An Electro-Assisted Powder Metallurgical Route for the Preparation of Porous Ti and NiTi in Molten CaCl 2
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I.
INTRODUCTION
TITANIUM (Ti) and Ti-alloys are good candidates used for aircraft and biomaterials owing to their light weight, good corrosion resistance, unique mechanical properties, and excellent biocompatibility.[1] In addition to being biostable and biosafe in terms of degradation and cytotoxicity, implants should have compatible structures to match the body environment surrounding the replaced failing tissues. To attain structural compatibility, the surface of the implants should be engineered to be in accord with the structure of the replaced living tissues that have a porous surface to allow the ingrowth of the tissue.[2,3] Thus, Ti and Ti-alloys should have a porous structure in order to be hosted in the human body.[4] Among various Ti-alloys, NiTi, which is named Nitinol, is a well-known shape memory alloy due to its extraordinary fatigue resistance and good biocompatibility.[5] To improve the mismatch of Young’s modulus between Ti or NiTi and natural bone, the use of porous Ti or NiTi is one of the major strategies in overcoming this mechanical issue.[6,7]
XUE MA, HONGWEI XIE, JIAKANG QU, QIUSHI SONG, ZHIQIANG NING, HAIJIA ZHAO, and HUAYI YIN are with the School of Metallurgy, Northeastern University, Shenyang 110819, P.R. China. Contact email: [email protected] Manuscript submitted August 29, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
There are two major routes to prepare NiTi,[8–10] i.e., casting and powder metallurgy. The casting route uses arc, induction, or electron beam to melt and remelt Ti and nickel ingots under vacuum or in an argon (Ar) atmosphere. The powder metallurgy uses either metals or alloy powders as raw materials to fabricate near-netshape components. The highly alloyed composition can be obtained without fully melting the species and avoiding segregation, and the machinability can be enhanced by the uniformity and fine microstructures of the starting metal powders. To date, various approaches have been applied to prepare porous NiTi,[11] i.e., plasma spraying, metal powder/fiber sintering, vapor deposition, combustion synthesis, gas injection into the molten metals or alloys, additive manufacturing, etc. The use of Ti and Ni powders is one of the straightforward and cost-effective ways to prepare porous Ti and NiTi.[12,13] Owing to the high affinity of oxygen with Ti, the processing routes require high-vacuum conditions or a high-pure inert gas atmosphere. Moreover, the sintering temperature of NiTi is usually maintained at a high temperature (1200 C to 1400 C) to speed the diffusion rate between Ti and Ni.[5,6] However, it is extremely hard to remove the native oxide layer on the surface of Ti and Ni, which results in the production of brittle oxide[14] (e.g., Ti4Ni2Ox, 0 < x £ 1) that slows the diffusion between Ti and Ni and reduces the mechanical properties of the obtained NiTi alloys. Moreover, some eutectic alloys of low melting point existing in the alloy during the manufacturing process could lead to the
heterogeneity of the sintered NiTi. Therefore, removing the oxi
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