Microstructure and Mechanisms of Ion-Simulated Irradiation-Induced Creep of Nickel

The microstructure of cold-worked, high-purity nickel has been investigated following ion-simulated irradiation-induced creep with 22-MeV deuterons and 70-MeV α-particles. The irradiations were conducted at 224°C (435°F), at stresses between 170 and 345 MPa, and at displacement rates between 13 and 30 × 10^-8 displacements per atom per second (dpa/s). Transmission electron microscopy (TEM) procedures were used to prepare, observe, and photograph the microstructure of the ion-irradiated uniaxial creep specimens and companion unirradiated specimens. Examination of the ion-irradiated microstructure revealed no substantial differences between the deuteron and α-particle irradiated specimens. In all cases, a heterogeneous distribution of defect clusters or small dislocation loops and network dislocations, or both, were observed. A significant reduction in dislocation density from the unirradiated values was seen for the irradiated specimens. It was found that the small loops and defect clusters provided effective obstacles to dislocation motion as evidenced by the bowing of dislocations between adjacent defects.

The microstructural results were evaluated in terms of the theoretical mechanisms proposed for irradiation-induced creep and the previously reported creep simulation results for nickel by Hendrick et al. A model based on the climb-controlled glide of dislocations over dispersed obstacles was found to be consistent with the microstructural results and the experimental creep data.