SIC PARTICLE CRACKING IN POWDER-METALLURGY PROCESSED ALUMINUM-MATRIX COMPOSITE-MATERIALS

Particle cracking is one of the key elements in the fracture process o f particulate-reinforced metal-matrix composite (MMC) materials. The p resent study quantitatively examined the amount of new surface area cr eated by particle cracking and the number fraction of cracked particle s in a series of SiC-reinforced aluminum-matrix composite materials. T hese composite materials were fabricated by liquid-phase sintering and contained 9 vol pct of 23, 63, or 142 mu m SiC. The matrix properties were varied by heat treating to either an underaged or peak-aged cond ition. In general, the new surface area created by particle cracking ( S-V) and the number fraction of cracked particles (F-no) were linearly dependent on the local strain along the tensile specimen. Multiple cr acks were frequently observed in the composites containing large parti cles. It was found that the new surface area created by particle crack ing per unit strain was higher for the case of high-strength matrices and was not systematically affected by particle size within the range studied. The number fraction of cracked particles was affected by both particle size and matrix strength. A higher number fraction of partic les cracked in the composites reinforced with large particles and with high matrix yield strengths. These results are interpreted in terms o f the size of the particle defects, which is a function of particle si ze, and the critical flaw size necessary to crack a given particle, wh ich is a function of the stress on the particle. The new surface area created by cracking and the fraction of cracked particles were related and are in good agreement for the large and medium sized particles.