Modeling the reaction synthesis of shock-densified titanium-silicon powder mixture compacts

The reaction behavior of shock-consolidated Ti-Si powder mixture compacts, densified at 5 to 7 GPa pressure, was investigated to determine conditions required for solid-state reaction synthesis leading to the formation of dense Ti5Si3 intermetallic compounds with fine-grained microstructure. It was observed that at temperatures greater than 1000 °C, the heat released following reaction initiation in the solid state exceeds the rate of heat dissipation causing a self-propagating combustion-type reaction to take over the synthesis process forming highly porous reaction products. A reaction synthesis model was developed to allow the prediction of optimum conditions necessary to ensure that the bulk of the reaction in dynamically densified Ti-Si powder compacts occurs by rapid solid-state diffusion and without being taken over by the combustion process. The model incorporates mass and heat balance with the kinetics evaluated using experimentally determined apparent activation energies for solid-state and combustion reactions. Considering the decrease in activation energy (as measure of degree of shock activation), average particle size, and compact porosity as the main variables, the model plots the fraction reacted as a function of time for various postshock reaction-synthesis temperatures, illustrating the dominant reaction mechanism and kinetics. The results show that although changes in average particle size and compact porosity influence the synthesis temperature above which the reaction may be taken over by the combustion-type process, lowering of the activation energy via shock-compression influences the time for reaction completion in the solid state.