MECHANICAL ALLOYING OF TANTALUM BORIDES AND MODELING OF BORON SOLID SOLUTIONS IN TANTALUM

Abstract

Different mechanisms of mechanochemical solid-phase reactions are observed in milling the powder mixtures of tantalum with boron under identical conditions but at different concentrations of initial products. The decomposition of the substitutional solution at 50 at.% B in the starting mixture stimulates the self-propagating mechanochemical synthesis reaction with the formation of the TaB phase. The supersatu-rated interstitial solid solution forms at a boron concentration of 66 at.%, which is dependent on the diffusion of boron atoms into tantalum, and gradually decomposes to form the TaB₂ phase. The formalism is proposed for estimating the free Gibbs energy of the interstitial and substitutional solutions with use of the regular solution model. The use of this formalism shows that milling of the tantalum and boron powders in a planetary-ball mill leads to a combined solid solution— interstitial and substitutional (two atoms of boron substitution an atom of tantalum)—at the initial stage. Both the substitutional and interstitial mechanisms lead to decrease of the Gibbs energy in this solution. When the substitutional solution prevails over the interstitial solution, the Gibbs energy of the solution becomes minimal at 50 at.% B in tantalum and leads to its disintegration.