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Intermetallics 46 (2014) 156e163

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Intermetallics

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Effect of Nb addition on microstructure evolution and nanomechanicalproperties of a glass-forming TieZreSi alloy

S. Abdi a,b,*, M. Samadi Khoshkhoo a,b, O. Shuleshova a, M. Bönisch a,b, M. Calin a,L. Schultz a,b, J. Eckert a,b, M.D. Baró c, J. Sort d, A. Gebert a

a IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germanyb TU Dresden, Institut für Werkstoffwissenschaft, D-01062 Dresden, GermanycDepartament de Física, Facultat de Ciències, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spaind Institució Catalana de Recerca i Estudis Avançats and Departament de Física, Facultat de Ciències, Universitat Autònoma de Barcelona, E-08193 Bellaterra,Spain

a r t i c l e i n f o

Article history:Received 4 August 2013Received in revised form7 November 2013Accepted 11 November 2013Available online

Keywords:B. Glasses, metallicB. Mechanical properties at ambienttemperatureC. Rapid solidification processingC. NanocrystalsD. MicrostructureG. Biomedical applications

* Corresponding author. IFW Dresden, P.O. Box 27many. Tel.: þ49 351 4659 749; fax: þ49 351 4659 45

E-mail address: s.abdi@ifw-dresden.de (S. Abdi).

0966-9795/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.intermet.2013.11.010

a b s t r a c t

The glass-forming Ti75Zr10Si15 alloy is regarded as a potential material for implant applications due to itscomposition of non-toxic, biocompatible elements and some interesting mechanical properties. Theeffects of partial substitution of 15 at.% Ti by Nb on the microstructure and the mechanical behaviourhave been investigated by X-ray diffraction, scanning electron microscopy with energy-dispersive X-rayanalysis, transmission electron microscopy and nanoindentation techniques. Copper mold casting andmelt-spinning methods have been applied to study the influence of the cooling rate on the properties ofboth alloys, Ti75Zr10Si15 and Ti60Zr10Nb15Si15. As a result of different cooling rates, significant micro-structural variations from multiphase crystalline states in cast rods to nanocomposite structures inribbons were observed. The limited glass-forming ability (GFA) of the Ti75Zr10Si15 alloy results for melt-spun ribbons mainly in nanocomposite structures with b-type nanocrystals being embedded in a glassymatrix. Addition of Nb increases the glass-forming ability. Raising the overheating temperature of themelt prior to melt-spinning from 1923 K to 2053 K yields for both alloys a higher amorphous phasefraction. The mechanical properties were investigated using compression tests (bulk samples) and thenano-indentation technique. A decrease of hardness (H), ultimate stress and reduced Young’s modulus(Er) is observed for Ti60Zr10Nb15Si15 rods as compared to Ti75Zr10Si15 ones. This is attributed to an increaseof the fraction of the b-type phase. The melt-spun ribbons show an interesting combination of very highhardness values (H) and moderate reduced elastic modulus values (Er). This results in comparatively veryhigh H/Er ratios of >0.075 which suggests these new materials for applications demanding high wearresistance.

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1. Introduction

Titanium and (a þ b)- or b-type Ti alloys are well-known ma-terials for implant applications due to their relatively low density,superior mechanical properties and good corrosion behaviourcompared to conventional stainless steel or CoeCr-based alloys.The spontaneously forming barrier-type oxide film (mainly TiO2) atthe surface of these materials insulates the inner material from theexternal environment, resulting in high corrosion resistance [1,2].This film is defined as bioinert and therefore, a Ti-based surface

0116, D-01171 Dresden, Ger-2.

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cannot easily bond to bone tissue. A consequence may be looseningof the implant after long-term resorption of bone tissue. Surfacemodifications are applied to improve the bone-bonding capabilityby inducing bioactivity while preserving the properties of the bulkmaterial [3,4]. Numerous approaches of surface modificationsinclude mechanical, physical and chemical methods, which resultin different surface topographies from micro- to nano-scale [2e4].

