Strain Rate Effect on Strain Localization in Alloy 718 Ni-Based Superalloy at Intermediate Temperature

Tensile tests on Alloy 718 Ni-based superalloyNi-based superalloy at 650 \(\,^{\circ }\) C at different strain ratesStrain rate revealed a strain-rate dependency on the fracture mode. A change from intergranular to transgranular fracture was observed in air as the strain rateStrain rate increased, mainly when Portevin-Le-ChatelierPortevin-Le-Chatelier (PLC) mesoscopic deformation bands were present. To better understand the link between strain rateStrain rate and fracture mode, a description of the strain localization in the early deformation stage is needed. In this study, high-resolution digital image correlationDigital image correlations (HR-DIC) was carried out at the onset of strain localization, a low strain rateStrain rate (LSR, \(\dot{\epsilon }\) = 10 \(^{-4}\)  s \(^{-1}\) ) and at high strain rateStrain rate (HSR, \(\dot{\epsilon }\) = 10 \(^{-2}\)  s \(^{-1}\) ). This latter condition aimed at investigating the microplasticityMicroplasticity development within PLC bands. The in-plane and out-of-plane displacement components of each single plastic event were measured to accurately assess and distinguish morphological sliding at grain boundariesGrain boundary (i.e., grain boundary sliding) andGrain boundary sliding dislocation slip. The deformation within the PLC bands was examined at macro, meso, and microscales. Statistical analyses highlighted the distribution and partitioning of these strain localization events related to different microstructural features, including grains, and grain and twin boundaries. Grain boundary slidingGrain boundary sliding was found to be more prominent at LSR. Interestingly, events near and parallel to twin boundaries are particularly intense regardless of the strain rateStrain rate. At HSR, grain boundary slidingGrain boundary sliding is less pronounced, and a high density of intragranular slip bands developed within the PLC bands; based on observations before and after the occurrence of the PLC band.