Multifunctional Shape Memory Alloys via Nanoscale Phase Transformation

 

Aslan Ahadi

ICYS-Sengen researcher

 

Superelastic NiTi shape memory alloys (SMAs) are used in a variety of fields such as biomedical and energy absorption applications [1]. 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 [2]. 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 [3]. 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 Youngfs 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.

Fig. 1 Field-cooling/zero-field cooling (left) and DMA (right) experiments showing the existence of glass transition temperature in nanocrystalline NiTi.

References

[1] X. Ren and K. Otuska, Progress in Materials Science Vol. 50 511 (2005)

[2] M. Xia, PhD thesis, The Hong Kong University of Science and Technology (2015).

[3] A. Ahadi and Q.P. Sun, Acta Materialia Vol. 76 186-197 (2014).