Multiphase reaction diffusion in transition metal-boron systems

The formation of various diffusion layers in selected transition metal-boron systems was investigated by means of pack-boriding. Samples of commercially pure titanium, molybdenum and tantalum were packed in pre-calcined amorphous boron powder and annealed for 1-4 h at temperatures between 1200 and 1500 degrees C. Experiments were carried out in a high-frequency furnace in inert gas atmosphere (less than or equal to 500 mbar Ar) or in rotary-pump vacuum (less than or equal to 5 x 10(-2) mbar). Layered boride phases were observed metallographically and identified by XRD as a function of depth. In a series of experiments for the Ta-B-system, small amounts of KBF4 (varied between 1 and 7.5 wt%) were added to the boron powder in order to activate a transport reaction supplying boron atoms to the sample surface via the gas phase. This was found to improve layer growth rates significantly. Also evidence was found that boron diffusivities in certain boride phases, like MoB or TaB2, might be higher than in the corresponding less boron-rich phases (Mo2B, TaB). (C) 1997 Elsevier Science S.A.