Exploring the correlation between microscopic mechanisms and macroscopic behaviour in creep of a directionally solidified tungsten-free γ/γ′ CoNi-base superalloy

Abstract

We describe in detail the deformation micromechanisms operative during the compressive creep of a directionally solidified (DS) tungsten-free γ/γ' Cobalt-base superalloy across a range of stresses and temperatures and their manifestation on the macroscopic creep behaviour. Creep experiments were carried out at 800 °C, 850 °C, and 900 °C under different stress levels between 200 MPa and 500 MPa. We observe two low and high-stress domains with stress exponent values of 2.3–3.2 and 6.8–9.5, and the estimated activation enthalpies of ∼352 KJ/mol (at 200 MPa) and ∼657 KJ/mol (at 400 MPa), respectively. Transmission electron microscopy and atom probe tomography were used to understand operative micromechanisms. In the low-stress regime, dislocations are confined to the γ matrix channels and form γ/γ' interface networks with enrichment of Mo solute at the dislocation cores implying that a viscous drag may influence dislocation bow out processes from the interface network leading to a stress exponent of 2–3. The high-stress regime is characterized by γ′ precipitate shearing along with the formation of superlattice-extrinsic stacking faults and micro-twins at 800 °C. Compositional analysis reveals segregation at these faults and twin boundaries. At higher temperatures (850 °C and 900 °C), γ' shearing is dominated by the formation of antiphase boundaries rich in Co and depleted in Mo. Accordingly, we identify three distinct deformation mode domains within the stress versus temperature space associated with interface network formation and low stress exponent creep, γ′ precipitate shearing by SESFs/micro-twinning, and shearing by APB.