Anisotropy of yielding in a Zr-2.5Nb pressure tube material
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NTRODUCTION
IN contrast to cubic materials, hcp metals are highly anisotropic during plastic deformation. In cubic metals, the amount of strain needed for the flow to be fully plastic is small, due to the availability of five independent slip systems of the same type. In general, these materials do not exhibit a strong plastic anisotropy in the flow stress if tested in different directions. In contrast, in hcp materials, there are fewer than the required five slip systems of the prismatic type, and additional systems of the basal and pyramidal type are required to accommodate the imposed strain. As a result, the elastoplastic transition takes place over a larger amount of strain and the observed flow stress varies significantly, depending on the direction of testing. In addition, the large anisotropy in the plastic properties of the single crystal, the strong crystallographic texture of these alloys, and the presence of large intergranular strains contribute greatly to the observed plastic anisotropy of the hcp polycrystalline aggregates. Zirconium alloys are used extensively in CANDU pressurized heavy-water power reactors, and their behavior under different states of stress can be highly anisotropic. In these reactors, the Zr-2.5Nb alloy is used in the production of pressure tubes, and its creep and fracture properties are a strong function of temperature, applied stress, and texture.[1,2] The creep and fracture properties of this alloy are also a strong function of the plastic behavior, particularly when the mechanical response of these components is N. CHRISTODOULOU, Senior Research Scientist, Deformation Technology Branch, and M. RESTA LEVI, Research Scientist, Materials and Mechanics Branch, are with the CRL, AECL, Chalk River, ON, Canada K0J 1J0. P.A. TURNER, Professor, is with IFIR, 2000 Rosario, Rosario, Argentina. E.T.C. HO, Principal Research Engineer, is with the Zirconium Metallurgy Department, Ontario Hydro Technologies, Toronto, ON, Canada M8Z 5S4. C.K. CHOW, Research Scientist, is with the Fuel Channel Engineering Branch, AECL, Mississauga, ON, Canada L5K 1B2. Manuscript submitted May 4, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
required under “off-normal” or “accident” conditions. In the present study, the anisotropy in the yield stress of this alloy is analyzed as a function of temperature, strain rate, and texture by means of a viscoplastic self-consistent model. In the present model, the deformation rate of the single crystal is assumed to be proportional to the resolved shear stress raised to the power of n, which is inversely proportional to the strain-rate-sensitivity coefficient (m). Experimental results from tensile and torsion tests on specimens made from this alloy are used to derive the value of the critical resolved shear stress (CRSS) of the slip systems that are activated during plastic flow and to determine the dependence of CRSS on temperature and strain rate.
II. EXPERIMENTAL PROCEDURE AND MATERIALS The fabrication procedures and the resulting microstructures of the extruded
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