LJournal of Alloys and Compounds 329 (2001) 264–271www.elsevier.com/ locate / jallcom

Effect of thermomechanical processing on microstructure of aTi–13Nb–13Zr alloy

a b , b b b*M. Geetha , A.K. Singh , K. Muraleedharan , A.K. Gogia , R. AsokamaniaSathyabama Engineering College, Chennai 600 098, India

bDepartment of Physics, Anna University, Chennai 600 025, India

Received 30 March 2001; accepted 15 May 2001

Abstract

This work describes the effect of mechanical working and heat treatment on the microstructure of the alloy Ti–13Nb–13Zr using X-raydiffraction and metallography. Lattice parameters of the constituent phases in as cast and different heat-treated conditions have beencalculated. The b solution treatment and water quenching (WQ) results in the a9 martensite while the a1b solution treatment and WQleads to three-phase microstructures consisting of a0, a and b. Only a and b phases are observed under aged condition irrespective of theprior solution treatment conditions. The effect of various heat-treatments on the hardness of the alloy has been presented. 2001Elsevier Science B.V. All rights reserved.

Keywords: Transition metal alloys; Microstructure; X-Ray diffraction; Mechanical properties

1. Introduction momechanical treatment. In the present work, the effect ofmechanical working and heat treatment on the structure

The use of titanium and its alloy as biomaterial is has been explored in a Ti–13Nb–13Zr alloy using X-rayincreasing due to their low modulus, superior biocom- diffraction and metallography. The effect of various heatpatibility and enhanced corrosion resistance when com- treatments on the hardness of the alloy has also beenpared to more conventional stainless steel and cobalt-based presented.alloys [1]. These attractive properties have been a drivingforce for the early introduction of commercially puretitanium and Ti–6Al–4V alloy. However, in recent years, 2. Experimental detailsvanadium has been found to cause cytotoxic effects [2] andadverse tissue reactions [3], while aluminium has been Pancakes of 600 g of the alloy Ti–13Nb–13Zr wereassociated with potential neurological disorders [4,5]. melted by non-consumable arc melting six to seven timesHence, new alloys that do not contain these elements are to ensure chemical homogeneity. The analyzed composi-receiving a great deal of attention. Alloys containing tion of the alloy is given in Table 1.elements such as Nb, Zr, Ta, Pt and Ti are being evaluated The b transformation temperature of this alloy is 7358Cextensively since these are the only five elements which [1]. Accordingly, a pancake was thermomechanicallyhave been identified as producing no adverse tissue processed by hot rolling in the a1b phase field at 6808Creaction [2,3]. Ti–13Nb–13Zr is one such alloy which has to obtain a 4-mm-thick sheet. The specimens cut from theshown great promise [6]. It possesses low modulus, rolled sheets were subjected to a variety of heat treatments.improved biocompatibility, improved corrosion and wear The heat treatments constituted a higher temperatureresistance.

Apart from the composition, performance of the alloys Table 1Chemical composition of the alloy Ti–13Nb–13Zris greatly influenced by the microstructure. The micro-

structure in titanium alloys can be controlled by ther- Element

O N C Fe Zr Nb Ti*Corresponding author.

Wt% 0.13 0.007 0.018 0.08 13.7 13.6 Bal.E-mail address: aks@dmrl.ernet.in (A.K. Singh).

0925-8388/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0925-8388( 01 )01604-8

M. Geetha et al. / Journal of Alloys and Compounds 329 (2001) 264 –271 265

Table 2Heat treatment details of the alloy

Heat treatment Designation Observed Hardnessphases 1000 g HV

7608C/1 h1WQ b ST WQ a9 2357608C/1 h1AC b ST AC a, b 2887608C/1 h1FC b ST FC a, b 250b ST WQ15008C/4 h1AC b ST WQA a, b 2841

b ST WQ15508C/4 h1AC b ST WQA a, b 2742

b ST WQ16008C/4 h1AC b ST WQA a, b 2673

b ST AC15508C/4 h1AC b ST AC A a, b 2752

b ST FC15508C/4 h1AC b ST FC A a, b 2432

6808C/1 h1WQ a1b ST WQ a0, a, b 2306808C/1 h1AC a1b ST AC a0, a, b 2656808C/1 h1FC a1b ST FC a, b 251a1b ST WQ15008C/4 h1AC a1b ST WQA a, b 2981

a1b ST WQ15508C/4 h1AC a1b ST WQA a, b 2742

a1b ST WQ16008C/4 h1AC a1b ST WQA a, b 2723

a1b ST AC15508C/4 h1AC a1b ST ACA a, b 2812

a1b ST FC15508C/4 h1AC a1b ST FCA a, b 2422

solution treatment in the b phase field (b ST) or in thea1b phase field (a1b ST) followed by an ageing

