Mechanical, microstructural and thermal properties of a 50:50 at.% nickel-titanium alloy subjected to a dieless drawing process

It is widely recognized that hot working, grain structure, degree of deformation and crystal orientation can have a significant effect on the thermomechanical properties of Nitinol, the shape memory alloy of nickel and titanium, widely used in the biomedical device industry. In the current work, a novel hot-working process was applied to Nitinol rods with subsequent material hardness testing, electron backscatter diffraction (EBSD) and differential scanning calorimetry (DSC) analysis performed to determine the mechanical properties, grain texture and transformation temperatures of the processed Nitinol. The properties of the processed alloy, as ascertained by the aforementioned techniques, were compared to the as-received material. It was established that processing the material had the beneficial effect of softening the material from the as-received condition, making the material suitable for subsequent forming operations, such as cold drawing. EBSD results showed a strong preference towards a {1 0 1} texture in the processed material with some preference also towards a {1 1 1} texture. The results of the DSC analysis, which measured the transformation temperatures of the processed Nitinol material, showed a significant reduction in the austenite finish transformation temperature of the processed material compared to that of the as-received material. Tensile tests carried out to determine changes in the superelastic behaviour of the material due to the dieless drawing process corroborated and emphasized the results of the hardness tests, EBSD and DSC analyses. The process has resulted in a material with lower transformation temperatures, higher ductility and reduced tensile flow stress. The upper loading plateau begins at a lower strain and is present over a longer strain range.