In-situ observations of titanium metal-matrix composites under transverse tensile loading

The transverse stress-strain behavior of several titanium metal-matrix composites (TiMMCs) has been studied in-situ. Debonding of 1140+/Ti-6-4 composites occurs over a range of stresses. The sharpness of the first “knee” is affected by the fiber volume fraction and by the relative moduli of the matrix regions and the reinforced composite. It has been observed that debonding occurs mainly at the interface between two sublayers of carbon/carbon coatings in 1140+/Ti-6-4 composites and mainly at the interface between the carbon/reaction zone in the as-processed and peak-aged 35 pct SCS-6/Tiβ21s composites. At surface positions, this process starts at very low stresses (≥50 MPa) from the positions with sharp changes of curvatures (or undulations), voids, or debris at the periphery of the interface. Cracking of the outermost carbon sublayer and of the reaction zone in the 1140+/Ti-6-4 composites and the reaction zone in the SCS-6/Tiβ21s composites occurs during elastic deformation of the matrix. This has been directly observed in a field-emission gun (FEG)-scanning electron microscope (SEM) under incremental loading. Although these cracks are arrested and blunted by the matrix material, they cause local stresses and, thus, stimulate local plastic deformation of the matrix and subsequent development of a second knee on the stress-strain curve. The in-situ observations are discussed in terms of the effects of fiber volume fraction and fiber type on the loci and dynamic processes of interfacial debonding, cracking of carbon coatings and reaction zones, and plastic deformation of the matrix.     

Fatigue crack growth resistance of unidirectional fiber-reinforced titanium metal-matrix composites under transverse loading

The transverse fatigue crack growth resistance of unidirectional 8 and 35 pct 1140+/Ti-6-4 fiber-reinforced composites has been investigated. It has been found that, at a low fiber volume fraction, the transverse fatigue crack growth resistance of metal-matrix composites (MMCs) is improved with respect to the monolithic matrix alloy. This occurs because “holes” from debonded interfaces can trap the crack and reduce the average fatigue crack growth rates by periodically increasing the effective crack-tip radius. However, an increase of fiber volume fraction from 8 to 35 pct decreases the fatigue crack growth resistance dramatically, due to the significant increase of the frequency of interaction and coalescence between the main crack, the debonded interfaces, and microcracks.     

Effect of reinforcement-particle-orientation anisotropy on the tensile and fatigue behavior of metal-matrix composites

The effect of extrusion-induced particle-orientation anisotropy on the mechanical behavior of metal-matrix composites (MMCs) was examined. In this study, we have shown that this anisotropy has a significant influence on the tensile and fatigue behavior SiC particle-reinforced Al alloy composites. The preferred orientation of SiC particles was observed parallel to the extrusion axis, with the extent of orientation being highest for the lowest-volume-fraction composites. The composites exhibited higher Young’s modulus and tensile strength along the longitudinal direction (parallel to the extrusion axis) than in the transverse direction. The extent of anisotropic behavior increased with increasing volume fraction, because of the increasing influence of the SiC reinforcement on the Young’s modulus and tensile properties. The preferred orientation also resulted in anisotropy in the fatigue behavior of the composite material. The trends mirrored those observed in tension, with higher overall fatigue strengths for both orientations and a higher anisotropy with increasing volume fraction of particles. The influence of particle-orientation anisotropy and the resulting tensile and fatigue damage mechanisms is discussed.     

Deformation behavior of skeletal composites under mechanical loading

The deformation behavior of ceramic-metal composites with a bulk-linked metal skeleton made of highly porous cellular nickel and a stainless wire mesh is examined. It is established that samples of skeletal ceramics remain intact after all types of destructive tests and show high plasticity and strain and fracture energies. A technique is developed for producing ceramics with a bulk-linked metal skeleton by filling a highly porous framework with a ceramic powder and subsequent pressing and sintering of the composite. The technique is tested for two types of skeletal ceramics: structural ceramics such as pyroceramics-highly porous cellular nickel and bioceramics such as hydroxyapatite-highly porous stainless wire skeleton. The hydroxyapatite-based composite has lower bending strength and shock bending energy but higher plasticity and nondestructability than the pyroceramics-based composite. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 7–8 (456), pp. 82–90, 2007.     

