Shape Memory Behavior of Porous NiTi Alloy

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DOI: 10.1007/s11661-015-3318-1  The Minerals, Metals & Materials Society and ASM International 2016

NiTi shape memory alloys (SMAs) have been successfully used in engineering, especially in medical applications due to their excellent properties, such as shape memory effect (SME), pseudoelesticity or superelasticity (SE), corrosion resistance, and biocompatibility which make these alloys suitable for surgery and brackets, implantation, and hard tissue replacement.[1–4] Shape memory and pseudoelesticity properties of these

MEHMET KAYA, Associate Professor, and O¨MER C¸AKMAK, Master Student, Research Assistant, are with the Metallurgy and Material Engineering Department, Engineering Faculty, Adıyaman University, Adıyaman 02040, Turkey. Contact e-mails: mkaya@ adiyaman.edu.tr, [email protected] Manuscript submitted April 23, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A

alloys occur due to phase transformations in their microstructures. Phase-transformation behaviors in nickel titanium alloys have been studied extensively with the most emphasis being placed on the near-equiatomic Ni–Ti compositions concerning aforementioned material responses: ‘‘shape memory’’ and ‘‘pseudoelasticity.’’ The ‘‘shape memory’’ refers to the transformation of the material from martensite to austenite upon heating to a temperature exceeding the austenite start temperature. ‘‘Pseudoelasticity’’ is the forward transformation upon loading and the reverse transformation upon unloading at temperatures above the austenite finish temperature. Recently, porous NiTi SMAs have been fabricated as a promising bio-material for hard tissue replacement, in particular for hip implantation and femur repair. These porous alloys have the adjustable mechanical properties, reduced weight, and increased biocompatibility due to their porous structure allowing in-growth of the human tissue, medicament transportation, and nutrition exchange within human bodies.[3–5] Also, some properties such as superelasticity, lightweight, and adjustable mechanical properties of the porous NiTi SMAs can decrease the stiffness mismatches between human bones and implant, and thus the wearing of bones is prevented.[6–8] Porous NiTi SMA used in this study was fabricated using self-propagating high-temperature synthesis (SHS). Several other powder metallurgy methods such as element powder sintering,[9] spark plasma sintering (SPS),[10] hot isostatic pressing (HIP),[11] capsule-free hot isostatic pressing (CF-HIP),[11] mechanical alloying (MA),[12] and metal injection molding (MIM)[13] have also been used for the fabrication of porous NiTi SMAs, and the details of these processes are readily available in the literature. These methods can be used to avoid the problems associated with casting, such as extensive grain growth or segregation, and have some advantages such as easy realization of complex part shapes and precise control of composition.[14] The SHS method offers more advantages compared to the aforementioned powder metallurgy methods during fabrication of porous NiTi SMA

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