The effect of interfaces on the strength of metal-matrix composite joints

The role of metal-ceramic interfaces in the mechanical performance of joints and transitions involving Al-matrix composites reinforced with continuous polycrystalline alumina fibers is discussed. Model joints consisting of thin metal interlayers at varying scarf angles between composite subelements were designed, manufactured and tested to study the relevant deformation and failure phenomena. Materials were produced by pressurized infiltration of a molten Al-4.5%Mg alloy into fiber preforms with prescribed discontinuities, to ensure matrix continuity through the interlayer and the minimization of processing defects normally introduced in actual joining operations. Reference composite specimens with volume fractions in the 60 +/- 5 vol% range were fabricated in the same manner and exhibited tensile strengths on the order of 700 MPa, compared with a matrix yield strength, sigma(o), of similar to 100 MPa. Measured strengths were in the range of 250-320 MPa for butt joints with interlayer thicknesses of 60 and 500 mu m and breadth:thickness ratios of 17 to 64, with no apparent correlation between joint dimensions and strength. For a fixed aspect ratio, the joint strength was relatively insensitive to the scarf angle between 0 and 45 degrees, where it remained at similar to 310 +/- 10 MPa, but increased at higher angles reaching over 480 MPa at 75 degrees. The predominant mode of failure was interfacial debonding at the fiber ends, apparently by microvoid nucleation and growth. The results are reasonably consistent with simple models that assume debonding when the stress normal to the macroscopic composite-interlayer boundary reaches a critical level. An upper limit to the joint strength was identified as arising from unstable cavitation in the metal interlayer, expected at a stress of similar to 4-5 sigma(o). Possible avenues to increase the joint strength to a level consistent with that of the composite are discussed in light of the understanding of failure mechanisms emerging from these model configurations.