An Abrupt Transition to an Intergranular Failure Mode in the Near-Threshold Fatigue Crack Growth Regime in Ni-Based Supe
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An Abrupt Transition to an Intergranular Failure Mode in the Near-Threshold Fatigue Crack Growth Regime in Ni-Based Superalloys J. TELESMAN, T.M. SMITH, T.P. GABB, and A.J. RING Cyclic near-threshold fatigue crack growth (FCG) behavior of two disk superalloys was evaluated and was shown to exhibit an unexpected sudden failure mode transition from a mostly transgranular failure mode at higher stress intensity factor ranges to an almost completely intergranular failure mode in the threshold regime. The change in failure modes was associated with a crossover of FCG resistance curves in which the conditions that produced higher FCG rates in the Paris regime resulted in lower FCG rates and increased DKth values in the threshold region. High-resolution scanning and transmission electron microscopy were used to carefully characterize the crack tips at these near-threshold conditions. Formation of stable Al-oxide followed by Cr-oxide and Ti-oxides was found to occur at the crack tip prior to formation of unstable oxides. To contrast with the threshold failure mode regime, a quantitative assessment of the role that the intergranular failure mode has on cyclic FCG behavior in the Paris regime was also performed. It was demonstrated that even a very limited intergranular failure content dominates the FCG response under mixed mode failure conditions. https://doi.org/10.1007/s11661-018-4685-1 This is a U.S. government work and its text is not subject to copyright protection in the United States; however, its text may be subject to foreign copyright protection 2018
I.
INTRODUCTION
SAFE operation of gas turbine disks is enhanced using the damage tolerance methodology needed to meet engine certification requirements for rotating components. Of particular importance for achieving accurate damage tolerance life predictions is the ability to predict threshold fatigue crack growth (FCG) behavior, which consumes the bulk of the crack growth lives. Yet some important aspects of the threshold FCG behavior in nickel-based turbine components are still not well understood. In a recent publication,[1] we reported on an unusual near-threshold high-temperature FCG behavior of two advanced nickel disk alloys. The article detailed the so-called ‘‘crossover’’ effect in the near-threshold region where the conditions that produce higher FCG rates in the Paris crack growth regime can also produce higher resistance to crack growth in the near-threshold regime (Figure 1). While the unusual crossover effect in the threshold FCG region has been reported previously by others,[2–6] the most surprising finding was the
J. TELESMAN, T.M. SMITH, T.P. GABB, and A.J. RING are with the NASA Glenn Research Center, Cleveland, OH 44135. Contact email: [email protected] Manuscript submitted March 12, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
identification of a sudden failure mode transition between the Paris regime and the near-threshold region. While the microstructural failure mode in the Paris regime was mostly transgranular, the failure mode
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