Dose rate effects in nickel-ion-irradiated nickel

Publisher Website
Google Scholar
Abstract
Nickel samples have been irradiated at two different dose rates with the same dose of 2.8-MeV 58Ni+ to a peak damage level of 13 dpa. The peak damage dose rates were ∼7 × 10−2 dpa sec−1 (HDR) and ∼7 × 10−4 dpa sec−1 (LDR). An upward shift of ∼75°C in the low-temperature limit for swelling has been observed for this increase of a factor of 100 in dose rate. The HDR results showed a peak swelling of 1.2% at 625°C, while the LDR results gave a peak swelling of 2.4% at 550°C. The LDR samples contained larger voids at lower densities than the HDR samples at all temperatures at which voids were observed. The results were compared with models for void nucleation and growth from which it was concluded that the HDR and LDR swelling curves both exhibited a low temperature regime which was void- growth-limited and a high temperature regime which was void-nucleation-limited. When temperature independent void and dislocation parameters were employed, the Brails ford and Bullough (BB) void growth model predicted that both LDR and HDR swelling would extend to much higher temperatures than was observed. When the experimentally measured parameters were employed at each point, the temperature dependence of the calculated swelling rate and that of the observed swelling agreed well for the LDR data but the resulting curves differed by ∼50°C for the HDR results. Thus, the BB theory has little quantitative predictive capacity because at present the temperature and dose rate variations of the void and dislocation parameters must be determined experimentally and even the use of these data does not necessarily give quantitative agreement. The various BB temperature shift models similarly lack quantitative predictive value since similar void and dislocation parameters enter these expressions. The temperature dependences of the void number density, the mean void diameter, and the dislocation density were only qualitatively similar when compared for the two dose rates, and a simple temperature shift did not produce reasonable coincidence between the values of these quantities obtained at the two different dose rates. This direct measurement of dose rate effects in nickel-ion-irradiated nickel indicates that, for the present, experimental measurement of dose rate effects appear to be needed in a given material before reasonably accurate predictions of swelling behavior as a function of damage dose rate can be made; further evaluation and improvement of theories of swelling is called for.