Technical University of Cluj-Napoca, Muncii Ave., Cluj-Napoca, Romania

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1 Materials Science Forum Online: ISSN: , Vol. 672, pp doi: / Trans Tech Publications, Switzerland Elaboration of Titanium-Nickel Alloy with Special Properties through Mechanical Milling Ionuţ Gligor a, Viorel Cândea b, George Arghir c and Cătălin Popa d Technical University of Cluj-Napoca, Muncii Ave., Cluj-Napoca, Romania a ionut.gligor@stm.utcluj.ro, b viorel.candea@stm.utcluj.ro, c george.arghir@stm.utcluj.ro, d catalin.popa@stm.utcluj.ro, Keywords: Nitinol, mechanical milling, biocompatibility. Abstract. Shape memory Titanium-Nickel alloys, also known as Nitinol, are amongst the most utilized materials with special properties in the medical field. Together with the properties of shape memory and superelasticity, these alloys have a very good biocompatibility. In this study, the equiatomic Ti-Ni alloy was obtained in the form of alloyed powder, starting from elemental high purity powders, through mechanical alloying. Specimens for testing the mechanical characteristics of the material, as well as smaller samples for biocompatibility tests were manufactured. The latter ones were prepared for implantation on live tissue, on Wistar rats and Guinea pigs. The structure of samples was studied by microscopy and X-Ray diffraction analysis. All the results have demonstrated the presence of the TiNi intermetallic compound as the quantitative dominant phase. After applying an adequate thermo-mechanical treatment, the tested samples displayed measurable shape memory effect and superelasticity. The in vivo biocompatibility tests, done according to international standards, demonstrated the material s bio-inertness in relation with living tissue. The obtained results have shown the possibility to elaborate Ti-Ni biocompatible alloys by mechanical milling and sintering. Introduction Titanium-Nickel alloys, also known as Nitinol, are amongst the most utilized materials with special properties in the medical field. Together with the properties of shape memory and superelasticity, these alloys need to have a very good biocompatibility, properties that need to be demonstrated. There are several ways to employ the shape memory alloys, SMAs, for biomedical applications inside or outside the human body i.e. for dental applications, orthopaedics, clamping, less invasive surgery tools, endoscopes, stents, lumbar discs, different kind of pumping actuators such as heart valves, drug delivery, etc [2]. Due to this wide application range their working requirements vary a lot. Stress input or output, straining, working frequency and connected fatigue as well as possible temperature variations are details affecting the behaviour of the SMA element. There are still some problems associated with the TiNi SMAs such as corrosion and fatigue resistance, as well as their high elastic modulus and mechanical properties compared with human bone tissues. Corrosion and nickel ion release out of TiNi in the body s environment result in the decreased biocompatibility of the alloy. On the other hand, because of the bioinert surface nature TiNi implants are generally encapsulated after implantation into the living body by fibrous tissue, which isolates the alloy from the surrounding bone [3]. Through a number of studies it has been shown that a rough or porous surface at the bone/implant interface provides a substantially stronger adhesion between bone tissue and the implant [4]. In this study, the equiatomic TiNi alloy was obtained in the form of alloyed powder, starting from elemental high purity powders, in a 50/50 atomic ratio, through mechanical milling. Materials and methods For the synthesis of TiNi alloyed powder the following powders were used: titanium powder with particle size between 40 and 100 μm, and nickel powder with particle size between 40 and 100 μm, for which the chemical composition is presented in table 1. The mixture atomic ratio was 50/50, All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-12/05/16,23:52:12)

2 Weight% 122 Researches in Powder Metallurgy which is equivalent to 55.1 wt% Ni, to ensure the formation of the TiNi compound. The milling process was conducted using a high energy planetary ball mill Fritsch Pulverisette 4 in two 500 ml stainless steel milling containers. To avoid powder oxidation argon gas was used as protective atmosphere. The milling parameters were varied to study their influence on the intermetallic compounds formation, main disk speed rot/min, containers speed rot/min. The milling time was up to 75 hours, with a pause at every 3 hours for intermediary sampling. X-Ray diffraction analysis was carried out on powder samples taken every 3 hours. To eliminate the internal stresses caused by the milling process, the alloyed powder was annealed at different temperatures between 450 and 600 C under vacuum. Specimens for mechanical characteristics testing, as well as smaller samples for biocompatibility tests were manufactured by die pressing and sintering. The sintering experiments were conducted at 1000, 1100 and 1150 C, for periods of 1, 2, 3 and 4 hours respectively. The small samples were specially prepared for implantation on Wistar lab rats and Guinea pigs. The samples were implanted in the connective subcutaneous tissue and muscular tissue of the thigh. The structure of all the samples was studied by electronic microscopy and X-Ray diffraction analysis. Results and discussion To ensure the reversible austenite to martensite transformation, which is responsible for the special characteristics of the alloy, the TiNi intermetallic compound has to be present in a large quantity, usually more than 30%. The variation of the milling parameters had the goal of finding the solution to increase the quantity of the TiNi compound. From the X-Ray diffraction diagrams analysis it was established that the powder mechanically alloyed for 30 hours had the highest content of TiNi compound. Inevitably other TiNi compounds form during the milling process, such as Ti 2 Ni, and TiNi 3, and their quantity is intended to be reduced at minimum. Also the quantity of free Ni and Ti phases decreases with the increase of the milling time, but are not completely eliminated, table 1. Quantitative results Al Si Ti Cr Fe Ni Fig.1. a) X-Ray diffraction pattern of the TiNi powder after 30 hours of milling, b) chemical composition of the alloyed powder. 0 Table 1. Phase distribution in the TiNi powder samples at different stages. Phase Ni Ti TiNi 3 TiNi Ti 2 Ni Milling Time, hours

