Mark R. Daymond

Nano-scale Mechanical Properties and Microstructure of Irradiated X-750 Ni-Based Superalloy

This study investigates the effect of irradiation on the mechanical properties of a Ni-based superalloy, X-750. 40 MeV Ni+ ions were used to irradiate the X-750 up to 1 dpa with and without 5000xa0appm helium pre-implantation at room temperature and 400xa0°C. Nano-indentation hardness tests were carried out at room temperature in the depth range of 200 to 1400xa0nm before and after irradiation. Cross-sectional TEM observations were performed on the irradiated materials to correlate the mechanical results with the microstructural evolution. The results show that helium pre-implantation enhances the irradiation-induced hardening due to generating a high density of small cavities and promoting the formation of larger Frank loops. In addition, nano-scale mechanical tests reveal that changing the subsequent Ni ion irradiation temperature from room temperature to 400xa0°C, leads to changing of the mechanical response from a softening behavior to an irradiation-induced hardening. The γ′ precipitates became disordered after irradiation at room temperature, whereas the γ′-phase remained ordered during irradiation at 400xa0°C. The softening effect of the γ′ instability outweighed the hardening impact of irradiation-induced defects such as cavities and Frank loops, leading to a hardness reduction for the room-temperature-irradiated material. Three different obstacle-hardening models were employed to assess the individual impact of each type of defect on the material’s overall strength enhancement. Furthermore, the superposition principle was used for each model to estimate the overall irradiation-induced strengthening, which is compared to the results from the nano-hardness measurements.

Advanced Characterization of Hydrides in Zirconium Alloys

The mechanical properties of zirconium alloys are affected by the presence of hydrides. The strain fields around hydrides, which are affected by the size, orientation, and hydride phase, are believed to influence the apparent hysteresis between solubility limits on heating and cooling. TEM characterization of dislocation fields near hydrides in Zircaloy-4 specimens, which were exposed to 300 °C primary-water conditions for 600 h, was performed both before and after a heating and cooling cycle. In addition, EELS characterization is provided before heating. In situ TEM imaging/recording and nano-diffraction allowed monitoring of the morphology of dissolving hydrides throughout the temperature cycling. No dislocations in the matrix surrounding the hydrides were visible prior to heating; however, when the hydrides dissolved, dislocations were visible in the space the hydrides had previously occupied, providing a map of the original hydride distribution. These dislocation ‘nests’ are likely the preferential sites for subsequent hydride precipitation and elucidate the so-called ‘memory effect’. Advancing the understanding of hydride formation kinetics, hydride morphology, and hydrogen solid solubility limits can help to reduce uncertainties and conservatism when addressing the risks of hydrogen embrittlement and hydride cracking in zirconium components.