ζ-Ta4C3 lamellae growth behavior and its effects on microstructure and fracture toughness of TaC0.6-Cu composites

Abstract

TaC0.6-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-Ta4C3-x lamellae. Cu addition did not change the reaction sequence of TaC + Ta → Ta2C + TaCy → ζ-Ta4C3-x to form the targeted ζ-Ta4C3-x phase, but enhanced growth of ζ-Ta4C3-x lamellae by introducing a dissolution-reprecipitation mechanism due to presence of Cu liquid at high temperatures. ζ-Ta4C3-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-TaCy grains to initiate extension of ζ-Ta4C3-x lamella outside the γ-TaCy grains, thus grew large ζ-Ta4C3-x lamellae with a uniform thickness in the microstructure of the TaC0.6-Cu composites. The TaC0.6-Cu composites with large ζ-Ta4C3-x lamellae in the microstructures showed increase in fracture toughness.