Moreover, there are still some significant problems with thebiomechanical compatibility of conventional Ti-based materialsarising from their high stiffness and the concomitant mismatchbetween Young’s modulus values of bone (E ¼ 10e30 GPa) and theimplant material (E ¼ 110e120 GPa for cp-Ti and Ti-6Al-4V). Thisresults in stress shielding effects causing implant loosening withthe consequence of re-surgery [5,6]. Another major problem is thelimited strength and hardness of these alloys, which can cause

S. Abdi et al. / Intermetallics 46 (2014) 156e163 157

implant fracture. Limited wear resistance can also create theincidence of allergy and inflammatory cascades because of therelease of particles and/or toxic metallic ions [1,5,7]. Consequently,the development of new materials with improved properties, i.e.lower Young’s modulus, higher strength and hardness combinedwith excellent corrosion resistance and cell biological compati-bility is an urgent, but challenging demand. In this regard, a newgeneration of cast low modulus b-type Ti-based alloys, such as Ti-(40-45)Nb is in the focus of fundamental studies. However, theirlimited strength causes the need for complex thermo-mechanicalpost-processing [8].

Recently, glassy Ti-based alloys have attracted increasing inter-est as advanced materials for biomedical applications because oftheir high strength (in the order of 1800e2500 MPa) and lowYoung’s modulus values (in the order of 80e100 GPa) [9,10]. Thesevery promising mechanical performance data result from the lackof long-range order and the simultaneous absence of microstruc-tural defects such as grain boundaries and dislocations in thesingle-phase material [11e14].

A critical aspect in developing biomaterials is to avoid the useof any kind of harmful, toxic elements, such as Ni, Cu, V, Al, Be etc.[15e21]. However, up to date, most of the Ti-based alloys withbulk glass-forming ability contain Cu and/or Ni. Qin et al. inves-tigated Ti47.5Zr2.5þxCu37.5�xPd7.5Sn5 (x ¼ 0, 5, 7.5) alloys andshowed that the pitting potential of the alloys shifted to lowervalues with increasing Cu content. Therefore, for these alloys alimited corrosion resistance in synthetic body fluids was identifiedas a critical issue [22]. Only a very limited number of Ti-basedglassy alloys without harmful elements have been developed sofar, e.g. in the systems TieZr-(Ta/Pd)eSi [14,23], TieSi, TieZreSiand TieNbeSi [24e27]. Due to the lack of late transition elementstheir glass-forming ability (GFA) is not high and thus, limitssample production to melt-spun ribbons. Nevertheless, in view oftheir expected superior properties, which can be very useful forbiomedical applications, these materials are worth being furtherinvestigated and developed.

Recently, we started to explore the TieZreSi system and toanalyse the effect of substitution of up to 15 at.% Ti by Nb leading tothe development of Ti75Zr10Si15 and Ti60Zr10Nb15Si15 alloys [28].Overheating of the melt appeared to be a key factor for the phaseformation upon rapid cooling by melt-spinning. For the ternaryalloy, a nanocomposite structure with b-type nanoparticlesembedded in a glassy matrix phase seemed to be the typicalmicrostructural state of melt-spun material. Also, Nb seemed to beeffective for increasing the GFA. Preliminary Vickers hardness testsrevealed higher hardness data for the ternary base alloy withnanocomposite structure than for a nearly fully glassy state of a Nb-containing melt-spun alloy. Remarkably increased corrosion resis-tance for the melt-spun Ti75Zr10Si15 and Ti60Zr10Nb15Si15 alloys wasdetected in synthetic body fluid, as compared to cp-Ti.

In the present paper we report on amore detailed analysis of theeffect of the overheating temperature on the microstructure evo-lution of melt-spun ribbons. To understand the phase formationprocesses independence of cooling conditions, studies on cast rodswere also carried out. Special emphasis is given to a description ofthe effect of Nb on the phase formation upon slow and rapidcooling. Nanoindentation is used to assess locally mechanicalproperties of different phases, as a basis for an in-depth clarificationof structureeproperty relationships.

