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.

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

This paper studied the combined effects of particle size and distribution on the mechanical properties of the SiC particle reinforced Al–Cu alloy composites. It has been shown that small ratio between matrix/reinforcement particle sizes resulted in more uniform distribution of the SiC particles in the matrix. The SiC particles distributed more uniformly in the matrix with increasing in mixing time. It has also been shown that homogenous distribution of the SiC particles resulted in higher yield strength, ultimate tensile strength and elongation. Yield strength and ultimate tensile strength of the composite reinforced by 4.7 μm sized SiC particles are higher than those of composite reinforced by 77 μm sized SiC particles, while the elongation shows opposite trend with yield strength and ultimate tensile strength. Fracture surface observations showed that the dominant fracture mechanism of the composites with small SiC particle size (4.7 μm) is ductile fracture of the matrix, accompanied by the “pull-out” of the particles from the matrix, while the dominant fracture mechanism of the composites with large SiC particle size (77 μm) is ductile fracture of the matrix, accompanied by the SiC particle fracture.

Effects of particle size and distribution on the mechanical properties of SiC reinforced Al–Cu alloy composites

Fatigue of particle-and whisker-reinforced metal-matrix composites

Finite element analyses of the overall stress-strain response of metal-matrix composites are carried out using axisymmetric and plane strain unit cell formulations. The metal matrix is characterized as an isotropically hardening elastic-plastic solid and the ceramic reinforcement is taken to be isotropic elastic. Perfect bonding between the matrix and the reinforcement is assumed. The focus is on the effects of reinforcement shape, size and spatial distribution. Under monotonic loading, the stress-carrying capacity in the plastic range increases in the following order for the reinforcement shapes considered: double-cone → sphere → truncated cylider → unit cylinder → whisker. The extent of the Bauschinger effect under reversed loading increases in the same order for particle reinforced composites. The effects of reinforcement size and distribution are analyzed by considering a plane strain model with two sizes of reinforcing particles. For certain distributions, it is found that the smaller family of particles plays virtually no role in affecting the stress-strain response. Thermal residual stresses are also considered and their effects are seen to persist far into the plastic range. The predicted plastic stress-strain behavior can be rationalized in terms of the evolution of matrix field quantities and, in particular, in terms of the effect of the constraint on plastic flow.

Effective plastic response of two-phase composites

Enhanced performance of nano-sized SiC reinforced Al metal matrix nanocomposites synthesized through microwave sintering and hot extrusion techniques

Residual stresses induced by thermal expansion mismatch in metal-matrix composites are studied by three-dimensional (3-D) elastic-plastic finite element analyses. Typically, the stress-free state is 150 to 300 K above room temperature. The coefficient of thermal expansion of the matrix is 3 to 5 times larger than that of the ceramic inclusion, resulting in compressive stresses of order 200 MPa in the inclusions. Both compressive and tensile stresses can be found in the matrix. Since the stress may exceed the matrix yield strength near the particles, plastic flow occurs. The authors find a significant influence of this flow on the elastic and plastic properties of the composite. The calculated residual strains in TiC particles due to thermal expansion mismatch and external loads compare well with recent neutron diffraction experiments (Bourkeet al.) The present work is the first reported three-dimensional analysis of spherical inclusions in different arrays (simple cubic (sc) and face-centered cubic (fcc)) that permit a study of particle interactions.

https://link.springer.com/content/pdf/10.1007/BF02646527.pdf

Residual stresses and their effects on deformation

The effects of composite microstructure (e.g. particle size and volume fraction) on the mechanical properties of 2080 Al matrix composites with SiC particulates are considered. Finite element calculations on unit cell representations are compared to tensile measurements. For fine particles, good agreement is obtained for inclusion volume fractions as large as 30%. Coarse particle materials show evidence of particle cracking, which degrades the modulus and yield stress. In a new computational approach, the unit cell of a cracked particle and surrounding matrix is embedded in an effective medium with properties of a composite with intact particles. By appropriate choice of the portion of cracked particles, experiments on materials with a broad distribution of particle sizes are reproduced by the model calculations.

Microstructure and strengthening of 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.

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

Accurate prediction of the elastic moduli of a composite material consisting of particles in a continuous matrix is difficult for several reasons. Even for the simplest situation where all particles are spheres of the same size, the randomness of the spatial distribution of the particles is not easy to include in an exact calculation. Further difficulties arise when other realistic effects, such as clustering of particles, irregular particle shape, and nonuniform particle sizes, occur. It is the purpose of this communication to show that for the technologically important metal-matrix composites (A1/SiCp), sufficiently accurate results can be obtained over the useful composition range by considering third-order bounds on the bulk and shear moduli, rl-4] Typically, these bounds differ by only a few percent. Because they contain information on the phase geometry, they are nearly an order of magnitude better than the well-known Hashin-Shtrikman bounds, tSJ which depend only on volume fraction. Torquato and co-workers [~-4] have evaluated the thirdorder bounds and have tabulated results for the threepoint parameters characterizing the particle distribution, which are necessary for the calculation of the moduli. Fortunately, they simplified the procedure so that the bounds are easy to compute. In the present work, it is shown that by averaging the upper and lower bounds for the bulk modulus K, as well as for the shear modulus G, one can obtain an estimate of Young's modulus E that is in error by less than 4 pct for volume fractions up to 50 pct. Such accuracy is well within the current experimental variability. For completeness, the mathematical expressions for the bounds on K and G are given below in terms of the moduli of the matrix (phase 1) and the particles (phase 2) and their volume fractions, ~b~ and ~b2:

Third-order bounds on the elastic moduli 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.

L. C. Davis

Papers

Residual stresses and their effects on deformation

Microstructure and strengthening of metal matrix composites

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

Third-order bounds on the elastic moduli of metal-matrix composites

Micromechanics effects in creep of metal-matrix composites