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Densification and microstructural behavior on the sintering of blended elemental Ti-35Nb-7Zr-5Ta alloy Taddei, E.B.1; Henriques,V.A.R2.; Silva, C. R.M.2; Cairo, C. A A.2 1. Instituto Tecnológico de Aeronáutica (ITA), Centro Técnico Aeroespacial, São José dos Campos -SP, CEP 12228-904, São José dos Campos, Brazil 2. AMR - Divisão de Materiais - Instituto de Aeronáutica e Espaço (IAE), São José dos Campos -SP, CEP 12228-904, São José dos Campos, Brazil
[email protected] Keywords : Powder metallurgy, densification, biocompatibility Abstract. Beta titanium alloys, e.g., are now the main target for medical materials. Ti35Nb-7Zr-5Ta alloy were manufactured by blended elemental (BE) powder method, which appears to be one of the most promising technique for titanium parts production at reduced cost. The process employs hydrided powders as raw materials with low production costs and oxygen content. Among the titanium alloys recently developed, Ti-35Nb-7Zr-5Ta is distinguished for presenting low modulus of elasticity, high mechanical resistance and superior biocompatibility. Samples were produced by mixing of initial metallic powders followed by uniaxial and cold isostatic pressing with subsequent densification by sintering among 800 at 1500 °C, in vacuum. Sintering behavior was studied by means of dilatometry. Sintered samples were characterized for phase composition, microstructure and microhardness by X-ray diffraction, scanning electron microscopy and Vickers indentation, respectively. Density was measured by Archimedes method. In this work, an alternative blending technique (with planetary mill) was used. The samples presented a good densification and a totally β-type microstructure, with complete dissolution of alloying elements in the titanium matrix with the temperature increase with low pore content. Introduction Ideally, a bone implant such as a hip implant should be such that it exhibits an identical response to loading as real bone and is also biocompatible with existing tissue. The compatibility issue involves surface compatibility, mechanical compatibility and also osteocompatibility [1]. The combination of good mechanical properties and biochemical compatibility makes titanium alloys a desirable class of implant materials for orthopedic applications. However, the elastic modulus of bio-titanium alloys currently in use is still not ideal compared with that of human bone, which may lead to premature failure of the implant [2]. The tissue reaction studies have identified Ti, Nb and Zr as non-toxic elements as they do not cause any adverse reaction in human body, and in addition Nb is found to reduce the modulus of elasticity when alloyed with titanium in certain preferred quantities.
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Thus, the attractive properties of Nb and Zr as mentioned above are the driving forces to introduce the new beta titanium alloys in the field of biomaterials. Beta-rich Ti–35Nb–7Zr5Ta alloy is one such implant material developed whose modulus is 55 GPa. As the modulus of this alloy is much closer to that of bone (30 GPa) it is considered to be a better alternate when compared to the conventional alloys like AISI Type 316L Stainless Steel with a modulus of 220 GPa and Co–Cr–Ni alloy with a modulus of 240 GPa [1,3]. Porous materials allow variable degrees of soft tissue ingrowth, decreased capsular contracture, and long-term immobility. Porosity and pore size both at the macroscopic and the microscopic level, are important morphological properties of a biomaterial scaffold for bone regeneration due to macrophages via oxidation and/or hydrolysis [4, 5]. The technique, which has been used to manufacture the porous implants is the powder metallurgy, that is techniques economical, and compared to the conventional processes they offer reduced material loss, as well as production costs [6]. The powder metallurgy can be divided in three steps to produce the solid part: powder production, compaction and sintering. Most compacts are prepared by applying pressure to the powder to increase density and provide the adequate shape. Uniaxial or isostatic pressing are commonly used. The pressure can be applied at room or high temperatures. Heat treatments are sometimes necessary to optimize mechanical properties of the final compact [7, 8]. Sintering is a thermal treatment for bonding particles into a coherent and predominantly solid structure via mass transport events that often occur on the atomic scale. The initial stage involves rearrangement of particles and initial neck formation at the contact point between each particle. In the second stage of sintering, referred to as intermediate, the size of the necks between particles grows. Porosity decreases and the center of the original particles move closer together. The third stage of sintering is called to as final sintering. It involves the final removal of porosity. The bonding leads to improved strength and lower system energy [7]. The two major techniques used for the titanium alloys production by powder