Microstructural characterization of Ti-based alloys

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MATRIB 2012, VELA LUKA
OTOK / ISLAND KORČULA, HRVATSKA
20-22. lipnja / June 2012.
MICROSTRUCTURAL CHARACTERIZATION OF Ti-BASED ALLOYS
MIKROSTRUKTURNA KARAKTERIZACIJA LEGURA NA BAZI TITANA
Ljerka Slokar*, Tanja Matković, Prosper Matković
University of Zagreb Faculty of Metallurgy, Department for Physical Metallurgy
Aleja narodnih heroja 3, 44103 Sisak, Croatia
*e-mail: slokar@simet.hr
Abstract:
In a few last decades development of biomedical titanium alloys without undesirable
elements, aluminium and vanadium, has continued to increase. This paper shows
microstructural characterization of two groups of titanium-based alloys. First, titanium was
alloyed with chromium and molybdenum. In second type, niobium and zirconium were added
to titanium. Purpose of this investigation was to examine the microstructure and hardness of
Ti-Cr-Mo and Ti-Nb-Zr alloys with potential for biomedical use. Chemical compositions of
investigated alloys were selected fro, the same corner of ternary titanium rich diagram and
according to the similar biomedical Ti-based alloys. Three samples with different composition
of each alloy type were laboratory prepared by an arc melting method. Their microstructure
was examined by scanning electron microscope with energy-dispersive spectrometer (EDS)
by point and line analysis. Hardness was determined by Vickers method. Results show that Ti-
Cr-Mo alloys have two-phases microstructure containing β and  or ” phase, while Ti-Nb-Zr
alloys were nearly β single-phase with traces of -phase. EDS analysis indicates that  and β
have similar chemical composition which is in a good agreement to alloy composition.
Vickers hardness of Ti-Cr-Mo alloys increases with molybdenum content, while those for Ti-
Nb-Zr alloys increases with niobium content. According to the closely single-phase
microstructure and lower hardness, Ti-Nb-Zr alloys have adventage as potential biomedical
materials.
Key words: Ti-based alloys, biomedical material, microstructure, Vickers hardness
Sažetak:
U zadnje vrijeme sve se više razvijaju legure titana za biomedicinsku primjenu koje ne
sadrže nepoželjne elemente, aluminij i vanadij. Ovaj rad prikazuje mikrostrukturnu
karakterizaciju dvije skupine legura na bazi titana. U prvoj je titan legiran s kromom i
molibdenom, a u drugoj su titanu dodani niobij i cirkonij. Svrha ovog rada je ispitati
mikrostrukturu i tvrdoću Ti-Cr-Mo i Ti-Nb-Zr legura za potencijalnu primjenu u biomedicini.
Kemijski sastavi istraživanih legura odabrani su iz istog dijela ternarnog faznog dijagrama
bogatog titanom, te prema sličnim biomedicinskim legurama na bazi titana. Po tri uzorka
različitog sastava od svake skupine legura laboratorijski su pripremljeni u lučnoj peći.
Njihova mikrostruktura je ispitana skenirajućim elektronskim mikroskopom s energijsko-
disperzivnim spektrometrom. Analiza sastava provedena je analizom u točki i linijskom
analizom. Tvrdoća je određena Vickersovom metodom. Rezultati pokazuju da su Ti-Cr-Mo
legure dvofazne, odnosno da sadrže β i  ili “ fazu, dok su Ti-Nb-Zr legure uglavnom β-
jednofazne s -fazom u tragovima. EDS analiza je pokazala da  (“) i β faze imaju vrlo

