Multifunctional Shape Memory Alloys via Nanoscale Phase Transformation
Superelastic NiTi shape memory alloys (SMAs) are used in a variety of fields such as biomedical and energy absorption applications . However, the conventional coarse-grained and nanocrystalline SMAs with grain size >60 nm face significant limitations such as a narrow temperature window of superelasticity, poor fatigue response, weak high-frequency cyclic stability, caused by heat accumulation, and significant jumps in mechanical and thermal fields in non-linear vibration applications . Recent studies have shown that grain size reduction to nanoscale (<60 nm) is a practical way to achieve novel properties such as small hysteresis, extended temperature window of superelasticity, improved high-frequency cyclic stability, weak heat accumulation, and improved stress-controlled fatigue response . The physical mechanisms behind such improvements have been fully explained based on the non-local Ginsburg-Landau theory and existing heat transfer models.
However, recent experiments have shown that nanocrystalline NiTi with average GS of 5 nm shows anomalous behaviors such as Invar (Invariance of thermal expansion with temperature), Elinvar (Invariance of Youngfs modulus with temperature), extended temperature window of elastocaloric effect and a novel transition temperature known as glass transition temperature. These observations are highly peculiar and offer a unique way to expand the repertoire of SMAs by nano-crystallization/defect engineering.
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Fig. 1 Field-cooling/zero-field cooling (left) and DMA (right) experiments showing the existence of glass transition temperature in nanocrystalline NiTi.
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