Microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with different prestrain levels

Abstract

Ni47Ti44Nb9 shape memory alloy (SMA) is a promising material in the aerospace field due to its wide transformation hysteresis. The application of shape memory effect depends on multistep thermomechanical loading, viz., low-temperature deformation and subsequent heating to recovery. Low-temperature deformation prestrain plays a pivotal role in shape memory properties tailoring of SMA components. However, microstructure evolution and deformation mechanisms of Ni47Ti44Nb9 SMA subjected to various prestrain levels are still unclear. To this end, microstructure evolution and shape memory behaviors of Ni47Ti44Nb9 alloy subjected to multistep thermomechanical loading with prestrain levels of 8% - 16% at -28 °C (Ms + 30 °C) were investigated. The results demonstrate that the stress-strain curve of the specimen exhibits four distinct stages at a maximal prestrain of 16%. Whereas stage II and stage III end at prestrains of ∼8% and ∼12%, respectively. In stage II, the stress-induced martensitic transformation is accompanied by the dislocation slip of the NiTi matrix and β-Nb inclusions. In stage III, in addition to the higher density of dislocations and further growth of stress-induced martensite variants (SIMVs), (0 0 1) compound twins are introduced as a result of the (0 0 1) deformation twinning in stress-induced martensite. More {2 0 -1} martensite twins are gradually introduced in stage IV. Correspondingly, after subsequent unloading and heating, a higher density of {1 1 4} austenite twins form in the specimen with a larger prestrain of 16%. With increasing prestrain from 8% to 16%, the recoverable strain εreT upon heating increases first and then decreases. The εreT obtains a maximum of 7.03% at 10% prestrain and decreases to 6.17% at 16% prestrain. The increase of εreT can be attributed to the formation of new SIMVs, the further growth of existing SIMVs, and the recoverable (0 0 1) compound twins. While the decrease of εreT is mainly associated with the irrecoverable strain by {2 0 −1} martensite twins. The effect of β-Nb inclusions on the evolution of SIMVs is also found herein that deformed β-Nb inclusions can significantly hinder the growth and recoverability of adjacent stress-induced martensite.