Mechanical Properties of TiTaHfNbZr High-Entropy Alloy Coatings Deposited on NiTi Shape Memory Alloy Substrates
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ry alloys (SMAs) are widely used in medical devices, such as orthopedic implants and prostheses, cardiovascular stents, and dental instruments, owing to their superior properties of superelasticity and shape memory effect.[1–5] Despite their superior mechanical properties, the biocompatibility of NiTi alloys has been a topic of debate.[2,3] Specifically, the long-term biocompatibility, bioactivity, and wear properties of NiTi SMAs give rise to concerns about the safety of their long-term utility in clinical
A. MOTALLEBZADEH, and D. CANADINC are with the Advanced Materials Group (AMG), Department of Mechanical Engineering, Koc University, Istanbul 34450 Turkey and also with the Surface Science and Technology Center (KUYTAM), Koc University, Istanbul 34450 Turkey. Contact e-mail: dcanadinc@ ku.edu.tr M.B. YAGCI is with the Surface Science and Technology Center (KUYTAM), Koc University. E. BEDIR and C.B. AKSOY are with the Advanced Materials Group (AMG), Department of Mechanical Engineering, Koc University. Manuscript submitted February 01, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
applications[6,7]: out-diffusion of harmful Ni ions from the NiTi SMA during prolonged use inside human body poses several health risks.[2,8,9] In order to eliminate these risks, surface treatment of NiTi SMAs, namely the high-temperature oxidation, passivation, oxygen ion implantation, sol–gel, and PVD coating are commonly employed to introduce a layer of corrosion-resistant TiO2 on the surface, such that the allergic and toxic effects due to nickel release can be avoided.[3,10–12] However, the protection provided by the biocompatible yet brittle TiO2 layer is very short term in applications that are subjected to repeated and/or complex loads,[13,14] mainly due to the easy formation of cracks followed by Ni ion released from the freshly exposed NiTi SMA underneath the broken TiO2 layer.[15] In order to address this issue, attempts have been made to find suitable alternatives to conventional coating.[16] High-Entropy Alloys (HEAs), a new class of metallic materials with equimolar or near-equimolar compositions, constitute an example to this case, and they exhibit unprecedented excellent structural and functional properties that have recently attracted significant interest.[15,17] HEAs are usually composed of at least five principal elements, with concentrations between 5 and 35 at. pct,[18] and because of the corresponding high mixing entropy, they usually form simple face-centered cubic (fcc) or body-centered cubic (bcc) solid-solution phase, and sometimes nanoscale or even amorphous structures.[19] Their simple structures and versatile physical and mechanical properties stem from their high mixing entropy, lattice distortion, sluggish diffusion, and cocktail effect.[20] Especially the HEAs composed of biocompatible constituents are potential candidates for utility as a new class of coatings on metallic biomaterials due to their superior mechanical properties,[21] which are also expected to overcome the coating–substrate property mismatch
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