Phase Transformation Behavior of Porous TiNi Alloys Produced by Powder Metallurgy Using Magnesium as a Space Holder
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DUCTION
SUPERELASTICITY and shape memory effect are two main characteristics that cause TiNi alloys to gain functionality and to be renowned. Equiatomic and near equiatomic TiNi alloys exhibit both behaviors depending on temperature. Recently, it was found that porous TiNi alloys also display superelasticity[1–5] as well as shape memory[6,7] to some extent. Material composition, amount of deformation, and heat treatment affect both superelasticity and shape memory behavior by altering the corresponding transformation temperatures of the alloys, crucial in terms of practical applications. TiNi foams retain their distinctive shape memory and superelasticity properties together with high strength and ductility while exhibiting considerable toughness and good corrosion resistance besides biocompatibility in porous form. Thus, TiNi foams have the potential to be used in different areas,[8] such as energy absorbers under impact conditions, separators or filters, connectors, actuators, and especially in dentistry and bone implants in orthopedics, where their high cost is compensated. The inelastic strain recovery capability of porous TiNi alloys enables easier implant insertion in conjunction with better mechanical compatibility with the host tissue[8] when compared to alternative metallic foams TARIK AYDOG˘MUS¸, Research Assistant, formerly with the Department of Metallurgical and Materials Engineering, Middle East Technical Universtiy, 06531 Ankara, Turkey, is now with the Department of Mechanical Engineering, Yu¨zu¨ncu¨ Yıl University, _ 65080 Van, Turkey. Contact e-mail: [email protected] ELIF TARHAN BOR, Specialist, is with the Advanced Material Characterization Research and Development-Training Center, Central Labora_ BOR, Professor, is tory, Middle East Technical University. S¸AKIR with the Department of Metallurgical and Materials Engineering, Middle East Technical Universtiy. Manuscript submitted October 7, 2010. Article published online May 5, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
of stainless steel, titanium, Ti6Al4V alloy, or tantalum. Moreover, bulk TiNi alloys have the lowest stiffness, except that of magnesium, among the ceramic and metallic materials used frequently in biomedical applications. In fact, TiNi alloys in the martensitic state have a lower stiffness than magnesium. As a result, the requirement of elastic modulus levels similar to those of the bones for the minimization of the stress shielding effect may be attained by adjusting the porosity content without considerable loss in strength. The high melting point, 1583 K (1310 C), and extreme reactivity of TiNi with atmospheric gases and crucible materials make its foaming in liquid state substantially difficult, despite the expensive high vacuum and high-temperature equipment required. Powder metallurgy techniques, on the other hand, provide the flexibility of low-temperature processing as well as close composition control, as well as mechanical and physical property modifications via adjustment of the processing parameters and characteristics o
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