Table 3treatment. The details of heat treatments are given in TableLattice constants of the constituent phases in different heat treated2. The samples for optical metallography and scanningconditions

electron microscopy were prepared using standard tech-˚Sample Heat treatment Phase Lattice constants (A)niques and etched with Kroll’s reagent (5 ml HF, 10 ml

no.HNO and H O). X-Ray diffraction (XRD) patterns of all a b c3 2

specimens were taken using a Philips 3121 diffractometer. 1 As cast (bulk) a 2.987 – 4.709The cell constants of as cast, hot rolled and heat-treated b 3.294 – –

2 As cast (powder) a 2.961 – 4.723samples were calculated using the standard program Celnb 3.294 – –[7]. The XRD measurement of as cast material was also

3 b ST WQ a9 2.985 – 4.724carried out on powder samples. The powder was prepared4 b ST AC a 2.966 – 4.702

by filing and subsequent annealing in vacuum to relieve 5 b ST FC a 2.964 – 4.719the stress. The hardness of all the specimens was de- b 3.301 – –

6 b ST WQ1A a 2.971 – 4.725termined using a Leitz Wetzlar micro hardness tester. 1

8 b ST WQ1A a 2.972 – 4.7112

b 3.304 – –9 b ST WQ1A a 2.968 – 4.71833. Results and discussion b 3.305 – –

10 b ST AC1A a 2.967 – 4.7192

b 3.305 – –Microstructures and corresponding XRD patterns of the11 b ST FC1A a 2.966 – 4.7162as cast, hot rolled and heat treated specimens have been

b 3.308 – –discussed in detail. This has been supplemented by calcu-12 a1b ST WQ a0 3.047 5.093 4.756

lation of lattice parameters of a and b phases of all the a9 2.947 – 4.727specimens (Table 3) and hardness measurements (Table b 3.289 – –

13 a1b ST AC a0 2.973 5.054 4.7682).a 2.964 – 4.711b 3.286 – –3.1. As cast condition

14 a1b ST FC a 2.961 – 4.738b 3.302 – –

The microstructure of the as cast alloy is acicular in 15 a1b ST WQ1A a 2.965 – 4.7251

b 3.291 – –nature as shown in Fig. 1. This shows two-phase micro-16 a1b ST WQ1A a 2.952 – 4.7382structures. XRD patterns of this specimen in bulk and

b 3.295 – –powder form reveal the presence of a and b phases (Fig.17 a1b ST WQ1A a 2.965 – 4.70732), the a phase being the major constituent. The lattice b 3.303 – –

parameters of both a and b phases calculated from bulk 18 a1b ST AC1A a 2.965 – 4.7082

b 3.298 – –and powder specimens are given in Table 3. The lattice19 a1b ST FC1A a 2.962 – 4.7132parameters of both a and b phases in as cast material are

b 3.307 – –slightly different in bulk and powder form. The lattice

266 M. Geetha et al. / Journal of Alloys and Compounds 329 (2001) 264 –271

Fig. 1. Microstructure of the as cast alloy. Fig. 3. Microstructure of the hot rolled specimen.

constants obtained in powder samples are considered to be 3.2. Hot rolledmore accurate since texture and microstructure relatedeffects are eliminated in powder samples. The microstructure of hot rolled specimen is shown in

Poggie et al. [8] have reported the XRD data (2u and d Fig. 3. Fine elongated a phase is observed in the b matrixvalues) of this alloy in bulk form and oxygen hardened which is typical of a worked structure. The XRD study ofconditions. We have calculated the lattice parameters of this sample confirms the presence of a and b phases.this alloy for both a and b phases from their data. These

˚ ˚are, a52.987 A, c54.735 A for the a phase, and a53.273 3.3. Solution treatmentA for the b phase, respectively. These values are veryclose to those obtained in the present study. It is important The hot rolled specimens have been solution treated into mention here that both a and c parameters of this alloy the b phase and the a1b phase field and followed byare higher than those of other titanium alloys [9–11]. This water quenching (WQ), air cooling (AC), and furnacecan be attributed to the large content of Zr, which has a cooling (FC).higher atomic radius (both in terms of a and c parameters)than that of pure titanium [12]. 3.3.1. b solution treatment

The microstructures of b solution treated specimenswith different cooling rates are shown in Fig. 4. The WQspecimen exhibits a martensitic microstructure (a9) whilethe FC specimen shows a basket weave type of micro-structure. These structures are typical of b solution treatedtitanium alloys. The XRD studies of these specimens (Fig.5a) show the presence of a9 phase in WQ specimen. TheAC and FC specimens reveal the presence of the a phasewith a very weak peak of the b phase (Fig. 5b,c).

