Effect of Annealing Temperature on Annealing Twins and Shape Memory Effect in Hot-Forged Co-23.9Ni-5.6Si Alloy

Effects of annealing temperature on shape memory effect (SME), annealing twins, and deformation behavior in a hot-forged Co-23.9Ni-5.6Si alloy were studied. The results showed that a low density of the annealing twin boundaries tended to cause a good SME in the hot-forged Co-23.9Ni-5.6Si alloy. The best SME was obtained in the hot-forged Co-23.9Ni-5.6Si alloy annealed at 1073 K. A high recovery strain of 4 pct was realized when deformed at 77 K. The low density of the annealing twin boundaries and lots of introduced stacking faults were responsible for this large recovery strain. The pre-existence of too many tangled dislocations was the main reason why the SME was poor in the as-forged alloy and the alloy annealed below 1073 K although their densities of annealing twin boundaries were also low. The significant improvement of SME after deformation at 77 K resulted from the remarkable increase of the critical stress for slip deformation due to the drop in the thermal activation. When the critical temperature of thermal activation (T₀) was above the forward martensitic transformation temperature (M_s), a negative temperature dependence of the critical stress for stress-induced martensitic transformation occurred between the M_s and T₀ temperatures.