Zhongwen Yao
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 defects analysis are used for studying the resulting structures after a displacement cascade, and results show that the largest number of stacking faults is surprisingly obtained with the potential giving the highest stacking fault energy (300 mJ·m-2). An SFT-like structure appears close to the core of cascades and also an isolated interstitial loop are found with Cleri-Rosato potential. It appears that the defects size, configuration and density relates more to the displacement threshold energy than to the stacking fault energy. It appears that a significant irradiation hardening occurs starting at the lowest dose of 10-3 dpa, at room temperature and 250°C. With increasing dose, the yield shear stress increases. The dose dependence of this increase varies from an irradiation temperature of room temperature to 250°C. In fact the irradiation hardening shows a significant temperature dependence. The thermal activation energy was calculated by measuring the activation volume, which is obtained from relaxation tests. In unirradiated Ni, an activation energy of about 0.3 eV can be extrapolated at RT. In case of irradiated Ni, the unsuccessful fitting with a linear relationship between the thermal energy and temperature suggests that multiple deformation mechanisms are operating simultaneously or in turn over the considered temperature range of –196 °C to 423 °C. The dispersion barrier hardening model is used to interpret the irradiation induced hardening in irradiated Ni. According to the equation, Δτ = αμb(Nd)1/2, with α, the obstacle strength. It appears that α is 0.12 for the room temperature irradiation induced hardening, similar to what was previously found for irradiated Cu, and is 0.22 for the hardening following irradiation at 250°C, which is much larger than previous values. This difference in α is suggested to be related to the presence of voids at high temperatures. It is suggested that voids are stronger obstacles than SFTs or dislocation loops. In situ TEM observations were made in irradiated Cu, in order to assess the strength of irradiation induced defects as obstacles to gliding dislocations. It appeared that the strength of obstacles is approximately 100 MPa. Considering that 90% of the damage consists in SFTs, it is suggested that the obstacles presenting a resistance of 100 MPa are SFTs. In particular, the pulling out of a string from the interaction between what was assumed to be an SFT and a moving dislocation could be recorded for the first time. This reaction implies that the gliding dislocation is strongly pinned by SFTs. TEM observation of the microstructure of the irradiated Ni following plastic deformation shows that defect-free channel formation is the deformation mechanism at the beginning of yielding. The channeling explains the observed softening. As deformation proceeds, a dislocation cell structure is initiated. It takes different forms depending on whether the sample is irradiated or not. In the irradiated case, cell formation initiates from mobile dislocation in the defect free channels. This implies that the eventual cell configuration will retain part of the dimensionality of the channels, for the least their width of 100 nm for the cell size, because of the geometrical constraint. In the unirradiated case the dislocation cell size is larger. The higher flow stress observed in the irradiated case is thus related to the dislocation cell size.
EPFL2005
