On the impact of nanoscale γ’’ precipitates on the Bauschinger effect in single- and polycrystal Ni-base superalloy 718

Abstract

Tension compression experiments were performed on Ni-base superalloy 718 single and polycrystals to investigate the effects of nanoscale γ’’ precipitates on cyclic deformation behavior and in particular the Bauschinger effect. γ’’ precipitates led to a more pronounced Bauschinger effect in the single crystal, but to a less pronounced one in the polycrystal. A potential mechanism is suggested to explain variations in the Bauschinger effect based on the analysis of slip traces and dislocation density distribution during cyclic deformation. It was found that an increasing amount of γ’’ precipitates promotes the activation of additional slip systems. In the single crystal, this leads to latent kinematic hardening and the formation of a substructure with local deformation gradients. The resulting increase in long-range and slip system-related back stresses is believed to be responsible for the increase in Bauschinger effect. In polycrystals, by contrast, the activation of additional slip systems leads to a more homogeneous deformation behavior and a reduction of back stresses at grain boundaries. Macroscopically, this results in a reduction of Bauschinger effect with increasing γ’’ precipitation amount.