Damage evolution in dynamic deformation of silicon carbide

Damage evolution was investigated in silicon carbide by subjecting it to dy namic deformation in (a) a compression Hopkinson-Kolsky bar (compressive st resses of 5 GPa), and (b) high-velocity impact under confinement (compressi ve 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 wa s observed through a significant increase in the 6H-SiC phase at compressiv e 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 o f this frontal layer is proposed to be related to the high lateral confinem ent, imposed by the surrounding material. It is shown that plastic deformat ion 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 wer e added as sintering aids. Several mechanisms are considered for the initia tion of fracture: (a) dilatant cracks induced by mismatch in the effective elastic moduli between two adjacent grains, leading to internal tensile str esses and creating transgranular fracture. Finite element calculations show that high tensile stresses are generated due to elastic compatibility stra ins. (b) Zener-Stroh cracks nucleated by the piled up dislocations along gr ain boundaries, and resulting in intergranular fracture; (c) cracks due to existing flaws connected with grain-boundary phases, voids, etc.; and (d) s tress concentrations due to twinning and stacking faults. The high dislocat ion density observed in the impacted specimen is consistent with existing m odels of microplasticity. (C) 2000 Acta Metallurgica Inc. Published by Else vier Science Ltd. All rights reserved.