ζ-Ta₄C₃ lamellae growth behavior and its effects on microstructure and fracture toughness of TaC_0.6-Cu composites

TaC_0.6-Cu composites were fabricated by hot pressing of TaC, Ta and Cu powder mixtures. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, scanning-transmission microscopy, X-ray energy-dispersive spectroscopy, and selected area electron diffraction were used to investigate formation process and growth behavior of the ζ-Ta₄C_3-x lamellae. Cu addition did not change the reaction sequence of TaC + Ta → Ta₂C + TaC_y → ζ-Ta₄C_3-x to form the targeted ζ-Ta₄C_3-x phase, but enhanced growth of ζ-Ta₄C_3-x lamellae by introducing a dissolution-reprecipitation mechanism due to presence of Cu liquid at high temperatures. ζ-Ta₄C_3-x could directly nucleate and grow on h-Ta₂C grain surface, meanwhile C immediately reacted with Ta solutes in Cu liquid after the C atoms diffusing along the {111}_TaCy stacking faults out of parental γ-TaC_y grains to initiate extension of ζ-Ta₄C_3-x lamella outside the γ-TaC_y grains, thus grew large ζ-Ta₄C_3-x lamellae with a uniform thickness in the microstructure of the TaC_0.6-Cu composites. The TaC_0.6-Cu composites with large ζ-Ta₄C_3-x lamellae in the microstructures showed increase in fracture toughness.