The Influence of Microcleanliness On the Fatigue Performance of Nitinol

The presence of carbide and oxide inclusions in Nitinol and their influence on the fatigue performance has been an important consideration in the medical device community. Micro-cleanliness, as determined by inclusion size and inclusion density, has often been cited as a potential factor influencing the fatigue behavior of the material. However no comprehensive study has been conducted to determine their impact on the fatigue behavior. The following effort compares the fatigue behavior of Nitinol wires produced by six different melt processes exhibiting equivalent ingot transformation temperatures. The inclusion size and density in the 2 mm re-draw wires from each supplier were quantified using SEM-BEI method. The re-draw wires obtained from the different suppliers were then uniformly processed into fine wire by a third party wire drawer utilizing consistent drawing parameters. The fine wires were uniformly heat treated and tensile tests, calorimetric measurements and rotating bending fatigue tests were conducted on the wires. Fracture surfaces of the wires were studied post failure using SEM. In the present study the only difference between the wires was the material supplier, their melt practice, and consequently their carbide and oxide content. Effort was taken to maintain all the other processing (i.e. cold work, heat treatment) and testing parameters identical to minimize the comparative variables.