Effects of sample stoichiometry of thermite-based SHS reactions on formation of Nb-Al intermetallics

Preparation of various niobium aluminides (Nb3Al, Nb2Al, and NbAl3) was conducted by self-propagating high-temperature synthesis (SHS) of the thermite type using the compressed samples from the powder mixtures of Al and Nb2O5 at a broad range of stoichiometries. For the reactant compacts with a molar ratio of Al/Nb2O5 = 4.0-6.67, self-sustaining combustion was achievable with a sufficiently high reaction temperature leading to massive melting of the burned sample. Due to a decrease in the reaction exothermicity, the flame-front propagation velocity and combustion temperature were declined by increasing the Al content in the reactant mixture. As a result, the reaction ceased to propagate in the samples with Al/Nb2O5 greater than 7.0. On account of different densities between the synthesized compounds, the final product comprises a cast metallic core extruded from a glassy shell made largely of the by-product Al2O3. Based upon the XRD analysis, two Nb-rich aluminides Nb3Al and Nb2Al were obtained in almost pure form from the powder compacts of Al:Nb2O5 = 12:3 and 13:3, respectively. For the samples with stoichiometric ratios of Al/Nb2O5 between 5.0 and 6.67, an aluminide composite composed of Nb2Al and NbAl3 was produced and the fraction of NbAl3 in the composite increased with Al content in the reactant mixture.