Nickel-based superalloy 718 holds significant importance in the aerospace and gas turbine sectors. Outstanding strength and corrosion resistance, even at elevated temperatures up to 650°C, enable the application in critical rotating turbine parts, e.g. shafts, blades and discs. The high strength is primarily achieved by nanometer-sized γ’’ precipitates, which have a disc-shaped form and are aligned parallel to the \{001\} planes in the fcc crystal lattice. Their effect on the plasticity behavior in polycrystals has already been extensively investigated [1]. However, their influence in single-crystal materials has hardly been recognized, especially regarding the cyclic plasticity behavior and the associated Bauschinger effect. The Bauschinger effect describes a reduction in strength of metallic materials under load reversal [2]. It is influenced by the elastic-plastic anisotropy of multi-crystal arrangement and resulting back stresses due to the influence of grain boundaries and precipitates on dislocation plasticity.
In order to investigate the impact of γ’’ precipitates on cyclic plasticity behavior and the Bauschinger effect, tension compression experiments combined with quasi in-situ electron backscatter diffraction (EBSD) measurements were performed on Ni-base superalloy 718 single- and polycrystals. The investigation revealed that a greater γ’’ volume fraction intensifies the Bauschinger effect in single crystals, while in polycrystals, it tends to decrease. The reason for this contradictory effect is assumed to be due to the influence of γ’’ precipitates on dislocation slip and hardening. Slip trace analyses show that γ’’ promotes the activation of additional slip systems in both single crystal and polycrystal. This leads to latent kinematic hardening and the formation of a cell structure in the single crystal. The locally different dislocation density generates back stresses, which are assumed to increase in the Bauschinger effect. In polycrystals, on the other hand, the activation of additional slip systems results in a more uniform deformation, reducing back stresses at grain boundaries. This, at the macroscopic level, leads to a decrease in the Bauschinger effect as the amount of γ’’ precipitation increases. [3]

[1]     N.C. Ferreri, S.C. Vogel, M. Knezevic, Determining volume fractions of γ, γ′, γ′′, δ, and MC-carbide phases in Inconel 718 as a function of its processing history using an advanced neutron diffraction procedure, Mater. Sci. Eng., A 781 (2020), 139228

[2]     J. Baushinger, Ueber die Veraenderung der Elastizitaetsgrenze und des Elastizitaetsmoduls verschiedener Metalle, Met. Civ. NF 27 (1881) 289.

[3]    M. Kreins, K. Büßenschütt, S. Wesselmecking, U. Krupp, On the impact of nanoscale γ’’ precipitates on the Bauschinger effect in single- and polycrystal Ni-base superalloy 718, Mater. Sci. Eng., A 856 (2022), 144008