Analysis of New Nitinol Ingot Qualities

Nonmetallic inclusions are suspected to be one of the main microstructural triggers for fatigue failure of Nitinol vascular implants. Chemical composition, size, number and distribution of inclusions are substantially defined by the Nitinol melting process. Recent fields of application of Nitinol vascular implants with a high significance regarding fatigue performance, such as self-expanding percutaneous heart valves, lead to tighter requirements concerning Nitinol ingot purity. As a result, commercial Nitinol ingot suppliers place material with reduced inclusion number and size on the market.

In this study new ingot qualities, such as Wah Chang ELI and SAES Redox, were compared with existing ones. Finished components as well as diamond shape samples representing typical dimensions of two different implant product groups – stents and heart valve frames – were produced using Nitinol tubing drawn from the new ingot qualities.

Metallographic cross sections were prepared and analyzed to determine inclusion size distributions of the various ingot qualities. Radial force and uniaxial tensile tests were used to determine the mechanical properties of fully processed material and tubing, respectively. Transformation temperatures of processed materials and raw tubing were measured by differential scanning calorimetry and deformation-and-free-recovery testing. Diamond shape fatigue testing of processed material was performed to evaluate the strain-life characteristics of the new ingot qualities.

Results of this study are compared to ADMEDES historical data from standard Nitinol materials to gain an assessment of the new ingot qualities with regard to the production of Nitinol vascular implants. The presentation highlights the latest developments in Nitinol ingot quality and shows the results of the comparison from a technical point of view.