Two different types of martensitic structures i.e. a9

(hexagonal) and a0 (orthorhombic) are observed intitanium alloys depending upon the content of b alloyingelements [9–13]. Higher b alloying content favors forma-tion of a0 martensite in preference to a9. The criticalcomposition limit of the b alloying elements in binaryTi–X (Nb, Mo, Ta, W, etc.) alloys for the formation of a0

have been reported in the literature [9–13]. The a9 /a0

boundary in the binary Ti–Nb system is around 11 wt%Nb [12]. The formation of a9 martensite in this alloysuggests that the presence of Zr probably shifts the a9 /a0

boundary to higher Nb levels.The lattice parameters a and c have the highest value in

the WQ specimen while the AC and FC specimens exhibitFig. 2. X-Ray diffraction patterns of cast alloy, (a) bulk material, and (b)powder. slightly lower and nearly the same values of the lattice

M. Geetha et al. / Journal of Alloys and Compounds 329 (2001) 264 –271 267

Fig. 5. XRD patterns of (a) b ST WQ, (b) b ST AC and (c) b ST FCspecimens.

of b phase precipitates during cooling. This results in adecrease in Nb and Zr in the a phase. As a result, thelattice constants a and c are slightly smaller compared tothat observed in the WQ specimens.

3.3.2. a1b solution treatmentThe microstructure of (a1b) solution treated specimens

is shown in Fig. 6. The overall microstructural features inWQ, AC and FC specimens appear to be similar in nature.The microstructure consists of equiaxed a (primary a) andtransformed b phases. The amount of a phase increaseswith decrease in the cooling rates (Fig. 6). XRD studies ofthese specimens reveal different trends compared to thoseof b solution treated samples (Fig. 7). The WQ and ACspecimens exhibit three phases namely a, orthorhombic(a0) and b. The lattice constants of all three phases aregiven in Table 3. The FC specimen shows the presence ofonly a and b phases. The lattice parameters of the a0

phase are higher than those reported in other titaniumalloys [9–11]. This can probably be again attributed to thehigher zirconium content present in this alloy.

The presence of a0 martensite under a1b ST and WQconditions as opposed to hexagonal a9 under b STcondition is due to enrichment of the b phase in Nb asseen in Table 4. This also suggests that a9 /a0 boundary in

Fig. 4. Microstructures of the (a) b ST WQ, (b) b ST AC and (c) b ST this alloy may be between 13 and 16 wt% Nb. On slowerFC specimens. cooling from a1b ST, there is further enrichment of b

phase in Nb which leads to stabilization of b phase atroom temperature.

constants. The high lattice constants observed in the WQspecimen are due to the martensitic transformation wherein 3.4. Aging treatmentthe b phase is completely transformed into the a9 phase.This a9 phase contains almost all of the Nb and Zr present The aging treatment has been carried out at threein the alloy. In the AC and FC specimens a small amount different temperatures 500, 550 and 6008C for solution

268 M. Geetha et al. / Journal of Alloys and Compounds 329 (2001) 264 –271

Fig. 6. Microstructures of the (a) a1b ST WQ, (b) a1b ST AC and (c) a1b ST FC specimens.

treated and WQ specimens. The AC and FC specimens The aging at 5008C reduces the sharpness of a9 plates (Fig.have been aged only at 5508C and air cooled. All the 8a) and also introduces globularisation. The aging at 5508Csamples have been air cooled from the aging temperatures. clearly leads to a fine globular a phase (Fig. 8b) which has

further grown at 6008C aging temperature (Fig. 8c). The3.4.1. b solution treated martensitic plates have completely disappeared in the 550

The aging of the WQ specimen shows a gradual change and 6008C aged specimens. The a9 phase is known toin the microstructure with increasing aging temperature transform to a and b phases (a9 →a1b) during aging(Fig. 8). The WQ specimen shows sharp martensitic plates. [14]. The lattice constants of the aged specimens are

M. Geetha et al. / Journal of Alloys and Compounds 329 (2001) 264 –271 269

Fig. 7. XRD patterns of (a) a1b ST WQ, (b) a1b ST AC and (c) a1b

ST FC specimens.

shown in Table 3. It can be clearly seen that the aparameter of the aged specimen is slightly lower than thatof the WQ specimen whereas the c parameter remainsalmost similar. The decrease in a parameter is due to acomposition change of the a phase. However, the latticeparameters of the aged specimens at the three differenttemperatures are nearly the same.

