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

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 & DEG;C (Ms + 30 & DEG;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 & SIM;8% and & SIM;12%, respectively. In stage II, the stress-induced martensitic transformation is accompanied by the dislocation slip of the NiTi matrix and fi-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 & epsilon;Tre upon heating increases first and then decreases. The & epsilon;Tre obtains a maximum of 7.03% at 10% prestrain and de-creases to 6.17% at 16% prestrain. The increase of & epsilon;Tre 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 & epsilon;Tre is mainly associated with the irrecoverable strain by {2 0 -1} martensite twins. The effect of fi-Nb inclusions on the evolution of SIMVs is also found herein that deformed fi-Nb inclusions can significantly hinder the growth and recoverability of adjacent stress-induced martensite.& COPY; 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.