The effect of cryogenic cooling on the tensile properties of metal-matrix composites

Tensile specimens machined from metal-matrix, oriented-fiber composites (aluminum alloy reinforced with high strength stainless steel wire) were heated to 260°C and cooled in air to produce a tensile residual stress state in the matrix. Some of the test pieces were cooled to the temperature of boiling nitrogen, held at temperature for fifteen minutes, and then air warmed to room temperature. All test pieces were subsequently strain cycled in tension and the resulting stress-strain behavior was recorded. The results indicated cryogenic refrigeration extended the first stage (totally elastic) behavior of these materials. It was shown that the beneficial effects of the cryogenic treatment resulted from an alteration of the residual stress state brought about by plastic flow of the matrix. Finally, it was shown that these effects could be computed by rigorous analytical methods.     

Damping behavior of discontinuously reinforced ai alloy metal-matrix composites

High damping materials allow undesirable mechanical vibration and wave propagation to be passively suppressed. This proves valuable in the control of noise and the enhancement of vehicle and instrument stability. Accordingly, the scientific community is continually working toward the development of high damping metals (hidamets) and high damping metal-matrix composites (MMCs). The MMCs are particularly attractive in weight-critical applications when the matrix and reinforcement phases are combined to provide desirable property combinations, such as high damping and low density. Inspection of the available scientific literature, however, reveals that an understanding of the precise correlation between the presence of secondary phases (either reinforcements or precipitates) and material damping has eluded investigators, partly as a result of the superposition of multiple mechanisms. As a step toward the clarification of damping phenomena in discontinuously reinforced MMCs, this article describes the damping behavior and mechanisms that are present in discontinuously reinforced MMCs, with particular emphasis on particulate-reinforced Al alloy MMCs processed using spray atomization and deposition. The operative damping mechanisms in the particulate-rein-forced MMCs are discussed in light of the data obtained from microstructural studies and damping capacity measurements. This article is based on a presentation given in the Mechanics and Mechanisms of Material Damping Symposium, October 1993, in Pittsburgh, Pennsylvania, under the auspices of the SMD Physical Metallurgy Committee.     

Deformation of whisker-reinforced metal-matrix composites under changing temperature conditions

A numerical technique for simulating the plastic response of whisker-reinforced metal-matrix composites under conditions of changing temperature and applied stress is developed. The model simulates an elastic-plastic (diffusion-controlled power-law creep) matrix and elastic whiskers, with variable whisker length and spacing. To test this model, the mechanical behavior of a metal-matrix composite of 6061 aluminum, reinforced with 20 vol pct discontinuous, oriented silicon carbide whiskers was studied under conditions of repeated temperature cycling and isothermal creep. The results of the thermal-cycling experiments are compared to those of the model. Both the experiments and the model demonstrate that the composite flw stress may be significantly reduced by thermal cycling (relative to isothermal, elevated temperature behavior) and that under appropriate conditions, the composite strain rate is proportional to the applied stress. Also, agreement between the experimental results and the first-principles model is very good in terms of both magnitude and trends, despite simplifications in the model.     

A Theory for the mechanical properties of metal-matrix composites at ultimate loading

A theory describing the strain, ultimate strength, and work during uniform strain to ultimate loading of metal-matrix composites deformed in tension parallel to the reinforcement is presented. These quantities may be calculated for composites of arbitrary volume fraction using only the component stress-strain curves. The theory is based on the systematic application of a macroscopic principle commonly used to predict the ultimate strength of ductile monolithic materials—namely, that necking occurs when the load borne by the material is maximized. For brittle reinforcing elements, the results are identical to those of previous workers. For ductile reinforcing elements, necking strains intermediate between those of the components and ultimate strengths increasing smoothly with volume fraction from that of the matrix to that of the reinforcement are predicted. The theory can be used to predict the variation of composite ultimate properties with any parameter of interest. In this paper the variation with volume fraction and yield strength of the matrix are studied, with both exact solutions and useful approximations being derived.     

金属基复合材料的发展及其应用

金属基复合材料是近几年来复合材料研究中的热点.文章综述了金属基复合材料的分类、性能特点、制备方法,总结了其主要进展及应用.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Facture” 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.     

