Fracture of Ti-8Al-1Mo-1V Alloy Fan Blade by Stress Corrosion Cracking and Fatigue

Abstract

Similar fractures occurred in two gas turbine engine fan blades made of Ti-8Al-1Mo-1V alloy. Their analyses and a supplementary test were performed with the aid of scanning electron microscope (SEM) and transmission electron microscope (TEM) fractography to identify the common mechanism(s). The initial fracture surface of each fan blade exhibited an area of subcritical crack growth. The area in one fan blade contained three blue discolored zones along the crack initiation side, whereas that in the other fan blade was discolored to white-gray. Intergranular separation and transgranular cleavage were evident at the three discolored zones in one fan blade and near the crack initiation site in the other. Cleavage-like facets were predominant in the remaining area of subcritical crack growth. On some of those facets, striations were seen. A hot salt stress corrosion cracking test of Ti-8Al-lMo-lV alloy plates resulted in discoloration, intergranular separation, and transgranular cleavage, similar to those observed in one fan blade. The mechanism of initial crack growth is stress corrosion cracking at an elevated temperature in one fan blade and at ambient temperature in the other. The mechanism of subsequent subcritical crack growth is fatigue at ambient temperature in both fan blades.

Environmentally assisted fatigue crack nucleation in Ti–6Al–2Sn–4Zr–6Mo

“…The same authors were able to reproduce the discoloured features by deposition of salt slurry on to a four-point bend loaded specimen. They concluded the initial 340 subcritical crack growth region in one of the fan blades to have initiated from salt in a fingerprint, but later to transition to a mechanism of fatigue crack growth [20]. Other studies discuss similar findings of blue discoloured fracture surfaces.…”

“…A study from the 1950s previously examined the cracking mechanism of gas turbine fan blades manufactured from Ti-8Al-1Mo-1V [20]. In one case, the 335 subcritical crack growth area was described to be flat and containing a number of blue discoloured semi-elliptical zones.…”

“…These "blue spots" were 20 observed on the fracture surface at the fatigue crack origin, and were found to be around 100 µm in size. Blue is a so-called temper colour of titanium that can be associated with a TiO 2 oxide film of similar thickness to the wavelength of visible light.…”