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Electrochimica Acta 53 (2008) 2809–2817

Effects of Zr content on microstructure and corrosion resistance ofTi–30Nb–Zr casting alloys for biomedical applications

Danielle Q. Martins, Wislei R. Osorio, Maria E.P. Souza, Rubens Caram, Amauri Garcia ∗Department of Materials Engineering, State University of Campinas, UNICAMP, P.O. Box 6122, 13083-970 Campinas, SP, Brazil

Received 3 September 2007; received in revised form 24 October 2007; accepted 24 October 2007Available online 4 November 2007

bstract

Titanium alloys show attractive properties for biomedical applications where the most important factors are biocompatibility, corrosion resistance,ow modulus of elasticity, very good strength-to-weight ratio, reasonable formability and osseointegration. The aim of this study was to investigatehe effects of Zr content (7.5 and 15 wt%) on microstructure and corrosion resistance of Ti–30Nb–Zr as-cast alloy samples. The corrosion testsere carried out in a 0.9% NaCl (0.15 mol L−1) solution at 25 ◦C and neutral pH range. A horizontal centrifuged solidification set-up was used

o obtain as-cast samples with a fine dendritic arrangement. In order to evaluate the corrosion behavior, electrochemical impedance spectroscopyEIS), polarization curves and an equivalent circuit analysis were used. It was found that the as-cast microstructure tends to have the dendrite arm

pacings reduced with increasing Zr content. The results have shown that the addition of Zr to a Ti–30Nb alloy has been efficient to stabilize theeta phase. The resulting impedance parameters and passive current densities have shown that the Ti–30Nb alloys sample without Zr additionresents a higher surface reactivity which can induce a better osseointegration than those containing Zr.

2007 Elsevier Ltd. All rights reserved.

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eywords: Ti–Nb alloys; Centrifugal casting; ElS; Bone ingrowth; Osseointegr

. Introduction

Titanium alloys are among the most studied metallic bioma-erials, particularly those with vanadium- and aluminum-freei alloys since these elements exhibit high cytotoxicity andegative tissue response in vivo and may induce senile demen-ia, neurological disorders and allergic reactions [1–4]. It isell known that recent biomaterials research has focused on �-

itanium alloys due to increased biocompatibility and decreasedoung’s modulus. The former provides biological acceptance of

he implant by the body and the latter promotes a load sharingetween the implant and natural bone [4,5]. Some studies havehown that Ti–Nb alloys ranging from 20 to 50 wt% Nb exhibitmodulus of elasticity of about 60 GPa [5–7], which is closer

o that of bone when compared to those of other conventional

lloys applied to orthopedic implants. The increase on Nb con-ent tends to decrease the modulus of elasticity and stabilizeshe �-titanium phase. It has also been reported that the yield

∗ Corresponding author. Tel.: +55 19 3521 3320; fax: +55 19 3289 3722.E-mail address: amaurig@fem.unicamp.br (A. Garcia).

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trength increases and the elongation decreases with increasingb content [8].The Ti–35Nb–7Zr–5Ta alloy has a lower modulus of elastic-

ty (55 GPa) than other metallic alloys typically used for implantomponents [5,8–11]. This alloy is a �-type titanium alloy withon-toxic and non-allergic elements, which can be considered toe one of the best choices for orthopedic implants [5,8]. Ti–Nblloys have presented superior corrosion resistance when com-ared to the traditional Ti–6Al–4V alloy [12]. Uniform surfaceorrosion of Ti–Nb alloys provides a passive film which can beormed by �-Ti2O3, TiO2 and Nb2O5 [13–15].

The high cost of titanium goods is not only due to the pricef raw titanium but, mainly due to fabrication processes. Anpproach to reduce cost may be found by applying Ti alloy cast-ng processes, especially when complex shapes are necessary.

