Anisotropic thermal creep of internally pressurized Zr–2.5Nb tubes

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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 ∼99.54 kJ/mol in the temperature range 300–400 °C. The stress exponent increased to ∼6 for transverse stresses from 250 to 325 MPa. From this data an experimental regime of 350 °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, ε ˙ A / ε ˙ T ) exhibited strong dependence on crystallographic textures of the Zr–2.5Nb tubes. It was found that the values of ( ε ˙ A / ε ˙ 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 ( ε ˙ A / ε ˙ T ) and tubes with a strong transverse texture exhibited the positive values of steady-state creep rate ratio ( ε ˙ A / ε ˙ 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 polycrystalline 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.