Nickel-titanium alloys are commonly used for the manufacture of intraluminal biomedical devices, such as self-expandable stents, stent grafts, embolic protection filters, and stone extraction baskets. Such devices may exploit the superelastic or shape memory behavior of equiatomic or near-equiatomic nickel-titanium alloys, which are commonly referred to as Nitinol. As a result of the poor radiopacity of nickel-titanium alloys, however, such devices may be difficult to visualize from outside the body using non-invasive imaging techniques, such as x-ray fluoroscopy. Visualization is particularly problematic when the intraluminal device is made of fine wires or thin-walled struts. Consequently, a clinician may not be able to accurately place and/or manipulate a Nitinol stent or basket within a body vessel.
Current approaches to improving the radiopacity of nickel-titanium medical devices include the use of radiopaque markers, coatings, or cores made of heavy metal elements. In addition, noble metals such as platinum (Pt), palladium (Pd) and gold (Au) have been employed as alloying additions to the improve the radiopacity of Nitinol, despite the high cost of these elements. In a more recent development, it has been shown (e.g., U.S. Patent Application Publication 2008/0053577, “Nickel-Titanium Alloy Including a Rare Earth Element,” which is hereby incorporated by reference in its entirety) that rare earth elements such as erbium can be alloyed with Nitinol to yield a ternary alloy with radiopacity that is comparable to if not better than that of a Ni—Ti—Pt alloy.
Ternary nickel-titanium alloys that include rare earth or other alloying elements are commonly formed by vacuum melting techniques. However, upon cooling the alloy from the melt, a brittle network of secondary phase(s) may form in the alloy matrix, potentially diminishing the workability and mechanical properties of the ternary alloy. If the brittle second phase network cannot be broken up by suitable homogenization heat treatments and/or thermomechanical working steps, then it may not be possible to find practical application for the ternary nickel-titanium alloy in medical devices or other applications.
As stated in U.S. Patent Application Publication 2008/0053577, the nickel-titanium alloy has a phase structure that depends on the composition and processing history of the alloy. The rare earth element may form a solid solution with nickel and/or titanium. The rare earth element may also form one or more binary intermetallic compound phases with nickel and/or with titanium. In other words, the rare earth element may combine with nickel in specific proportions and/or with titanium in specific proportions. Without wishing to be bound by theory, it is believed that most of the rare earth elements set forth as preferred ternary alloying additions will substitute for titanium and form one or more intermetallic compound phases with nickel, such as, for example, NiRE, Ni2RE, Ni3RE2 or Ni3RE7. In some cases, however, the rare earth element may substitute for nickel and combine with titanium to form a solid solution or a compound such as TixREy. The nickel-titanium alloy may also include one or more other intermetallic compound phases of nickel and titanium, such as NiTi, Ni3Ti and/or NiTi2, depending on the composition and heat treatment. The rare earth addition may form a ternary intermetallic compound phase with both nickel and titanium atoms, such as NixTiyREz. Some exemplary phases in various Ni—Ti-RE alloys are identified below in TABLE 1. Also, in the event that one or more additional alloying elements are present in the nickel-titanium alloy, the additional alloying elements may form intermetallic compound phases with nickel, titanium, and/or the rare earth element.
TABLE 1Exemplary Phases in Ni—Ti-RE AlloysAlloyExemplary PhasesNi—Ti—DyDyNi, DyNi2, DyxTiy, α(Ti), α(Ni), NixTiyDyzNi—Ti—ErErNi, ErNi2, ErxTiy, α(Ti), α(Ni), NixTiyErzNi—Ti—GdGdNi, GdNi2, GdxTiy, α(Ti), α(Ni), NixTiyGdzNi—Ti—LaLaNi, La2Ni3, LaxTiy, α(Ti), α(Ni), NixTiyLazNi—Ti—NdNdNi, NdNi2, NdxTiy, α(Ti), α(Ni), NixTiyNdzNi—Ti—YbYbNi2, YbxTiy, α(Ti), α(Ni), NixTiyYbz
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