M A T E R I A L S C H A R A C T E R I Z A T I O N 8 4 ( 2 0 1 3 ) 1 0 5 – 1 1 1

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Microstructure and beta grain growth behavior of

Ti–Mo alloys solution treated

Jin-Wen Lua,b,⁎, Yong-Qing Zhaoa,b,⁎, Peng Geb, Hong-Zhi Niub

aSchool of Materials and Metallurgy, Northeastern University, Shenyang 110819, ChinabNorthwest Institute for Nonferrous Metal Research, Xi'an 710016, China

A R T I C L E D A T A

⁎ Corresponding authors at: No.96 Weiyang RE-mail addresses: lujwen@163.com (J.-W.

1044-5803/$ – see front matter © 2013 Elseviehttp://dx.doi.org/10.1016/j.matchar.2013.07.0

A B S T R A C T

Article history:Received 5 November 2012Received in revised form 15 July 2013Accepted 22 July 2013

The microstructure and grain growth kinetics of Ti–Mo binary alloys in the β phase havebeen studied. The results show that the phase composition and grain growth kinetics of thesolution-treated alloys are sensitive to Mo content. Ti–1Mo alloy mainly containeshexagonal α phase, while Ti–4Mo alloy is composed of dominated β phase and someinner acicular α′. When Mo content increases to 10 wt% or higher, the retained β phasebecomes the only phase. The β grain size is not only related to temperature and solutiontime, but also relates to Mo content. The grain growth rate of Ti–4Mo alloy during solutiontreatment is much faster than that of Ti–20Mo alloy, and the time exponent 0.42 of the Ti–4Mo alloy is higher than that of Ti–20Mo alloy due to the low dislocation density. However,the grain-growth activation energy of Ti–20Mo alloy is 272.16 kJ/mol, which is much higherthan that of Ti–4Mo alloy, due to the “solute drag effect” of Mo. The room-temperaturestrength and plasticity of Ti–Mo alloys increase with the increasing of Mo content anddecreasing of grain size.

© 2013 Elsevier Inc. All rights reserved.

Keywords:Ti–Mo alloysMicrostructureSolution treatmentGrain growthTensile properties

1. Introduction

Titanium and titanium alloys, with high specific strength,high corrosion resistance, low elastic modulus and excellentbiocompatibility, are extensively used in aerospace, petro-chemical and biomedical industries [1–5]. Because of thereleasing of toxicity of V and Al icons from the Ti–6Al–4 Valloy, the latest research activities are oriented to titaniumalloys composed of non-toxic elements [6–8]. Mo is one of theelements which have been judged to be non-toxic andnon-allergic, and it has been selected as one of the safealloying elements to develop low elastic modulus titaniumalloys with high strength for biomaterials. It is also wellknown that Mo has high β phase stabilization effect and isable to form the infinite solid solution in β-Ti [9,10]. Alloyed byelement Mo, the β-transus temperature of titanium alloy isdecreased, and the properties such as strength, corrosionresistance, high-temperature performance and formability

oad, Xi'an, Shaanxi 71001Lu), trc@c-nin.com (Y.-Q.

r Inc. All rights reserved.14

can be improved [11]. Therefore, it plays an important role intitanium alloys designed for biomedical application.

A small grain size of structural alloy is usually required tooptimize the strength and plasticity, so that a good understand-ing of grain growth is a prerequisite for controlling themicrostructure and properties of titanium alloy [12,13]. It iswell known that the driving force for grain growth is thereduction of the free energy associated with the decreasing ofthe grain boundary area [14]. Grain growth takes place bydiffusion, when the temperature is high enough and the time ofsolution treatment is long enough. The mechanism of graingrowth process includes the normal grain growth and abnormalgrain growth or secondary recrystallization. There have been atleast two different types of approaches to study grain growthbehavior by now. The first one emphasizes the research of graingrowth kinetics by measuring average grain size of the wholesamples in the micrograph, and the other researches thefeatures of microstructural morphology by using the advanced

6, P.R.China. Tel.: +86 29 86231078; fax: +86 29 86360416.Zhao).

Table 1 –Mo content of experimental Ti–Mo alloys (mass%).

