Biaxial creep of textured zircaloy I: Experimental and phenomenological descriptions

doi:10.1016/0025-5416(85)90278-2

Materials Science and Engineering

Volume 70, April 1985, Pages 169-180

https://doi.org/10.1016/0025-5416(85)90278-2 Get rights and content

Abstract

Creep of Zircaloy-4 was studied using biaxially loaded tubes at 673 K. Pronounced creep anisotropy is attributed to the presence of strong preferred orientations (texture) Stress and temperature dependences for steady state creep were determined from closed-end internal pressurization tests. The derived activation energy and area suggest that the climb of edge dislocations is the rate-controlling mechanism for creep in Zircaloy. A single relationship is derived which correlates the temperature and stress dependence of primary creep behavior over the range of experimental investigation. Anisotropic creep behavior as a function of loading state is modelled using Hill's anisotropic formulation modified for constant levels of creep potential. This approach is successful for both cold-worked stress-relieved materials and partially recrystallized materials.

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The evolution of internal stresses in plastically deforming Zircaloy were confirmed experimentally by in situ neutron diffraction measurements [7]. A different approach has been pursued in a wide class of studies focused on cladding creep behavior with practical applications in mind [12–20]. The proposed models describe the secondary (steady state) and primary (transient) creep strains as additive contributions.
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Dislocation mechanisms in a zirconium alloy in the high-temperature regime: An in situ TEM investigation
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The reason for this unsatisfying situation is obviously that the dislocation mechanisms likely to occur above room temperature are not sufficiently understood. Different interpretations found in the literature are all based on the analysis of the temperature dependence (activation energy), stress dependence (activation volume, power-law exponent) of the deformation rate and post-mortem TEM observations [1–21]. The temperature dependence will not be discussed here.
Dislocation mechanisms responsible for the high-temperature mechanical properties of a Zr alloy have been investigated using in situ straining experiments between 250 °C and 450 °C. At 250 °C and 300 °C, the results show a steady and homogeneous dislocation motion in prismatic planes, with little cross-slip in the pyramidal and/or basal planes. At 350 °C, the kinetics of mobile dislocations becomes very jerky and inhomogeneous, in agreement with a dynamic strain aging mechanism. Above this temperature, the motion is again steady and homogeneous. Extensive cross-slip forms super-jogs which are efficient pinning points against the glide motion. These super-jogs move by glide along the Burgers vector direction, never by climb. The glide velocity between super-jogs is linear as a function of the total driving stress (applied stress minus line-tension stress due to dislocation curvature), in agreement with the solute dragging mechanism. The origin of the stress–strain rate dependence with an exponent larger than unity is then discussed.
Biaxial creep deformation behavior of Fe-14Cr-15Ni-Ti modified austenitic stainless steel fuel cladding tube for sodium cooled fast reactor
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Hexagonal close packed materials such as zircaloys used in thermal reactors are known to exhibit anisotropy in the creep properties i.e. creep strength is different in the hoop and axial directions of the internally pressurized tube, since they develop preferred orientations or texture during the fabrication process. Several studies have been reported in the literature on the biaxial creep deformation behavior of zircaloys [4–10], stainless steels [11] and titanium alloys [12]. Face centered cubic materials such as austenitic stainless steels in annealed condition are not expected to show anisotropy in the mechanical properties.
Twenty percent cold worked Fe–14Cr–15Ni–Ti modified austenitic stainless steel is used as the cladding tube material for the fuel pins of the Prototype Fast Breeder Reactor in India. Biaxial creep properties of the tubes have been studied at 973 K by carrying out creep tests by internally pressurizing the tubes. Hoop and axial components of creep strain were measured and found to be significantly different. For a given gas pressure, steady state hoop rate was higher than the axial rate. Steady state hoop and axial creep rates followed Norton's power law with the same stress exponent n = 7. Steady state hoop rates determined from biaxial creep tests agreed with the steady state creep rates determined from uniaxial creep tests. For a thin walled closed tube under internal pressure, significant axial deformation along with hoop deformation is indicative of anisotropic deformation of the material.
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The time-dependent deformation of materials or creep governs the useful life of many engineering structures. It assumes even higher significance in the case of structures constituting a nuclear reactor, wherein materials bombarded with neutrons develop defects that assist faster diffusion leading to greater plastic deformation. As a result, an understanding of the creep deformation process and factors controlling it is necessary for gauging the usefulness of materials in a nuclear reactor as well as for predicting life-times of various structures. Thus in this work we discuss the various mechanisms of creep, the rate controlling factors, deformation mechanism maps and useful life prediction methodologies. We also identify a few cases where direct application of simple creep correlations might not be feasible. Finally, we discuss the various factors that control the creep behavior of materials in light water reactors.

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Biaxial creep of textured zircaloy I: Experimental and phenomenological descriptions

Abstract

Scr. Metall.

Scr. Metall.

Mater. Sci. Eng.

J. Mech. Phys. Solids

J. Nucl. Mater.

J. Nucl. Mater.

J. Nucl. Mater.

Can. Met. Q.

Zirconium in Nuclear Applications

ASTM Spec. Tech. Publ.

Phys. Met. Metallogr. (Engl. Transl.)