2. Experimental

Master alloys with nominal compositions Ti75Zr10Si15 andTi60Zr10Nb15Si15 were prepared by arc-melting a mixture of theconstituent elements with high purity (99.9%) in an argon

atmosphere. The ingots were re-melted 5 times to achieve goodchemical homogeneity. Rods with dimensions of 4 mm in diameterand 50 mm in length were prepared by copper mold casting. Asingle Cu-roller melt-spinning device was used to produce melt-spun ribbons with w50 mm thickness and 3e4 mm width usingdifferent temperatures of the melt in the range of 1900e2053 K.The melting temperatures of the Ti75Zr10Si15 and Ti60Zr10Nb15Si15alloys as determined from DSC measurements (not shown here) asliquidus temperatures of the endothermic event are 1620� 2 K and1715 � 2 K, respectively. Therefore, 1900e2053 K are overheatingtemperatures of the melt, which were employed to obtain a single-phase molten state without any residual high melting phases thatcould act as heterogeneous nuclei during solidification. Casting andmelt-spinning were carried out under highly purified argonatmosphere.

Structural characterization of rod and ribbon samples wasperformed by X-ray diffraction (XRD, using a D3290 PANalyticalX’pert PRO with Co-Ka radiation) and scanning electron micro-scopy (SEM, Gemini 1530 microscope) equipped with an energydispersive X-ray spectrometer (EDX) for elemental mapping. EDXanalysis was implemented to determine the chemical compositionof the intermetallic phases observed in the SEM micrographs. Inorder to have a good statistics, the chemical composition wasmeasured at more than 10 spots on the different regions.Furthermore, high-resolution structural analysis of melt-spunribbons was conducted with transmission electron microscopyTEM: a Philips TEKNAI F30 microscope (300 kV). Dark- and bright-field images were taken together with selected area electrondiffraction (SAED) patterns in order to investigate whether thestructure is fully amorphous or is a mixture of glassy phase andnanocrystals. This was further assisted using high-resolution TEMinvestigations, which is capable to identify the presence of verysmall nanocrystals (<10 nm).

The mechanical properties were investigated using an Instron8562 testing facility with a maximum load of 100 kN under qua-sistatic loading conditions at room temperature. Cylindrical sam-ples with a length/diameter ratio of 2.0 (6 mm length, 3 mmdiameter) were prepared from the lateral area of the extrudedrods. Compression testing was performed at a cross head speed of0.001 mm/s (strain rate of 6 � 10�5e1.5 � 10�4 l/s). Both ends ofthe specimens were polished to make them parallel to each otherprior to the compression test. All the given features are averagedvalues of at least three samples. Ultrasonic measurements wereconducted to assess elastic properties. Ultrasonic pulser-recievermodel 5072PR and oscilloscope model TDS 2022B Textronix,along with density assessment (Archimedes’ method) was used.The standard formalism for isotropic solids, based on Aleksan-drov’s method, was employed. The total error of measuring elasticconstants is about 5% which include the error of the acousticmeasurements (error of the oscilloscope delay time), the error ofthe sample thickness measurements (normally �1 or 2 microns)and the error in density measurements (�0.02 g/cc). Besides,hardness and reduced elastic modulus of samples were evaluatedby nanoindentation in load control mode using exactly the sameconditions for rods and ribbons, in order to reliably compare theresults. An UMIS device from Fischer-Cripps Laboratories equippedwith a Berkovich pyramidal-shaped diamond tip was employed.The samples were prepared by embedding them in polyester resinand subsequent polishing to a mirror finishing state using 1 mmdiamond paste. Indentation was implemented along the cross-section of ribbons. The indentation function consisted of aloading segment of 40 s, to a maximum force of 30 mN, followedby a load holding segment of 20 s, and an unloading segment of40 s. The results represent the average of a total of at least 40indentations for rod samples and 20 for ribbons.

S. Abdi et al. / Intermetallics 46 (2014) 156e163158

3. Results and discussion

The following section discusses the microstructural differencesbetween the melt-spun ribbons produced by rapid solidificationusing different overheating temperatures of the melt and the castrods formed under conditions of relatively slow cooling rates.