It is well known that the resulting microstructure of metal-ic alloys has an important role on mechanical properties andorrosion behavior of as-cast components [16–19]. Generally,

ne-structure castings are desirable since smaller grain sizenhances mechanical strength and toughness, the microstructurend general properties are more isotropic and species segregations reduced. In this work, a refined microstructure was obtained

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y using a centrifuged casting process in a massive copper cru-ible. The centrifugal force favors the thermal contact betweenolten metal and mold wall and promotes a copious nucleation

f grains which will lead to a fine equiaxed structure. On thether hand, as far as the corrosion resistance of metallic alloyss concerned, it is also very important to analyze the dendriticcale inside the grains, the solute distribution in the interden-ritic region and the anodic/cathodic behavior of the segregatedolute with respect to the dendritic matrix [20]. The centrifugedasting process can provide two different structures: a ‘band-ng’ structure (in which the special structural discontinuity iseatured) or a typical structure that usually contains a chill zone,columnar zone and a central equiaxed zone [21].

The aim of this study was to investigate the effects of Zrontent (7.5 and 15 wt%) on the resulting microstructure and onhe electrochemical corrosion behavior of Ti–30 wt% Nb as-castlloy samples in a 0.9% NaCl (0.15 mol L−1) solution at 25 ◦Cnd a neutral pH range.

. Experimental procedure

Ti–30Nb and Ti–30Nb–Zr alloy samples were prepared fromommercially pure metals: Ti (99.86 wt%), Nb (99.99 wt%) andr (99.99%). Ti–30Nb–Zr alloys with two different Zr contents

7.5 wt% Zr and 15 wt% Zr) were melted in an arc-meltingurnace with a non-consumable tungsten electrode and water-ooled copper hearth under an ultra-pure argon atmosphere.nitially, vacuum of 10−3 atm was created and then ultra-purergon was injected. In order to homogenize the sample compo-ition, at least five remelting steps were carried out and in eachperation the sample was slipped into the hearth. In all cases,rgon atmosphere and slight vacuum conditions were employed.his procedure guarantees the minimization of undesirable oxi-ation. More details about the experimental procedure can beound in a previous article [22].

The centrifuged casting set-up consists of a permanent cop-er mold and a copper crucible positioned in a centrifugal arm,hich rotates around a central axis, as shown in Fig. 1. A 30 g

ample is arc-melted and the homogenized alloy sample is posi-ioned at the crucible and remelted in situ. After, it is immediatelynjected into a permanent copper mold by the centrifugal force

y using a rotation of about 1000 rpm.

The resulting ingots were sectioned, ground by using siliconarbide papers up to 1200 mesh, polished and etched to reveal theicrostructure (Kroll’s etchant 5 mL of HF, 30 mL of HNO3 and

Fig. 1. Schematic representation of the centrifuged casting set-up.

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a Acta 53 (2008) 2809–2817

5 mL of H2O). Microstructural characterization was performedy applying optical microscopy (Olympus BX60M) and SEMJMS T20 of Jeol Co., Japan). X-ray diffraction patterns werebtained utilizing a Rigaku diffractometer (DMAX 2200) oper-ted at 40 kV and 30 mA with Cu K� radiation and a wavelength,, of 0.15406 nm.

Electrochemical impedance spectroscopy (EIS) tests werearried out in samples extracted from the centrifugally solid-fied ingots by using a solution of 0.9% NaCl (0.15 mol L−1)t 25 ◦C under a neutral pH range (of about 6.80). Such solu-ion was used to simulate the human body fluid [13,14]. EISeasurements began after an initial delay of 30 min for the sam-

le to reach a steady-state condition. A potentiostat coupled tofrequency analyzer system, a glass corrosion cell kit with a

latinum counter-electrode and a saturated calomel referencelectrode (SCE) were used to perform the EIS test. The work-ng electrodes consisted of Ti–30Nb–Zr alloy samples whichere positioned at the glass corrosion cell kit, leaving a circu-

ar 0.33 cm2 metal surface in contact with the electrolyte. Theotential amplitude was set to 10 mV in open-circuit potentialnd the frequency range was set from 10 mHz to 100 kHz.

