Anisotropic thermal creep of internally pressurized Zr-2.5Nb tubes

The anisotropy of creep of internally pressurized cold-worked Zr-2.5Nb tubes with different crystallographic textures is reported. The stress exponent n was determined to be about three at transverse stresses from 100 to 250 MPa with an activation energy of similar to 99.54 kJ/mol in the temperature range 300-400 degrees C. The stress exponent increased to similar to 6 for transverse stresses from 250 to 325 MPa. From this data an experimental regime of 350 degrees C and 300 MPa was established in which dislocation glide is the likely strain-producing mechanism. Creep tests were carried out under these conditions on internally pressurized Zr-2.5Nb tubes with 18 different textures. Creep strain and creep anisotropy (ratio of axial to transverse steady-state creep rate, epsilon(A)/epsilon(T)) exhibited strong dependence on crystallographic textures of the Zr-2.5Nb tubes. It was found that the values of (epsilon(A)/epsilon(T)) increased as the difference between the resolved faction of basal plane normals in the transverse and radial directions (f(T) - f(R)) increases. The tubes with the strongest radial texture showed a negative axial creep strain and a negative creep rate ratio (epsilon(A)/epsilon(T)) and tubes with a strong transverse texture exhibited the positive values of steady-state creep rate ratio (epsilon(A)/epsilon(T)) and good creep resistance in the transverse direction. These behaviors are qualitatively similar to those observed during irradiation creep, and also to the predictions of poly-crystalline models for creep in which glide is the strain-producing mechanism and prismatic slip is the dominant system. A detailed analysis of the results using polycrystalline models may assist in understanding the anisotropy of irradiation creep. (C) 2010 Elsevier B.V. All rights reserved.