Studies on the formation of aluminides in heated Nb–Al powder mixtures

Abstract

The formation of aluminides during the heating of Nb–Al powder mixtures with different initial compositions (25, 33.3 and 75at.% Al) has been studied using a differential scanning calorimeter. The effect of parameters like particle size, compaction and heating rate on the onset temperature of reaction has been determined. The results show that an increase in heating rate leads to an increase in onset temperature for compacted as well as loose powder samples in the particle size range considered. For Al-rich mixtures, compaction increases the onset temperature irrespective of particle size. For all samples, finer aluminum particles and slower heating rates resulted in a decrease in onset temperature while higher aluminum contents in the mixture led to a higher reaction temperature. In Nb-rich samples, compaction led to a decrease in the onset temperatures. NbAl3 was the first compound to form in all the mixtures, irrespective of the initial composition. After heating to 1000°C, EDS and XRD analyses confirmed the formation of only NbAl3 in Al-rich samples and a mixture of NbAl3 and Nb2Al along with unreacted niobium particles in Nb-rich samples. A subsequent heat treatment was necessary to obtain a single aluminide corresponding to the initial composition. These observations can be explained on the basis of niobium dissolution in molten aluminum and subsequent precipitation of NbAl3 in Al-rich samples and solid state diffusion through Nb3Al and Nb2Al phases in Nb-rich samples. For NbAl3 formation through the reaction between aluminum melt and niobium particles, an enthalpy of −153±15kJmol−1 and an activation energy of 255±26kJmol−1 have been calculated from DSC data.