Potentiodynamic measurements were also carried out in a.9% NaCl solution at 25 ◦C using a potentiostat at the sameositions where the afore-mentioned EIS tests were carried out.hese tests were conducted by stepping the potential using acanning rate of 10 mV/s from −0.800 mV (SCE) to +3000 mVSCE). Using an automatic data acquisition system, the poten-iodynamic polarization curves were plotted and both corrosionate and potential were estimated by Tafel plots by using bothnodic and cathodic branches. Duplicate tests for EIS and poten-iodynamic polarization curves were carried out. In order toupply quantitative support for discussions of these experimen-al EIS results, an appropriate model (ZView® version 2.1b) forquivalent circuit quantification has also been used.

. Results and discussion

.1. Microstructures

Typical optical micrographs of the alloys experimentallyxamined are shown in Fig. 2. It can be seen that the cen-rifuged casting process has induced a fine dendritic patternue to the inherent high cooling rates during solidification.t can also be observed that the addition of Zr has a strongefinement effect on the secondary dendrite arm spacing whichecreases significantly as the Zr content is increased. As previ-usly mentioned, the centrifuged casting increases the numberf nucleation sites and hence fine equiaxed grains are obtained.he final microstructure is then constituted by fine equiaxedrains, in which the fineness of the dendritic pattern is associ-ted not only with the high cooling rate of the centrifuged castingrocess but also with the Zr content of the alloy. Generally,oth mechanical and fatigue properties increase with centrifuged

asting applications [23].

The final microstructure, immediately after solidification inhe centrifuged apparatus will be formed by a dendritic arrayhere the dendritic matrix is formed by a Nb-rich phase with

D.Q. Martins et al. / Electrochimica Acta 53 (2008) 2809–2817 2811

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In order to analyze the effects of Zr content on the general(uniform) corrosion resistance of the Ti–30Nb–Zr centrifugedcast samples, EIS diagrams and polarization tests were carried

Fig. 2. Typical microstructures of Ti–30 wt% Nb–Zr alloys

i-rich phases located in the interdendritic region (orthorhom-ic �′′ martensite and � athermal phases) since Nb distributionoefficient is bigger than unity. In Fig. 2(a), the light regions associated with the Nb-rich dendritic matrix and the darkegion is the interdendritic Ti-rich solution with a secondaryendritic arm spacing (λ2) of about 29 �m. The addition of Zrtrongly reduces λ2: 12 and 5 �m for the Ti–30Nb–7.5Zr andi–30Nb–15Zr alloy samples, respectively, as shown in Fig. 2(b)nd (c). Dendrite arm spacings are of high importance, sincehey affect not only the mechanical properties and corrosionesistance of castings but also the subsequent manufacturing ofomponents from ingots [17–19,23].

Fig. 3 shows typical X-ray diffraction patterns (XRD) ofi–30Nb, Ti–30Nb–7.5Zr and Ti–30Nb–15Zr alloy samples.esides the typical dendrite array of the Ti–30Nb alloy, a finecicular �′′ martensitic phase is clearly observed throughout thenterdendritic regions. Further, a slight precipitation of the �-hase has been detected in the XRD patterns, as shown in Fig. 3.he literature reports that the amount of �′′-phase increases with

ncreasing cooling rate due to the transformation of �-phase into′′ [16,24,25]. On the other hand, only �-phase peaks have beenetected for the Ti–30Nb–Zr alloys, as shown in Fig. 3. This rein-orces the effect of zirconium as a �-phase stabilizer element ini alloys [2,3,16]. It is well known that �-type Ti–Nb based

lloys have reduced Young’s modulus. For the alloys exam-ned in the present investigation, the Young’s modulus is about00 GPa for theTi–30Nb alloy, while for those Zr-containinglloys it lies between 60 and 70 GPa. These values were deter-

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ined by ultrasonic methods [16]. The higher Young’s modulusor the Ti–30Nb alloy can be attributed to the �-phase precipi-ation.

.2. EIS measurements and equivalent circuit analysis

ig. 3. X-ray diffraction (XRD) patterns for Ti–30Nb–15Zr, Ti–30Nb–7.5Zrnd Ti–30Nb alloy samples.

