Macro- and micro-mechanical perspectives on creep-fatigue interaction in Type 316L stainless steel

Creep-fatigue of Type 316L stainless steel under asymmetric waveforms (specifically slow tension-fast compression, S-F, and fast tension-slow compression, F-S) has been understudied, despite its significant implications as demonstrated in this work. This study bridges macro- and micro-mechanical perspectives through a combined approach, involving high-temperature testing, scanning electron microscopy, X-ray computed tomography, neutron diffraction, and crystal plasticity modelling. Macro-mechanical tests revealed distinct deformation behaviours under S-F and F-S waveforms with and without a 1-hour tensile dwell at 550 °C, with S-F reducing lifespan in both fatigue and creep-fatigue conditions. Post-mortem analyses revealed distinct fracture morphologies induced by tensile dwell, with creep-fatigue S-F specimen exhibiting more pronounced intergranular-dominant fracture and higher internal defect volume. It also exhibited the highest number fraction of medium-sized (10–40 μm) microcracks, which correlates with its shortest fatigue life and more creep damage accumulation. Higher grain-level deformation incompatibility was observed during tensile dwell in the S-F load waveform. Crystal plasticity modelling revealed that the higher tensile stress amplitudes during S-F loading stem from increased dislocation density, with average densities at peak tensile strain during the saturation cycle reaching 186 μm⁻² for S-F and 147 μm⁻² for F-S waveforms. These findings establish a strong link between macroscopic and microscopic behaviours under asymmetric loading, emphasising the potential of S-F waveforms for cost-effective creep-fatigue experiment design. Furthermore, for the asymmetric waveforms studied, creep-fatigue life assessment using the ductility exhaustion method demonstrates greater accuracy than those based on the time fraction method.