Abstract
Here we consider our four-point flexure and compression creep results obtained under Ar protection at 1800 °C to predict the tensile creep behavior of a ZrB2–20 vol% SiC ultra-high temperature ceramic. Assuming power law creep, and based on four-point bend data, we estimated the uniaxial creep parameters using an analytical method present in the literature. Both predicted and experimental compressive stress exponents were found to be in excellent agreement, 1.85 and 1.76 respectively, while observation of the microstructure suggested a combination of diffusion and grain boundary sliding creep mechanisms in compression. Along with the microstructural evidence associated with the tensile regions of the flexure specimens, the predicted tensile stress exponent of 2.61 exceeds the measured flexural value of 2.2. We assert an increasing role of cavitation to the creep strain in pure tension. This cavitation component adds to the dominant grain boundary sliding mechanism as described below and elsewhere for flexural creep.
Highlights
Ultra-high temperature ceramics (UHTCs), namely ZrB2 and HfB2, are designed to operate at temperatures as high as 2400 °C due to their high melting points, oxidation resistance, and mechanical properties [1,2,3,4,5,6]
The analytical solution proposed by Chuang coupled with our compression and flexure data were used to determine the tensile power law creep constants for ZrB2–20 vol% SiC composite at 1800 °C
The new geometry used in compression tests showed similar strain rates as the cubic specimens but allowed for higher strains to be reached
Summary
Ultra-high temperature ceramics (UHTCs), namely ZrB2 and HfB2, are designed to operate at temperatures as high as 2400 °C due to their high melting points, oxidation resistance, and mechanical properties [1,2,3,4,5,6]. Improved oxidation behavior as well as enhanced fracture strength and toughness can be achieved by addition of 20–30 vol% of SiC [2,7]. Creep behavior of these materials is important for several applications such as re-entry vehicles and hypersonic aircraft, which involve exposure to high temperatures for extended periods of time [4,8]. Four-point bending creep tests and compression tests were conducted on ZrB2–20 vol% SiC at 1800 °C. We showed the solution proposed by Chuang [17] to successfully predict the compressive stress exponent which we measured here. We propose an enhanced specimen geometry for compression creep testing that allows higher creep strains to be reached prior to the onset of catastrophic specimen instability
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