Cyclic behavior of a γ’ precipitation-hardened nickel-base superalloy: Fundamental strengthening mechanisms impacted by hydrogen

• Hydrogen exerts an antagonistic control over cyclic behavior, governed by the precipitate state. • Short-range internal stresses are selectively modified by hydrogen in shearing versus Orowan/Hirsch bypass regimes. • Hydrogen lowers APB energy, enabling γ′ shearing and amplifying elastic shielding within shear bands. • Hydrogen trapped at γ/γ′ interfaces near GNDs suppresses Orowan/Hirsch-type bypass mechanisms. • Long-range internal stresses are reduced by hydrogen via an elastic shielding mechanism, independent of precipitate size and evolving under cyclic loading. • Crack initiation is dictated by a critical slip-irreversibility threshold, which hydrogen markedly lowers irrespective of strain localization. Waspaloy® is a γ′-precipitation-hardened Ni-base superalloy used in turbine disks, where combined cyclic loading and hydrogen exposure can be service-critical. Here we quantify how dissolved hydrogen (8–25 wppm) modifies room-temperature low-cycle fatigue across five aging conditions—under-aged, peak-aged, transitory, and two over-aged states—spanning γ′ radii from 5 to 110 nm and covering shearable and non-shearable regimes. Hydrogen produces opposite macroscopic responses depending on the deformation regime: it promotes cyclic softening for shearable γ′ (under-aged/peak-aged) and cyclic hardening for non-shearable γ′ (transitory/over-aged). Hysteresis-loop partitioning combined with TEM links these trends to distinct changes in short- and long-range internal stresses. In the shearable regime, hydrogen decreases the effective stress, consistent with a reduced effective APB energy and enhanced precipitate shearing; this softening diminishes with cycling, consistent with solute drag and defect-assisted pinning within slip bands. In the non-shearable regime, hydrogen increases the effective stress, consistent with preferential trapping at γ/γ′ interfaces and GND-rich regions that impedes bypass (Orowan) and may activate additional interface-controlled bypass pathways. In contrast, hydrogen decreases the back stress across all states, indicating a systematic modification of the long-range incompatibility mechanisms governing strain localization. A precipitate-size-dependent framework incorporating slip-band morphology, dislocation density contrast, and defect storage (Orowan loops vs white-band-type structures) captures the observed back-stress evolution. Overall, hydrogen shifts the balance between short- and long-range stress contributions in a microstructure-dependent manner and modifies cyclic irreversibility and the conditions for fatigue crack initiation.