The entering behavior of environmental gases into the plastic zone around fatigue crack tips in titanium

Fatigue crack growth tests were performed on titanium in vacuum, a nitrogen gas, inert gases (helium and argon), and air. Fracture surface morphologies were different from each other even if these environments were mild. Microcracks which were parallel to the striations were observed on the fracture surfaces, and the frequency of them increased with the environment becoming active (He < N₂ < Air). These results suggest that the environment has some effects on the deformation behavior in the plastic zone at the crack tip. The concentration of nitrogen in the plastic zone around a crack surface tested in nitrogen was analyzed using electron probe microanalysis (EPMA). The concentration of nitrogen in the plastic zone, especially in the cyclic plastic zone, increased significantly. The result indicates that nitrogen may be adsorbed on the fresh surfaces produced at the crack tip during loading and diffuse into the cyclic plastic zone with cyclic dislocation movement. Considering all the results, it can be thought that atoms of environmental gases including argon, as well as nitrogen, have some effects on the chemical composition of the cyclic plastic zone.