Effects of Heat Treatment on the Magnetic Properties of Nitinol Devices

Abstract

The shape memory alloy Nitinol is considered safe for magnetic resonance imaging (MRI) after electropolishing. However, oxidized Nitinol has been implanted and utilized in non-implanted devices. The use of oxidized devices in MR fields motivates the need to understand the effects of heat treatment on magnetic susceptibility to develop standards. This work characterizes the impact of surface treatment and thermally-produced oxidation on the averaged magnetic susceptibility of Nitinol (including surface layers). It was found that thin oxide layers—not visible to the human eye—like those resulting from surface preparation techniques or low-to-moderate temperature heat treatment, can alter the averaged magnetic susceptibility of Nitinol devices by several percent. Importantly, higher temperature and longer duration heat treatments (900 °C for 300 min) can result in strong ferromagnetic characteristics, accompanied by an increase in magnetic susceptibility measurements of more than 100 fold. This result was attributed to a ferromagnetic nickel layer in a multi-layer surface oxide that formed during heat treatment. Heavy oxidation created a sufficiently large ferromagnetic Ni subsurface layer such that an ordinary neodymium magnet could levitate a Nitinol wire specimen. There are currently no standards to prevent companies from heavily oxidizing components, potentially creating MRI safety hazards.