Reaction behavior and evolution of phases during the sintering of Ta–Al powder mixtures

Abstract

Intermetallic compounds based on tantalum aluminides are of considerable interest in various industrial applications. In this work, the formation of tantalum aluminides has been studied in elemental powder mixtures containing 25, 50 and 66.7 at% Ta. A differential scanning calorimeter (DSC) was used to heat the samples up to 1500 K at 15 K min−1. Phase evolution was studied by heating a few samples to temperatures below and above the observed DSC peaks. The heat treated samples were analyzed using scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results suggest an exothermic reaction between tantalum particles and molten aluminum, which leads to the formation of Al3Ta compound as the initial product. This reaction reached completion for the aluminum-rich samples and the corresponding DSC peak was very broad, containing two distinct steps which indicated the effect of a diffusion barrier during the reaction. In these samples, the Al3Ta product was stable upon further heating. A different behavior was observed for the equiatomic and tantalum-rich samples, an incomplete reaction with a considerable amount of unreacted tantalum. These samples were associated with a narrower reaction peak in the DSC plots, followed by a mildly exothermic peak at higher temperatures. The latter was found to correspond to the formation of Al69Ta39 phase in the solid state, together with minor amounts of σ (Ta-rich) and φ (near equiatomic) phases. The Al69Ta39 phase showed a tendency to disappear on prolonged heating. The σ and φ phases were observed to dominate as the major phases in tantalum-rich and equiatomic samples, respectively.Increasing the heating rate shifted the reaction peak for Al3Ta formation to higher temperatures and the apparent activation energies were estimated as 383 ± 13 kJ mol−1 and 439 ± 22 kJ mol−1 for the initial and final stages of this reaction. The heat of formation of Al3Ta was also estimated as −36 ± 7 kJ mol−1 in the interval 1050–1350 K. Studies on the effect of particle sizes of the reactants showed that, in most cases, the reaction peak shifted to lower temperatures on decreasing the tantalum particle size. A similar behavior was observed for aluminum in the tantalum-rich samples, while an inverse effect was seen in equiatomic and aluminum-rich samples.