Influence of Annealing and Aging Process on Superelastic Behavior and Martensitic Transformation of Ni-rich NiTi Shape Memory Wires

In the present study, the influence of annealing heat treatment and aging treatment after annealing process on the superelastic behavior of 40 and 60% cold worked Ni-rich NiTi shape memory alloy have been investigated. Results showed that at the presence of the fine grains due to the recrystallization process, superelastic behavior improved; however, some strains about 0.2% remain in the strain-stress curves of the alloy. With the formation of recrystallized fine grains, due to annealing heat treatment as well as Ni4Ti3 precipitates, due to the aging treatment after the annealing process, the best superelasticity behavior achieved. It is observed that the variation of grains size result in variation of precipitates distribution within microstructure. This, therefore, leads to variation of transformation temperatures. Differential scanning calorimetry (DSC) analysis was also used to evolution the transformations temperatures and behavior of the samples. Consequently, it is observed that, with variation of grains sizes during annealing process and with aging treatment after that, transformation temperatures can be controlled for the different thermal applications.