HIGH-STRAIN-RATE DEFORMATION OF GRANULAR SILICON-CARBIDE

Silicon carbide powders with three particle size distributions (averag e sizes of 0.4, 3 and 50 mu m) were subjected to strain-controlled, hi gh-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 deforma tion. The powder, with initial density of 33-59% of theoretical densit y, was densified to densities between 73 and 94% of theoretical densit y in the first stage. The densified powders were subjected to a global effective strain of approximate to-0.27 in the second stage. Their re sponse to the imposed constraints occurred through both homogeneous de formation (82-100%) and shear localization (0-18%), depending on the p article size. In the coarse powder (50 mu m), the shear localization p rocess was primarily due to particle break-up (comminution) and rearra ngement of the comminuted particles, through a similar mechanism to th e 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 localiz ation at sufficiently high displacement (> 150 mu m) leads to the form ation 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 (usi ng an assumed shear strength of 2 GPa and linear thermal softening), w hich explain the bonding. An analytical model is developed that correc tly predicts break-up of large particles and plastic deformation of th e smaller ones. It is based on the Griffith fracture criterion and Wei bull distribution of strength, which quantitatively express the fact t hat the fracture is generated by flaws the size of which is limited by the particle size. (C) 1998 Acta Metallurgica Inc.