Enhancement of recovery stress by annealing and prestraining of NiTiNb wires

NiTiNb shape memory alloy has attracted much attention for its use in prestressing and remedying structural steel and concrete members due to its unique shape memory effect. However, the recovery stress values reported in the literature vary significantly, as the interplay between recovery stress and treatment parameters is not fully understood. In this study, we carefully investigated the effects of annealing and prestraining on the martensitic transformation behavior and mechanical properties of NiTiNb alloy and clarified their correlations to the recovery stress. Our results showed that the yield strength and superelasticity decreased while the ductility and retained stress-induced martensite increased with increasing annealing temperature. We found that high recovery stress can be achieved by annealing the sample above the recrystallization temperature while keeping the annealing temperature as low as possible to maintain high strength. Prestraining the sample to beyond the stress plateau but before yielding was found to be most effective for enhancing the recovery stress. By using these criteria, we were able to obtain large recovery stresses of greater than 550 MPa from 20 degrees C to 200 degrees C through a simple one-step annealing at 700 degrees C and a 10% prestrain using commercial superelastic NiTiNb wire products.