Novel oxide-dispersion-strengthened copper alloys from rapidly solidified precursors: Part 2. Creep behavior
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I.
INTRODUCTION
THE creep of dispersion-strengthened (DS) alloys has been described in terms of three regimes that depend on the applied stress and temperature. ~,2] At high moduluscompensated stresses, these materials exhibit creep behavior similar to their non-DS counterparts, with relatively low stress exponents, 4 < n < 8, and activation energies on the order of that for bulk diffusion in the material. With decreasing stress, the creep exponents become very large, 20 < n < 100, and the activation energy for creep becomes significantly larger than that of bulk diffusion in the matrix. At high homologous temperatures and relatively low stresses, the stress exponent returns to relatively small values. The transition from the low to the intermediate stress regime has been described in terms of the presence of a "threshold stress" below which creep essentially (and in many cases for all practical purposes) ceases in the material. Observations indicate that this threshold stress is in the range of a few tenths of the Orowan stress of the material, ~ror, and increases with decreasing test temperature. This has led to the hypothesis that the creep rate in the second regime is controlled by the time required for dislocations to bypass the dispersoids by a climb mechanism. Shewfelt and Brown [3] suggested that dislocations might "locally" climb over the dispersoids only in their near vicinity, with the remainder of the dislocation remaining in its glide plane. Threshold stresses MICHAEL S. NAGORKA, formerly Graduate Research Assistant, High Performance Composites Center, Materials Department, College of Engineering, University of California at Santa Barbara, is Staff Research Engineer, Kaiser Aluminum and Chemical Corporation, Pleasanton, CA 94566. CARLOS G. LEVI, Professor of Materials and Mechanical Engineering, and GLENN E. LUCAS, Professor of Nuclear and Mechanical Engineering and Materials, are with the High Performance Composites Center, Materials Department, College of Engineering, University of California at Santa Barbara, Santa Barbara, CA. Manuscript submitted April 25, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
of 0.70"Or and 0.4o%r were calculated with this model for climb over cuboidal and spherical particles, respectively. Arzt and Ashby t4j discussed an alternate "general" climb geometry in which the length of the climbing segment is on the scale of the dispersoid spacing. Values of the threshold predicted by their model are in the range 0.04 to 0.08 OrOr14]. Analysis of these models reveals significant shortcomings. The dislocation configuration assumed by Shewfelt and Brown is unlikely to evolve in the absence of attractive dislocation/dispersoid interactions. Lagneborg, [Sj and later Rrsler and Arzt, [6l argued that the sharp bends in the dislocation line near the particle dictated by the local climb model would tend to relax by diffusional flow of vacancies and produce a line geometry more like that of general climb. Conversely, the later climb model predicts threshold stresses that are signif
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