Effect of Particle Size of Titanium and Nickel on the Synthesis of NiTi by TE-SHS
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MATELY, equimolar NiTi alloy called nitinol is the most widely used shape memory alloy. Shape memory effect in this alloy is connected with the transformation between high-temperature cubic austenite (B2 structure) and low-temperature monoclinic martensite (B19‘structure).[1,2] Due to its exceptional properties, the NiTi alloy is applied in both medical (dental implants, stents, scaffolds)[3–5] and technical applications (actuators, robotics, etc.).[6] As an alternative to conventional production methods of NiTi alloy[7] (vacuum induction melting and vacuum arc remelting), powder metallurgy processes have been developed.[8,9] There are several works over the decades, dealing with the production of nitinol from prealloyed powder by hot isostatic pressing (HIP),[10] metal injection molding (MIM),[11] or net-shape laser processing.[12] Alternative powder metallurgy process for the production of ceramics or intermetallics is reaction synthesis. In this process, the compressed mixture of elemental PAVEL NOVA´K, Associate Professor, PETR DVORˇA´K, Assistant Professor, TOMA´Sˇ VESELY´ and VLADIMI´R VOJTEˇCH, Students, IVO MAREK, Research Worker, and PAVEL SALVETR, Ph.D. Student, are with the Department of Metals and Corrosion Engineering, University of Chemistry and Technology, Prague, Technicka´ 5, 166 28 Prague 6, Czech Republic. Contact e-mail: [email protected] MIROSLAV KARLI´K, Professor, and PETR HAUSˇILD, Associate Professor, are with the Department of Materials, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Trojanova 13, 120 00 Prague 2, Czech Republic. JAROMI´R KOPECˇEK, Research Worker, is with the Institute of Physics of the ASCR, v. v. i., Na Slovance 2, 182 21 Prague 8, Czech Republic. Manuscript submitted August 5, 2015. Article published online January 27, 2016. 932—VOLUME 47B, APRIL 2016
powders is transformed to intermetallics thermally activated exothermic reactions. During the reactions, the heat is generated, which sustains and propagates the reaction through the body of the reactants. Therefore, the process is called Self-propagating High-temperature Synthesis (SHS).[13] This reaction can be initiated by heating of the whole reaction mixture (thermal explosion mode—TE) or by ignition of one side of the compressed powder by a localized source of heat—e.g., filament, spark discharge, or laser (plane wave propagation mode—PWP).[13] In the case of NiTi alloy, the specific method for reaction synthesis had been patented already in 1988.[14] The process consisted of two steps—heating at the temperature between 580 K and 923 K (307 °C and 650 °C) in high vacuum and subsequent heating to 1103 K to 1173 K (830 °C to 900 °C) for 10 to 240 minute. In this patent application, the utilization of fine-grained powders was claimed (Ni < 2.2 lm and Ti < 55 lm). This process was proposed to minimize the oxygen content in the material and to achieve homogeneous composition of the product.[14] Whitney et al.[15] investigated the mechanism of reactive sintering of NiTi alloys and the
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