Effect of Applied Stress during annealing on the Mechanical Behaviors of NiTiNol Wires

Nitinol has been widely used in medical devices due to its unique superelasticity and shape memory effect.  For the application of Nitinol-based medical devices, the lower plateau strength (LPS) is the actual stress used during the service.  A bunch of studies have been explored to optimize the mechanical properties of Nitinol alloys.  However, there is very limited attention to the LPS.  In the present study, the effect of applied stress on a nominal 0.053 mm cold drawn wire during straight annealing on the mechanical behaviors of Nitinol wires is investigated.  The applied stress varies from zero to 500 MPa was employed.  After annealing, the transformation temperatures from both differential scanning calorimetry (DSC) and bend and free recovery (BFR), the mechanical and fatigue behaviors of the heat treated wires were investigated.  It was found that the LPS firstly keeps constant, and then increases when the applied stress is in the range of 200~300 MPa.  When the applied stress is above 350 MPa, the LPS saturates again.  However, the variation of the upper plateau stress (UPS) is very limited.  Additionally, the stress has no effect on the austenite finish temperature (Af).  The energy dissipation decreases abruptly when the applied stress is between 300 and 400 MPa.  This study will shed some light on the optimization of Nitinol wires.