Effect of rapid thermomechanical fatigue pre-damage on the residual isothermal low-cycle fatigue behavior of a DD6 single-crystal superalloy with a film-cooling hole

Abstract

Turbine-blade materials often experience sequential thermomechanical fatigue (TMF) and low-cycle fatigue (LCF) loading during service, yet the mechanism governing residual LCF resistance after TMF pre-damage remains unclear. DD6 nickel-based single-crystal superalloy specimens with a film-cooling hole (FCH) and smooth round-bar (RB) geometries were subjected to TMF preloading followed by LCF testing at 980 °C. Residual LCF resistance was evaluated using normalized life fractions, and the damage-transfer process was investigated through fractography, longitudinal-section microscopy, γ/γ′ characterization, EBSD analysis, and microstructure-informed γ/γ′ two-phase cyclic-plasticity simulations. The results show that residual LCF life does not follow a linear life-fraction relationship. At a TMF pre-damage fraction of approximately 25%, the normalized residual LCF life fractions were approximately 0.260 for the FCH specimen and 0.070 for the RB specimen under their respective baseline-normalized conditions. In the FCH specimen, TMF-induced damage was mainly localized near the hole edge, and subsequent LCF failure involved further crack propagation and damage accumulation from this localized region. In contrast, the RB specimen developed a more spatially distributed damage field across the gauge section, resulting in multiple potential sites for crack initiation, interaction, and coalescence during subsequent LCF loading. When the TMF pre-damage fraction reached approximately 45% or higher, both specimen types exhibited very low residual LCF resistance as accumulated damage became sufficiently severe to dominate subsequent failure. These findings demonstrate that residual LCF resistance after TMF preloading is governed not only by the consumed TMF life fraction but also by the spatial localization and connectivity of TMF-induced microstructural damage.