Shape memory alloys can retain and recover their initial shape after deformation through heating at temperatures above transformation temperature. Over the past several decades, the unique shape memory and super-elastic behaviour of NiTi alloy make it an ideal candidate to produce sensitive tools and components. In addition, high corrosion resistance and biocompatibility as well as great mechanical and tribological properties of the NiTi alloy have led to the extensive biomedical, aerospace and military applications. In this study, the NiTi shape memory alloy was fabricated using copper boat vacuum induction melting method. The influence of thermo-mechanical treatment such as cold rolling and subsequent annealing on microstructure, mechanical, super-elastic and shape memory properties of NiTi alloy was investigated. The samples prepared by induction melting inside the copper boats were homogenized at 900 o C for 4 h, after which they were subjected to hot rolling and subsequent cold rolling with 20-70% reduction in thickness at room temperature. Microstructural characterization carried out by transmission electron microscopy showed a nanocrystalline austenite structure with 20 nm grain size as well as deformation bands and amorphous structure in the 70% cold-rolled sample. The XRD analysis confirmed the presence of austenite phase in the cold rolled specimens. The density of dislocations for 70%-rolled sample was found to be ~ 10 13 cm -2 . Annealing at 300 o C mainly decreased the density of dislocations, whereas the amorphous bands were still present in the sample microstructure. Annealing at 400 o C led to the crystallization of amorphous phase and growth of austenite nano-grains to a range of 20-70 nm. In addition, annealing at 500 o C caused grain growth, and lowering of the dislocation density, leading to the stability of martensite phase. The results derived from hardness and shear punch tests revealed an increasing trend for hardness and strength with increasing the cold roll percentage; meanwhile, lower hardness and strength were obtained with increasing the annealing temperature executed after cold rolling. The super-elastic properties of the thermo-mechanically treated specimens, derived from the triple-point bending test at 100 o C, showed a promising behaviour of the samples annealed at 400 o C and existence of strain for the samples annealed at 500 o C. The shape memory test results revealed that thermo-mechanical treatment yields to an increase in the shape recovery percentage from 85% to over 94%. As well, for 40% and 70%-rolled samples which were annealed at 400 o C, the shape recovery percentage was measured to be ~100%. Keywords : NiTi Shape memory alloy; Nanostructured alloy, Thermo-mechanical treatment; Shape memory properties
