Tensile Creep and Creep-Recovery Behavior of a SiC-Fiber─Si3N4-Matrix Composite

The tensile creep and creep-recovery behavior of a unidirectional Sic-fiher/Si,N,-matrix composite was investigated at 1200°C in air. A primary objective of the study was to determine how various sustained and cyclic creep loading histories would influence the creep rate, accumulated creep strain, and the amount of strain recovered upon specimen unloading. The key results obtained from the investigation can be summarized as follows: (1) A threshold stress of 60 MPa was identified, helow which the creep rate of the composite was exceedingly low s-I). (2) Periodic fiber fracture was identified as a fundamental damage mode for sustained tensile creep at stresses of 200 and 250 MPa. (3) Because of transient stress redistribution between the fibers and matrix, the creep life and failure mode at 250 MPa were strongly influenced by the rate at which the initial creep stress was applied. (4) Very dramatic creepstrain recovery occurred during cyclic creep; for cyclic loading between stress limits of 200 and 2 MPa, 80% of the prior creep strain was recovered during 50-h-creep/ 50-h-unloading cycles and over 90 % during 300-s-creep/ 300-s-unloading cycles. (5) Cyclic loading significantly lowered the duration of primary creep and overall creep-strain accumulation. The implications of the results for microstructural and component design are discussed.