THE EFFECT OF FIBER-MATRIX REACTIONS ON THE INTERFACE PROPERTIES IN A SCS-6/TI-24AL-11NB COMPOSITE

The interfacial structure/property relationships of a representative composite system consisting of SiC (SCS-6) fibers in a Ti3Al + Nb intermetallic alloy have been investigated. Two samples were fabricated at 1040-degrees-C with different exposure times in order to vary the amount of fiber-matrix reaction. This resulted in samples with reaction zone thicknesses (delta) of 1.1 and 1.7 mum, while ensuring roughly the same residual stress state. A pushout test was used to determine the debond strength (tau(d)) and sliding resistance (tau(s)) of both interfaces. An increase in the interface debond strength and sliding resistance with reaction zone thickness was observed and has been correlated with a change in debond path. Pushout analysis of the delta = 1.1 mum sample (where debonding occurred between the fiber's SCS carbon coating and the reaction product) revealed a debond fracture energy, GAMMA(i) almost-equal-to 0-0.9 J m-2, a coefficient of friction (assuming simple Coulomb friction), mu almost-equal-to 0.5-0.95, and a radial residual stress, sigma(r), almost-equal-to 100-190 MPa. A similar analysis on the delta = 1.7 mum sample proved unsuccessful using either a simple Coulomb or Coulomb plus constant friction law. This is believed to be due to multiple, debond path branching between the SiC/inner SCS, inner SCS/outer SCS and outer SCS/reaction product interfaces. The transition to this mode of sliding is deleterious to composite properties and suggests the importance of minimizing the integrated thermal exposure associated with the consolidation process.