metallurgy are: the "prealloyed" (PA), and the " blended elemental " (BE) approaches [9] The present work was aim to investigate the properties of Ti-35Nb-7Zr-5Ta produced from blended elemental powder. Materials and Methods The blended elemental powder metallurgy method was used to fabricate that alloy. All the powders were obtained by hydriding method and sintered in hydrided state. For the titanium hydrided powder production, the hydriding was carried out at 500 °C, in a high vacuum furnace. After reaching the nominal temperature, the material was hold for 3 hours, under a positive hydrogen pressure. After cooling to room temperature, the friable hydride was milled in a titanium container in vacuum (10-2 Torr). Nb, Zr and Ta hydrided powders were obtained using the same route; however, hydriding temperatures were significantly higher (800 °C). Table 1 presents the chemical composition of these powders. In this work was used a different blending method of the former researches with the same alloy (blended in double-cone mixer) [10-13]. In the new method, the starting powders were weighed (30 grams) and dried for one hour in stove and blended for 30 minutes in a planetary mill with six drips of alcohol. After blending, the powders were cold uniaxially pressed (60 MPa), in cylindrical 15 mm dia.-dies. Afterwards, samples were
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encapsulated under vacuum in flexible rubber molds and cold isostatically pressed (CIP), at 350 MPa, during 30 s, in an isostatic press. Sintering was carried out in niobium crucible in high vacuum condition (10-7 torr), between 800 and 1600 °C, with heating rates of 20 °C/min. After reaching the nominal temperature, samples were hold at the chosen temperature for 2 h and then furnace-cooled to room temperature. Metallographic preparation was carried out using conventional techniques. Specimens were etched with a Kroll solution: 1,5mL HF: 2,5mL HNO3: 100 mL H2O to reveal its microstructure. Microhardness measurements were carried out in a Micromet 2004 equipment (Buehler) with a load of 0.2 kgf. The micrographs were obtained using a SEM LEO model 435VPi. The density of the sintered samples was determined by Archimedes method. The micrographs were obtained using a SEM LEO model 435VPi. The specific mass of the sintered samples was determined by Archimedes method, ASTMC744-74. Particle size distribution was determined by means of laser-scattering equipment (Cilas model 1064). Table 1.Chemical composition of the elemental powders used in this investigation. Elemental powder Ti Nb Zr Ta
N 0,872 0,038 0,080 0,150
Impurity content (%) O C Si Fe 0,349 0,073 0,025 0,620 0,020 0,450 0,028 0,550 0,033 0,030
0,040 0,040 0,030 0,030
Results A great variation of the mean particle sizes between zirconium powders (~3µm) and niobium, titanium and tantalum powders (~14, ~31 and ~25 µm) was observed. This fact has influence on the sintering mechanisms involving the dissolution of the particles, phases stabilization and it is responsible for the final porosity in the samples. The microstructural evolution indicates that the homogenization of the alloy is diffusion-controlled (Figure 1). At 800 °C, the microstructure consists of large angular Ti and Nb smaller Ta and Zr particles, at this temperature begins the formation of the β phase regions, starting, mainly from the niobium particles, that act as β-phase nucleator agent. It can be observed that the β structure grows from the niobium diffusion in the neighboring areas consisted of α-titanium particles. The beginning of the stabilization of homogeneous β-areas can be observed at 1100 ºC. These areas present a large amount of Nb. The microstructure shows that the β-phase formation (plates) has an important role on the samples densification, promoting the bond among the particles and consequently, shrinkage. At 1300 ºC, large areas of α phase (titanium originally from particles) are still present indicating that the homogenization of the alloy is still incomplete. These areas present Widmanstatten (α + β) involved with porosities, indicting that the sintering mechanics is slower. With the continuous dissolution of Nb particles these titanium areas are found almost dissolved in the core of a stabilized area and some acicular β structures are observed.
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The specimen sintered at 1500°C presented the best results when compared to the microstructure found in commercial samples, with a β-homogeneous microstructure and with low porosity. Due the complete dissolution of the alloys elements in the titanium matrix, a good combination of microstructure, mechanical properties and densification could be reached. The specimens processed at lower temperatures (below 1500 °C) did not develop a β-like microstructure distributed throughout the samples. These results indicate that there was not enough time for mutual diffusion (homogenization) and further formation of a β microstructure throughout the specimens. The EDS analysis confirmed that the Nb is present in larger amount in β-areas.
