Macrokinetics of thermal explosion in a niobium-aluminum system. II. Phase-formation dynamics

The thermal explosion in a gas-free mixture of metal powders has been studied for the first time using the method of dynamic x-ray structural analysis. The dynamics of formation of new crystalline phases at all stages of the process is determined. It is shown that melting of aluminum is followed by dissolution of niobium in the melt and crystallization of the intermediate product NbAl$_3$, and the melt does not wet niobium particles. A dramatic improvement in wetting is observed at a temperature of $\approx$ 1040 K (apparently, due to the destruction of oxide films). This results in a thermal explosion, and the Nb$_2$Al phase is formed. The reaction during the thermal explosion also follows the dissolution-crystallization mechanism.