The aged microstructure of the AC specimen exhibitsfine globular features similar to that of the WQ and agedspecimen. The FC and aged specimen exhibit coarse a

plates. The fine globular features are also seen in thisspecimen to be present to a lesser extent than in the WQand AC aged specimens. The XRD results show almost thesame values of the lattice constant in the AC and FC agedspecimens.

3.4.2. a1b solution treatedThe aged microstructures of (a1b) ST specimens are

shown in Fig. 9. The aging at 5008C does not change thesize and shape of the primary a phase. However, fine scalemicrostructural changes may be occurring in the trans-formed b phase, which can only be observed by transmis-

Table 4The Nb and Zr contents in a and b phases in a1b ST conditions

Fig. 8. Microstructures of b ST WQ and aged at (a) 5008C, (b) 5508C,obtained by electron probe microanalysis (EPMA)and (c) 6008C specimens.

Heat treatment Phases Nb (wt%) Zr (wt%)condition

sion electron microscopy. XRD studies of this specimena1b ST WQ a 6.135 10.476

show the presence of a and b phases. It is important tob 16.033 13.266mention that the orthorhombic phase which appeared undera1b ST AC a 6.580 10.814

b 15.442 13.664 a1b ST/WQ and a1b ST/AC conditions has disap-a1b ST FC a 5.864 11.596 peared during aging. The a0 phase can transform to a1b

b 21.488 15.043 (Ms.room temperature) or b (Ms5room temperature)

270 M. Geetha et al. / Journal of Alloys and Compounds 329 (2001) 264 –271

transformation of a0 to a9 by aging at a stabilizingtreatment temperature (803 K for 6 h) without anyprecipitation of the b phase. It is very difficult to concludeanything about the transformation of a0 to a or a1b in thepresent study since some amount of b phase is alreadypresent in the as quenched specimen.

The aging of a1b ST AC and a1b ST FC specimensshows similar microstructural features to those of a1b STWQ and aged specimen. The XRD of AC and FCspecimens exhibits a and b phases. The orthorhombicphase observed in AC phase has also disappeared duringaging.

3.5. Hardness

Hardness is one of the important properties whichdetermines suitability of the material for a biomedicalapplication. The hardness of the alloy varies from 230VHN to 298 VHN (Table 2) and critically depends uponthe heat treatment. Under solution treated conditions, thehardness is maximised at an intermediate cooling rate(such as air cooling). Under aged conditions, lower tem-perature-aging results in higher hardness than highertemperature aging and can be associated with finer micro-structural features obtained at lower temperature aging.However, the hardness values in the alloys are quite low,and heat treatment increases the hardness only marginally.An appropriate surface hardening treatment thus appears tobe essential for its applications. Diffusion hardeningappears to have shown significant improvement in thealloy [1].

4. Conclusion

1. The lattice constants of the a phase of the alloy are˚ ˚found to be a52.961 A and c54.723 A. These are

sensitive to solution treatment and aging temperatures.2. The b solution treated and WQ specimen has a single

phase a9 martensitic structure while the (a1b) ST WQand AC specimens exhibit three-phase a0, a and b

microstructures.3. The lattice constants of the a0 phase observed in this

˚ ˚alloy are found to be a53.0468 A, b55.0931 A and˚c54.756 A.

4. The b ST AC and FC specimens exhibit a and b

phases, respectively.Fig. 9. Microstructures of a1b ST WQ and aged at (a) 5008C, (b) 5508C, 5. Only a and b phases are observed under aged con-and (c) 6008C. ditions, irrespective of the prior ST condition.

[14] where Ms is the martensitic start temperature. Atransformation of the a0 to the a phase without b Acknowledgementsprecipitation has also been reported [10]. Young et al. [10]have observed the transformation of the a0 phase through The authors acknowledge the financial support of theprecipitation of a needles rather than through the precipi- Defence Research Development Organisation, Governmenttation of b particles. Singh et al. [9] have reported the of India. Thanks are also due to the members of the

M. Geetha et al. / Journal of Alloys and Compounds 329 (2001) 264 –271 271

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