Quasi-static and high-rate mechanical behavior of aluminum-based MMC reinforced with boron carbide of various length scales

We report on an investigation of the quasi-static and dynamic mechanical behavior of an ultrafine-grained (UFG) aluminum composite reinforced with nanoscale boron carbide under uniaxial compression. Aluminum composites reinforced with either nanometric or half-micron sized B₄C exhibit high strength ~900MPa and considerable strain-to-failure under compression (>15pct.) at quasi-static (10^-3s^-1) and dynamic (10³s^-1) strain rates. Microstructural analyses of the composites and unreinforced aluminum alloy were performed to determine the strengthening mechanisms and plastic behavior that govern the structure-property relationships for this class of materials. Our results reveal that the flow stress of the composites does not depend strongly on strain rate if high rate data are included, while strain rate jump tests via instrumented nanoindentation indicate that the strain rate sensitivity exhibits similar behavior as the unreinforced matrix. The unreinforced alloy undergoes strain hardening at both quasi-static and dynamic strain rates, while the composites show strain softening at dynamic strain rates and elastic nearly-perfectly plastic behavior at quasi-static rates. Adiabatic shear band formation was evident in the composite samples during dynamic loading, whereas no shear banding was observed in the unreinforced alloy for the strain rates studied herein. The notion of adiabatic shear band toughness was adopted to identify the increased propensity to shear localization in the composites vis-a-vis the unreinforced material. An attempt is made to account for the effect of the boron carbide reinforcement size on the strengthening mechanism and overall plastic response for these metal matrix composites.