Deformation mechanisms at the tip of internal fatigue cracks in vacuum and in the presence of an air environment in a Ti alloy

Ultrasonic fully reversed tension fatigue tests have been performed in the Very High Cycle Fatigue (VHCF) regime ( N R > 1 0 7 − 1 0 8 c y c l e s ) on Ti-6Al4V specimens containing a controlled internal notch. Two sets of samples have been used. The first one contains a central chimney along the specimen longitudinal axis which brings air to the internal notch; in the second series the notches are not connected to the surface. The microstructure present below the fracture surface of the broken specimens has been studied by electron microscopy (EBSD, TKD and TEM). The formation of nanograins and nanovoids was observed below the surface of the cracks growing in a vacuum environment but not below the surface of cracks connected with ambient air. In the latter case extensive striations were observed. Below each striation the formation of tensile {10 1 ̄ 2} twins was observed.