In Situ TEM Investigation of the Thickness Effect On the Deformation Behaviors In Nanocrystalline NiTi Alloys

A thin film tensile deformation technology in transmission electron microscopy (TEM) was developed to in situ study the deformation behaviors of nanocrystalline NiTi alloys in nanoscale. NiTi strips with thickness ranging from 60-500 nm were subjected to tensile deformation in TEM. The results showed that the thickness of the strip play an important role on the deformation behaviors of NiTi alloys. When the thickness was refined to 140 nm, a transition from stress-induced martensitic transformation to slip of dislocations occurs. The results indicated that a critical thickness existed comparing with the critical transformation stresses for nucleation of martensite and dislocations. For specimen with thickness larger than 140 nm, two martensitic variants with a same value of shear angle to the loading axis were triggered in the grains. Intragranular fracture was observed for all of NiTi strips.