Cyclic creep and recovery behavior of Nextel™720/alumina ceramic composite at 1200 °C

Abstract

The cyclic creep and recovery behavior of an oxide–oxide continuous fiber ceramic composite was investigated at 1200°C in laboratory air and in steam environments. The composite consists of a porous alumina matrix reinforced with laminated, woven mullite/alumina (Nextel™720) fibers, has no interface between the fiber and matrix, and relies on the porous matrix for flaw tolerance. The main objective of this study was to assess the influence of various sustained 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. Cyclic creep-recovery tests were performed for maximum stress levels of 100 and 125MPa with creep and recovery periods ranging from 3min to 1h. In laboratory air, lifetimes produced in cyclic creep and recovery tests significantly exceeded those obtained in sustained creep tests. Introduction of intermittent periods of unloading and recovery at near zero stress into the creep loading history resulted in an appreciable improvement in creep lifetime. Presence of steam considerably degraded the material performance. In steam, lifetimes produced in cyclic creep and recovery tests were close to those obtained in sustained creep tests. Composite microstructure, as well as damage and failure mechanisms were investigated.