Delayed mechanical failure of silver-interlayer diffusion bonds
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INTRODUCTION
IT has
long been known that thin (e.g., 1 /zm to 1 mm) interlayer welds, bonds, or brazes between stronger base materials may have high ultimate tensile or rupture strengths despite the relatively low strength of the filler metal. The high strength of the joint results from the mechanical constraint provided by the nondeforming base metals, which restricts transverse contraction of the interlayer. The constraint produces a triaxial state of stress and reduces the effective stress, thus reducing the tendency for the joint to plastically deform. H-a[ The degree of mechanical constraint in the joint is generally known to increase with decreasing thickness-to-diameter ratio of the interlayer. [5.6.7]Other factors, such as plasticity of the base metal, reduce the constraint. [7[ Higher joint strength is associated with higher constraint. Recent work by the authors 18'91 has verified an earlier observation [~~ of delayed or time-dependent tensile failure at stresses substantially less than the ultimate tensile strength. In those studies, the solid-state bonds were prepared using silver interlayers deposited by the hot hollow cathode vapor deposition process I8,9~and by interlayer foils of different compositions. [~~ The tests were performed at ambient and elevated temperatures. The preliminary conclusions I8,91 indicated that the ambient-temperature time-dependent plasticity or creep of the base metals (e.g., to plastic strains of about 0.01 or less) relieves the constraint, and the joint strength may be correspondingly degraded. That is, base metal creep induces concomitant shear within the interlayer under a state of high triaxial
M.E. KASSNER, Associate Professor, is with the Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331. R.S. ROSEN and G.A. HENSHALL, Metallurgists, are with the Physical Metallurgy and Joining Section, Lawrence Livermore National Laboratory, Livermore, CA 94550. Manuscript submitted January 11, 1990. METALLURGICAL TRANSACTIONS A
stress that causes ductile failure within the interlayer. Failure occurs at the center plane of the interlayer or near the base metal/interlayer interfaces after relatively small plastic strains. Therefore, the creep rate of the base metal controls the time-to-failure. The delayed failure phenomenon for the case of interlayer bonds or welds utilizing base metals that deform only elastically was not investigated. The purpose of this work was threefold. First, the validity of the "base metal-accelerated" delayed failure theory for bonds utilizing plastic base materials was checked. Creep-rapture tests were performed on diffusionwelded specimens, using silver interlayers deposited by planar-magnetron sputtering (PMS), a physical vapor deposition process. Bonded specimens were loaded to various tensile stresses below the ultimate tensile strength. The creep rates of the base metals and the silver interlayer were measured. There should be some correspondence between these creep rates if a base metal control explanation of failur
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