COMBUSTION SYNTHESIS IN THE TI-C-NI-MO SYSTEM .1. MICROMECHANISMS

Combustion-wave at-resting experiments were conducted on Ti-C-Ni and Ti-C-Ni-Mo powder mixtures. The reactant powder mixtures were placed within a conical hole machined in a Cu block. The reaction was initiated at the base of the cone and proceeded down the cone axis, toward the apex, until the heat loss to the Cu block was sufficient to arrest the reaction. This enabled the postreaction characterization of the three distinct regions of the combustion wave: unreacted, partially reacted, and fully reacted. The unreacted region is characterized by removal of a surface scale on the Ti particles and Ti alpha --> beta solid-state phase transformation. The partially reacted region is characterized by a number of physical processes and a distinct interface with the unreacted region. These processes include the formation of Ti-Ni phases, Ti-Ni melt, TiCx layer on the C particles, and TiCx spherules. The TiCx layer is composed of coarsening TiCx precipitates which are ejected into the progressively Ni-rich Ti-Ni melt. These TiCx spherules vary in size with apparent diameters of approximately 0.2 to 1 mu m. No distinct interface exists between the partially and fully reacted regions. Final consumption of C is followed by TiCx spherule growth by combined Ostwald ripening and grain coalescence mechanisms resulting in an apparent diameter of 2.5 mu m. The addition of Mo does not significantly affect the processes occurring within the partially reacted region. It is apparent that Mo enters into solution with the Ti-Ni melt at a rate much slower than that characteristic of the other processes (i.e., Ti-Ni melt mixing or Ti-C reaction).