On the faulting and twinning mediated strengthening and plasticity in a gamma ' strengthened CoNi-based superalloy at room temperature

Microscopic defects and their mutual interactions are fundamentally important in deciding the load carrying capacity of structural materials. In this context, the present work reports an unprecedented precipitate shearing mechanism in CoNi-based superalloys at room temperature where L1(2) ordered gamma ' precipitates are embedded within a disordered CoCrNi-rich matrix. A combined electron channelling contrast imaging (ECCI) and scanning transmission electron microscopy (STEM) analysis revealed stacking fault (SF) and SF coupled nano-twin formation that rendered a significant work hardening rate. The observed twinning induced plasticity (TWIP) effect correlates with an excellent combination of yield strength, ultimate tensile strength (UTS) and plasticity of 910 +/- 30 MPa, 1360 +/- 40 MPa and similar to 18.5 (%), respectively, as exhibited by the peak aged Co-30Ni-12Cr-7Al-4Ti-2Nb-0.006B alloy at room temperature. Rigorous center of symmetry (COS) analysis on atomic resolution STEM images from the deformed samples unambiguously establishes precipitate shearing via single alpha/6 < 112 >{111} partial dislocations resulting in twin formation, succeeding the intrinsic stacking fault (ISF) and complex stacking fault (CSF) of gamma and gamma ', respectively. Through thermodynamic and first principles density functional theory calculations, it is demonstrated that the experimentally observed deformation mechanism is directly correlated with the negative stacking fault energy of. matrix and low CSF energy of the gamma ' precipitates. Based on these new findings, the present work demonstrates the flexibility of tuning the composition of CoCrNi containing super-alloys to widen the alloy spectrum for designing alloys with improved properties.

Automated summary

This study reports an unprecedented precipitate shearing mechanism in CoNi-based superalloys at room temperature, which leads to significant work hardening rate. The observed twinning induced plasticity (TWIP) effect correlates with an excellent combination of yield strength, ultimate tensile strength (UTS) and plasticity. The deformation mechanism is directly correlated with the negative stacking fault energy of the matrix and low CSF energy of the gamma ' precipitates.