Alloy Ti–1Mo Ti–2Mo Ti–4Mo Ti–15Mo Ti–20Mo

Mo contents 1.15 2.15 3.96 14.11 20.30

106 M A T E R I A L S C H A R A C T E R I Z A T I O N 8 4 ( 2 0 1 3 ) 1 0 5 – 1 1 1

measuring equipments and analysis techniques [15]. Over thepast few decades, the microstructure and mechanical propertyof the Ti–Mo alloys have been investigated by many scientistsall over theworld. Ho [16] andOliveira et al. [17,18] have studiedmicrostructures and properties of cast binary Ti–Mo alloys, andthe cold-rolled Ti–Mo alloys for dentistry have been investigat-ed by Chen et al. [19] and Zhou et al. [20,21]. Furuhara's [22]result showed that α precipitate was formed in the β matrix inthe Ti–10, 20 and 30 Mo alloys (wt%), and grew into the interiorof β grains at the aging temperature range between 923 K and773 K. Nevertheless, little attention has been paid to theinfluence of Mo contents on the grain growth process of Ti–Moalloys, so it is necessary to understand the relationship betweenthe grain growth and the mechanical property.

In the present study, the microstructure and kinetics ofgrain growth in Ti–Mo binary alloys after solution treatmentwith different Mo contents are analyzed, and the influence ofthe grain size on the mechanical properties have been alsoinvestigated. The aim of this work is to understand themechanism of the beta grain growth behavior with increasingthe Mo contents, and to provide experimental evidence for therole of alloying element Mo in titanium alloy design.

2. Materials and Experimental Procedure

TheMo content of Ti–Moalloys selected for this study include 1,2, 4, 15 and 20 wt% and the kinetics of grain growthwas studiedin β phase of the Ti–4Mo and Ti–20Mo alloys. All the materialswere prepared from raw titanium (99.9% pure) and molybde-num (99.9% pure) using a commercial arc-melting vacuumpressure casting system. The melting chamber was firstlyevacuated and then purged with argon. An argon pressure of1.5 kgf/cm2 was maintained during melting process. Appropri-ate amounts of each kind of the metal were melted in aU-shaped copper hearth with a tungsten electrode. All thealloys were re-melted three times at 1273 K for 10.8 ks toimprove the chemical homogeneity and then given around 60%reduction by hot-forging into quadrate blocks of 70 mmwide × 30 mm thick × 600 mm length followed by air cooling.The actual chemical compositions were determined by energydispersive spectrometry, and the results are listed in Table 1.

Table 2 – Heat treatment temperatures and times.

Material Phase Temperatures (°C)

Ti–1Mo α 900Ti–2Mo α 900Ti–4Mo α′ 900, 950, 1000, 1050Ti–15Mo β 800Ti–20Mo β 650, 700, 750, 800,900

The β-transus temperatures of the tested Ti–4Mo and Ti–20Moalloys were identified by solution treating specimens in thetemperature range of 650 ~ 900 °C and examining the micro-structure (metallographic techniques), and their β-transustemperatures were found to be 865 and 710 °C, respectively.Cubic specimens (10 mm × 10 mm × 10 mm) were spark ma-chined from the quadrate blocks, thenwere placed in the argonatmosphere in the tubular furnace to perform different heattreatments (Table 2). The specimens after solution treatmentwere removed from the furnace according to the time sequenceand then quenched in air.

The specimens were then grounded, polished, and etched insolution composed of 5 vol.% HF, 10 vol.% HNO3, and 85vol.%H2O. X-ray diffraction for phase analysis was conductedon Rigaku diffractometer (Rigaku D-max IIIV, Rigaku Co., Tokyo,Japan) in the typical 2θ range of 20–90° and operated at 30 kV and200 mA, The phase was identified by matching each character-istic peak with JCPDS files. Moreover, the dislocation density ofthe initial states of the Ti–Mo alloys was also determined usingX-ray diffractometry according to the equation of Dunn[23]. Thedislocationdensity forTi–4MoandTi–20Moalloys before solutiontreatment was 1.05 × 1014 and 2.64 × 1014 m−2, respectively. Themicrostructures of Ti–Mo alloys were observed on the opticalmicroscopy (OM, OLYMPUS GX71) and the grain size wascalculated by the Metal-methods for estimating the averagegrain size (Planimetry, GB/T 6394-2002, CHINA). Tensile speci-menswith a gage lengthof 40 mmandacross sectionof 5 mmindiameter were fabricated by machining from these prestrainedand heat-treated specimens after removal of surface oxides.Uniaxial tensile tests were carried out at a cross-head speed of1 mm/min−1 at room temperature. The ultimate tensile strength,yield strength, elongation and the area of reduction at fracturewere accordingly determined.

