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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. 

Molybdenum and its alloys are of interest for applications with extreme thermomechanical requirements such as nuclear energy systems, electronics, aerospace vehicles, and hypersonic vehicles. In the present study, pure molybdenum and samples with added hafnium carbide (HfC) grain refiners were produced using field assisted sintering technology (FAST). The molybdenum and HfC reacted with oxygen to produce MoO2 and HfO2, and increased HfC content from 1 wt% to 5 wt% decreased grain size while the microhardness correspondingly increased. Room temperature three-point bending tests were conducted, and finite element modeling was used to define HfC-dependent bilinear material models. The presence of oxygen most severely affected pure molybdenum, which exhibited little strength and limited ductility, whereas for samples with added HfC, HfO2 was present, resulting in increased toughness hypothesized to be due to microcrack toughening. The samples with 1 wt% added HfC had the greatest energy absorption capability.