The effect of environment on fatigue mechanisms in aerospace titanium alloys
Creators: name: Chapman, Tamara
Publisher: name: Imperial College London
Year: 2015

description: Titanium alloys are widely used in the aerospace industry, owing largely to their high specific fatigue strength at temperatures approaching 550C. A highly adherent and repassivating oxide layer makes titanium alloys highly corrosion resistant in many environments; their presumed immunity to corrosion-enhanced fatigue in typical aerospace environments was another initial attraction. The inaccuracy of this assumption was first revealed in the 1960s. Later, numerous studies found titanium to be vulnerable to stress corrosion cracking, particularly in the presence of molten salts. This thesis concerns the fatigue behaviour of titanium alloys used for aeroengine gas turbine compressor discs, specifically Ti-6246 and IMI 834. This research originally arose from the unexpected premature cracking of a spinning rig test component, suffered by Rolls-Royce plc, with a mysterious blue spot at the fatigue crack origin. The same macroscopic appearance of the crack origin could also be found in some test specimens held within the company's specimen archives. Initial discussions led to a parallel investigation of (i) the blue spot origins and (ii) the rate of crack growth in subsurface, naturally initiated cracks. Chemical characterisation of the blue spot origin was undertaken using focussed ion beam-secondary ion mass spectrometry and scanning-transmission electron microscopy based energy dispersive X-ray analysis as the principle techniques. The blue spot cracking phenomenon is found to be due to a hot salt stress corrosion cracking mechanism. Evidence from chemical analysis on the fracture surface and adjacent specimen surface suggests that in the presence of moisture, stress and elevated temperatures, NaCl deposits react with and disrupt the protective titanium oxide scale, producing byproducts of sodium titanate and HCl(g). In subsequent reactions the HCl attacks the newly exposed bare titanium metal, forming volatile titanium chlorides and atomic hydrogen, as well as a regenerating cycle of gaseous HCl. The resulting hydrogen segregating to the crack tip causes the crack to advance in a brittle manner, until the finite supply of corrodant is exhausted leading to a transition back to conventional low cycle fatigue. Importantly, it is inferred that because HSSCC requires both low pressures (so the alloy chlorides are volatile) and high temperatures, this is a mechanism that will operate in spin rig tests and under laboratory conditions, but not in compressors during flights where the localised air pressure is much higher. Post-mortem examination of electron transparent specimens lifted directly from the fracture surface enabled comparisons of the dislocation morphology and density beneath a hydrogen assisted origin (blue spot), low cycle fatigue origin and low cycle fatigue propagation region. A distinct change in dislocation mechanism is observed in the presence of hydrogen, where a lower dislocation density is observed compared to the LCF origin and propagation region. The results are consistent with the hydrogen enhanced localised plasticity (HELP) mechanism, and reference is also made to the competing theories of hydrogen enhanced decohesion (HEDE) and adsorption induced dislocation emission (AIDE). X-ray microtomography was used to monitor the growth of naturally initiated surface and subsurface fatigue cracks in air and vacuum environments at elevated temperatures. Surprisingly, this appears to be the first time that naturally initiated subsurface fatigue cracks have been examined using synchrotron X-ray microtomography. It is found that subsurface cracks grow more slowly than surface breaking cracks, even in vacuum, whilst air-exposed cracks grow fastest of all. In all three cases, cracking initiates at the primary alpha grains. The topic is of interest, as while it has long been known that cracks grow more slowly in laboratory vacuum than in air, the vacuum found in a subsurface crack would be much better, and potentially so good that hydrogen could be desorbed from the matrix material.