matrix alloys and, as in solid solution alloys, there is a division into control by viscous glide (class A) and climb (class M), respectively. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.