Creep behavior of nickel-copper laminate composites with controlled composition gradients
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I.
INTRODUCTION
THE
high-temperature creep behavior of inhomogeneous materials which possess fine-scale microstructural and compositional gradients reflects the composite effects of local variations in creep resistance within the gradients. Inhomogeneous materials include solidified weld metal and castings, welded structures, materials with multiphase microstructures, and composites. In assolidified microstructures, local composition gradients, with a dimensional scale on the order of the interdendritic arm spacing, are controlled by the partition coefficient for the particular alloy system. In welded structures, transitions in microstructure from the solidified weld metal, through the heat-affected zone, and into the base metal develop as a result of the thermal history associated with welding. In contrast to materials discussed above with compositions and/or microstructures which vary continuously with position, composites, both artificial or natural (e.g., directionally solidified eutectics), exhibit abrupt changes in properties between constituents. Creep of inhomogeneous materials depends on the magnitude of the local gradient. Several studies |1,2,31 have used specialized techniques, such as the impression creep test,t2'3] to directly measure the creep behavior within a gradient. However, the majority of creep tests still evaluate the effects of gradients on overall properties. For example, in a recent study of the effects of postweld heat treatment on the creep behavior of MONEL* alloy 400 *MONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
weld metal, tq the minimum creep rate decreased and the rupture life increased with heat treatment. The heat treatment, which tended to homogenize the solidification segregation, increased the creep resistance. It was hypothesized t4] that the creep resistance of weldments and other inhomogeneous materials could be evaluated based on composite modeling techniques, if the positiondependent creep resistance within the gradient is known.
I.D. CHOI, formerly Graduate Research Assistant, Colorado School of Mines, is with the Michelin Tire Company, Clermont, France. D.K. MATLOCK, Charles F. Fogarty Professor, and D.L. OLSON, Professor, are with the Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. Manuscript submitted July 10, 1989. METALLURGICAL TRANSACTIONS A
Theoretical treatments of high-temperature creep in inhomogeneous systems have primarly involved the application of standard composite modeling techniques, e.g., rule of mixtures for composites subjected to an isostrain loading condition, in which the strengths of the individual constituents are expressed as strain rate dependent parameters. Efforts have been made t5'6'71 to model the properties of actual solidified composite structures, such as directionally solidified superalloys and eutectic composites, with composite creep theory. The effects of fiber radius and phase stability on the creep properties have been, to this point, the primary co
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