Some reactive processing aspects of high-temperature aluminides Nb3Al and RuAl

In the present investigation elemental powder mixtures with nominal composition of Nb-25at. % Al and Ru-50at.% Al corresponding to Nb₃Al and RuAl intermetallic phases were prepared. Green compacts from these mixtures were reactive sintered in vacuum or hot isostatically pressed after being encapsulated and reacted in quartz capsules. In the absence of pressure, reactive sintering at temperatures up to 1800 °C did not lead to densification, and swelling effects were observed. Reactions of formation of Nb₃Al and RuAl were found to take place at different temperature regions characteristic for each aluminide system, with clear dependence of the reaction temperature on the heating rate of the powder mixtures. Dilatometry studies showed that swelling occurred instantaneously at temperatures corresponding to the reaction temperature and led to a volume increase of about 25% and 42% for Nb₃Al and RuAl, respectively. The activation energy for the reactions was estimated from the differential thermal analysis. Similar data for the NiAl intermetallic phase were also obtained for comparison. The densification behavior and the phase composition of the reaction products were investigated and correlated with the heating rate, the mutual solubility of the reactants, and the amount of liquid phase available during the reaction.