The relationship between the irradiation induced damage and the mechanical properties of single crystal Ni

Irradiation is known to lead to a degradation of the mechanical properties of materials. This is particularly crucial in the case of materials that will be used in the future thermonuclear fusion reactor, where extremely high irradiation doses are expected. In the quest of a better understanding between the irradiation induced defects and the mechanical properties Ni single crystal specimens have been irradiated with 590 MeV protons to doses ranging from 10-3 dpa to 0.3 dpa, at room temperature, 250°C and 350°C. The irradiation induced microstructure has been characterized by transmission electron microscopy, and the mechanical properties have been assessed by mechanical testing. Molecular dynamics (MD) simulations have been conducted in Ni and Cu in order to understand the defect formation and accumulation following a displacement cascade. The following conclusions are established. Following irradiation at room temperature, 43% to 55% of the irradiation induced defects in Ni consist in stacking fault tetrahedra (SFTs), 31%∼41% in loops and about 10 % in unidentified black dots. In the case of Ni irradiated at 250 °C, 44%∼53% of the irradiation induced defects consists in SFTs, 35%∼51% in loops, and less than 5% in black dots. In the case of Ni irradiated at 350 °C, 50% of the irradiation induced defects consists in voids. The remaining 50% include ∼ 14% SFTs and 36% loops. Moreover, it appears that these ratios are independent from the irradiation dose, contrary to what was found in the literature. These results allow clarifying a long standing issue, namely that in fact in Ni there is no transition in the ratio between SFTs and loops with increasing dose. In addition, it appears that in fact the irradiation induced defect density is similar to the one found in other irradiated fcc metals for RT irradiation. The data of the irradiation at 250°C are in agreement with previous published results on a neutron irradiation at 230 °C. It appears that the size of SFTs is independent of the irradiation dose, similar to what is found in irradiated Cu. It depends, however, on the irradiation temperature. MD simulations have been performed in order to understand the influence of the interatomic potential's parameters on the formation of defects following displacement cascades. Starting with a known defect configuration, namely the SFT, 4 different potentials are tested. Results of the formation of an SFT from a triangular platelet of vacancies show that i) the platelet of 6 vacancies did not collapse to SFT regardless of the annealing conditions, but formed a void above 800K, for all selected potentials; ii) the platelet of 15 vacancies collapsed into an SFT when simulated with Farkas-I and Farkas-II potentials, which have low stacking fault energies; iii) the platelet of 66 vacancies easily collapsed to an SFT at 500K for all applied potentials, even with a high stacking fault energy (SFE) of 300 mJ·m-2. The common neighbor analysis and Wigner-Seitz defect