EFFECT OF FIBER COATING ON CREEP-BEHAVIOR OF SIC FIBER-REINFORCED TITANIUM ALUMINIDE MATRIX COMPOSITES

The effect of fiber coating on the creep behavior and damage mechanisms of unnotched SCS-6 fiber-reinforced Ti3Al matrix composites under longitudinal and transverse loading was investigated at 700 degrees C. Stresses ranging from 700 to 900 MPa and 200 to 400 MPa were used for longitudinal and transverse loading, respectively. An Ag/Ta duplex layer was coated onto the SCS-6 fiber prior to consolidation via physical vapor deposition. The microstructure of the crept composites was examined to determine the creep deformation mechanisms. The creep cracking behavior of the notched composites was also studied at initial stress intensity factors, K-i, ranging from 15 to 20 MPa-m(1/2). Microstructural observation revealed that multiple fiber fracture (at low to medium stress levels), microcracking along the reaction zone/matrix interface (at medium stress levels), and matrix cracking extending from the broken fiber ends (at high stress levels) were the major damage mechanisms during quasi-steady state creep under longitudinal loading. The results show that the Ag/Ta duplex coating significantly improved the creep resistance and flexural strength of the composite under transverse loading. The Ag/Ta duplex coating was also shown to significantly prolong the creep rupture life of SiC fiber-reinforced Ti3Al composites.