Investigation of low level neutron radiation on tantalum alloys for radioisotope power system applications

Tantalum alloys have been used by the U.S. Department of Energy (USDOE) as structural materials for space nuclear power systems such as Radioisotopic Thermoelectric Generators (RTG) since the 1960s. The thrust of this thesis research is to investigate the effects of low-level neutron radiation on the mechanical and microstructural properties of two tantalum alloys, Ta-10%W and T-111 (Ta-8%W-2%Hf), which have been used to encapsulate radioactive fuel for space nuclear power systems. Ta-10%W and T-111 test specimens were exposed to a neutron fluence level (1.2 x 1015 nvt) at temperatures less than <0.2 Tm, which is equivalent to the cumulative fluence associated with the 30-year mission life of a RTG. This fluence level resulted in an atomic displacement damage of approximately 3.0 x 10-7 dpa in both alloys.; In T-111 test specimens, this level of atomic displacement damage produced an approximate 6.6% reduction in the tensile ductility, an approximate two-order of magnitude increase in the stress rupture time, and a two-order of magnitude reduction in steady state creep rate. These observations are statistically significant at the 0.05 significance level. Through the employment of Transmission Electron Microscopy it was determined that the interaction of the defects produced by atomic displacement damage with moving ao/2<111> screw dislocations increased the magnitude of cross-slip of the screw dislocations, thus leading to a five-fold increase in dislocation density and a pronouncement of the ordering of dislocations into mosaic patterns of cellular or subgranular arrangements at the boundaries of cells/subgrains. In addition, the experimental results determined that an atomic displacement damage of approximately 3.0 x 10-7 dpa did not produce a statistically significant effect on the mechanical or microstructural properties of Ta-10%W when subjected to a strain rate of 0.0017 s-1.; The culmination of this research is the development of a phenomenological model based on the experimental data that can be used to determine the minimum creep rate of irradiated T-111 within the parameters bounded by this research. The results of this research are significant because they provide a basic understanding of the strength mechanisms in two tantalum alloys (Ta-10%W and T-111) resulting from neutron radiation at temperatures <0.2 Tm. This greatly enhances the level of understanding of potential irradiation hardening mechanisms in both alloys. Thus, space power system designers can more confidently extrapolate relevant mechanical-property trends of T-111 and Ta-10%W encapsulation materials as a function of neutron irradiation doses and internal pressure levels which correspond to actual RTG service lifetimes.