Strain Rate-Dependent Anomalous Work Hardening of a Single-Crystalline CoNi-Base Superalloy

Abstract

The deformation mechanisms operating in superalloys depend on different parameters such as composition, temperature and deformation rate. So far, the transition from shearing by APB-coupled dislocations to shearing under the formation of stacking faults has been studied exclusively as a function of temperature but not as a function of the strain rate. Therefore, interrupted compression tests with strain rates between 10–3 s−1 and 10–5 s−1 were performed on the single-crystalline CoNi-base superalloy ERBOCo-4 at a temperature of 850 °C. The evolution of the defect structures has been analyzed by conventional transmission electron microscopy. A change of the deformation mechanism from APB-coupled dislocation shearing to stacking fault shearing is found to depend on the strain rate. At lower strain rates, an increased stacking fault density is associated with a higher yield strength and higher work hardening rates at the early stages of plastic deformation due to a very high stacking fault density. After approximately 2.0 pct plastic strain, the stress reaches a plateau and decreases subsequently, which is associated with the formation and thickening of twins as shown by high-resolution scanning transmission electron microscopy. At higher strain rates, the work hardening rate is significantly lower in the early deformation stage. The role of segregation to planar defects and the influence of local phase transformations (LPT) at SESFs is further discussed in reference to the influence of the strain rate. The segregation of W as an η stabilizing element is found to be crucial for the formation of a local phase transformation in ERBOCo-4. At higher strain rates the phase transformation is hindered by insufficient W segregation, resulting in a higher twin density.

Segregation-induced strength anomalies in complex single-crystalline superalloys

“…Recently, elemental segregation to dislocations at the γ/γ′ interfaces was also proposed to influence the type of the occurring shearing mechanism 22,34 . Based on the results in the present and a previous study 22,24 , the acting shearing mechanism transitions from athermal shearing by APB-coupled dislocation pairs to shearing under stacking fault formation with increasing temperature, increasing plastic strain and/or decreasing strain rate, i.e. conditions with increasing effective diffusion length.…”

“…Interestingly, the transition from shearing by APBcoupled dislocations to stacking-fault shearing does not lead to a significant decrease of the yield strength in Co-base and CoNibase superalloys 17,[20][21][22] and might even be associated with an anomalous increase 23 . As reported recently 22,24 , this transition depends not only on the temperature but also on the test duration (or strain rate), suggesting that diffusive processes play an important role.…”

“…In further support of this argument, our recent study 24 reveals that APB-coupled dislocation pairs remain the dominant deformation mechanism at 850 °C, a strain rate of 10 −3 s −1 and a higher plastic strain of about 1%. Intriguingly, despite a higher applied stress of 745 MPa at a lower strain rate of 10 −5 s −1 (compared to 655 MPa at 10 −3 s −1 ), APB-coupled dislocation pairs are only very rarely observed.…”

“…During high temperature deformation of γ′ strengthened superalloys, various deformation mechanisms can occur depending on numerous factors such as alloy composition, grain orientation, applied stress, temperature and deformation rate 24,27,32,[38][39][40][41][42] . Recently, elemental segregation to dislocations at the γ/γ′ interfaces was also proposed to influence the type of the occurring shearing mechanism 22,34 .…”