An overview of tailoring strain delocalization for strength-ductility synergy

A356 Al–fly ash particle composites were fabricated using stir-cast technique and hot extrusion. Composites containing 6 and 12 vol.% fly ash particles were processed. Narrow size range (53–106 μm) and wide size range (0.5–400 μm) fly ash particles were used. Hardness, tensile strength, compressive strength and damping characteristics of the unreinforced alloy and composites have been measured. Bulk hardness, matrix microhardness, 0.2% proof stress of A356 Al–fly ash composites are higher compared to that of the unreinforced alloy. Additions of fly ash lead to increase in hardness, elastic modulus and 0.2% proof stress. Composites reinforced with narrow size range fly ash particle exhibit superior mechanical properties compared to composites with wide size range particles. A356 Al–fly ash MMCs were found to exhibit improved damping capacity when compared to unreinforced alloy at ambient temperature.

Synthesis of fly ash particle reinforced A356 Al composites and their characterization

The mechanisms of deformation and failure in a 2618 Al alloy reinforced with 15 vol pct SiC particilates were studied and compared with those of the unreinforced alloy, processed by spray forming as well. Tensile and fracture toughness tests were carried out on naturally aged and peak-aged specimens. The broken specimens were sliced through the middle, and the geometric features of fractured and intact particulates were measured. The experimental observations led to the conclusion that failure took place by the progressive fracture of the particulates until a critical volume fraction was reached. An influence of the particulate size and aspect ratio on the probability of fracture was found, the large and elongated particulates being more prone to fail, and the fracture stress in the particulates seemed to obey the Weibull statistics. The dif- ferences in ductility found between the naturally aged and peak-aged composites were explained in terms of the number of broken particulates as a function of the applied strain. Numerical simulations of the deformation process indicated that the stresses acting on the particulates are higher in the peak-aged material, precipitating the specimen failure. Moreover, the compressive residual stresses induced on the SiC during water quenching delayed the onset of particulate breakage in the naturally aged material.

Particulate fracture during deformation of a spray formed metal-matrix composite

To study the effect of particle shape on thermal residual stress and strain distributions in composites, five two-dimension SiCp/6061Al model composites reinforced with spherical, hexagon, square, triangle and shuttle-shaped particles, respectively were analyzed by finite element method (FEM). The results showed that the particle shape has a great effect on thermal residual stress and strain fields in composites. There are a residual plastic strain concentration in the matrix around a pointed particle corner and a serious residual stress concentration in the pointed particle corner. The two concentrations increase with decreasing pointed corner degree. When an external stress is applied, the plastic strain concentration in the matrix around the pointed particle corner and the serious stress concentration in the pointed particle corner which will fracture on a relative low level of applied stress can decrease ductility of the composite. Two 6061 aluminium alloy matrix composites reinforced with general SiC particles and blunted SiC particles were studied on the basis of the FEM analyses. It was found that most very pointed particle corners are eliminated after SiC particles being blunted. Replacing general SiC particles with blunted ones can reduce residual plastic strain concentrations in the matrix and serious residual stress concentration in the particle. Therefore, blunted SiC particles reinforced composite shows a higher ductility than general one. (C) 1999 Elsevier Science S.A. All rights reserved.

The effect of particle shape on ductility of SiCp reinforced 6061 Al matrix composites

The effects of heat-treatment, matrix microstructure, and reinforcement size on the evolution of damage, in the form of SiCp cracking, during uniaxial tension testing of an aluminum-alloy based composite have been determined. A powder metallurgy Al-Zn-Mg-Cu alloy reinforced with 15 vol pct of either 5 or 13 μm average size SiCp was heat treated to solution annealed (SA), underaged (UA), and overaged (OA) conditions. The SA treatment exhibited lower yield strength and higher ductility for both particulate sizes compared to the UA and OA conditions. The evolution of damage, in the form of SiCp fracture, was monitored quantitatively using metallography and changes in modulus on sequentially strained specimens. It is shown that the evolution of SiCp fracture is very dependent on particulate size, matrix aging condition, and the details of the matrix-reinforcement interfacial regions. SiCp fracture was exhibited by the UA and OA treatment over a range of strains, while a preference for failure near the SiCp/matrix interfaces and in the matrix was exhibited in the OA material. While thepercentage of cracked SiCp at each global strain typically was equal or somewhat lower in the material reinforced with 5 μm average size SiCp, theabsolute number of cracked SiCp was always higher at each global stress and strain in the material containing 5 μm average size SiCp, for each heat treatment. Damage(e.g., voids) in the matrix and near the SiCp/matrix interfaces was additionally observed, although its extent was highly matrix and particle-size dependent. It was always observed that increases in stress (and strain) produced a larger amount of fractured SiCp. However, neither a global stress-based nor a global strain-based model was sufficient in converging the amount of SiCp fractured for all heat treatments and particle sizes tested.

Effects of heat treatment and reinforcement size

Tensile tests of SiC-6061 Al composites containing various volume fractions of whiskers or particles (20, 5 and 0 vol.%) showed that for samples containing a high volume fraction (20 vol.%) the fracture process was very localized, i.e. a very narrow neck. As the volume fraction of whiskers or particles decreased, the deformed region spread out. One might expect that the microstructure should correspond to the macroscale changes. In the highly deformed region the dislocation density is expected to be higher, in the less deformed regions the dislocation density should be lower, and if the deformation is very localized, then the high dislocation density should also be limited to a very narrow region. Overall, there is good agreement between the microstructure (dislocation density) change and the macroscale deformation of SiCAl composite tensile samples. The mechanism proposed to account for this change in deformation behavior as a function of volume fraction of SiC in aluminum is related to the expansion of the plastic zone (due to differences in thermal coefficients of expansion between SiC and aluminum) when the external stress is applied. Also, the localized deformation is related to localized clusters of SiC particles. There is a cooperative effect which leads to a region of very localized plastic deformation.

Localized deformation of SiCAl composites

Neutron powder diffraction was employed to investigate the deformation-induced changes of matrix residual stresses in an annealed 20 vol pct SiC whisker-reinforced 6061 Al composite. It was found that the changes are asymmetric in response to a uniaxial external tensile or compressive load applied along the longitudinal whisker axis. In order to investigate this unusual asymmetric behavior and understand the underlying mechanisms, the finite element method (FEM) was employed and a parametric study was conducted. It was found that a gradient of plastic flow in the matrix is responsible for the asymmetric changes.

Deformation-induced residual stress changes in SiC whisker-reinforced 6061 Al composites

Residual microstress was measured, using X-ray and neutron diffraction, in SiCAl composites with SiC in the form of whiskers, platelets and spheres. Average whisker and sphere stresses are similar, with platelet values considerably lower, in disagreement with finite element method analyses. Reasons for the discrepancy, and a comparison of the X-ray and neutron results, are discussed.

Residual stresses in discontinuous metal matrix composites

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Papers

Localized deformation of SiCAl composites

Deformation-induced residual stress changes in SiC whisker-reinforced 6061 Al composites

Residual stresses in discontinuous metal matrix composites