2812 D.Q. Martins et al. / Electrochimica Acta 53 (2008) 2809–2817

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ig. 4. Experimental and simulated EIS diagrams obtained by the ZView® sofaCl solution at 25 ◦C.

ut in a 0.9% NaCl solution. Fig. 5 shows experimental andimulated EIS diagram results for Ti–30Nb, Ti–30Nb–7.5 andi–30Nb–15Zr alloy samples.

It can be seen in Fig. 4, that the moduli of impedance (Z)or those Zr-containing Ti–Nb alloys are higher than that of thei–30Nb alloy. These qualitative results and other impedancearameters shown in Table 1 (capacitances and resistances) arendicative of a nobler electrochemical behavior of Ti–Nb alloysith Zr addition. The impedance parameters supply quantitative

upport for discussions of EIS results and were obtained by theView® software, by adopting the well known equivalent circuit

nitially proposed by Pan et al. and other researchers [26–30],s shown in Fig. 5.

The agreement between experimental and simulated resultsndicates that the experimental results are well fitted to the pro-

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for: (a) Ti–30Nb, (b) Ti–30Nb–7.5Zr and (c) Ti–30Nb–15Zr alloys in a 0.9%

osed equivalent circuit. The fitting quality was evaluated byhi-squared (χ2) [20,26] values of about 10−4, as shown inable 1.

The physical significance of the elements of the proposedquivalent circuit has been intensively reported in the literature20,24–30]. The model assumes that the oxide layer on the threei alloys experimentally examined consists of a barrier-like inner

ayer and a porous outer layer. In this model, Rel corresponds tohe resistance of the electrolyte (0.9% NaCl solution) which inode diagram is expressed in a high frequency limit (F > 1 Hz).1 and R2 are the resistances of porous and barrier layers [24–29]

hich are associated to the charge transfer resistance through theorous layer and the participation of adsorbed intermediates.CPE(1) corresponds to the capacitance of the porous layer andCPE(2) to the capacitance of the barrier layer which seem to

D.Q. Martins et al. / Electrochimica Acta 53 (2008) 2809–2817 2813

Table 1Impedance parameters for Ti–30Nb and Ti–30Nb–xZr centrifuged cast alloys

Parameters Ti–30Nb Ti–30Nb–7.5Zr Ti–30Nb–15Zr

Rel (� cm−2) 159.7 170.8 169.4ZCPE(1) (106 � s−n cm2) 15.94 (±1.9) 10.55 (±0.8) 7.44(±0.3)ZCPE(2) (106 � s−n cm2) 18.02 (±2.1) 0.98 (±0.3) 2.61 (±0.2)n1 0.95 0.89 0.89n2 0.81 0.96 0.66R1 (� cm−2) (porous) 0.4 × 103 7.6 × 103 27 103

R2 (� cm−2) (barrier) 1.26 × 106 2.15 × 106 2.35 × 106

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e associated to the double layer formation. A constant-phaselement representing a shift from an ideal capacitor was usednstead of the capacitance itself, for simplicity. The impedancef a phase element is defined as ZCPE = [C (jω)n]−1, where C ishe capacitance, ω is the frequency and −1 ≤ n ≤ 1. The valuef n seems to be associated with the non-uniform distribution ofurrent as a result of roughness and surface defects [26].

Comparing the capacitances ZCPE(1) and ZCPE(2) for Ti–30Nb,i–30Nb–7.5Zr and Ti–30Nb–15Zr alloys samples, it is possible

o observe that the Ti–30Nb alloy has the highest capaci-ances (ZCPE(1) and ZCPE(2) of about 16 106 � s−n cm2 or 16nd 18 �F cm−2, respectively) when compared with those of thei–Nb–Zr alloys. On the other hand, the polarization resistancesf the inner barrier layer (R2) for all three alloys are signifi-antly higher than R1 (porous layers). This indicates that therotection is predominantly provided by the inner barrier layer,s also observed by Assis et al. for Ti-alloys [26]. The ZCPE(2)alues must be analyzed with association with the R2 values. Iteems that the component ZCPE(2) is responsible for very highhase angles obtained at low frequencies. For the Ti–30Nb alloyample, ZCPE(1) and ZCPE(2) are similar (of about 20 �F cm−2);owever, the barrier layer resistance (R2) is superior by a fac-or of about 103 times. For the Ti–30Nb alloy sample with Zrdditions, the capacitance of the porous layer (ZCPE(1)) is higherhan the barrier one (ZCPE(2)) by a factor of about 2–20 times.