Transverse creep and stress-rupture of borsic-aluminum composites and borsic-aluminum composites containing stainless steel and titanium

The transverse creep and stress rupture behavior of a number of Borsic®-aluminum composites was investigated at temperatures from 200° to 400°C. The cpmposites studied consisted of nominally 50 vol pct Borsic fiber and included matrices of 6061, 2024, 2219, and 5052 aluminum alloys. The effect of heat treatment was studied in the heat-treatable alloys. Where transverse composite behavior differed from matrix alloy behavior, the difference was found to be due primarily to a change in fracture mode at higher matrix strength levels from matrix failure to one which involves longitudinal fiber splitting. Of the four basic matrix alloys tested, the best creep resistance was obtained with the 2024 matrix. Additional improvement of transverse creep and stress rupture resistance was realized by incorporating transverse reinforcements such as SAP alloy foil, titanium alloy foil, and 0.002 in. stainless steel wire in the composites. These reinforcements made possible good transverse properties at 400°C with density increases of ≤15 pct. The two best additions were 21 pctβ III titanium foil and 6 pct AFC-77 stainless steel wire. A transverse fracture mode incorporating longitudinal fiber splitting was documented and characterized, and its effect on composite behavior determined. The use of nonsplitting fibers such as 5.6 mil B and 5.7 mil Borsic in preventing this fracture mode was investigated.     

The quasi-static and cyclic fatigue fracture behavior of 2014 aluminum alloy metal-matrix composites

In this article, the quasi-static and cyclic fatigue fracture behavior of aluminum alloy 2014 discontinuously reinforced with fine particulates of aluminum oxide are presented and discussed. The discontinuous particulate-reinforced 2014 aluminum alloy was cyclically deformed under fully reversed, tension-compression loading over a range of strain amplitudes, well within the plastic domain of the engineering stress-strain curve, resulting in cyclic fatigue lives of less than 10⁴ cycles. The influence of both ambient and elevated temperatures on cyclic stress and cyclic stress-strain response is highlighted. The underlying mechanisms governing the fracture mode during quasi-static and cyclic fatigue are discussed and rationalized in light of the concurrent and mutually interactive influences of intrinsic composite microstructural features, deformation characteristics of the metal matrix and reinforcement particulate, cyclic strain amplitude and resultant fatigue life, and test temperature. 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.     

Solidification of particle-reinforced metal-matrix composites

The solidification behavior of ceramic particle-reinforced metal-matrix composites (MMCs) is different from that of the bare matrix, not only because of the presence of the ceramic particles, but also due to their redistribution in the melt that results in nonhomogeneous thermophysical properties. The MMCs comprised of 10-to 15-μm SiC particles of varying volume fractions, dispersed uniformly in a modified aluminum A356 alloy by the melt stirring technique, were solidified unidirectionally in a thermocouple-instrumented cylindrical steel mold. The cooling rates were continually monitored by measuring temperatures at different depths in the melt, and the solidified MMCs were sectioned into disks and chemically analyzed for SiC volume fraction. The results point out that the cooling rate increased with increasing volume fraction of SiC particles. A small increase in the bulk SiC volume fraction of the cast MMC was observed due to particle settling during solidification. A one-dimensional enthalpy model of MMC solidification was formulated, wherein particle settling occurring in the solidifying matrix was coupled to the enthalpy equation by means of the Richardson-Zaki hindered settling correlation. A comparative study of simulations with experiments suggested that the thermal response of SiC particles used in this study was similar to that of single crystals, and their presence increased the effective thermal conductivity of the composite.     

Transverse and cyclic thermal loading of the fiber reinforced metal-matrix composite SCS6/Ti-15-3

The transverse properties of a SiC fiber reinforced Ti alloy matrix composite subjected to transverse mechanical and cyclic thermal loading have been investigated. Fibers and matric have a mismatch in the coefficients of thermal expansion that induces thermal stresses in addition to those caused by mechanical loading. When fluctuations occur in the operating temperature the thermal stresses change and this could cause an incremental accumulation of plastic strain or increase in creep rate. The composite under consideration has a modest mismatch and it was found that the strain accumulation is caused by creep deformation in the matrix at the high temperature portion of the thermal cycles. In the early stages of the deformation for low transverse loading the interface is in compressive contact and the creep rate is accelerated by the cyclic thermal stresses. After debonding has occurred the cyclic thermal stress component is diminished and the creep rate is given by a matrix with holes.     

