Connolley, T., Starink, M.J. and Reed, P.A.S.,
Effect of oxidation on high temperature fatigue crack initiation and short crack growth in Inconel 718
Pollock, T.M., Kissinger, R.D., Bowman, R.R., Green, K.A., McLean, M., Olson, S. and Scbirra, J.J. (eds.)
In Proceedings of the 9th International Symposium on Superalloys.
The Minerals, Metals & Materials Society (TMS)., .
Full text not available from this repository.
Fatigue tests at 600°C in air were performed on U-notch
specimens of wrought IN718, using l-l-l-l and 1-20-l-l
trapezoidal cycles. SEM examination and acetate replication was
used to study crack initiation and growth. In support of the fatigue
tests, thermal exposure experiments were performed on
unstressed specimens to study surface and sub-surface primary
carbide oxidation. Compared to a 1 second dwell, a dwell time of
20 seconds at maximum load had a beneficial effect on fatigue
lifetime. Polishing the U-notch to remove broaching marks also
had a beneficial effect on fatigue life. Multi-site crack initiation
along the root of the U-notch was observed in fatigue specimens.
Many of the cracks initiated at bulge-like features, associated with
the oxidation of sub-surface primary carbides. Crack initiation
was first observed as early as 12% of the total fatigue life, with
further crack initiation occurring as tests progressed. Short cracks
in the U-notches grew at a constant rate, except when interactions
between parallel cracks resulted in crack arrest or coalescence.
Significant crack coalescence occurred towards the end of the
fatigue life, producing a dominant defect which propagated
rapidly to failure. Surface eruptions and localised surface
deformation were observed in fatigue and unstressed thermal
exposure specimens, demonstrating a considerable volume
expansion when primary (Nb,Ti)C carbides oxidised. It is
proposed that the misfit strains due to primary carbide oxidation
were superimposed on the plastic strain field in the U-notch due
to external loading. This would create local strains high enough to
cause rupture of the matrix, hence initiating a fatigue crack.
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