Recovery stress of Ni-Ti-Nb wide-hysteresis shape memory alloy under constant strain and thermomechanical cycling

Abstract

Ni-Ti-Nb alloy is a new type of shape memory alloy with a wide transformation hysteresis ( A ' s M ~ = 130-150 °C) [1]. Parts made of the alloy can be stored and shipped at ambient temperature without keeping in liquid nitrogen, which is very convenient for engineering. Therefore, this new alloy has great potential usage as mechanical components for joining, fastening and sealing. The microstructure and effect of deformation on transformation hysteresis of the alloy have been studied [2-5]. However, the recovery stress of Ni-Ti-Nb shape memory alloy has not been reported. Therefore, the purpose of the present work was to investigate the effect of deformation and thermomechanical cycling on the recovery stress of Ni-Ti-Nb alloy. The composition of the alloy was 44.7 at % Ni, 46.3 at % Ti and 9 at % Nb. The Mr, Ms, As and Af temperatures of the alloy were determined to be -36, 14, 45 and 60 °C, respectively, by means of electrical resistance versus temperature measurement. The resultant ingot was hot-swaged and rolled at 850 °C to strips of approximately 0.5 mm thickness. Specimens for all tests having a tensile gauge length of 40 mm and a width of 5 mm were cut from the strips and annealed at 850°C for 1.8 x 103s under 0.1 MPa vacuum and then furnace cooled. For measuring recovery stress, tensile tests were performed on a Shimadzu Autograph DDS-10T-S machine with fixture to allow heating the specimen for temperature control with a strain rate of 4.1 x 10 -4 s -1. The schematic stress-strain curve in the experiments is shown in Fig. 1. The tensile specimens were deformed at various temperatures (Ta) to different total strain (eT) and then the load was removed. The specimen was then slowly heated