Rate Dependence of Tensile Properties and Stress-Strain Behavior of an Oxide/Oxide Ceramic Matrix Composite at Elevated Temperature and the Effects of Low-Magnitude Sustained Loading on Composite Microstructure

Advanced aerospace structures and components require materials that exhibit high strength at high operating temperatures. Nextel ™ 720/Alumina (N720/A) is an oxide/oxide, porous-matrix ceramic-matrix composite developed for load bearing applications at elevated temperatures. The current research investigates the rate dependence of tensile properties of the N720/A CMC at 1200°C as well as the creep behavior of this composite under applied stresses in the 1-25 MPa range. In addition, influence of prior loading rate on the creep response of N720/A under applied stresses above 50 MPa was evaluated at 1200°C. Tensile tests conducted at loading rate of 0.0025 and 25 MPa/s revealed a strong influence of rate on the ultimate tensile strength, elastic modulus and failure strain. Several samples subjected to additional heat-treatments were tested to ascertain whether the stress-strain behavior observed in the 0-30 MPa stress range in tests conducted at 0.0025 MPa/s was an artifact of incomplete processing of fibers in the as-received material. Additional heat treatment had no effect on stress-strain behavior at 0.0025 MPa/s. Negative creep (i. e. decrease in strain under constant stress) was observed in creep tests conducted with the applied stresses \u3c 30 MPa. Microstructural investigation using scanning electron microscope together with the results of mercury porosimetry suggest that a decrease in matrix porosity and matrix densification may be taking place in the N720/A CMC exposed to loads \u3c30 MPa for prolonged periods of time. Prior loading rate was found to have no appreciable effect on creep behavior at stresses \u3e50 MPa