High-strain-rate deformation of granular silicon carbide

Silicon carbide powders with three particle size distributions (average sizes of 0.4, 3 and 50 mu m) were subjected to strain-controlled, high-strain-rate deformation ((epsilon) over dot approximate to 3 x 10(4)/s) in a cylindrical geometry which imposed simultaneous compressive stresses. The experiments involved two explosive stages to (a) densify the powder and to (b) subject the densified granules to large deformation. The powder, with initial density of 33-59% of theoretical density, was densified to densities between 73 and 94% of theoretical density in the first stage. The densified powders were subjected to a global effective strain of approximate to-0.27 in the second stage. Their response to the imposed constraints occurred through both homogeneous deformation (82-100%) and shear localization (0-18%), depending on the particle size. In the coarse powder (50 mu m), the shear localization process was primarily due to particle break-up (comminution) and rearrangement of the comminuted particles, through a similar mechanism to the bulk and prefractured SIC (Shih, C. J., Nesterenko, V. F. and Meyers, M. A., Journal of Applied Physics. 1998, 83, 4660). Comminution was observed in the medium powder (3 mu m), but was never seen in the fine powder (0.4 mu m). In medium and fine granular SiC, the shear localization at sufficiently high displacement (> 150 mu m) leads to the formation of a thin layer (5-20 mu m) of well-bonded material. Calculated temperatures in the centers of the bands are up to 2300 degrees C (using an assumed shear strength of 2 GPa and linear thermal softening), which explain the bonding. An analytical model is developed that correctly predicts break-up of large particles and plastic deformation of the smaller ones. It is based on the Griffith fracture criterion and Weibull distribution of strength, which quantitatively express the fact that the fracture is generated by flaws the size of which is limited by the particle size. (C) 1998 Acta Metallurgica Inc.