3.1. Microstructure of cast alloy samples

Ti75Zr10Si15 and Ti60Zr10Nb15Si15 rods with 4 mm diameter wereproduced by copper mold casting. Since cooling rates were too lowfor glass formation in these systems, fully crystalline states wereobtained. Fig. 1(a) shows a XRD pattern taken from the cross-sectional area of the Ti75Zr10Si15 rod. The hexagonal (Ti,Zr)5Si3intermetallic phase and the hexagonal a-Ti phase can be indexed inthe multi-phase system comprising also traces of additional non-identified phases. Previous studies on arc-melted TieZreSi alloysrevealed that within the composition range of interest two silicidephases, hexagonal (Ti,Zr)5Si3 (S1) and (Ti,Zr)2Si (S2) can formduringsolidification along with the high temperature b-Ti phase with bccstructure, which upon cooling transforms to a hexagonal a-Ti viasolid-state reaction [29,30]. According to the XRD pattern of the rodsample, the formation of the S2 phase was suppressed under thepresent conditions, which is certainly due to a higher cooling raterealized during Cu mold casting compared to that of arc melting.

Fig. 1(b) illustrates a typical back-scattered SEM image of themicrostructure of the cross-sectional area of the Ti75Zr10Si15 rod. Inthe microstructure, a faceted intermetallic phase is observed as a

Fig. 1. X-ray diffraction patterns (a) and back-scattered SEM image (b) taken from across-section of a cast Ti75Zr10Si15 rod (4 mm diameter).

primary phase surrounded by a disperse eutectic region consistingof a-Ti and S1. The average chemical composition of the interme-tallic regions measured by EDX for the ternary alloy (Ti ¼ 46 at.%,Zr ¼ 15 at.%, Si ¼ 39 at.%) is in good agreement with the chemicalcomposition of the S1 phase.

A XRD pattern of the Ti60Zr10Nb15Si15 4 mm rod is shown inFig. 2(a). The addition of Nb resulted in the stabilization of a b-Tiphase at room temperature. However, traces of a-Ti are also pre-sent. Phase analysis results revealed that the existing intermetallicphase is of the S1-type. Fig. 2(b) shows a SEM image of the typicalmicrostructure of the Ti60Zr10Nb15Si15 rod. Similar to the ternaryalloy, the microstructure is composed of an eutectic and an inter-metallic S1 phase. The b-phase nucleates on the primary interme-tallic phase. Due to the fast growth kinetics (typical for solidsolution phases), the b-phase spreads around the primary S1 phaseand even starts to develop dendrites (a cellular morphology wasfound in the microstructure). In addition, the residual melt trans-fers to an eutectic composed of S1- and b-phase. EDX analysis data(Ti¼ 41 at.%, Zr¼ 13 at.%, Nb¼ 8 at.%, Si¼ 38 at.%) illustrate that themean values of element concentrations in the intermetallic phasefit well with that of the S1 phase.

According to the results of elemental EDX mapping (shown inFig. 3), the addition of Nb increases the inhomogeneity of theelement distribution within the sample. During solidification Nb isaccumulated at the S1-liquid interface, which is due to its limitedsolubility in the intermetallic phase. Apparently, the residual liquiddoes not have sufficient time for homogenization, resulting in theconcentration gradient through the eutectic.

Fig. 2. XRD patterns (a) and back-scattered SEM micrograph (b) taken from a cross-section of a cast Ti60Zr10Nb15Si15 rod (4 mm diameter).

S. Abdi et al. / Intermetallics 46 (2014) 156e163 159

3.2. Microstructure of melt-spun alloy samples

In a previous study, investigations regarding the effect of over-heating the melt on the microstructure of melt spun ribbons werebegun [28]. We have now continued the work and additionallyconsidered the cooling rate gradient across the ribbon cross-section. We separately analysed the different sides of the ribbons,i.e. the air-side which has a lower cooling rate compared to thewheel-side, which has been in contact with the quenching wheel.

