Phase formation in the SHS of a $\mathrm{Ti}$–$\mathrm{B}$ mixture with the addition of $\mathrm{Si}_3\mathrm{N}_4$

The structure and phase composition of a material obtained via SHS in the combustion of a $87\%\mathrm{Ti}+13\% \mathrm{B}$ powder mixture with the addition of $\mathrm{Si}_3\mathrm{N}_4$ is under study. The phase formation mechanism in this system is discussed. It is established that the material contains $64\%$ of the $\mathrm{TiB}$ phase with an orthorhombic structure and $36\%$ of the solid boron solution in titanium $\alpha$-$\mathrm{Ti}[\mathrm{B}])$. The boron content in $\alpha$-$\mathrm{Ti}$ significantly exceeds its equilibrium content according to a state diagram. In the case of combustion of an $87\%\mathrm{Ti}+13\% \mathrm{B}$ mixture with the addition of $5\%\mathrm{Si}_3\mathrm{N}_4$, the finite product contains $\mathrm{Ti}\mathrm{B}$, $\alpha$-$\mathrm{Ti}[\mathrm{B}]$, $\mathrm{TiB}_2$, and $\mathrm{Ti}_5\mathrm{Si}_3$ phase. The $\mathrm{TiB}$ phase is present in the form of two modifications: orthorhombic and cubic. The completeness of a structural transition of the cubic modification of $\mathrm{TiB}$ into an orthorhombic one is determined by the cooling rate of a sample. It is assumed that the combustion of the mixture with the addition of $\mathrm{Si}_3\mathrm{N}_4$ forms the dispersed discharges of $\mathrm{TiN}$, which are the crystallization centers of cubic $\mathrm{TiB}$.