Vacuum Levels Needed to Simulate Internal Fatigue Crack Growth in Titanium Alloys and Nickel-Base Superalloys: Thermodynamic Considerations

Prior studies have examined fatigue growth of surface cracks in vacuum to simulate subsurface growth in Ti alloys and Ni-base superalloys. Even with the highest vacuum level attained using state-of-the-art pumps, it is unclear if conditions of internal crack growth are truly simulated. We consider thermodynamics of the oxidation process to help answer this question. This consideration helps explain a previously reported anomalous behavior of longer life in air than in vacuum under certain material/test conditions.