Residual strains in HY100 polycrystals: Comparisons of experiments and simulations
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I. INTRODUCTION
BY their nature, plastic deformations alter the structural state of a metal and, in so doing, modify its derivative mechanical properties. This may be done intentionally, so as to render a material with superior properties, or it may be unintentional or unwanted, with possibly very detrimental effects. Important to the mechanical response of a metal is the internal stress state of its constituent crystals, a condition that is determined in large measure by the prior thermomechanical history imposed on the metal. Such stress may exist in a metal even when the external tractions are effectively zero and is then referred to as residual stress. The focus of this article is on residual stress at the crystal level in a polycrystalline metal following various amounts of plastic deformation. Residual stress will exist after unloading if the elastic strain during loading does not constitute a compatible field. Residual stress arises naturally when a polycrystal is unloaded following plastic straining, whether or not the macroscopic* elastoplastic deformation is uni*In this article, macroscopic quantities such as the stress or deformation rate are averages of corresponding crystal quantities taken over volumes of material which are much larger than the volume of an individual crystal.
form. The magnitude of this stress depends on the degree of the single-crystal anisotropy and the strength of the crystallographic texture and may be substantial for many alloys. Should the macroscopic deformation vary over the dimension of a component, the residual stress state may vary from one position to another. In this case, there exists a macroscopic state of residual stress that varies spatially over
PAUL DAWSON, Professor, and DONALD BOYCE, Research Associate, are with Cornell University, Ithaca, NY 14853. STUART MacEWEN, Principal Scientist, is with Alcan International, Limited, Kingston, ON, Canada. RONALD ROGGE, Assistant Research Officer, is with the Neutron Program for Materials Research, National Research Council, Chalk River Laboratories, Chalk River, Ontario, Canada. Manuscript submitted July 7, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
the dimension of the component and that coincides at each position with a nonvanishing crystallographic average stress. Attention here is directed at deformations that are uniform macroscopically and that produce no macroscopic residual stress. By limiting our attention to this case, we can investigate better the occurrence of residual stress at the intercrystalline level without introducing the additional complexity associated with the macroscopic stress variations. Neutron diffraction offers an effective means of measuring the spacing of atomic planes in crystal lattices, with sufficient precision to determine the lattice distortions and the associated elastic strain. This can be done without destroying the specimen and even when the specimen is under load. Neutrons can penetrate deeply into a metal volume and diffract off particular sets of grains, thereby facilitat
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