High work-hardening rate and deformation twinning of Co-Ni-based superalloy at elevated temperatures

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Abstract
In order to investigate changes in the work-hardening rate for a fcc-type Co-Ni-based superalloy with temperature, tensile tests have been conducted in the temperature range from room temperature to 1123 K. A serrated flow on the stress-strain curve, caused by dynamic strain ageing (DSA), appears in the temperature range from approximately 673 to 1073 K. A linear work-hardening behaviour is observed, followed by fracture without appreciable dynamic recovery over a temperature range from room temperature to 943 K (T p) where the DSA exhibits a peak, indicating that stage III work hardening does not occur below T p. Transmission electron microscopy observations and phase stability considerations leads us to conclude that, in the temperature range where the DSA operates, the so-called Suzuki segregation into stacking faults bounded by the Shockley partials can occur so that dislocation dissociation is extended, thereby causing suppression of the cross-slip event. Since it is difficult for the stress relief mechanism due to the cross-slip event to occur in the temperature range where DSA operates, deformation twinning begins as an alternative mechanism to reduce the internal stress which is increased by plastic deformation. In the Co-Ni-based alloy, DSA is responsible for the higher work-hardening rate and the occurrence of the deformation twinning at elevated temperatures.

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