Fig. 4(a) shows X-ray diffraction patterns of melt-spunTi75Zr10Si15 ribbons prepared at different overheating tempera-tures ranging from 1923 K to 2053 K (liquidus temperatures ofTi75Zr10Si15 and Ti60Zr10Nb15Si15 are 1620� 2 K and 1715� 2 K). Foreach overheating temperature, diffraction patterns were recordedfor both air- and wheel-sides. As a general observation, XRD pat-terns are composed of sharp peaks superimposed on a diffuse haloindicating the presence of a nano-crystalline phase, identified asbcc b-Ti, dispersed within a glassy matrix phase. It is also observedthat the relative intensities of the b-Ti reflections are stronger in thepatterns of the air-sides compared to those in the patterns of thewheel-side. The higher cooling rate at thewheel-side of the ribbonsobviously retards the b-phase formation and supports vitrification.

In addition, as the overheating temperature increases from1923 K to 2053 K, the crystalline reflections reduce in intensity forboth ribbon sides, indicating that the volume fraction of thenanocrystalline b-Ti phase is reduced. According to the XRD pat-terns, the Ti75Zr10Si15 sample that was quenched from a tempera-ture of 2053 K has a fully amorphous structure on the wheel-side.However, reflections corresponding to nanocrystalline b-Ti arestill present in the XRD pattern of the air-side of the ribbon.

For melt-spun Ti60Zr10Nb15Si15 alloy samples (Fig. 4(b)), similartrends in phase evolution were observed. For both sides of theribbons a decrease of the intensities of the XRD reflections of thecrystalline phase was detected when increasing the overheatingtemperature. Moreover, the presence of Nb in the alloy increasesthe fraction of the glassy phase. Accordingly, the XRD patternstaken from the wheel-side of the ribbons that were melt-spun at1993 K and 2053 K, show completely amorphous features. How-ever, even at the highest melting temperature (2053 K), a minor

Fig. 3. SEM/EDX mapping results for rod microstructures of Ti75Zr10Si15 (a) secondary electro(d) elemental (Ti, Zr, Si) map (e) element (Nb) map.

volume fraction of bcc b-Ti nanocrystals may exist in the glassymatrix, which is not detectable by XRD.

Fig. 5 shows characteristic TEM results obtained for the wheel-sides of both alloy compositions produced by melt-spinning at anoverheating temperature of 1993 K. According to the bright-field(BF) image displayed in Fig. 5(a), the structural state of thewheel-side of the Ti75Zr10Si15 alloy is a mixture of nanocrystalsembedded in an amorphous matrix. The two-phase nature isconfirmed by the SAED pattern displayed as an inset in the BFimage which shows the diffuse rings of an amorphous phasetogether with Bragg-reflections corresponding to crystalline pre-cipitates. Indexation of the Bragg-reflections showed that theirposition matches well with the ring pattern of b-Ti. The interfacebetween amorphous matrix and b-Ti nanocrystalline regions isillustrated well in the high-resolution image (Fig. 5(b)).

In contrast, Fig. 5(c) and (d) demonstrates that no crystallineprecipitates were found in the wheel-side of quaternaryTi60Zr10Nb15Si15 ribbons produced at the same overheating tem-perature of 1993 K. According to the BF and high-resolution TEMmicrographs (Figs. 5(c) and (d), respectively), as well as confirmedby the SAED diffraction pattern (inset of Fig. 5(c)), the wheel-side ofthe ribbon is fully amorphous. The results clearly show that sub-stitution of 15 at.% Ti with Nb improves the GFA of the ternaryTi75Zr10Si15 alloy.

The effect of the overheating temperature on the structure evo-lution of melt-spun ribbons can be discussed considering differentaspects. Increasing this temperature results in a better homogeni-zationof themelt, including a completemeltingof high temperaturephases present in themulti-phase crystalline pre-alloy, which couldact as nucleation agents during rapid cooling. Also, the overheatingtemperature affects the cooling rate within the ribbon. It has beenreported that by increasing this temperature, the cooling rate isdecreased and hence, causes an increase of the crystalline volumefraction [31]. Tkatch et al. [32] reported that in Fe40Ni40P14B6 ribbonsthe cooling rate decreases by increasing the overheating melt tem-perature. This was attributed to an increase of the roughness at thewheel-sideof the ribbon [32,33]. Increasing the surface roughness ofFe64Co21B15 by increasing the melting temperature also stated byFerrara et al. [33]. However, as we observed for the Ti(-Nb)eZreSi

n image (b) elemental (Ti, Zr, Si) map and Ti60Zr10Nb15Si15(c) secondary electron image

S. Abdi et al. / Intermetallics 46 (2014) 156e163160

system, a decrease of the nanocrystalline volume fraction occurswhen increasing the overheating temperature. So it seems that thesensitivity to variation of the cooling rate is not pronounced.Consequently, our results strongly suggest that amorehomogenizedmelt and therefore, a lower fraction of heterogeneous nucleationsites is the main factor determining the microstructural differenceswhen the overheating temperature is varied.