similar behavior has also been recently reported in the lit-rature [26]. Low capacitances can be associated with both anncrease of the passive layer thickness [28,29] and a decrease

f the oxide film dielectric constant (due to variation in ratio ofhe electrolytic solution volume/oxide film) [30]. Low capaci-ances associated to high polarization resistances are conduciveo a nobler electrochemical corrosion behavior [20,27,28]. Thus,

ig. 5. Proposed equivalent circuit for modeling impedance parameters fori–Nb and Ti–Nb–Zr alloy samples in a 0.9% NaCl solution.

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he impedance parameters shown in Table 1 permit to con-lude that the addition of Zr to a centrifuged cast Ti–30Nb alloyncreases the corrosion resistance in a 0.9% NaCl solution. In theresent investigation, zirconium has shown to provide a noblerharacteristic to the alloy. It has also favored the �-phase stabi-ization and has provided a refinement of the dendritic network.t seems that the synthesis of such factors has been responsi-le for the resulting better corrosion resistance of Ti–30Nb–Zrlloys.

.3. Polarization results

Fig. 6 shows experimental polarization curves in a 0.9% NaClolution, for the three studied alloys. The corrosion current den-ities (ICorr) were obtained from the polarization curves by Tafellots using both cathodic and anodic branches of the polar-zation curves. Such results reinforce the corrosion resistanceendency favoring the Ti–30Nb centrifuged cast alloys with Zrdditions. It can be seen that ICorr decreases with increasing Zrontent, with the highest ICorr being observed for the Ti–30Nblloy.

A partial stabilization on the current density is observedetween 2 and 3 × 10−8 A cm−2 for all the three studied alloyshich is associated to an oxide film formation. However, thisxide film becomes more stable of about 0.1 V for all the threelloys. Different passive current densities (IPP) are associatedo such corrosion potential for each alloy. In Fig. 6, the highestPP is exhibited by the Ti–30Nb alloy (of about 15 �A cm−2),he intermediate value is that of the Ti–30Nb–7.5Zr (of about�A cm−2), and the lowest that of the Ti–30Nb–15Zr alloy

of about 3.5 �A cm−2). The association of values of IPP,Corr and impedance parameters permits to conclude that thei–30Nb–15Zr alloy has a nobler electrochemical corrosionehavior when compared with those of the other examinedlloys.

At potentials of about 1300, 1700 and 2300 mV (SCE),ccurrence of passive film breakdown can be observed for thei–30Nb–15Zr, the Ti–30Nb–7.5Zr and the Ti–30Nb castinglloys, respectively. Although very protective Ti oxide films are

ormed on the surface of such centrifuged cast samples, the highalues of IPP indicate that defects in the oxide film can be formed,.e., there is a tendency of more irregular or porous oxide layerormation [26].

2814 D.Q. Martins et al. / Electrochimic

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ig. 6. (a) Experimental polarization curves for Ti–30Nb, Ti–30Nb–7.5Zr andi–30Nb–15Zr centrifuged cast samples in a 0.9% NaCl solution at 25 ◦C andb) magnification evidencing the breakdown potential occurrence.

The development of a stable bone-implant interface is crit-cal to the successful use of orthopedic and dental prostheses.everal approaches have been developed to stimulate favorableost-implant interactions, such as creating implant surfaces withacroscopic irregularities, porous surfaces, and osteoconduc-

ive coatings (e.g. hydroxyapatite or similar calcium phosphateoatings) [31]. Biological advantages of texture surfaces mighte gained by the capacity of these surfaces to create a microme-hanical anchorage with bone interlocking, which turns out toe a stronger implant fixation than for smooth surfaces. Bonenterlocking requires the conjunction of a minimal roughnessith peaks and/or valleys of adequate shape that permit bone

ngrowth and bone retention [31,32].Since biocompatibility, good corrosion and mechanical resis-

ances, very good strength and low modulus of elasticity are

ecognized as intrinsic characteristics of Ti-Nb alloys, the con-rol of surfaces with more porous aspect can be very usefulor planning both micromechanical anchorage between bone-mplant and ingrowth of the bone in order to achieve long-life

