THE STRENGTH OF METAL-MATRIX COMPOSITE JOINTS

The mechanical performance of model butt joints between Al-matrix composites reinforced with continuous alumina fibers has been investigated. The joints consisted of thin metal interlayers of varying geometries and thicknesses, normal to the fiber direction in the composites. Materials were produced by pressurized metal infiltration into fiber preforms with prescribed discontinuities in them, thus ensuring matrix continuity through the interlayer and minimization of processing defects normally introduced in actual joining operations, Tensile strengths for reference composite specimens with volume fractions in the 60 +/- 5 vol. % range were on the order of 700 MPa, compared with a matrix yield strength of sigma(0) approximate to 100 MPa. Measured joint strengths were similar to 250 MPa for interlayers of uniform thickness (60 and 300 mu m) wherein plane strain conditions predominated, and similar to 170 MPa for interlayers of variable thickness (less than or equal to 6.4mm) wherein conditions changed from plane strain to plane stress. The predominant mode of failure was interfacial debonding at an estimated local stress of similar to 260 MPa. 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 sigma(0). 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.