Influence of stoichiometry on the high-temperature strength of zirconium carbide

To understand the impact of stoichiometry on the thermomechanical performance of zirconium carbide (ZrC), 3 mm diameter, 0.25 mm thick, truncated disc specimens with varying C/Zr ratios (0.84, 1.05, and 1.2) were subjected to three-point flexure at temperatures between 300 and 1673 K and strain rates between 0.0001 and 0.01/s in a high-vacuum environment. Although the sub-stoichiometric (C/Zr ∼0.84) composition exhibited higher flexural strengths because of fine grains and increased microstructural and compositional homogeneity, a significant decrease in strength with increasing temperature was observed as a result of the onset of plasticity. Further evidence of plastic behavior beyond 1270 K in this composition was indicated by drastically decreasing strain rate sensitivity. Although samples with higher carbon contents (C/Zr ratios of approximately 1.05 and 1.2) exhibited overall lower strengths compared with the sub–stoichiometric composition, the dependence of the strength and the strain rate sensitivity on the test temperature was less pronounced. The overall reduction in strength and the preservation of brittle behavior even at high temperatures in ZrC_1.05 and ZrC_1.2 could be attributed to the carbon-rich phases in these samples. Microstructural observations of the fractured specimens indicated strong stoichiometry dependence of the cleavage facet morphology. As the carbon content increased, cleavage steps became finer as the cleavage facets appeared more fragmented. This led to an overall increase in the fracture surface roughness, indicating the preservation of brittle behavior.