Quasi-static and high strain-rate mechanical behavior of FP™ (α-alumina) long fiber reinforced magnesium and aluminum metal matrix composites

Abstract

The mechanical response of an FP long fiber (35%) Mg composite has been determined in the transverse and longitudinal directions in compression. Results were also compared with those of a similar composite of Al matrix. It was found that the composite in the transverse direction exhibited strain rate sensitivity of the flow stress and maximum stress within the studied strain rate range (10-4 to 1x103s-1). However the increase in strain rate decreased the failure strain. Microscopic observations on the failed samples have shown that the composite failed predominantly by shear banding. Near to the fracture surface DRX grains were observed within the shear band and it was proposedthat the lower ductility of the composite at increasing strain rates was due to the early DRX grain formation which softened thecomposite and resulted in lower ductility. Although twinning was observed in the deformed cross-sections of the samples at all strain rates particularly near the shear band region, it was proposed that the main deformation mechanism was slip which was evidenced by the slip lines on the fracture surface. The strain rate sensitivity in fracture stress of the composite in transverse direction was also found to be similar to that of the Al composite tested in the same direction. In the longitudinal direction, the composite failed by kink formation at quasi-static strain rates, while kinking and splitting at high strain rates. The maximum stress in the axial direction was however foundto be strain rate insensitive. In this direction similar to transverse direction DRXgrain formation was observed in the kink region. The lack of strain rate sensitivity in this direction was attributed to DRX grain formation at high strain rates combined with adiabatic heating and the brittle nature of the composite leading to fluctuatitonin the compressive strength.