sličan kemijski sastav, koji odgovara sastavu legure. Tvrdoća prema Vickersu Ti-Cr-Mo
legura povećava se s udjelom molibdena u leguri, dok se ona za Ti-Nb-Zr legure povećava s
udjelom niobija. Zbog skoro β-jednofazne mikrostrukture i niže tvrdoće u odnosu na Ti-Cr-
Mo legure, Ti-Nb-Zr legure imaju veći potencijal za primjenu kao biomedicinski materijali.
Ključne riječi: legure na bazi titana, biomedicinski materijali, mikrostruktura, tvrdoća prema
Vickersu
1. INTRODUCTION
Due to their light weight, excellent mechanical properties and corrosion resistance,
titanium alloys are popularly used today in many medical applications. The most widely used
titanium biomedical alloy is Ti-6Al-4V. But, since aluminium and vanadium are known to
might cause some long-term health problems, alternative Ti-based alloys without toxic
elements have been developed [1-3].
The goal of this study was to prepare a new titanium-based alloys with potential for
biomedical use. Chromium is a β-eutectoid element, which stabilizes β-phase of titanium.
Besides, it is cheap and decreases Young's modulus in a manner similar to niobium as β-
isomorphous element in a Ti-Nb alloys. Zirconium suppress the athermal ω-phase produced
during quenching and as a result decreases Young's modulus to be similar to the cortical bone
(4-30 GPa) [4,5].
2. MATERIALS AND METHODS
In this paper two types of titanium alloys were prepared with purpose to examine and
compare their microstructure and hardness which would satisfy conditions for biomedical
applications.
First, titanium was alloyed with chromium and molybdenum. In second type, niobium and
zirconium were added to titanium. Cr and Mo were chosen as a β-stabilizers as well as
zirconium and niobium, which are not toxic and do not cause allergic reactions in body tissue
[6-8]. Chemical compositions of experimental alloys were selected according to the similar
biomedical Ti-based alloys [4,10,11]. Their positions in the portion of isothermal sections of
ternary diagrams at the room temperature are presented in Figure 1.
(a) Ti-Cr-Mo alloys (b) Ti-Nb-Zr alloys
Figure 1. Chemical compositions of investigated alloys

Alloys were prepared by melting a pure elements (with purity better than 99.9%) in
vacuum arc-furnace under argon atmosphere (Fig.2.). Because of large difference in melting
temperature of elements in Ti-Cr-Mo system, first chromium and molbdenum were melted
into a „button-shape“ (Fig.2.). These buttons were remelted for a three times, and then
titanium was added. Such samples were remelted for another four times to ensure their
homogeneity. In the similar way Ti-Nb-Zr alloys were prepared. Namely, niobium and
zirconium were remelted for three times, and then titanium was added.
Figure 2. Laboratory arc-furnace and „button-shaped“ alloys in copper mould
Alloys in forms of buttons of approximately 7g were casted in the same equipment by
means of specially constructed water-cooled copper anode, which served as a casting mould.
In this way, as-cast cylindrical specimens with dimensions 8 mm x 25 mm were produced and
they were sectioned using a Buehler Isomet low-speed diamond saw. After embedding in
conductive resin Conductomet, specimens were metallographically prepared by grinding and
polishing. The microstructure of two Ti-Cr-Mo alloys (No. 2 and 3) was remarked easily, but
there was a need for etching another samples. Microstructural analysis was performed using a
scanning electron microscope (SEM) Tescan Vega TS 5136 MM with Bruker energy-
dispersive spectrometer (EDS) by point and line analysis. Hardness of as-cast experimental
alloys were determined by Vickers method with load 19.60N for 10 s.
3. RESULTS AND DISCUSSION
Microstructure and hardness of Ti80Cr10Mo10, Ti70Cr20Mo10, Ti70Cr10Mo20 and Ti80Nb10Zr10,
Ti70Nb20Zr10, Ti70Nb10Zr20 respectively, were examined. SEM analysis of three Ti-Cr-Mo as-
cast alloys showed that all of them have two-phases microstructure (Fig.3). Alloy Ti80Cr10Mo10
with highest titanium content after etching revealed a white needles of ''-phase in a matrix of
β-phase. Other two non-etched alloys showed a dark dendrites of -phase in a matrix of β-
phase.

