Investigation about the Stress Corrosion Cracking of Ti-6Al-4V

Although the titanium alloys represent an optimal solution for several applications, they suffer from the detrimental effect related to the stress corrosion cracking (SCC) which can take place also in the most used Ti-6Al-4V. This study is focused on the characterization of the SCC on this alloy and it has been performed through the mechanical fracture tests realized within an environment of synthetic sea water featured by two levels of NaCl concentration. The identification of the Ti-6Al-4V behaviour in these situations is very significant because it can be often used for the substitution of more expensive alloys (i.e. cupronickel, monel etc.). The toughness tests realized in air and in the sea water solutions have been performed through the use of CT type specimens taken from a hot rolled plate. The fatigue crack propagation has been performed in air along the direction perpendicular to the rolling one (L-T specimens). The macroscopic analysis of the fracture phenomenon revealed the change of the propagation direction as a function of the NaCl concentration and the applied deformation rate. The unstable crack becomes parallel to the rolling direction as the deformation rate lowers and as the NaCl concentration increases. The same environmental and chemical condition produces a significant decrease of the KISCC value. The SEM-EBSD observation has pointed out a clear dependence of the crack propagation pattern on the crystallographic orientations induced in the alloy by the former plastic deformation process.