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Ultra-high temperature ceramics (UHTCs) are refractory transition-metal carbides, nitrides, and borides with the highest melting temperatures known materials, making them prime candidates for applications in aerospace and hypersonic vehicles. Of the UHTCs, tantalum carbide (TaC) and hafnium carbide (HfC) feature the highest melting temperatures. We investigated the binderless consolidation of HfC/TaC powder blends using Field Assisted Sintering Technology (FAST). Powders consisting of 90/10, 50/50, and 10/90 vol% HfC:TaC were sintered to high densities (>94 %). Bulk and nanomechanical, chemical, and microstructural characterization revealed substantially greater strength, hardness, and stiffness for ternary alloys. Mechanical properties correlated with physiochemical analysis indicated trace oxygen phases, solid-solution strengthening, and nonstoichiometric carbon were the key mechanisms driving the peak property enhancement of the 50 vol% solid-solution sample, despite lower densities. This study provides insight into optimizing the compositional design of HfC-TaC alloys by balancing influences from solid solution strengthening and the thermodynamic effects of oxygen/carbon stoichiometry. [ABSTRACT FROM AUTHOR]