Microstructural damage assessment in alloy 617M near high cycle fatigue threshold at elevated temperature

High cycle fatigue (HCF) behavior of Ni superalloy 617M is investigated at 973 K and R-ratio −1 on a resonance-based fatigue testing system at 85 Hz frequency. The alloy experiences an abrupt drop in fatigue life within a narrow domain of 2.5–5 MPa near its fatigue strength at 320 MPa. Electron microscopy and diffraction techniques were employed to thoroughly analyze the nominal fatigue damage. The characterization revealed the significance of precipitation of secondary phases M₂₃C₆, Ti (C, N), and γ′ phase in dictating the HCF strength of the alloy. Cyclic loading at high temperature causes γ-matrix hardening and secondary phase precipitation synergistically strengthening the material beyond its yield strength. Conjunctively, dynamic strain aging was also seen to play a major role in the evolution of fatigue damage. The work highlights the collective contribution of γ′-phase precipitation, carbides, and dynamic strain aging and their influence on the HCF behavior of alloy 617M.