Mechanical performance of seven low-inclusion Nitinol grades for medical device applications

Nitinol is widely used in biomedical applications due to its superelasticity and shape memory properties, but inclusions can compromise mechanical performance. This study evaluates the mechanical performance of seven low-inclusion grades (≤26 μm inclusion size), focusing on tensile properties, fatigue performance and the influence of inclusion size under simulated operational conditions. By minimizing inclusion size to varying degrees, these grades aim to enhance durability for critical applications such as cardiovascular stent frames. Raw materials, sourced from several suppliers with the same requirements such as transformation temperatures and percent cold work, were processed into test specimens at Gore using identical equipment and parameters. Characterization involved uniaxial tensile testing (ASTM F2516), fatigue testing (up to 10^8 cycles), and micro-cleanliness evaluation. Fatigue fracture initiating inclusions were further analyzed via scanning electron microscopy (SEM). Results indicate that smaller inclusion size significantly improves fatigue performance. However, additional factors such as surface quality can impact fatigue performance as well. Additionally, variations in inclusion size measurement methods will be compared to fatigue performance results. This study also highlights the mechanical performance variations among the seven Nitinol grades.