SHOCK-ENHANCED ALPHA TO BETA-PHASE TRANSFORMATION IN SI3N4 POWDERS

Shock-compression was used to make Si3N4 powder compacts (80-85% theor etical density) in order to investigate its effect on the alpha-beta p hase transformation occurring during post-shock thermal treatments. Cr ystallite size reduction to half of the starting powder size was attai ned, and residual strains in crystallites in excess of 10(-3)-10(-2), corresponding to dislocation densities of the order of 10(15)-10(17) c m(-2), were generated due to shock compression. Transmission electron microscopy showed evidence of defects in the form of irregular striati ons (fringes), hexagonal dislocation arrays, sub-grains, and vacancy c lusters in the interiors of the individual crystallites, with contrast -free amorphous regions along interparticle boundaries. Upon subsequen t annealing (sintering) of the shock-densified compacts, the alpha-bet a phase transformation was observed to occur at temperatures of approx imate to 200 degrees C below that in unshocked powders, with no obviou s change in density. The amount of alpha-beta phase transformation inc reased with increasing temperature and time, with up to 87% conversion to beta-phase in the compact heated for 5 h at 1650 degrees C. The ap parent activation energy for the alpha-beta phase transformation in th e shock-compressed silicon nitride powder compacts was determined base d on the measured fraction transformed as a function of time and tempe rature, and was found to be in the range of 154-286 kJ mol(-1). The re sults provide evidence that shock compression activates and enhances t he reactivity of Si3N4 powders, by creating defect sites for heterogen eous nucleation of the beta-phase, and thereby causing the alpha-beta phase transformation to occur in the solid state and at reduced temper atures. (C) 1998 Elsevier Science S.A. All rights reserved.