Macrokinetics of thermal explosion in a niobium–aluminum system. I. Basic macrokinetic stages

The basic macrokinetic stages of a gas-free thermal explosion in an Nb–Al system with different stoichiometric ratios of the reagents are determined using scanning differential calorimetry, electron metallography, and microanalysis. Chemical interaction begins when aluminum melts (993 K) and proceeds slowly up to a temperature of 1020–1040 K, at which dramatic self-heating and acceleration of the reaction (thermal explosion) occur. Migration of the melt from the center of the sample to the surface layers and growth of droplets on the sample surface are observed at the stage of slow reaction, and the drops penetrate again into the sample at the stage of thermal explosion. The phases NbAl$_3$ and Nb$_2$Al are formed in the thermal explosion regardless of the initial stoichiometry of the composition; the differences are manifested only in the ratio of these phases and in the amount of residual (nonreacted) niobium.