- The fully dense composites were obtained by vacuum infiltrating boron carbide compacts (80% green density) with molten AZ91 magnesium alloy (850°C) and with the melt of a 50/50 AZ91- silicon mixture (1050°C). The densities composites were, 2.44 g/cm3 and 2.54 g/cm3, respectively. The impact response of the composites was studied in a series of VISAR -instrumented planar impact experiments with velocities of W and Cu impactors ranging from 100 to 1000 m/s. The velocity history recorded for the composites produced by infiltration with the Mg-Si alloy contains a distinct elastic precursor front followed by a plastic ramp. In contrast, the velocity history of the composite infiltrated with AZ91 does not display any step-like front; the amplitude of the elastic wave grows gradually from zero level and transforms smoothly into the plastic front. The influence of the composites microstructure on their compressive and tensile behavior is discussed.