Creep and microstructure of Nextel™ 720 fiber at elevated temperature in air and in steam

Creep of Nextel™ 720 alumina–mullite fiber tows was investigated at 1100 and 1200 °C for tensile stresses of 100–400 MPa in air and in steam. Fiber microstructures were characterized after creep by transmission electron microscopy. At low stresses steam increased creep rates by up to an order of magnitude and reduced creep lifetimes. At high stresses creep rates in steam and air were similar. Cavitation was prevalent in steam but not in air. The creep-rupture data obtained at 1200 °C were analyzed in terms of a Monkman–Grant (MG) relationship. The MG parameters were independent of the test environment. Results reveal that the MG relationship can be used to predict creep rupture for Nextel™ 720 fibers and composites reinforced with these fibers at 1200 °C in air and in steam. In steam the mullite in the Nextel™ 720 fibers decomposed to porous alumina. Decomposition kinetics were linear and had an activation energy of ∼200 kJ mol−1. Intergranular films were not observed on alumina grain boundaries or alumina–mullite interphase boundaries after creep in steam. Creep mechanisms are discussed.