Stress-Aging Nitinol: The Surreptitious Factors Influencing Shape-Setting

Shape-setting is a vital process for contemporary medical devices containing Nitinol, where the nickel-titanium alloy undergoes a thermal-mechanical heat treatment (HT) to “memorize” a particular geometry and/or tune the mechanical properties. This study offers a holistic evaluation of the principles, methodologies, and paradigms that fortify the shape-setting process. Key independent variables influencing the process are systematically investigated, ranging from HT time (1 – 5 min), HT temperature (450 – 550 ˚C), number of shape-setting steps (1 – 3 steps), and cooling time delay (0.5 – 5.0 sec). Additionally, the lesser-known and infrequently studied input parameters of the shape-setting fixture strain-dependence (1 – 11%) and percent cold-work in the substrate (23 – 52% final area reduction) are also analyzed. A broad and well-balanced design of experiments (DOE) unravels HT effects on A_f temperature, percent shape retention, tensile properties, and micro-nano-scale alterations to grain morphology. With a statistical confidence interval in excess of 99.9%, the under-investigated aspect of fixture input strain is identified as one of the most influential factors governing the shape-setting process. In-depth mechanical testing, coupled with advanced microstructural analysis, fosters progressive refinement of fundamental knowledge by analyzing stress-aging conditions that are both practical and applicable to the manufacturing of modern Nitinol medical devices.