3.3. Nanomechanical characterization of crystalline andnanocomposite alloys

The mechanical properties of bulk crystalline Ti75Zr10Si15 andTi60Zr10Nb15Si15 samples have been firstly assessed by compression

Fig. 4. XRD patterns corresponding to the air- (A) and wheel- (W) sides of (a)Ti75Zr10Si15 and (b) Ti60Zr10Nb15Si15 melt spun ribbons prepared by using differentoverheating temperatures of the melt.

tests with b-type Ti-40Nb which was employed as reference ma-terial. Mean values of Young’s modulus, ultimate stress and plasticelongation are summarized in Table 1, and representative curvesare shown in Fig. 6. It is clear that with the addition of Nb, theYoung’s modulus decreases from about 150 GPa to a value com-parable to that of the commercial (a þ b)-type Ti-based implantalloy Ti-6Al-4V which is about 120 GPa [34]. Nevertheless, theobtained value is much higher than the Young’s modulus of b-typeTi-based alloys such as Ti-40Nb (Ti74.4Nb25.6) with about 60 GPa.However, both investigated crystalline alloys exhibit a similar highyield strength (around 1200 MPa), which is much higher than thatof Ti-40Nb measured under similar conditions and which is desir-able for specific load-bearing biomedical implant applications. Inaddition, high ultimate stress, i.e. over 1800 MPa for the ternaryalloy and over 1600 MPa for the Nb-containing alloy, are obtained.Furthermore, the addition of Nb to the ternary alloy does notsignificantly affect the compressive plasticity, which is considerablylower than that of Ti-40Nb.

Ultrasonic measurements were performed to provide more ac-curate values of the Young’s modulus for the crystalline alloys, ascompared to those from compression tests. Interestingly, similar tothe compression test results, adding Nb decreases the Young’smodulus significantly from 155.7 � 1.5 GPa for Ti75Zr10Si15 to116.8 � 2.5 GPa for Ti60Zr10Nb15Si15. For a detailed analysis of themechanical properties of the single phases constituting the rodsmicrostructure, nanoindentation tests were carried out. Theaverage value of hardness calculated from an array of nano-indentation tests on a sample cross-section is larger for theTi75Zr10Si15 alloy than that for the Ti60Zr10Nb15Si15 alloy (seeTable 2). This is in agreement with the results from macroscopiccompression in which yield stress indicates the same trend. Thishardness decrease while Nb addition can be attributed to thepresence of a larger fraction of bcc b-type phase in theTi60Zr10Nb15Si15 alloy, which is typically softer than the hexagonala-phase due to the larger number of slip planes and slip directions.In addition, the Young’s modulus is usually lower for a b-phase thanfor an a-phase, which is in agreement with the obtained results(Tables 1and 2, Fig. 6). The reduced Young’s modulus of theTi60Zr10Nb15Si15 alloy is larger than that of Ti-40Nb, but the hard-ness of the latter is considerably lower. Furthermore, in single-phase nanoindentation analysis the presence of Nb in the inter-metallic S1 phase of the crystalline Ti60Zr10Nb15Si15 alloy was foundto decrease Er locally as compared to the value for S1 in theTi75Zr10Si15 sample (see Table 3). This is to some extent expectedsince the Young’s modulus of Nb is lower than that of Ti.

Fig. 7 shows exemplary the loadedisplacement (Peh) nano-indentation curves for the intermetallic phase and the eutecticregions of a cast Ti75Zr10Si15 sample and of selected regions of thenanocomposite structure at the cross-section of its ribbons pro-duced at overheating temperatures at 1923 K and 2053 K. Thecurves related to the ribbons are serrated by a series of discretesteps (pop-ins). These pop-ins in the Peh curve correspond to shearband nucleation and propagation during the indentation as previ-ously suggested [35,36].

