Size Effect on the Martensitic Transformation of Ti−Nb Shape Memory Alloy

The effect of grain size on martensitic transformation was investigated by transmission electron microscopy in a nanocrystalline Ti–16Nb thin film with a broad range of grain sizes. The results show that the larger nanocrystals are filled with multiple variants with the {111}α" type I twinning relationship. As the grain size decreases to about 40 nm, the martensite changes to the single variant morphology, and the shape memory effect is believed to deteriorate significantly then. Finally, the martensitic transformation is totally suppressed and the β parent remains when the grain size drops below 25 nm.