*Invited* "Nanocrystalline NiTi Shape Memory Alloy: Really Something New?"

There are two potential size scales that may impact the transformation behavior of SMAs: the grain size d and the sample size D. However, the volume of existing work on the effect of either of these variables on structural and functional properties of SMAs remains small. There is thus a standing scientific and technological need for systematic investigations of size effects in SMAs.

Grain size has been known for a long time to represent an important variable in defining material strength, fracture toughness and fatigue strength since the pioneering work of E.O. Hall and N.J. Petch appeared in the late 1940s.

Recent studies reported that the transformation temperatures continue to decrease in nanocrystalline NiTi SMAs, until d reaches a critical size around 50 nm, when transformation to martensite is no longer observed. It was also claimed that the evolution of axial properties as a function of grain size followed a path predicted by the classic Hall-Petch theory: fine-grained material exhibited increased strength and reduced plasticity during cyclic testing. A large increase in both ultimate tensile stress and the level of the unloading plateau was found in the samples with grain sizes ranging from 50 to 100 nm, respectively. Moreover, the strain extension of the loading plateau during the stress-induced formation of martensite was found to depend on microstructure.

Aim of this work is to critically review the above mentioned literature and show that commercially available NiTi products are nanocrystalline. A comparison between TEM and XRD measurements on specimens of different size will be presented. Examples of the grain size evolution throughout processing, its effects and significance on shape memory and superelastic properties will also be reported.