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a Acta 53 (2008) 2809–2817

mplantation. In the present investigation, the results of corro-ion tests have indicated that, although Zr content significantlyncreases the electrochemical performance of centrifuged casti–Nb based alloys [13], the Ti–30Nb alloy samples has exhib-

ted an oxide film which is more irregular if compared to that ofhe other Ti–Nb–Zr alloys. This can be an important factor forhe improvement of osseointegration.

By analyzing the resulting microstructures before and afterorrosion tests, it seems that the refinement of the dendriterm spacing, which has been associated with Zr additions, hastrongly influenced the oxide film formation for both Ti–30Nbnd Ti–30Nb–xZr alloy samples. Experimental results of thelectrochemical corrosion behavior permit to affirm that finerecondary dendritic arm spacings induce better corrosion behav-or than coarser ones. From the electrochemical point of view,he Ti-rich regions have nobler corrosion behavior when com-ared with Nb-rich regions. Thus, Ti–30Nb alloys immersedn some aggressive environment have the Ti- and Zr-richegions (interdendritic regions) acting as anodic protective bar-iers “enveloping” the Nb-rich dendritic matrix. On the otherand, there is a cathodic protection in Nb-rich regions. Thus,ner dendritic structures with a consequent more homogeneousistribution of the nobler region along grain boundaries andnterdendritic regions will exhibit a more homogeneous oxidelm with a consequent improvement on the corrosion resistance

han coarser dendritic structures. Fig. 7 shows typical SEMmages of the resulting surface oxide films after polarizationycles in a 0.9 M NaCl solution for Ti–30Nb and Ti–30Nb–15Zrlloys. It is clearly observed that the presence of Zr has pro-ided a more homogeneous oxide film which is associated tohe impedance parameters results afore-mentioned. On the otherand, a non-homogeneous oxide layer is developed at the surfacef the Ti–30Nb alloy sample.

The impedance parameters shown in Table 1, specifically,apacitances and polarization resistances of both outer porousnd inner barrier layers have important roles to the understandingf the kinetics of oxide film formation and its electrochemicalehavior. The literature proposes a schematic representation fornner barrier and outer porous layer lattices, as shown in Fig. 8(a)24–30]. A number of procedures can be used to prepare theurface texture of an implant component such as acid exposi-ion, subsequent thermal or electrochemical treatments and someiocompatible coatings application. The former depends on thexposure time, nature and composition of the used acid whichoes not create a common topography and in industrial processs not fully reproduced [33]. The latter depends on the previ-usly formed oxide film when a spray surface coating techniques used.

It has been recently reported [30] that the anodizing treat-ent in a HF solution can produce both inner and outer oxide

ayers with porous aspect, increasing the inner layer corrosionesistance. In order to propose a mechanism for dual-oxideorous layer formation, some modification on the schematic

epresentation of Fig. 8(a) is made, as shown in Fig. 8(b). Inespite of the proposed modification, the interpretation of therevious proposed equivalent circuit is the same. It is wellnown that an electrolytic solution can reside into the porous

D.Q. Martins et al. / Electrochimica Acta 53 (2008) 2809–2817 2815

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acancies of the oxide layer. This can modify the oxide filmielectric constant and its thickness, and as a direct consequence,an modify the resulting capacitances (ZCPE) and polarizationesistances (R).

The literature reports that small pores (Fig. 8b and d) decreasehe capacitance due to a decrease of the film dielectric constant orhen the oxide film thickness is increased [29,30]. It is suggested

hat the barrier layer thickness can be significantly increased dueo the associated reactions between metastable oxide particles,.g. Ti2O3 and Nb2O5 [14,15] (resident in the outer layer) andhe electrolytic solution. As a consequence, some oxide particles

f the outer layer are transformed in more stable oxide particlesnd due to electronic and nature affinities (e.g., particle size andeometry) they are incorporated into the inner layer providingfiner porous layer. Associated to such mechanism, the inner

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Fig. 8. Schematic representation of the dual-oxide layer: (a) inner barrier

zation cycle in the Ti–30Nb and Ti–30Nb–15Zr alloy samples.

ayer can have its capacitance increased while the outer layerapacitance is not necessarily modified.

