Intergranular stresses in ZIRCALOY-2
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I. INTRODUCTION
RESIDUAL stresses are generated when samples undergo inhomogeneous plastic deformation. The stresses occur on three length scales. First, a macroscopic stress field, or type 1 stress, of primary interest to the engineer since it may lead to the failure of a component, has a length scale of the same size as the part. In the absence of an applied stress, the residual stresses in the part are in equilibrium. Second, whenever polycrystalline metals are loaded beyond the yield point, the anisotropy of the plastic and elastic deformation of the grains, for grains of different crystallographic orientation, always generates residual stresses. In this case, the stresses and the corresponding strains are different for each grain orientation and are termed intergranular, or type 2 stresses. The spatial scale of these stresses is of the order of the grain size. In a small volume of material sufficient to contain many grains, the intergranular stresses for different grain orientations balance each other and a measurement with a strain gauge is not sensitive to the effect. Only techniques, such as diffraction, which single out particular reflections and, hence, grain orientations, are sensitive to type 2 stresses. Finally, there are strain fields associated with extended defects within grains (type 3 effects), and these have a spatial scale on the order of 1000s ˚ . Type 1 stresses give rise to shifts in the diffraction of A peaks, as do type 2 stresses. In addition, because the response of different crystal orientations that present a given plane normal in a certain direction does vary, type 2 stresses also lead to line broadening. Type 3 stresses contribute to line broadening only. In this review, the emphasis is on using the intergranular strains, deduced from the shifts in diffraction lines, to learn about the deformation processes that have occurred. Since diffraction methods only measure elastic T.M. HOLDEN, Wheatley Scholar, Los Alamos Neutron Science Center, and J.W.L. PANG, Staff Scientist, Metals and Ceramics Division, are with Oak Ridge National Laboratory, Oak Ridge, TN 37831-6118. Contact email: [email protected] or [email protected] R.A. HOLT, Director, is with the Fuel Channels Division, Atomic Energy of Canada, Ltd., Chalk River, ON, Canada K0J 1P0. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
strains, a theoretical model is needed that considers both elastic and plastic deformation in the grains under applied stress. Then, the measured elastic strains may be compared with the theoretical elastic strains in
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