Investigation of susceptibility of titanium-grade 2 and titanium-grade 12 to environmental cracking in a simulated basalt repository environment

Abstract

Slow strain rate tests and fatigue crack growth rate tests were used to evaluate the relative susceptibility of titanium (Ti)-grade 2 and -grade 12 to cracking in a simulated repository environment. Slow strain rate tests were performed in air and simulated Hanford basaltic groundwater at 250/sup 0/C. Fatigue crack growth rate tests were done in Hanford groundwater, fluoride-ion-enhanced Hanford groundwater, and high-purity water at 90/sup 0/C. The following conclusions can be drawn: Ti-grade 2 and Ti-grade 12 exhibited strain-rate-dependent degradation of ductility in slow strain rate tests at 250/sup 0/C. In-air tests confirmed that the loss of ductility is not limited to the simulated repository environment and may be caused by an internal strengthening or embrittlement mechanism such as dynamic strain aging. The ductility degradation in Ti-grade 12 was found to be highly orientation dependent. (Only one orientation of grade 2 was tested). No fractographic evidence of an environmental cracking mechanism was found in the slow strain rate tests; all specimens failed by micro-void coalescence. The fatigue crack growth rate of Ti-grade 2 and -grade 12 was not affected relative to air or high-purity water by any of the environments used, and no frequency dependence was observed. This may indicate that no environmental cracking mechanism relative to a Hanford basalt repository is operating significantly under the conditions (high oxygen fugacity and a temperature of 90/sup 0/C).