Volume 8, Issue 2 (February 2011)
Polycrystalline Modeling of the Effect of Texture and Dislocation Microstructure on Anisotropic Thermal Creep of Pressurized Zr-2.5Nb Tubes
We have investigated the thermal creep of cold-worked Zr-2.5Nb tubes for a range of textures and stress states. The creep tests were performed on internally pressurized thin-wall standard (ratio of axial and transverse stress R≈0.5) and end-loaded (ratio of axial and transverse stress, 0.25<R<0.75) capsules 10 mm in diameter×40 mm long×0.5 mm wall-thickness at a stress of 300 MPa at 350°C where dislocation creep is the dominant operating mechanism. The tests showed an obvious correlation of anisotropic creep with the texture under different stress states. A self-consistent visco-plastic polycrystalline model based solely upon crystallographic texture showed a poor correlation with experimental results for radial basal textures, similar to that found previously for in-reactor creep. Hence texture alone is not sufficient to accurately predict the creep anisotropy. A modified self-consistent model is introduced to take into account different pre-existing anisotropic dislocation distributions (from cold-work) in different crystal orientations. Much better agreement with the experimental creep anisotropy was found, indicating that individual dislocation distributions in grains with different orientations are important in controlling the creep anisotropy. The predicted effect on creep anisotropy of introducing dislocation structures by a radically different strain path is very large.