The experimental and simulated EIS results and the result-ng oxide films shown in Fig. 7 give support to the proposed

echanism. The centrifuged casting process and the additionf a nobler solute to the Ti–30Nb alloy can be used as alter-ative ways to produce components with better characteristicsor medical implants. The former induces the formation of finequiaxed grains and influences the resulting dendritic array andhe latter affects the refinement of the dendritic microstructurend favors �-phase stabilization.

Although both in vivo and in vitro tests have not beenerformed to evaluate the practicability of biocompatibilityr osseointegration by using the studied Ti–30Nb alloy sam-les, the presented experimental corrosion tests carried out in

and outer porous layers and (b) variation of these dual-oxide layers.

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saline solution can be useful to demonstrate the potential ofi–30Nb–Zr alloy samples in biomedical applications. A recenttudy [34] has reported that electrochemical tests carried out inTi–Nb–Zr alloy after 410 days of immersion in a Hank’s solu-

ion has indicated a high corrosion resistance and both resistancend capacitance values have shown a propensity to stabiliza-ion after 3 days. Although the electrochemical tests carried outn the present study refers to a 30 min period of immersion,t is believed that the mechanism of oxide film formation androwth after longer immersion periods will also be significantlyependent on the dendritic arrangement, since the microstruc-ural pattern and the species distribution are defined previouslyuring the alloy solidification.

. Conclusions

The following main conclusions can be drawn from theresent investigation:

. Although the centrifuged casting has induced a finermicrostructure favoring the mechanical properties, it was ver-ified that Zr additions to the Ti–30Nb alloy has provided aconsiderable refinement on the dendritic array and has pro-vided �-phase stabilization (7.5Zr and 15Zr). Finer dendriticstructures with a consequent more homogeneous distributionof the nobler region (Ti-rich region) will provide barrier pro-tection along the grain boundaries and interdendritic regionsand will exhibit a more homogeneous oxide film with a con-sequent improvement on the corrosion resistance than coarserdendritic structures.

. The experimental electrochemical results have shown that thecentrifuged cast alloy samples present a passive oxide layerwhich can have a significant effect on biocompatibility. Thisis a potential indication that the centrifuged casting processis a prospective resource for the manufacturing of implantdevices.

. The experimental EIS diagrams, polarization curves andthe equivalent circuit parameters have shown that Zr addi-tions have improved the electrochemical corrosion behaviorof the Ti–30Nb alloy. In contrast, the Ti–30Nb cen-trifuged cast alloy sample has a higher surface reactivitywhich can induce a better osseointegration capacity whencompared to those of Ti–30Nb–Zr alloys. However, invivo and in vitro tests are necessary to evaluate thereal biocompatibility or osseointegration capability of suchTi-alloys.

. A mechanism in which the barrier layer thickness can be sig-nificantly increased due to the associated reactions betweenmetastable oxide particles and the electrolytic solution hasbeen proposed. As a consequence, some oxide particles ofthe outer layer are transformed in more stable oxide particlesand due to electronic and nature affinities they are incorpo-rated into the inner layer providing a finer porous layer. Ti–Nb

alloy samples are composed of a dual-layer oxide consistingof an inner barrier responsible for the corrosion protection,and an outer porous layer which apparently facilitates theosseointegration.

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a Acta 53 (2008) 2809–2817

. The centrifuged casting process and the addition of a noblersolute to the Ti–30Nb alloy can be used as alternative ways toproduce components with better characteristics for medicalimplants, i.e., with improved corrosion resistance, biocom-patibility and osseointegration.

cknowledgements

The authors acknowledge financial support provided byAPESP (The Scientific Research Foundation of the State of Saoaulo, Brazil), FAEPEX- UNICAMP and CNPq (The Brazilianesearch Council).

eferences

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