Carbon and Oxygen Levels in Nitinol Alloys and the Implications for Medical Device Manufacture and Durability

This paper will consider the impact of carbon and oxygen content on Nitinol for medical applications. Three Nitinol melts were chosen to yield varying levels of oxygen and carbon content and were subsequently drawn to yield representative wire and tubing sizes for typical medical device applications. Inclusion content was studied via scanning electron microscopy and a series of fatigue and processing trials were carried out in order to study their effect. Introduction The increasing number of permanently implanted devices employing Nitinol places great demands on manufacturing quality and on in vivo fatigue life. In the past, inclusion content within Nitinol has often been linked to low process yields and fatigue crack initiation. While several studies have attempted to link the presence, size and distribution of inclusions with fatigue life [1-3], none have reached definitive conclusions regarding their effect. The following paper reports on the results of a systematic study on the effect of different melt methods and their associated inclusion content on fatigue life and Nitinol component manufacture. Materials Melting techniques for NiTi include either multiple vacuum arc remelts (VAR), or a vacuum induction melt (VIM) process followed by VAR. The combination of the VIM and the VAR process will be referred to as VIM/VAR. The standard VAR process (VARStandard) is a series of four successive melts into increasingly larger water-cooled copper crucibles, ending with a finished ingot of 585 mm diameter. Input materials for the standard practice include alloy grade, Kroll-reduced, titanium sponge and electrolytic nickel. The electrode is made by standard compaction and welding techniques. This practice has proved to produce good homogeneity and reasonably uniform Af temperatures for largescale (3,000 kg) ingots. The VAR-ELI (Extra Low Inclusion) process incorporated three melts to finish with a 405 mm diameter ingot. Input materials for the ELI ingot included iodide-reduced titanium crystal bar produced by a Van ArkelProceedings of the International Conference on Shape Memory and Superelastic Technologies May 7–11, 2006, Pacific Grove, California, USA Brian Berg, M.R. Mitchell, and Jim Proft, editors, p 821-828 Copyright © 2008 ASM International® All rights reserved. DOI: 10.1361/cp2006smst821