The creep resistance of polycrystalline (PX) Ni-based superalloys is controlled by the complex dislocation mechanistic produced by a carefully design γ-γ´microstructure [1]. Dislocations can either climb or glide to form a rich variety of faults in the γ´ precipitates: antiphase boundaries (APBs), superlattice extrinsic and intrinsic stacking faults (SESFs and SISFs), or complex variants of the latter (CISF and CESF) [1]. The dominant mechanism determines the creep resistance of the alloy and it is influenced by the chemistry of the alloy, microstructure, the temperature and the stress conditions. Historically, focus have been put on rationalising the connection between temperature, stress and microstructure with dominant deformation mechanisms [1]. Recently, it has been found that chemistry of the γ´ precipitates and local diffusional processes play a critical role in determining the type of fault produced by dislocation shearing [2], thus determining the creep behaviour of the alloy. However, there is still not a full understanding of the chemistry effect on the mechanism dominance. Further systematic analysis is needed in this regard.

In this work, a carefully designed study has been performed to unveil the effect of chemistry on the creep strength of the alloy, with an emphasis on the composition-dependence of the microtwinning effect. Microstructure, stress and temperature conditions are fixed to identify the solely effect of the alloy composition on the deformation dominance. Two different disk-grade alloys (table 1) with different γ´/ γ stabilisers ratios are carefully selected and tested at 760°C-552MPa and 706°C-724MPa. The results show the effect of γ-stabilisers in promoting low creep resistant mechanism like microtwinning. This is rationalised using density functional theory, to quantify the effect of Co and Cr on lowering the energy penalty of forming the microtwin precursors, CISFs. The results presented in this study are of critical importance for design of future high-temperature disk-alloy developments.

References

[1] R.C. Reed and C.M.F. Rae. Physical metallurgy of the nickel-based superalloys. Physical Metallurgy (Fifth Edition), pages 2215–2290. Elsevier, Oxford, 2014. 

[2] D. Barba et al. On the microtwinning mechanism in a single crystal superalloy. Acta Materialia 135 (2017)