Characterization of Microstructure and Dynamic Mechanical Properties of Biomedical Nitinol Alloy

Nitinol, an equiatomic alloy of Nickel and Titanium, has broad applications in the biomedical industry. It is widely accepted that Nitinol is very biocompatible in the human body. It has been used mainly as a material for various kinds of stents. Much information exists for the mechanical response of Nitinol that has been loaded in tension, but little exists for Nitinol in compression. Furthermore, even less information is available for the compressive mechanical response of Nitinol under increasing strain rates encountered in various manufacturing processes. This study details the compressive mechanical response of Nitinol (50.9 at.%Ni-49.1 at.%Ti) under both quasi-static compression and high rate Split Hopkinson pressure bar testing conditions. Under quasi-static loading conditions, the elastic modulus in the austenite and martensite regions were found to be 48.28 GPa and 40.96 GPa, respectively. The ultimate compressive strength (UCS) for the quasi-static condition was found to be 2.34 GPa. The SHPB data showed a general trend of lessening the UCS while increasing the toughness with increased strain rate. Increasing the strain rate also tended to flatten out the transformation region between austenite and martensite. This region became unrecognizable in the stress-strain response at the highest strain rates. Optical images of this material showed a grain size on the order of 30 micron for the undeformed samples. Optical images were also taken to show the evolution of the grain structure with increased strain rate.

 

Keywords: Nitinol, implant, mechanical property, microstructure, compression