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NITINOL, which is the near equiatomic phase of nickel-titanium alloys, has unique shape-memory and super-elastic properties along with excellent biomechanical compatibility and corrosion resistance.[1] These properties enabled its use especially in the biomedical field (e.g., orthopedic implants, orthodontic wires, bone plates, bone screws, minimally invasive surgical devices, and medical stents) besides many other applications.[2,3] The fundamental mechanism for the shape-memory and super-elastic properties is the thermal or stress-induced solid-state, diffusionless, reversible phase transformation between ‘‘austenite,’’ the high-temperature phase and ‘‘martensite,’’ the low temperature phase, of nitinol. It is well known that it is difficult to process nitinol and fabricate complex geometries from it. Its rapid work-hardening property and super elasticity make the machining of nitinol by conventional techniques difficult.[4–6] Moreover, its properties are highly sensitive to the initial chemistry and processing history used to fabricate the final components. A change of 1.0 at. pct of nickel content can change the Ms (onset martensite transformation temperature) by more than 150 K ( 123 C).[7] A slight inclusion of impurities or oxygen also leads to a change in its properties. Currently, the most widely used commercial methods for the production of nitinol components are vacuum arc melting and vacuum induction melting, followed by casting, hot working, or cold working with intermediate PRATIK R. HALANI, Graduate Research Assistant, and YUNG C. SHIN, Professor, are with the Center for Laser-based Manufacturing, School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907. Contact e-mail: [email protected] Manuscript submitted April 29, 2011. Article published online September 23, 2011 650—VOLUME 43A, FEBRUARY 2012

annealing and finally shape-memory treatment.[3] The melting process has to be carried out in a vacuum or inert atmosphere because of the high affinity of titanium to oxygen. Vacuum arc melting requires remelting several times to ensure sufficient homogeneity of the material, and vacuum induction melting has the drawback of crucible contamination.[4] It is likely that casting would lead to segregation defects.[8] To overcome the problems related to the melting process and subsequent conventional machining, alternate production routes have been developed for nitinol. Powder metallurgical methods like hot-isostatic pressing (HIP)[9–12] metal injection molding,[10,13] self-propagating high-temperature synthesis (SHS),[14,15] and normal sintering[8,16] have been investigated. Metal injection molding is a combination of polymer injection molding and powder metallurgy.[13] Metal powder is mixed with a binder to obtain a feedstock for the injection molding devices to form green bodies, which are sintered to obtain final components.[13] The binder used, however, acts as a direct source of impurity, which is detrimental to the final properties of nitinol. Hot-isostatic pressing has been used to fabrica