Volume 7, Issue 5 (May 2010)
Mechanical Conditioning of Superelastic Nitinol Wire for Improved Fatigue Resistance
Metallic wire used in medical devices contains small defects that must be accounted for in design to guard against failure. Sites of probable failure are often constituent inclusion particles, pores, or surface defects that behave as crack-like, stress concentrators. The aim of this research is to examine the effects of mechanical overload conditioning applied to medical-grade nitinol wire on fatigue performance. A mechanical overload conditioning treatment comprising a single axial tensile strain cycle of 11.5 % was applied at room temperature (300 K) to nominally Ti 50.9 at. % Ni wires with active Af 280 K. The conditioning strain cycle was applied to both plain wire samples with only process and melt-intrinsic defects and to samples which were milled by focused ion beam to produce a transverse 10×0.5×3 μm notch. Transmission electron microscopy was used to probe the root of the milled notch before and after overload conditioning in order to ascertain microstructural parameters responsible for property changes. Evidence of a plasticity-locked, mixed-phase, microstructure at the sharp defect root was found after conditioning. Samples were loaded in a rotary beam fatigue apparatus and cycled in air at 60 s−1 to a maximum of 109 cycles. The fatigue strain limit was increased by more than 20 % at 107 cycles in the conditioned versus non-conditioned plain wire.