MATERIALS CHARACTERIZATION OF SILICON-CARBIDE REINFORCED TITANIUM (TISCS-6) METAL-MATRIX COMPOSITES .1. TENSILE AND FATIGUE BEHAVIOR/

Flexural fatigue behavior was investigated on titanium (Ti-15V-3Cr) me tal matrix composites reinforced with cross-ply, continuous silicon ca rbide (SiC) fibers. The titanium composites had an eight-ply (0, 40, 45, -45 deg) symmetric layup. Fatigue life was found to be sensitive t o fiber layup sequence. Increasing the test temperature from 24 degree s C to 427 degrees C decreased fatigue life. Interface debonding and m atrix and fiber fracture were characteristic of tensile behavior regar dless of test temperature. In the tensile fracture process, interface debonding between SIC and the graphite coating and between the graphit e coating and the carbon core could occur. A greater amount of coating degradation at 427 degrees C than at 24 degrees C reduced the Ti/SiC interface bonding integrity, which resulted in lower tensile propertie s at 427 degrees C. During tensile testing, a crack could initiate fro m the debonded Ti/SiC interface and extend to the debonded interface o f the neighboring fiber. The crack tended to propagate through the mat rix and the interface. Dimpled fracture was the prime mode of matrix f racture. During fatigue testing, four stages of flexural deflection be havior were observed. The deflection at stage I increased slightly wit h fatigue cycling, while that at stage II increased significantly with cycling. Interestingly, the deflection at stage III increased negligi bly with fatigue cycling. Stage IV was associated with final failure, and the deflection increased abruptly. Interface debonding, matrix cra cking, and fiber bridging were identified as the prime modes of fatigu e mechanisms. To a lesser extent, fiber fracture was observed during f atigue. However, fiber fracture was believed to occur near the final s tage of fatigue failure. In fatigued specimens, facet-type fracture ap pearance was characteristic of matrix fracture morphology. Theoretical modeling of the fatigue behavior of Ti/SCS-6 composites is presented in Part II of this series of articles.