Mechanical Synthesis of Tantalum Borides and Modeling of Solid Solutions of Boron in Tantalum

Abstract

Two mechanisms of mechanochemical solid-phase reactions are observed when tantalum and boron powder mixtures are ground in the same conditions but at different contents of the starting components. The substitutional solid solution decomposes at 50 at.% B in the starting mixture and stimulates the self-propagating mechanochemical synthesis reaction to form the TaB phase. The supersaturated interstitial solid solution, whose formation is controlled by the diffusion of boron atoms into tantalum, shows up at a boron content of 66 at.% and gradually decomposes to form the TaB2 phase. Formalism is proposed for estimating the free Gibbs energy of the interstitial and substitutional solid solutions with the regular solution model. The use of this formalism shows that grinding of the tantalum and boron powders in a planetary-ball mill produces a combined interstitial and substitutional solid solution (two boron atoms substitute one tantalum atom) at the initial stage. Both the substitutional and interstitial mechanisms lead to decrease in the solution’s Gibbs energy. When the substitutional solid solution prevails over the interstitial solution, the Gibbs energy of the substitutional solution becomes minimal at 50 at.% B in tantalum and causes its disintegration.