Unraveling Deformation Mechanisms during Shape Setting of Superelastic Nitinol

Superelastic Nickel–Titanium (Nitinol) shape memory alloys are extensively used in medical devices such as stents, which are typically manufactured from cold-worked tube or sheet stock via laser cutting. The laser-cut components are subsequently formed or shape-set to their final geometry through thermomechanical processes conducted at 400–600 °C. During these steps, a small degree of springback occurs, while a permanent shape change is achieved. However, the fundamental mechanisms governing this permanent deformation and springback behavior remain insufficiently understood.

It is hypothesized that high-temperature shape setting in NiTi alloys—yielding maximum dimensional stability with minimal springback—results from one or both of the following mechanisms: (1) dislocation activity and/or (2) preferential nucleation and reorientation of Ni₄Ti₃ precipitate variants. To investigate the relative contributions of these mechanisms, various shape-setting heat treatment conditions were applied to NiTi strip specimens, systematically varying temperature, duration, and mandrel size to isolate the effects of precipitate- and dislocation-based processes on permanent deformation. Preliminary results show that preferential Ni₄Ti₃ precipitate formation play significant role during shape setting step. Subsequently, samples were subjected to a secondary unconstrained heat treatment to evaluate dimensional changes during free heating and gain further insight into the governing mechanisms of permanent shape evolution.