Deformation substructure in the nickel-base alloy IN738LC under superimposed creep-fatigue loading

The nickel-base superalloy IN738LC containing a unimodal distribution of γ' precipitates was deformed under superimposed creep-fatigue loading at elevated temperatures. The alloy shows a stable cyclic deformation behavior under LCF portion of the loading and at the same time a cyclic softening effect in the periods of creep deformation. Transmission electron microscopic examination reveals different mechanisms of dislocation - γ' precipitate interaction under various loading conditions. At high hold stress levels and strain amplitudes stacking faults were frequently observed, indicating shearing of γ' precipitates by dislocations. At low hold stress levels and strain amplitudes the observed absence of dislocations in γ' precipitates suggests that they are bypassed by dislocations. Dislocation networks at γ/γ' interfaces were found in most of the specimens. A directional coalescence of the γ' precipitates was observed under certain testing conditions. The development of both these features with deformation could be followed. The softening effect is accompanied by both processes.