(a) Ti80Cr10Mo10 (b) Ti70Cr20Mo10
(c) Ti70Cr10Mo20
Figure 3. SEM micrographs of investigated Ti-Cr-Mo alloys
with EDS spectrum of phases
SEM micrographs of Ti-Nb-Zr alloys (Fig.4) show β-matrix and probably the presence of -
phase according to the reference [1].
(a) Ti80Nb10Zr10 (b) Ti70Nb20Zr10

(c) Ti70Nb10Zr20
Figure 4. SEM micrographs of investigated Ti-Nb-Zr alloys
with EDS spectrum of phases
Chemical compositions of all phases were determined by EDS point analysis (Table 1).
Table 1. EDS results obtained by point analysis
No. Alloy composition,
at.% Element Elements content in
β-phase, at.% -phase, at.%
1. Ti80Cr10Mo10
Ti 80 80 (α'')
Cr 9 9 (α'')
Mo 11 11 (α'')
2. Ti70Cr20Mo10
Ti 72 89
Cr 16 7
Mo 12 4
3. Ti70Cr10Mo20
Ti 70 92
Cr 10 3
Mo 20 5
1. Ti80Nb10Zr10
Ti 81 -
Nb 9 -
Zr 10 -
2. Ti70Nb20Zr10
Ti 72 -
Nb 19 -
Zr 9 -
3. Ti70Nb10Zr20
Ti 70 -
Nb 7 -
Zr 23 -
These results revealed very similar data for composition of β-phase which mainly corresponds
to the alloy composition. The composition of β and ’’-phases are equal for Ti80Cr10Mo10 alloy.
This suggests that during an alloys casting with the fast cooling rate the phase transformation
occurred, without change in composition (Fig.5a), as the evidence of martensitic
transformation β’’.

(a) Ti80Cr10Mo10 (b) Ti70Cr20Mo10 (c) Ti70Cr10Mo20
Figure 5. Elements line distribution in Ti-Cr-Mo alloys
In Ti70Cr20Mo10 and Ti70Cr10Mo20 alloys -phase contains much higher titanium ratio which
can be clearly seen in Figs.5b and 5c obtained by line analysis.
SEM and EDS analysis of Ti-Nb-Zr alloys showed that all of them have homogenous
structure of β-phase (Fig.4). Chemical composition determined in all points revealed the same
data, which corresponds to alloy compostion (Table 1). Also, line analysis (Fig.6) showed
uniformly line distribution of elements.
(a) Ti80Nb10Zr10 (b) Ti70Nb20Zr10 (c) Ti70Nb10Zr20
Figure 6. Elements line distribution in Ti-Nb-Zr alloys
The Vickers hardness measurements (Table 2) show the strong effect of chemical
composition and microstructure on hardness values. Obtained data for Ti-Cr-Mo alloys were
in the range of 449 to 555 HV2 and they were increased with increasing molybdenum content.
So, the highest value of 555 HV2 has alloy Ti70Cr10Mo20.
Table 2. Vickers hardness of experimental alloys
No. Alloy composition,
at.% HV2
1. Ti80Cr10Mo10 449
2. Ti70Cr20Mo10 542
3. Ti70Cr10Mo20 555

