Abstract
The shape recovery characteristics of titanium nickelide with an Ni content of 50.0 at % and 50.7 at % were studied in a wide range of structures obtained as a result of cold drawing with an accumulated true strain of e = 0.52 and subsequent annealing in the 250 to 700 °C temperature range. Shape memory effect (SME) inducing was carried out by bending using a non-isothermal loading mode, which made it possible to reveal implementing elastic strain in the equiatomic alloy up to 12% and thereby increase the total shape recovery by a factor of 1.5. The obtained results prove that the Ni content strongly affects the value and specific features of changes of the shape recovery characteristics with loading strain as well as grain/subgrain size. In equiatomic alloy, the total recovery strain manifests its maximum of 13.5–15% and the recovery strain of 9% at a loading strain range of 12 to 14%. In Ni-rich alloy, the total recovery strain manifests its maximum of 20% and the recovery strain of 14% at a loading strain range of 15 to 21%. The maximum two-way SME value correlates with the residual strain in both alloys and reaches its maximum of 3.0% in a material with a recrystallized structure. Varying the loading strain value under bending in the 11 to 21% range allows regulation of the temperature of shape recovery in Ni-rich alloy in the 45 to 80 °C range.
Highlights
Ti-Ni-based shape memory alloys (SMAs) of near-equiatomic compositions remain the most popular as functional materials with a unique set of functional properties
Metals 2021, 11, x FOR PEER REVIEWevolution and DSC plots for Ni-rich alloy are presented in Figures 2 and 3 respectively; similar studies of the equiatomic alloy were performed in [9]
15% in the range of grain/subgrain sizes of 0.03–0.6 μm and the recovery strain manifests its maximum of 9% in the grain/subgrain size range of 0.2 to 3 μm; In Ni-rich alloy, the total recovery strain manifests its maximum of 20% in the range of grain/subgrain sizes of 0.05 to 0.2 μm, and the recovery strain manifests its maximum of 14% in the range of grain/subgrain sizes of 0.2 to 3 μm; The use of the non-isothermal shape memory effect (SME) inducing mode made it possible for the first time to reveal the possibility of implementing elastic strain in the equiatomic alloy up to 12% and thereby increase the total shape recovery by 1.5 times
Summary
Ti-Ni-based shape memory alloys (SMAs) of near-equiatomic compositions (withNi content in the range of 0–50.9 at %) remain the most popular as functional materials with a unique set of functional properties. In fundamental research and development of various kinds of devices, the attention of scholars and engineers is aimed at ensuring high shape recovery properties: superelasticity (pseudoelasticity), one-way shape memory effect (SME), two-way SME (TWSME), and shape recovery temperatures. These properties are sensitive to structure, which determines the possibility of using various structural factors for their purposeful control [6,7,8,9,10]. Structure evolution under aging brings changes in the kinetics, sequence, and staging of martensitic transformations (MT) [17,18], which determine both shape recovery characteristics and the thermomechanical conditions for their implementation [18,19]
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