Characterization of Dynamic Mechanical Behavior and Microstructure of Biomedical Nitinol

Nitinol has broad applications in medical devices such as vascular stents due to its biocompatibility to human body and other implant materials.  The mechanical stress-strain response of Nitinol is much closer to that of tissues than traditional biometallic materials such as stainless steel.  The mechanical behavior of Nitinol in tension has been well studied, but little knowledge exists for the behavior of Nitinol in compression. Even less information is available for the compressive mechanical response of Nitinol under increasing strain rates.  Furthermore, it has been shown that Nitinol exhibits an asymmetric response when loaded in tension versus compression, so the mechanical behavior in compression mode will be very valuable in design and manufacture Nitinol devices.  This study investigates the mechanical response of biomedical Nitinol SE508 (50.8 at.%Ni - 49.2 at.%Ti) under both quasi-static and dynamic compression modes. Under quasi-static compression conditions, the elastic modulii in the austenite and martensite regions were found to be 48.28 GPa and 40.96 GPa, respectively.  The ultimate compressive strength (UCS) was found to be as high as 2.34 GPa.  The Split Hopkinson pressure bar (SHPB) testing data showed a general trend of decreasing the UCS while increased toughness with the increased strain rate.  The average grain size of the undeformed Nitinol samples was on the order of 30 microns, while grain sizes for the samples at strain rates of 0.001s^-1, 2600s^-1, and 3157s^-1, were 34µm, 32µm, and 26µm, respectively.

Keywords: Nitinol, biomaterial, implant, mechanical behavior, microstructure