The boronization of refractory transition metals

Summary

• 1. An investigation was made of the diffusional saturation process of some d-transition metals with boron. The phase composition of the boride layers forming on the metals was determined, and their structure resulting from different boronizing conditions was studied.

• 2. It is shown that growth of the layers occurs as a result of one-sided diffusion of boron atoms from the boron-containing medium, via the phases being formed, into the metal. The kinetics of the process are described by a law which, is close to that of a quadratic parabola.

• 3. On the basis of a metallographic analysis, it was established that, with increasing thickness of the diffusion layers, the latter's adhesion to the base metal becomes weaker, their brittleness increases, and cracks in them grow in quantity and size. When selecting practical boronizing conditions, one should aim at the minimum temperature (1100–1200°C) and holding time which secure layers of the required thickness.

• 4. Determinations were made of the reaction diffusion constants and of the effect of temperature on the coefficients of diffusion of boron into titanium, zirconium, columbium, tantalum, molybdenum, and tungsten. The view expressed in [6], according to which the processes of diffusion of nonmetals into d-transition metals are governed by the acceptor capacity of the d-electron shells of the metallic atoms, was confirmed.