The physical metallurgy of nitinol for medical applications

Abstract

OverviewThe purpose of this paper is to review the current processing and resultant properties of Nitinol for medical device applications. The melting and fabrication of Nitinol present a number of unique challenges because of the strong sensitivity of the alloy system to chemistry and processing. The first part of this paper will summarize the effect of alloy fabrication on key material properties, vacuum-melting techniques, hot working, and cold working. The effects of the final shape-setting heat treatments on transformation temperature and mechanical properties for medical devices will also be reviewed.

Functional textiles driven by transforming NiTi wires

“…In order to restore the plain shape, the textile is shape set i.e. it is flattened, constrained, and exposed to temperatures usually ranging from 300°C to 500°C for 10-30 minutes [8]. Fig.…”

“…The use of thin NiTi wires in textiles is further seen beneficial for functional properties such as temperature driven reversible changes of Young's modulus through martensitic transformations [6] (switching between stiffer hot austenite phase ~75 GPa and softer cold R-phase or martensite phases ~40 GPa), pseudoplastic behaviour of cold martensite phase [7] (recoverable deformation up-to 10 % due to martensite detwinning), and thermally induced shape memory effect [7] (recovery of detwinned martensite upon heating) may provide textiles with advanced functional properties. Finally, cold drawn thin NiTi wires can be shape set [8] in complex curved shapes through annealing termed shape setting, which may be advantageously used as an additional textile processing step. To sum up, NiTi textiles may provide advanced functional properties stemming from the combined effect of textile patterns and textile-compatible outstanding functional properties of thin NiTi wires.…”

Experimental Study of Helical Shape Memory Alloy Actuators: Effects of Design and Operating Parameters on Thermal Transients and Stroke

“…The wire was then constrained using clamps and subsequently heated. From the literature, the necessary heat times and temperatures were estimated at 10 min and 500 °C, as these settings have been shown to adequately shape set the alloy without significantly changing the material's transition temperature and strength properties [16][17][18]27]. Conversely, it should be noted that these parameters are not completely agreed upon in the literature [11].…”

Nanoindentation of Annealed and As-Sputtered Thin Films of Nickel Titanium Shape Memory Alloys

“…Nickel titanium (NiTi) was discovered by the Naval Ordinance Laboratory in 1962 21 . NiTi is the most common SMA because it can exhibit shape memory or pseudoelastic properties with a range of transition temperatures depending on processing 22 . There are copperbased SMAs; but are not preferred over NiTi because they are less stable and more brittle.…”

“…Impurities in NiTi affect the transition temperature, usually decreasing the martensite start temperature 22 . Titanium strongly reacts with oxygen, nitrogen, and carbon to form oxides, nitrides, and carbides.…”