Modeling the Reaction Synthesis of Shock-Densified Titanium-Silicon Powder Mixture Compacts

摘要

The reaction behavior of shock-consolidated Ti-Sipowder mixture compacts, densified at 5 to 7GPa pressure, wasinvestigated to determine conditions required for solid-statereaction synthesis leaking to the formation of dense Ti_5Si_3intermetallic compounds with fine-grained microstructure. Itwas observed that at temperatures greater than 1000deg C, theheat released following reaction initiation in the solid stateexceeds the rate of heat dissipation causing a self-propagatingcombustion-type reaction to take over the synthesis processforming highly porous reaction products. A reaction synthesismodel was developed to allow the prediction of optimumconditions necessary to ensure that the bulk of the reaction indynamically densified Ti-Si powder compacts occurs by rapidsolid-state diffusion and without being taken over by thecombustion process. The model incorporates mass and heatbalance with the kinetics evaluated using experimentallydetermined apparent activation energies for solid-state andcombustion reactions. Considering the decrease in activationenergy (as measure of degree of shock activation), averageparticle size, and compact porosity as the main variables, themodel plots the fraction reacted as a function of time for variouspostshock reaction-synthesis temperatures, illustrating thedominant reaction mechanism and kinetics. The results showthat although changes in average particle size and compactporosity influence the synthesis temperature above which thereaction may be taken over by the combustion-type process,lowering of the activation energy via shock-compressioninfluences the time for reaction completion in the solid state.