EFFECTS OF ENVIRONMENT ON CREEP BEHAVIOR OF NEXTEL 720/ALUMINA-MULLITE CERAMIC COMPOSITE WITH ±45° FIBER ORIENTATION AT 1200℃

摘要

The tensile creep behavior of an oxide-oxide continuous fiber ceramic composite with ±45° fiber orientation was investigated at 1200℃ in laboratory air, in steam and in argon. The composite consists of a porous alumina-mullite 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 tensile stress-strain behavior was investigated and the tensile properties measured at 1200℃. The elastic modulus was 38.9 GPa and the ultimate tensile strength (UTS) was 37 MPa. Tensile creep behavior was examined for creep stresses in the 13-32 MPa range. Primary and secondary creep regimes were observed in all tests. Tertiary creep was observed at 32 MPa in steam and at stress levels ≥ 26 MPa in argon environment, Creep run-out (set to 100 h) was achieved in all test environments for creep stress levels ≤ 20 MPa. At creep stresses > 20 MPa, creep performance was best in laboratory air and worst in steam. The presence of either steam or argon accelerated creep rates and significantly reduced creep life. Composite microstructure, as well as damage and failure mechanisms were investigated. Matrix degradation appears to be the cause of early failures in argon and in steam.