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°C with different exposure times in order to vary the amount of fiber-matrix reaction. This resulted in samples with reaction zone thicknesses (δ) of 1.1 and 1.7 μm, while ensuring roughly the same residual stress state. A pushout test was used to determine the debond strength (τ_d) and sliding resistance (τ_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 δ = 1.1 μm sample (where debonding occurred between the fiber's SCS carbon coating and the reaction product) revealed a debond fracture energy, Γ_i ≈ 0-0.9 J m^-2, a coefficient of friction (assuming simple Coulomb friction), μ ≈ 0.5-0.95, and a radial residual stress, σ_r ≈ 100-190 MPa. A similar analysis on the δ = 1.7 μm 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.