Combustion synthesis of ultra-high-temperature solid solutions (ZrxNb1-x)B2. Part 1: The mechanisms of combustion and structure formation

Abstract

The self-propagating high-temperature synthesis (SHS) in Zr–Nb–B system has been thoroughly investigated with a focus on its macrokinetics and phase formation mechanisms. The activation energies were determined by analyzing the relationship between the combustion rate and temperature. For compositions NbB2–40%ZrB2 and NbB2–50%ZrB2, the increase in the initial temperature has resulted in a significant change in combustion rate's dependence on temperature, signifying a possible shift in the reaction mechanisms. The quenched combustion front technique (QCF) method in conjunction with thermodynamic and ab initio calculations were used to analyze the sequence of phase formation events, including the formation of niobium and zirconium borides through gas transport reactions involving volatile boron suboxide BO in the heating zone, followed by the emergence of a zirconium-boron melt in the combustion zone and formation of main fraction of niobium and zirconium borides. Interactions between the primary niobium and zirconium borides and the melt result in the formation of solid solutions; however, at sub-optimal combustion conditions multiple non-equilibrium solid solutions are retained in the products. To address this issue, a machine learning model was developed, attaining coefficients of determination (R2) of 89% for combustion temperature and rate predictions, thus enabling the fine-tuning of macrokinetic parameters of the SHS process in the system.