Processing for Improved Creep Behavior of Ultra-High Temperature Ceramics
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Ultra-high temperature ceramics have been considered for several extreme applications involving high temperatures and oxidizing atmospheres. Most notably, sharp leading edges and nosecones associated with hypersonic re-entry vehicles often benefit from advanced material performance under long exposure to severe conditions. ZrB2-based composites consistently compete with the other candidates for these applications due to their high melting temperatures (exceeding 3000°C), high temperature strength retention and good oxidation resistance. Long duty cycle aerospace applications particularly necessitate excellent creep deformation resistance reaching or exceeding 10-8s-1 in steady state creep rates. In the present work, ZrB2-SiC composite and ZrB2-WC alloy were selected for creep testing at 1800°C in protected environments. Two sets of experiments were performed on the hot pressed composite: four-point flexure tests at 16 and 20 MPa and compression tests under stresses ranging between 10 and 40 MPa. The data fit well to power law creep models (Norton) and based on four-point bend data, uniaxial creep parameters were determined using an analytical method present in the literature. Predicted and experimental compressive stress exponents were found to be in excellent agreement, 1.85 and 1.76 respectively. Stress exponent supported by observation of the microstructure suggest a combination of diffusion and grain boundary sliding creep mechanisms in compression. In tension, a stress exponent of 2.61, exceeds the flexural stress exponent of 2.2, suggesting an increased contribution from cavitation to the creep strain, contrasting with grain boundary sliding observed as the predominate creep mechanism for flexural creep at this temperature. Earlier work has shown the importance of grain boundary sliding on the creep deformation of ZrB2 at 1800°C, and also that this mechanism relies upon a critical accommodation mechanism involving dislocation activity in the grain boundary region. Therefore, the effect of WC addition on the creep behavior of ZrB2 was investigated in the context of a solute-dislocation interaction hypothesis in efforts to impede the accommodation event and thus retard the creep deformation. Four-point flexure experiments showed two orders of magnitude reduction in creep rates in the ZrB2-WC alloy compared to ZrB2-SiC composite when ~1.5 mol% of W dissolved in ZrB2 lattice. Additionally, a stress exponent drop from ~2.2 in ZrB2-SiC to ~1.2 in ZrB2-WC suggests transition from climb to glide controlled deformation accommodation due to effective solute interactions with gliding dislocations. The solubility of W in ZrB2 is not well known and was estimated here through two methods. First, experimentally, where ZrB2 was successfully densified by pressureless sintering to near full density at temperatures as low as 1850°C, with the addition of B4C as sintering additive. An experimental phase diagram was approximated as the samples were produced. Second, using a combination of density functional theory simulations and thermodynamic modeling based on a sublattice description of the solid solution phase. The calculated solvus line suggests no solubility of W in ZrB2 below ~1380°C and reduced solubility in the presence of C at higher temperatures.
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