Microstructural changes under heavy ion or proton irradiation

Heavy ion irradiation by Kr has been used to simu­late some aspects of fission neutron irradiation, such as high damage rate (up to 100 dpa). Gan et a/.173 irradiated TEM foils of commercial hot-pressed ZrC099 (the authors report a C/Zr ratio of 1.01, but the composition was corrected to reflect the impurity content of 1.9 wt% Hf, 0.19 wt% Ti, 0.21 wt% 0, and 0.61 wt% N, considering that the metals and nonmetals substitute for Zr and C on their respective sublattices). Irradiation was conducted at 298 or 1073 K to >1MeV Kr ions to a fluence of 2.5 x 1015—1.75 x 1016cm—2 (10-70dpa), with in situ TEM ofmicrostructural evolution during irradiation. Lattice parameter swelled by 0.6-0.7% (~2% volume increase) at 10 dpa (298 and 1073 K), 0.9% (~3% volume increase) at 298 K and 30 dpa, and 7% (21% volume expansion) at 1073 K and 70 dpa. Simulta­neously, precipitation of a fcc phase with 8% larger lattice parameter (5.09 A) than the matrix (4.71 A) was detected by ring patterns superimposed on the single-crystal ZrC electron diffraction pattern. Precipitate coarsening with temperature and fluence was observed. Energy-dispersive X-ray spectroscopy (EDX) detected no change in stoichiometry during irradiation. The authors linked the precipitate phase and the 7% lattice parameter increase at high tem­perature and fluence, but could not explain ade­quately its origin, hypothesizing that the expansion was related to Kr implantation. Cubic Zr02 formation is also plausible (a ~ 5.1 A). They acknowledged that the large ratio of surface area to volume in a TEM foil may permit larger lattice expansion than is possi­ble in the bulk. 0ther microstructural features noted were grain boundary cracking at high fluence, defect clusters at low temperatures and fluence, and dislo­cation segments at high temperatures. No irradiation — induced voids or amorphization were detected.

Because of very small irradiated volume (depth < 1 mm) produced by Kr ion irradiation, the authors later performed proton irradiation, asserting that protons provide a damage rate similar to the fast reactor core, with a more significant irradiated vol­ume (depth ~ 30 mm), though the achievable dose is limited (~10 dpa). Gan eta/.174 subjected the same commercial hot-pressed ZrC0.99 to irradiation at 1073 K by 2.6 MeV protons to a fluence of 2.75 x 1019cm—2 (1.8 dpa), subsequently preparing TEM foils. Lattice parameter change was assessed by higher order Laue zone (H0LZ) patterns in conver­gent beam electron diffraction, but no change within the uncertainty limit of 0.2% was detected. In con­trast, when the same material was irradiated by Yang et a/.175 at 1073 K in a 2.6 MeV proton fluence of 1 x 1019 or 2.3 x 1019cm—2 (0.7 or 1.5 dpa), XRD determined a lattice parameter expansion of 0.09% (0.27% volume expansion) for 0.7 dpa and 0.11% (0.33% volume expansion) for 1.5 dpa. Gan eta/.174 detected faulted dislocation loops on {111} planes, characteristic of irradiation of fcc metals, which were not seen for Kr irradiation. No ring pattern or pre­cipitation was detected, as in Kr irradiation.

Gosset et a/.176,177 irradiated commercial hot — pressed ZrC0.95 (containing <0.03 wt% 0) and sol-gel synthesized ZrC0.85O0.15 to irradiation at 298 K by 4MeV Au ions to a fluence of 1 x 1012—5 x 1015cm—2. In the carbide, XRD-determined lattice parameter expanded by 0.03-2% (0.09-6% volume expansion), increasing with fluence but saturating at about 1014cm—2, while the oxycarbide lattice param­eter expanded by 0.05% (0.15% volume expansion), independent of fluence. In both, fine precipitates formed, identified by electron diffraction as tetrago­nal ZrO2 (a~ 3.61 2A, c~ 5.19 A) and identified by tilting as adherent to the sample surface. The authors concluded that high oxygen content in ZrC did not modify the nature of the ion irradiation-induced defects. Faulted dislocation loops were identified in both. No amorphization was detected by XRD.

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *