Effect of 20 mol% Al addition on phase formation and mechanical properties of TaCx (x=0.5, 0.55, 0.6 and 0.7) ceramics

Highlights: • TaC_x (x = 0.5–0.7) ceramics with addition of 20mol%Al were prepared. • The composites were easy to densify due to presence of a transient Al–Ta liquid. • The Al–Ta liquid was replaced by formation of the ultimate refractory Ta₄AlC₃ and ζ-Ta₄C_3-z/h-Ta₂C. • Lamellar growth of the constituent phases imparted good fracture toughness to the composites. TaC_x (x = 0.5, 0.55, 0.6 and 0.7) ceramics with 20 mol% Al addition were prepared by hot pressing TaC, Ta and Al powder mixtures. Effects of the overall C:Ta ratios and the Al addition on constituent phases, microstructures and mechanical properties of the consolidated materials were examined. Results showed that O impurity contained in the starting powders was collected in form of Al₂O₃ due to strong affinity of O with Al. The TaC_x-Al composites were easy to densify due to presence of molten Al at its melting point (T_m = 660 °C) and a transient Al–Ta eutectic liquid at higher temperatures. After densification, the metallic liquids were eliminated ultimately by formation of refractory Ta₄AlC₃ via reaction of Al, Ta and TaC. Reactions between the remnant Ta and TaC after Ta₄AlC₃ formation resulted in ζ-Ta₄C_3-z or h-Ta₂C depending on the initial C:Ta ratio. The Ta₄AlC₃ compound was compatible with the tantalum carbides. Microstructures of the TaC_x-Al (x = 0.5–0.7) composites were consisted of ζ-Ta₄C_3-z and Ta₄AlC₃ fine lamellar grains to benefit the mechanical properties of the composites. The TaC_0.7–20mol% Al composition had satisfying mechanical properties (flexural strength 613 ± 28 MPa and fracture toughness 11.2 ± 0.8 MPa m^1/2). TG-DSC evidenced an increase in oxidation resistance of the TaC_x ceramics by the addition of 20 mol% Al.