Characterisation of short fatigue cracks in titanium alloy IMI 834 using X-ray microtomography

• Published in: Acta materialia Vol. 99; pp. 49 - 62
• Main Authors: Chapman, T.P., Kareh, K.M., Knop, M., Connolley, T., Lee, P.D., Azeem, M.A., Rugg, D., Lindley, T.C., Dye, D.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.10.2015
• Subjects: Crack initiation; Crack propagation; Evolution; Fatigue; Fatigue cracks; Fatigue failure; Fracture mechanics; Three dimensional; Titanium; Tomography; Vacuum; X-ray microtomography; Titanium; Tomography; Fatigue; Vacuum
• ISSN: 1359-6454; 1873-2453
• DOI: 10.1016/j.actamat.2015.07.069

[Display omitted] A first attempt at the three-dimensional evaluation of naturally initiated surface connected and internal fatigue cracks is presented. Fatigue crack initiation and growth in air and vacuum environments have been investigated through X-ray microtomography in air and vacuum environments at elevated temperatures (350°C), accompanied by post-mortem electron microscopy of the fracture surfaces. In vacuum (<10−5mbar), multiple internal and surface-connected crack initiation was observed, but only the surface-connected cracks grew. In contrast, fewer cracks formed in air, these were mostly surface-connected and all were observed to grow. In all instances the initiation features were associated with globular primary α. An improved fatigue life was found in vacuum, which was mostly a consequence of delayed initiation, but was also due to slower fatigue crack propagation. The non-propagation of internal cracks was taken to imply that even the good laboratory vacuum obtained here was insufficient to simulate the conditions obtained for an internal crack in a component. The crack shape evolved towards a semi-circular shape a/c=1 in air during fatigue crack growth, whilst the vacuum cracks remained semi-elliptical (a/c≃1.4). This was taken to imply that oxide-induced crack closure played a role in fatigue crack growth in air.