Cyclic Creep and Recovery Behavior of Nextel™ 720/Alumina Ceramic Matrix Composite at 1200° C in Air and Steam Environments

The cyclic creep and recovery behaviors of the N720/Al2O3 composite were investigated in this research. The ceramic matrix composite (CMC) contains a porous alumina matrix with laminated, woven mullite/alumina (NextelTM 720) fibers. The composite does not have an interface between the fiber and matrix. The CMC relies on the porous nature for flaw tolerance. The objective of this study the influences of monotonic creep and cyclic creep loading histories on the creep lifetime, creep strain rate, accumulated creep strain as well as on the recovery of creep strain at near zero stress. The cyclic creep and recovery tests were performed at 1200 °C with maximum creep stress levels of 100 and 125 MPa in air and in steam. The creep and recovery periods were ranged from 3 min to 30 h. The laboratory air tests significantly exceeded the life of the monotonic creep tests. Introduction of intermittent periods of unloading and recovery at near zero tress into the monotonic creep history resulted in one to two orders of magnitude improvement in the creep life and rate. The presence of steam greatly reduced the performance of the material. The results in steam were similar to those of the monotonic creep. The omposite microstructure, damage and failure mechanisms were also explored