3 Materials Science Forum Vol The first compound detected after 9 hours of milling is TiNi 3 in a fairly large quantity, 15%, which is undesirable. The desired TiNi compound formed after 18 hours of milling together with the third compound Ti 2 Ni, figure 1a. The TiNi compound reaches a maximum of 35% after 30 hours of milling, the diffraction analysis for the powder samples taken up to 75 hours showed no significant differences in phase distribution. The results of the diffraction analysis are presented in table 1. The chemical analysis of the alloyed powder shows small quantities of Fe and Cr due to contamination from the stainless steel milling containers and grinding balls. The powder used for further experimentation was thus the one milled for 30 hours. Before any further processing, the powder was annealed at different temperatures between 450 and 600 C to eliminate residual stresses caused by the milling process. The shape of the powder particles was studied by SEM microscopy, figure 2. It can be observed that after 3 hours of milling time, the particles are very large due to cold welding of the initial powder particles, while after 30 hours of milling the size of the particles is much smaller due to continuous fragmentation processes. a Fig.2. SEM images of the TiNi powder milled for a) 3 hours, b) 30 hours. b Samples and specimens were further fabricated by die pressing and vacuum sintering using different regimes, 400, 600 and 800 MPa pressing, and sintering at 1000 to 1200 C for 1 to 4 hours. The samples vacuum sintered at 1100 C for 2 hours indicated, on the X-ray diffraction analysis, figure 3, the highest content of TiNi inter-metallic compound, 64%. The sintering process also removed all traces of free Ni and Ti from the alloy. Fig.3. X-Ray diffraction of the Ni-Ti sample sintered at 1100 C for 2 hours. Mechanical tests were conducted on sintered specimens hot rolled at C for several times with subsequent quenching after each pass. The 1 mm thick rolled strips showed promising results, between 187 and 298 MPa tensile strength and a Young s modulus between 13.6 and 17 GPa. The mechanical properties are quite low compared to that of forged TiNi alloy of 850 MPa tensile strength and 70 GPa elastic modulus, but the obtained values are sufficient enough for medical applications.

4 124 Researches in Powder Metallurgy Given the destination of the future products, the biocompatibility of the alloy was tested. Small samples were cut from the rolled strips and prepared for implantation on live subjects. The samples were implanted in the subcutaneous conjunctive tissue and muscular tissue, according to international standards, ISO /1997. Clinical examinations demonstrated a very good postsurgical behaviour, while histological analysis proved a good toleration of the implants with development of a fibrocellular capsular structure specific to the bioinertial state of the implant, figure 4. Fig.4. Aspect of the implanted area and histological analysis of the surrounding tissue. Conclusion The TiNi alloy was obtained in the form of alloyed powder through mechanical milling. All the results have demonstrated the presence of the TiNi intermetallic compound as the quantitative dominant phase. After applying an adequate heat treatment, the powder was pressed and sintered to obtain mechanical test specimens. The mechanical characteristics proved to be significantly lower compared to those of TiNi alloys obtained through melting-casting method. Further experiments will be conducted to improve the mechanical properties of the alloy. In vivo biocompatibility tests indicated that the sample implants were very well tolerated by the subjects, a proof of that being the development of a fibrocelullar capsular structure specific to the bioinertial state of the implant. This proves that the elaborated alloy using the described technology is very well suited for medical applications. The obtained results have shown the possibility to elaborate TiNi biocompatible alloys by mechanical milling and sintering. References [1] V. Candea, I. Gligor, C. Popa, A. Popa, About the elaboration and characterization of a NiTi alloy with biomedical destination, International Congress Advanced Processing for Novel Functional Materials, Dresden, Book of Abstract, B2, 23, pp 126, [2] S.P. Hannula, O. Söderberg, T. Jämsä and V.K. Lindroos, Bioactive NiTi Implants Used for Bone Repairing Applications, Key Engineering Materials Vols (2005) pp [3] Z.D. Cuia, X.J. Yangb and S.L. Zhuc, Shape Memory Alloys for Biomedical Applications, Advances in Science and Technology Vol. 49 (2006) pp [4] I. Shishkovsky, Yu. Morozov, I. Smurov, Nanofractal surface structure under laser sintering of titanium and nitinol for bone tissue engineering, Applied Surface Science 254 (2007) pp

5 Researches in Powder Metallurgy / Elaboration of Titanium-Nickel Alloy with Special Properties through Mechanical Milling / DOI References [4] I. Shishkovsky, Yu. Morozov, I. Smurov, Nanofractal surface structure under laser sintering of titanium and nitinol for bone tissue engineering, Applied Surface Science 254 (2007) pp /j.apsusc