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.

Increasing demands on titanium alloys in aerospace applications have driven a push towards deeper understanding of their behaviour in service. This extends from component performance during planned operation to damage mechanisms and how parts may ultimately fail. Investigation of damage and failure requires a comprehensive framework of techniques in order to identify a root cause, and further the understanding of failure mechanisms. It is crucial to defining and improving component lifetimes via a design optimisation feedback loop. This paper presents an overview of the techniques used in state-of-the-art industrial titanium alloy failure investigation, highlighting the inherent reciprocal links to frontline research and the need for constant innovation.