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

Abstract

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 $$\beta $$ β -transus temperature and strength, but they decreased ductility, even in microstructures where some $$\beta $$ β 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.

Tensile properties of Ti-6Al-4V as-built by laser metal deposition: The relationship between heat affected zone bands, strain localization and anisotropy in ductility

“…The post-fabrication nitrogen contents are 0.01 µg/g and 0.09 µg/g in the SA and SB samples respectively. The nitrogen concentration found in the SB sample exceeds the nominal Ti-6Al-4V nitrogen content [59], and can result in a substantial effect on tensile and fatigue properties [60]. However, no significant difference was observed regarding microstructural features, which suggests that oxygen and nitrogen uptakes does not significantly influence the resulting microstructure.…”