Study of New Multifunctional Shape Memory and Low Elastic Modulus Ni-Free Ti Alloys
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NICKEL-TITANIUM alloys have extreme importance due to their capability of recovering the shape when thermal or load cycles are applied. The responsibility of this behavior is a thermoelastic phase transformation, which occurs from an ordered body centered cubic (called B2, austenite or beta) structure to a thermoelastic martensite phase (a¢¢). This transformation promotes interesting properties such as superelasticity, high damping capacity, and excellent wear behavior.[1–5] Also, such alloys present excellent corrosion resistance and biocompatibility,[6,7] which increment their interest for use in the biomedical field. There are a considerable number of manufactured products developed for orthopedic, cardiovascular, and dental applications, such as osteosynthesis plates, jaw plates, and orthodontic archwires using TiNi alloys;[2,5] however, the issue of material biocompatibility is still controversial, as Ni-ions release from the implant to the surrounding tissue has been reported to produce adverse M. ARCINIEGAS, Doctoral Student, J.M. MANERO, Senior Research, F.J. GIL and J.A. PLANELL, Professors, are with the Biomaterials and Biomechanics Division, Department of Materials Science and Metallurgy, Technical University of Catalonia, Av. Diagonal 647, 08028 Barcelona, Spain. Contact e-mail: milena. [email protected] J. PEN˜A, Associated Professor, Biomaterials and Biomechanics Division, Department of Materials Science and Metallurgy, Technical University of Catalonia, Av. Diagonal 647, is with the Department of Materials Science, Escola Superior en Disseny ELISAVA, C/ Ample 11–13, 08002 Barcelona, Spain. Manuscript submitted February 1, 2007. Article published onlined February 28, 2008 742—VOLUME 39A, APRIL 2008
reactions.[3,8–10] Two alternative methods of avoiding diverse tissue reactions are being investigated. One is Ti-oxide surface modifications, which change the implant-tissue interface,[11] and the other is the investigation of new Ni-free Ti shape memory alloys, which appear to be the surest way for substitutive materials for biomedical applications.[12–19] Another relevant aspect in the biomedical field is the research for optimizing the bone-implant coupling in order to avoid the stress-shielding effect. Bone requires a mechanical stimulus for growth; hence, in a boneimplant coupling, it is essential to use materials with rigidity properties similar to bone for improving the load-transfer and favoring bone healing and remodeling.[5] The most important implant materials used for substituting hard tissue and forming bone-implant coupling are the Ti alloys due to their high mechanical and fatigue resistance, low density, high corrosion resistance, and excellent biocompatibility properties.[20–22] Moreover, the Ti alloys present a lower rigidity than other implant materials such as CoCr alloys or stainless steels.[20] Due to the lower elastic modulus exhibited by the b phase of titanium,[23–27] compared with the values presented by the a phase, b-Ti alloys are a relevant field of investigation. However, the
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