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Figure 1. Microstructural evolution of Ti-35Nb-7Zr-5Ta alloy during sintering (for 2 hours and heating rate equal 20 ºC⋅min-1 for all temperatures). The values of hardness varied in function of the sintering temperature, staying in the range from 100 to 345 HV, while the hardness commonly reported for hot wrought alloys is about 350 HV [14]. The samples presented high densification, varying between 91 and 93 %, after sintering, with homogeneous microstructure. The theoretical density of Ti-35Nb7Zr-5Ta alloy is 5, 72 g/cm3 [14]. The results of microhardness and specific mass of Ti35Nb-7Zr-5Ta in function of the sintering temperature were showed in Table 2. In general, the relative specific mass and consequently the microhardness of sintered materials increase
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when the sintering temperature is raised, because sintering at higher temperatures promotes additional particle-to-particle bonding and more complete alloying due to the higher diffusion rates. Table 2. Values of microhardness and specific mass in function of the sintering temperature of Ti35Nb-7Zr-5Ta. Temperature (ºC) 800 900 1000 1100 1200 1300 1400 1500
Microhardness (HV) 100 145 205 206 338 342 344 345
Specific mass 4,65 5,02 5,11 5,33 5,40 5,47 5,58 5,68
Figure 2 a) shows the expansion/contraction behavior of Ti-35Nb-7Zr-5Ta from 200 to 1500 ºC. The curve is smooth up to 440 ºC. The contraction begins from 440 ºC. Densification continued up to 1200°C and overall contraction exceeding 11% was achieved. The contraction starts in a low temperature when compared with others titanium alloys sintered from dehydrided powders [15]. This fact indicates the influence of hydrogen atoms in the sintering mechanisms providing a contraction even in low temperatures. X-ray diffraction patterns of the samples sintered between 800 and 1500 ºC are presented in Fig. 2 b). The analysis revealed α and β phase peaks only, not being identified peaks related to the hydride, oxide or intermetallics. α peaks were not detected in samples sintered at high temperatures ( >1300 ºC) due to the β-phase homogenization, since the β phase and β stabilizators elements peaks are the same, it does not exclude the possibility of very fine particles in the nm-range coexist in the microstructure.
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Figure 2. a) Expansion contraction behavior of Ti-35Nb-7Zr-5Ta sintered at 1500 ºC and b) X-rays spectra of Ti-35Nb-7Zr-5Ta sintered among 800 at 1500 °C.
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Conclusions The blended elemental P/M process demonstrated to be efficient for the alloy production. The new blending process provide an optimized homogenization enabling a faster diffusion of the elements due to the reduced presence of agglomerates in relation to the former process. The hydrided powders provided activation in the sintering mechanism promoting contraction in low temperature (400 °C). The samples had presented a good densification with a β rich microstructure. The results show that a β−homogeneous microstructure is obtained in the whole sample extension with the increase of the sintering temperature and the dissolution of β-phase stabilizers particles (Nb and Ta). The hardness values observed in the samples are within the range used in parts produced by conventional techniques (350 HV). Acknowledgments The authors wish to thank FAPESP for the scholarship of Taddei, E. B. and DEMAR-FAENQUIL for the Nb and Ta supplying. References [1] K. S. Katti. Colloids and Surfaces B: Biointerfaces xxx–xxx (2004), p. xxx [2] S.J. Li, R. Yanga, S. Li, Y.L. Hao, Y.Y. Cui, M. Niinomi, Z.X. Guo. Wear 257 (2004) p. 869–876 [3] M. Geetha, U. Kamachi Mudali, A.K. Gogia, R. Asokamani, Baldev Raj. Corrosion Science xxx (2003) p. xxx–xxx [4] V. Karageorgiou, D. Kaplan. Biomaterials xxxx (2005) p. [5] J. K. Potter, E. Ellis III, J Oral Maxillofac Surg 62 (2004) p.1280-1297 [6] M. Can, A. B. Etemoglu. Filtration + Separation 41 (2004) p. 37-40 [7] Sintering Theory and Practice. Randall M. german. [8] F.H. Froes,D. Eylon., Powder Metallurgy International, 17, no. 4, (1985), p.47-54 [9] M.J. Donachie Titanium: a Technical Guide. ASM Metals Park, (1988) [10] Taddei, E. B, Henriques, V. A.R, Silva, C. R. M, Cairo, C. A. A. Sinterização A Vácuo Da Liga Ti-35Nb-7Zr-5Ta. In: Congresso Brasileiro de Aplicação de Vácuo na Indústria e na Ciência, 2003, Bauru. Anais...Bauru: Brasil, 2003,a. [11]Taddei, E. B, Henriques, V. A.R, Silva, C. R. M, Cairo, C. A.A. SEM Study of the Ti35Nb-7Zr-5Ta Sintering., In: Congresso da Sociedade Brasileira de Microscopia e Microanálise, 2003, Caxambu. Anais...Caxambú: Brasil, 2003,b. [12] Taddei, E. B, Henriques, V. A.R, Silva, C. R. M, Cairo, C. A.A. Production of new titanium alloy for orthopedic implants, In: II Encontro da Sociedade Brasileira de Pesquisa de Materiais, 2003, Rio de Janeiro. Anais...Rio de Janeiro: Brasil, 2003,c. [13] Taddei, E. B, Henriques, V. A.R, Silva, C. R. M, Cairo, C. A.A. Characterization of Ti-35Nb-7Zr-5Ta Alloy Produced By Powder Metallurgy. In: Fourth International Latin American Conference on Powder Technology, 2003, Guarujá. Anais…Guarujá: Brasil, 2003,d. [14] Allvac, An Allegheny Technologies Company, Catalogue [15] Fujita, T., Ogawa, A., Ouchi, C., Tajima, H. Materials Science and Engineering A 213, (1996) p. 148-153
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