Concerning the ribbons, it is worth noting that their H and Ervalues are lower than for the rods with the same composition,reaching Young’s modulus values of about 123 GPa for Ti75Zr10Si15and of about 108 GPa for Ti60Zr10Nb15Si15, which are similar tothose of commercial cp-Ti (w112 GPa) and Ti-6Al-4V (w120 GPa[37]). On the other hand, hardness values that are three times largerthan that of commercial reference materials (Ti z 2.7 GPa,Ti6Al4V z 4 GPa [37]) are preserved (Table 2). This is ascribed tothe presence of the glassy structure in the matrix in the ribbons.Indeed, metallic glasses often exhibit lower Young’s modulusvalues than the crystalline materials with analogous compositions

Fig. 5. (a) Bright field and (b) high resolution TEMmicrograph of the wheel-side of a Ti75Zr10Si15 ribbon, (c) bright field and (d) high resolution TEM micrograph of the wheel-side ofa Ti60Zr10Nb15Si15 ribbon; both ribbons were quenched from 1993 K.

S. Abdi et al. / Intermetallics 46 (2014) 156e163 161

[38e40]. Evidence for a reduction of the elastic constant accom-panying the loss of long-range order was also reported duringamorphisation of a Zr3Al glass by ion irradiation [41] or in Pd-based,Zr-based or rare-earth metallic glasses when compared to theirfully crystallized counterparts [38e40]. This effect has beenreferred to as “elastic softening”. The elastic softening has beenattributed to static atomic displacements and anharmonic vibra-tions resulting from the chemical and topological disorder of themetallic glass as well as to quenched-in free volume in the glassstructure [39]. Although the difference in the H and Er for thedifferent investigated ribbons is not very pronounced, there is aclear trend, particularly for H values, to increase with higheroverheating temperatures. This can be due to the reduction of thefraction of soft b-phase in the ribbons when prepared at higheroverheating temperatures. An effect of the Nb addition on thenano-mechanical properties of the melt-spun ribbons could not be

Table 1Experimental data of Young’s modulus (E), Yield Strength (sy), compressive strength(sdb) and plastic elongation ( 3p) for compression Tests of Ti75Zr10Si15,Ti60Zr10Nb15Si15 and b-type Ti-40Nb.

E (GPa) sy (MPa) sdb (MPa) 3p (%)

Ti75Zr10Si15- Rod 150 � 15 1231 � 41 1871 � 6.3 2.7 � 0.1Ti60Zr10Nb15Si15- Rod 120 � 7 1185 � 32 1684 � 27 3 � 0.2Ti-40Nb (Ti74.4Nb25.6) 62 � 7 544 � 66 1070 � 190 28 � 7

clearly identified in the error limit of the method, despite thefinding that Nb tends to improve the glass-forming ability.

Finally, the ratio H/Er, which gives an indirect assessment of thewear resistance [42e44], is much larger for all the investigated

Fig. 6. Representative room temperature compression engineering stress-engineeringstrain curves for cast crystalline Ti75Zr10Si15 and Ti60Zr10Nb15Si15 alloy samples;reference cast b-type Ti-40Nb.

Table 2Hardness, reduced modulus and H/Er results, measured by nanoindentation.

H (hardness,GPa)

Er (reducedmodulus, GPa)

H/Er

Ti75Zr10Si15-Rod 12.2 � 0.2 176.6 � 1.1 0.069Ti75Zr10Si15-Ribbon 2053 K 10.2 � 0.1 123.1 � 0.5 0.083Ti75Zr10Si15-Ribbon 1923 K 8.98 � 0.5 117.5 � 4.6 0.076Ti60Zr10Nb15Si15-Rod 8.9 � 0.3 143.6 � 2.9 0.062Ti60Zr10Nb15Si15-Ribbon 2053 K 8.8 � 0.2 108.01 � 1.4 0.084Ti60Zr10Nb15Si15-Ribbon 1923 K 8.52 � 0.1 113.0 � 1.9 0.075Ti-40Nb (Ti74.4Nb25.6) 2.8 � 0.1 77.8 � 4.6 0.036

Table 3Hardness, reduced modulus and H/Er measured by nanoindentation in intermetallicphase and eutectic matrix of crystalline Ti75Zr10Si15 and Ti60Zr10Nb15Si15 alloysamples.

