Formation of Ta-Al intermetallics by combustion synthesis involving Al-based thermite reactions

Thermite-based combustion synthesis was conducted to investigate in situ formation of tantalum aluminides (Ta2Al, TaAl, TaAl2, and TaAl3) and Al2O3 from the powder compacts of Al and Ta2O5 under a molar ratio ranging from Al/Ta2O5 = 13/3 to 28/3. Effects of the sample stoichiometry on the combustion behavior and degree of phase conversion were studied. Experimental evidence showed that upon ignition, the synthesis reaction proceeded in a self-sustaining manner featuring a planar combustion front or two localized reaction zones traveling spirally. The combustion front temperature varies between 1135 and 1460 °C. In agreement with composition dependence of the reaction exothermicity, the reaction front velocity in the axial direction increased from 5.8 to 7.2 mm/s with molar ratio of Al/Ta2O5 from 13/3 to 16/3, beyond which the flame velocity decreased substantially. The slowest reaction front with an axial velocity of 1.15 mm/s was observed in the sample of Al/Ta2O5 = 28/3. Based upon the XRD analysis, in situ formation of the composites composed of Al2O3 and tantalum aluminides was confirmed. For the samples with Al contents of Al/Ta2O5 = 13/3-18/3, an increase in the Al/Ta2O5 ratio improved the production of tantalum aluminides and caused the change in the dominant aluminide phase from Ta2Al, TaAl, to TaAl2. Due to lack of adequate reaction time, the phase conversion was not stoichiometrically achieved in the samples of Al/Ta2O5 = 13/3-18/3. However, it was found that the slower reaction front provided longer reaction time to complete the phase conversion almost stoichiometrically in the samples of Al/Ta2O5 = 22/3-28/3, which yielded the Al-rich phases TaAl2 and/or TaAl3 in the final products.