Theoretical calculation of anisotropie creep and stress-strain behavior for a class of metal-matrix composites

A unified microcontinuum theory is developed to calculate the development of the anisotropic creep strain and the stress-strain relations under a constant strain rate for a class of metal-matrix composites from the constitutive equations of its constituent phases. Here, the ductile matrix is strengthened with aligned, identically shaped, spheroidal inclusions, which may be disc-like, spheres, or whiskers, so that at a given volume concentration, its anisotropic properties will further depend on the inclusion shape. The principle of stress transfer from the ductile matrix to the reinforcing inclusions is established for both creep and constant strain-rate processes. The theoretical analysis points to enhanced response with reinforcement along the axial direction with whiskers, but disc-reinforcement is far superior along the transverse direction. It is also found that the stress-strain curve of the dual-phase system can reach a saturation stress under a constant strain rate. The simple theory developed here is intended for the low volume concentration and small creep strain range, and it is demonstrated that, within this range, the theoretical predictions for the development of creep strain of a Borsic/aluminum system and for the stress-strain curves of a silicon carbide/aluminum system are in close accord with the experimental observations. Formerly Graduate Student     

Pressureless infiltration of aluminum metal-matrix composites

Pressureless infiltration of ceramic preforms by molten aluminum is described. The preforms are SiC with varying amounts of particulate Al, Ti, and Ni. Infiltrants employed are pure Al and Al-12.5 wt pct Si. It is shown that a pressure differential within the preform is required for infiltration, and measurements are made of pressure changes in the preforms during infiltration. Results indicate that atmospheric pressure is essential for infiltration but that capillarity may play a role as well. T. NUKAMI, formerly Research Assistant, Massachusetts Institute of Technology     

Micromechanics effects in creep of metal-matrix composites

The creep of metal-matrix composites is analyzed by finite element techniques. An axisymmetric unit-cell model with spherical reinforcing particles is used. Parameters appropriate to TiC particles in a precipitation-hardened (2219) Al matrix are chosen. The effects of matrix plasticity and residual stresses on the creep of the composite are calculated. We confirm (1) that the steady-state rate is independent of the particle elastic moduli and the matrix elastic and plastic properties, (2) that the ratio of composite to matrix steady-state rates depends only on the volume fraction and geometry of the reinforcing phase, and (3) that this ratio can be determined from a calculation of the stress-strain relation for the geometrically identical composite (same phase volume and geometry) with rigid particles in the appropriate power-law hardening matrix. The values of steady-state creep are compared to experimental ones (Krajewskiet al.). Continuum mechanics predictions give a larger reduction of the composite creep relative to the unreinforced material than measured, suggesting that the effective creep rate of the matrix is larger than in unreinforced precipitation-hardened Al due to changes in microstructure, dislocation density, or creep mechanism. Changes in matrix creep properties are also suggested by the comparison of calculated and measured creep strain rates in the primary creep regime, where significantly different time dependencies are found. It is found that creep calculations performed for a timeindependent matrix creep law can be transformed to obtain the creep for a time-dependent creep law. 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.     

Work of fracture in aluminum metal-matrix composites

The effect of isothermal exposure and thermal cycling on the toughness of B/Al (1100), B/Al (6061), and A1₂O₃/A1 composites has been investigated. In B/Al (1100), isothermal exposure at 773 K for 45 × 10⁴ s (125 hours) reduced toughness, measured by the work of fracture, from 76 kJm^-2 to 10 kJm^-2, and a similar reduction occurred after equivalent thermal cycling. The corresponding reduction in toughness after isothermal exposure in B/Al (6061) was from 44.5 kJm^-2 to 8 kJm^-2; however, the effect of thermal cycling was less detrimental. In the FP-A1₂O³/A1 composite, the work of fracture was insensitive to both forms of thermal treatment. Changes in the toughness of the B/Al composites have been correlated with and analyzed in terms of modifications to matrix, fiber, and interface properties, in particular, matrix softening, interface reaction products, and fiber notch sensitivity. The latter currently on The latter currently on