The Alpha‐Case Formation Effect on Very High Cycle Fatigue and Fracture of Ultrafine‐Grained Near β Titanium Alloy

ABSTRACT Ultrasonic testing was conducted to investigate for the first time the impact of alpha‐case formation during air aging (450°C, 5 h) on the very high cycle fatigue (VHCF) and failure of an ultrafine‐grained (UFG) near β Ti‐5Al‐5V‐5Mo‐1Cr‐1Fe alloy. The research findings indicate that air aging results in the development of an alpha‐case layer, diminishing the fatigue properties of the UFG titanium alloy produced by radial shear rolling during VHCF testing. The fatigue limit of the alloy samples with an oxide layer decreased from approximately 620 to 520 MPa based on 10 9 cycles. Examination of the fatigue failure mechanisms revealed that the alpha‐case layer does not significantly impact crack initiation at stress amplitudes below 650 MPa but does accelerate crack propagation. In the alloy without an oxide layer, at all stress amplitudes, the crack initiation is internal. Oxidation leads to a change in the crack initiation mechanism from internal to subsurface at stress amplitudes above 650 MPa with low‐cycle fatigue failure.