The Effect of Dissolved Nitrogen on the Fatigue Behavior of Ti-6Al-4V

The effect of nitrogen additions on fatigue behavior has been examined in near-equiaxed, rolled Ti-6Al-4V bar. This is the first-time nitrogen content that has been systematically explored with respect to monotonic and cyclic properties in a Ti-6Al-4V alloy base composition. Nitrogen additions were found to increase the beta-transus temperature and strength, but they decreased ductility, even in microstructures where some beta phase remained. This carried across into both the low- and high cycle fatigue behavior; even small contents of 240 and 560 ppmwN caused reductions in both low cycle fatigue life and high cycle fatigue strength. In samples containing 240 and 560 ppmwN, a conventional striated fractographic appearance was observed, but a dramatic change to a macroscopically brittle fracture surface was observed at 1800 and 3600 ppmwN, but still with substantial evidence of plasticity at the microscale. Therefore, neither microstructure or fractographic examination, nor EDX-based compositional analysis in the electron microscope are necessarily a reliable indicator of an absence of deleterious nitrogen contamination. This is significant for the investigation of potentially nitrogen-contaminated surface-initiated cracks, either due to service or processing exposures.

Automated summary

The study showed that nitrogen additions increase beta-transus temperature and strength, but decrease ductility, leading to reductions in both low cycle fatigue life and high cycle fatigue strength. Even small amounts of nitrogen can cause these effects, and neither microstructure nor fractographic examination is necessarily a reliable indicator of the presence of harmful nitrogen contamination.