CONVERSION OF TITANIUM HYDRIDE TO TITANIUM NITRIDE DURING MECHANICAL MILLING

The response of a stable titanium hydride to severe mechanical milling treatments at room temperature was studied in different gaseous envir onments using different milling media. Structural details were monitor ed by Reitveld refinements using x-ray-diffraction data and hydrogen c ompositions were measured by decomposition at 850 degrees C in a const ant volume system. Milling in an argon atmosphere produces only nanocr ystalline TiHx with a reduction in hydrogen content proportional to th e milling time. Milling in steel and Co bonded WC vials in air gave a mon rapid loss of H and simultaneous formation of TiN. The lattice par ameters of both the TiHx and the Ti formed by decomposition support a mechanism based an solid solution of N in the TiHx followed by partiti on into two phases. This resembles a chemically enhanced version of th e ''solid-solution pumping'' mechanism previously described for the ni trogenation of pure Ti milled in air. Experiments with different milli ng media and minor additions of Fe and or Co suggest that 3d transitio n metal is necessary for the nitrogenation to occur, most likely by pr omoting dissociative chemisorption of N-2.