1. Ti80Nb10Zr10 379
2. Ti70Nb20Zr10 474
3. Ti70Nb10Zr20 411
Hardness of Ti-Nb-Zr alloys has lower values (379 – 474 HV2) when compared with Ti-Cr-
Mo alloys. This could be explained by fact that β-phase has lower hardness than -phase [12].
Measured values are similar to that of other titanium β-type biomedical alloys [11,13,14].
4. CONCLUSIONS
In this paper two types of titanium-based alloys for biomedical application were studied.
From the obtained results it can be concluded as follows:
SEM analysis shows two-phases microstructure of Ti80Cr10Mo10, Ti70Cr20Mo10 and
Ti70Cr10Mo20 alloys, which consists of β and  or ’’ phases.
As-cast alloys Ti80Nb10Zr10, Ti70Nb20Zr10 and Ti70Nb10Zr20 were single β-phases.
EDS indicated that chemical composition of β and ’’ phases are equal for
Ti80Cr10Mo10 alloy. This is the evidence that martensitic transformation β'' takes
place.
Vickers hardness of Ti-Cr-Mo alloys depends on molybdenum content, and that of
Ti-Nb-Zr alloys depends on niobium content.
Hardness of Ti-Nb-Zr alloys are lower than that for Ti-Cr-Mo alloys because the β-
alloys has lower hardness than β/ alloys.
According to the results showing single-phase microstructure and lower hardness, Ti-
Nb-Zr alloys have advantage in front of Ti-Cr-Mo alloys as potential biomedical materials.
5. REFERENCES
1. D.J. Lin, J.H. Chern Lin, C.P. Ju, Effect of chromium content on structure and
mechanical properties of Ti-7.5Mo-xCr alloys, Journal of Materials Science: Materials in
Medicine 14 (2003) 1-7
2. E. Eisenbarth, D. Velten, M. Müller, R.Thull, J. Breme, Biocompatibility of β-stabilizing
elements of titanium alloys, Biomaterials 25 (2004) 5705-5713
3. W-F. Ho, C-H. Pan, S-C. Wu, H-C. Hsu, Mechanical properties and deformation
behavior of cast binary Ti-5Cr-xFe alloys, Journal of Alloys and Compounds 472 (2009)
546-550
4. M. Geetha, A.K. Singh, R. Asokamani, A.K. Gogia, Ti based biomaterials, the ultimate
choice for orthopaedic implants – A review, Progress in Materials Science 54 (2009)
397-425
5. Y. Murayama, S. Sasaki, H. Kimura, A. Chiba, Phase stability and mechanical properties
of Ti-Cr based alloys with low Young's modulus, Materials Science Forum 654-656
(2010) 2114-2117
6. I. J. Polmear, Light Alloys, Metallurgy of the light metals, Third Edition, Arnold,
London, 1995.
7. L. J. Xu, Y. Y. Chen, Zh. G. Liu, F. T. Kong, The microstructure and properties of Ti-Mo-
Nb alloys for biomedical application, Journal of Alloys and Compounds 453 (2008) 320-
324

8. M. Lamirand, J-L. Bonnentien, G. Ferrière, S. Guérin, J-P. Chavalier, Relative effects of
chromium and niobium on microstructure and mechanical properties as a function of
oxygen content in TiAl alloys, Scripta Materialia 56 (2007) 325-328
9. M. Niinomi, T. Akahori, T. Takeuchi, Sh. Katsura, H. Fukui, H. Toda, Mechanical
properties and cyto-toxicity of new beta type titanim alloy with low melting points for
dental applications, Materials Science and Engineering C, 25 (2005) 417-425
10. W-F. Ho, S-C. Wu, H-H. Chang, H-C. Hsu, Structure and mechanical properties of Ti-
5Cr based alloy with Mo addition, Materials Science and Engineering C 30 (2010) 904-
909
11. Lj. Slokar, T. Matković, P. Matković, Alloying and heat treatment effects on the
microstructure and hardness of biomedical titanium alloys, International Conference
Proceedings of MATRIB2011, Vela Luka, Croatia, 2011, 450-456
12. I. Weiss, S.L. Semiatin, Thermomechanical processing of beta titanium, Materials
Science and Engineering A, 243 (1998) 46-65
13. M. Koike, M. Itoh, O. Okuno, K. Kimura, O. Takeda, T.H. Okabe, T. Okabe, Evaluation
of Ti-Cr-Cu alloys for dental applications, Journal of Materials Engineering and
Perfomance 14 (2005) 778-783
14. H-C. Hsu, S-C Wu, T-Y. Chiang, W-F. Ho, Structure and grindability of dental Ti-Cr
alloys, Journal of Alloys and Compounds 476 (2009) 817-825