Ti75Zr10Si15 Ti60Zr10Nb15Si15

IntermetallicS1 phase

Eutectic IntermetallicS1 phase

Eutectic

H (hardness, GPa) 13.7 � 1.6 11.8 � 0.37 8.85 � 1.9 8.1 � 0.63Er (reduced

modulus, GPa)215 � 11.2 172.7 � 2.3 156.1 � 7.7 129.4 � 3.9

H/Er 0.064 0.068 0.057 0.063

S. Abdi et al. / Intermetallics 46 (2014) 156e163162

alloys as compared to the ratio for commercial cp-Ti or Ti-40Nb(Table 2). Particularly large values of H/Er are obtained for theglassy ribbons. All these combinations of mechanical properties, i.e.large H, relatively low Er and high H/Er ratios, indicate the potentialof the investigated alloys to be used for specific biomedical appli-cations under load-bearing conditions.

4. Conclusions

Effects of the cooling rate on the microstructure evolution ofTi75Zr10Si15 and Ti60Zr10Nb15Si15 cast rods and melt-spun ribbonshave been investigated. The effects of Nb addition on the phaseformation and themechanical properties have been assessed. In thecase of Ti75Zr10Si15, conditions of slow cooling provide fully crys-talline rods consisting of the S1 intermetallic phase and an eutecticof S1 and a-Ti phase. The Nb addition yields a microstructurecomprising the S1 phase and an eutectic of S1 and b-Ti(Nb) phase.

Fig. 7. Representative loadedisplacement nanoindentation curves measured forTi75Zr10Si15 samples: the intermetallic and eutectic regions of the crystalline rod andselected region of nanocomposite ribbons prepared using two overheatingtemperatures.

Rapidly quenched ribbons of the ternary alloy exhibit a nano-composite structure of b-Ti nanocrystals being embedded in aglassy matrix. Results reflect also a cooling-rate-dependentmicrostructural gradient over the ribbon cross-section. Nano-crystalline b-Ti is mainly precipitated at the air-side due to the locallower cooling rate. Increasing the overheating temperature resultsin an increase of the amorphous phase fraction over the ribbonscross-section. Adding Nb to create the Ti60Zr10Nb15Si15 alloy in-creases the glass-forming ability of the system, as expressed in anincrease of the amorphous phase fraction at both sides of the rib-bon. At an overheating temperature of 2053 K, a nearly fully glassystructure is obtained. Mechanical property investigations revealedthat Nb has the main effect of decreasing the Young’s modulus ofthe crystalline alloy as well as its hardness which is due to thestabilization of a significant fraction of a b-type phase.

Although the limited GFA of the ternary alloy is improved by theaddition of Nb, formation of glassy structures remains restricted torapid cooling rates, i.e. thin foils and thin functional coatings madefrom gas-atomized powders can be realized. Usually, those rapidlyquenched alloys have nanocomposite structures with nanocrystalsbeing embedded in a glassy matrix phase and exhibit an interestingcombination of very high hardness implying a high wear resistancetogether with Young’s modulus values which are equal to those ofcommercial Ti-based crystalline materials but higher than those ofthe new generation of b-type alloys. Considering the non-toxiccompositions of the investigated alloys, these materials could bepotential candidates for forthcoming selected biomedicalapplications.

Acknowledgements

The authors thank S. Scudino and L. Giebeler for helpful dis-cussions, A. Helth for providing reference material for mechanicaltests and S. Donath, D. Lohse, M. Frey for technical support. Fundingby the European Commission within the framework of the FP7-ITNnetwork BioTiNet (PITN-GA-2010-264635) is gratefully acknowl-edged. Partial funding from the MAT2011-27380-C02-01 from theSpanish MINECO is also acknowledged.

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