Strain rate dependence of the flow stress and work hardening of γ′

The strain rate (ε) sensitivity of the flow stress (τ), β = (δτ/δ ln ε), of crystals of γ′ in the temperature (T) range of the yield stress anomaly obeys a Cottrell-Stokes law when plotted against τh = τ − τy, where τy is the yield stress at 0.01% strain. The slopes are approximately 1%, decrease with increasing T and are approximately independent of orientation. τh is due to work hardening and depends on the prestrain ε, ε and T. Transmission electron microscopy observations suggest that at 2% strain at 600 K and 720 K, τh and β are mainly controlled by forest obstacles. At 600 K, the forest consists largely of [101] dislocations on the (010) cross slip plane threading the (111) planes and generated by bowing of the [101] primary screws (Kear-Wilsdorf locks). At 720 K, the forest is non-uniform and consists of [101] on (010), primary cube [110] on (001) and secondary octahedral slip dislocations. At room temperature, the strength of the obstacles is weaker and it is suggested that they are mainly jogs on edge dislocations generated by cross slip of screw segments. At 2% strain, fine slip on (010) and (001) contributes increasingly to strain with increasing T and this correlates with the decrease in the work-hardening rate.