Assessment of the Kinetics of Local Plastic Deformation of Zr-2.5%Nb CANDU Pressure Tube Material

Constant-load pyramidal indentation creep tests and high precision micro-indentation strain rate change tests were performed to assess the effect of indentation depth, irradiation damage and temperature on the mechanical anisotropy and local plastic deformation parameters of the Zr-2.5%Nb CANDU pressure tube material. Polished samples aligned normal to the transverse (TN), axial (AN) and radial (RN) directions of the pressure tube were irradiated with 8.5 MeV Zr+ ions to simulate the effect of neutron irradiation. Constant-load pyramidal indentation creep tests performed at 25ºC show that the average indentation stress increases with decreasing indentation depth and increasing levels of irradiation. The ratio of indentation stresses on the TN plane relative to that on the AN and RN planes was 1.3 and 1.2 respectively before irradiation which reduced to 1.04 and 1.08 respectively after irradiation indicating a decrease in anisotropy as a result of irradiation hardening. The apparent activation energy of the obstacles that limit the rate of dislocation glide during indentation creep at 25ºC does not change with indentation depth and direction but increases with increasing levels of irradiation damage. Samples irradiated at 300°C with 8.5 MeV Zr+ ions show similar changes in indentation stress, anisotropy and activation energy with increasing levels of irradiation. However the values are lower than those exposed to Zr+ irradiation at 25°C indicating the effect of concurrent thermal annealing on the accumulation of irradiation damage. Micro-indentation creep tests performed on the non-irradiated samples over the temperature range from 25 to 400ºC show that the ratio of indentation stresses in the transverse direction relative to that in the radial and axial directions decreased with increasing temperature. The apparent activation energy increases with increasing temperature and is independent of indentation direction. Micro-indentation strain rate change tests show that the inverse apparent activation area of the deformation process followed a linear, Cottrell Stokes type, dependence upon the applied stress. The increase in the apparent activation work indicates that the irradiation induced damage act as strong obstacles to dislocation glide and thus increase both the magnitude and the strain rate sensitivity of the yield stress of the Zr-2.5%Nb alloy at 25ºC