A comparative study of the porous TiNi shape-memory alloys fabricated by three different processes

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I. INTRODUCTION

TINI-BASED shape memory alloys (SMAs) have been widely used in a number of different fields, such as mechanical and electrical applications, as well as aeronautics and astronautics fields.[1] In addition, it has also been found that TiNi SMAs exhibit good corrosion resistance and high wear resistance, making them a superior tribo-material.[2] In recent years TiNi SMAs have been extensively applied as metallic biomaterials in making biomedical apparatus and implant devices (such as stents, vena cava filters, artificial hip joints, etc.), due to their good biocompatibility and biological properties as well as their unique shape-memory effects and superelasticity.[3,4] As a biomaterial used for hardtissue replacement, however, TiNi SMAs may also induce some problems; for example, the bone resorption problem caused by stress shielding due to their higher stiffness than that of a human bones and the difficulty of human tissue ingrowth due to the rigid and dense surface of the SMAs. Therefore, tremendous effort has been made to solve the aforementioned problems and meet the requirements of better biocompatibility since the early 1990s. For porous TiNi SMAs, their stiffness and Young’s modulus can be easily adjusted by controlling the porosity, and the porous structure would allow the in-growth of human tissue, nutrition exchange, and medicament transportation. In addition, porous alloys are lightweight. All these factors have made porous TiNi SMAs one of the promising biomaterials for orthopedic implants and hard-tissue replacements.[3–6] Various aspects of dense TiNi SMAs, such as crystal structure,[7] shape-memory effect,[8] mechanical properties,[9] and biocompatibility,[10,11] have been extensively studied for many B. YUAN, X.P. ZHANG and M. ZHU, Professors, and M.Q. ZENG are with the College of Mechanical Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China. Contact e-mail: [email protected] C.Y. CHUNG is with the Department of Physics & Materials Science, City University of Hong Kong, Kowloon, Hong Kong. Manuscript submitted February 23, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

years. In contrast, however, the studies on porous TiNi SMAs are limited, preliminary, and scattered because of their special structure and preparation process.[12,13,14] Porous TiNi SMAs fabricated by different researchers using different processes showed diverse properties and behaviors. For example, porous TiNi SMAs fabricated by self-propagating high-temperature synthesis (SHS) show a large pore size and an anisotropic pore distribution, as well as unsatisfactory mechanical properties.[14,15,16] The porous TiNi SMAs fabricated by the conventional sintering (CS) process exhibit a small, irregular pore size and good pore connectivity.[13,17] While using a hot isostatic pressing (HIP) process, the prepared porous TiNi SMAs exhibit large-sized pores with an irregular pore shape[18] and good superelasticity.[19] However, all of these results cannot come to an entire viewpoi

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