The high temperature low cycle fatigue behavior of the nickel base alloy IN-617

Abstract

Fully reversed strain controlled LCF tests were performed on 4.5 mm thick sheet specimens IN-617 at 1033 K and 1144 K in air. The strain-life and cyclic stress-strain data were analyzed parametrically. While the inelastic strain amplitude-life relationships are similar at the two temperatures, the softer cyclic stress-strain relationship observed at 1144 K produces inferior fatigue resistance to that at 1033 K when comparisons are based on stress or total strain amplitude. Transmission electron microscope observations suggest that grain boundary sliding was the primary mechanism of deformation at 1144 K while at 1033 K intragranular slip produced a dense dislocation substructure which was stabilized by fine scale precipitation of M₂₃C₆. At 1144 K, grain boundary migration occurred within the specimen interiors, producing a cellular precipitation of M₂₃C₆ as well as the intragranular M₂₃C₆ observed at 1033 K. Cellular precipitation was not observed in the near surface regions. This is attributed to oxygen penetration along grain boundaries. Comparison of the data with mean stress-HCF data indicates that the mean load significantly reduces fatigue resistance at both temperatures.