Damage evolution in dynamic deformation of silicon carbide

Damage evolution was investigated in silicon carbide by subjecting it to dynamic deformation in (a) a compression Hopkinson-Kolsky bar (compressive stresses of 5 GPa), and (b) high-velocity impact under confinement (compressive stresses of 19-32 GPa) by a cylindrical (rod) tungsten alloy projectile. Considerable evidence of plastic deformation, as dislocations and stacking faults, was found in the fractured specimens. A polytype transformation was observed through a significant increase in the 6H-SiC phase at compressive stresses higher than 4.5 GPa (in the vicinity of the dynamic compressive failure strength). Profuse dislocation activity was evident in the frontal layer in the specimen recovered from the projectile impact. The formation of this frontal layer is proposed to be related to the high lateral confinement, imposed by the surrounding material. It is shown that plastic deformation is consistent with an analysis based on a ductility parameter (Delta = K-C/tau(y)root pi c). The microstructural defects and their evolution were found to be dependent on the concentration of boron and aluminum, which were added as sintering aids. Several mechanisms are considered for the initiation of fracture: (a) dilatant cracks induced by mismatch in the effective elastic moduli between two adjacent grains, leading to internal tensile stresses and creating transgranular fracture. Finite element calculations show that high tensile stresses are generated due to elastic compatibility strains. (b) Zener-Stroh cracks nucleated by the piled up dislocations along grain boundaries, and resulting in intergranular fracture; (c) cracks due to existing flaws connected with grain-boundary phases, voids, etc.; and (d) stress concentrations due to twinning and stacking faults. The high dislocation density observed in the impacted specimen is consistent with existing models of microplasticity. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.