Progress of the NbAl3 Combustion Synthesis Reaction

Parallelopipedal (60 × 10 × 10-mm) bars obtained by uniaxial compaction of Nb-Al powder mixtures with a stoichiometric NbAl₃ composition (75 a/o Al) were submitted to reaction synthesis using the SHS mode (self-propagating high-temperature synthesis). The experimental procedures used were such as to allow for the reaction ignition, propagation, and extinction at a certain plane along the bar length. The microstructure of the reacted bars was characterized by optical microscopy, scanning electron microscopy, and EDS. The various phases present along the bar length evidenced that the local composition was changed due to the presence of a molten aluminum flux during the reaction. All the observed phases are predicted by the Nb-Al equilibrium phase diagram. A possible explanation for the reaction sequence is advanced based on the experimental observations.