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

Abstract

Preparation of various niobium aluminides (Nb 3Al, Nb 2Al, and NbAl 3) was conducted by self-propagating high-temperature synthesis (SHS) of the thermite type using the compressed samples from the powder mixtures of Al and Nb 2O 5 at a broad range of stoichiometries. For the reactant compacts with a molar ratio of Al/Nb 2O 5 = 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/Nb 2O 5 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 Al 2O 3. Based upon the XRD analysis, two Nb-rich aluminides Nb 3Al and Nb 2Al were obtained in almost pure form from the powder compacts of Al:Nb 2O 5 = 12:3 and 13:3, respectively. For the samples with stoichiometric ratios of Al/Nb 2O 5 between 5.0 and 6.67, an aluminide composite composed of Nb 2Al and NbAl 3 was produced and the fraction of NbAl 3 in the composite increased with Al content in the reactant mixture.