3. Results and Discussion

3.1. Microstructures

The results obtained in this work show that the phasecomposition of the binary Ti–Mo alloys were sensitive to Mocontent. As shown in Fig. 1, Ti–1Mo alloy mainly containshexagonal α phase, while the solution treated (ST) Ti–4Mo alloyis composed of dominated β phase and some α′. When Mocontent increases to 10 wt% or higher (15 and 20 wt%), theretained β phase becomes the only phase. However, there arelittle peaks of α″ phase at 35 and 40° in X-ray diffraction profilesof the Ti–15Mo alloy, this is main because that the solution

Times (min) Cooling-down method

60 Air cooling6015, 30, 60, 120, 240, 3606015, 30, 60, 120, 240, 360

Fig. 1 – X-ray diffraction profiles of the Ti–Mo alloy aftersolution treatment.

Fig. 2 – Optical micrographs of the Ti–Mo alloy after solutiontreatment: (a) Ti–1Mo, (b) Ti–4Mo, (c) Ti–15Mo and (d) Ti–20Mo.

107M A T E R I A L S C H A R A C T E R I Z A T I O N 8 4 ( 2 0 1 3 ) 1 0 5 – 1 1 1

treated temperature of Ti–15Mo alloy (800 °C) approaches to itsβ-transus temperatures 795 °C and α″phaseneeds enough timeor higher temperature for phase transformation.

The microstructure of the series ST Ti–Mo alloys, as shownin Fig. 2, are nearly consistent with the XRD results. The ST Ti–1Mo exhibits a typical equiaxed α phase microstructure. WhenMo content is 4 wt%, the fine, acicular martensitic structure ofα′ phase and the retained β phase can be observed. When thealloy contains 10 wt% or more Mo (15 and 20 wt%), equiaxed βphase becomes the only phase.

Similar results were found by Zhou et al. [20,21], Ho et al.[16] and Davis et al. [24], they have analyzed the binary Ti–Moalloys after solution treatment. When Mo content was 10 wt%, a new ω phase except β was detected and the ST Ti–20Moalloy was composed of single β phase. Ho et al. [16] provedthat a minimum of 10 % Mo was required to completelystabilize the β phase at room temperature. When the Mocontent was within the range of 4 and 10 wt%, the phase andcrystal structure of the ST Ti–Mo alloys were sensitive to theMo concentration and solution temperature. Davis et al. [24]has reported that, in the cast Ti–Mo system, the martensiticstructure changed from hexagonal (α′) to orthorhombic (α″) ataround 6 wt% Mo. Those studies have an exceedingly impor-tant effect for us to understand the phase transformation andthe mechanical properties of the alloy.

3.2. β Grain Growth Kinetics

In this section, the kinetics of β grain growth in binary Ti–4Moand Ti–20Mo alloys was analyzed and compared. All thegrains of binary Ti–4Mo and Ti–20Mo alloys was equiaxedafter solution treatment. Fig. 3 presents the microstructure ofTi–4Mo alloy solution treated at 900 °C for various times, Fig. 4presents the microstructure of Ti–20Mo alloy solution treatedat 800 °C for various times. From Figs. 3 and 4, it can beobserved that the β grains in more Mo content alloys aremuch smaller than those in less Mo content alloys around 4%.

The grain size as a function of solution time is plotted inFig. 5 for Ti–4Mo and Ti–20Mo alloys. As shown, the grain sizeincreases with increasing the solution treatment time, and the

grain growth rate of Ti–4Mo alloy ismuch faster than that of Ti–20Mo alloy in the same condition. It is also worth to note thatthe grain growth takes place at a very fast rate in the first 15–60 min during the solution treatment, and the growth rate

Fig. 3 – Opticalmicrographs of the Ti–4Moalloy solution treatedat 900 °C for various time: (a) 60 min, (b) 120 min, (c) 240 minand (d) 360 min.

Fig. 4 – Optical micrographs of the Ti–20Mo alloy solutiontreated at 800 °C for various time: (a) 60 min, (b) 120 min,(c) 240 min and (d) 360 min.

108 M A T E R I A L S C H A R A C T E R I Z A T I O N 8 4 ( 2 0 1 3 ) 1 0 5 – 1 1 1

decreases as time goes on. This phenomenon can be explainedby the increasing of the grain size, which leads to a decrease inthe grain boundary area per unit volume. This may mean that

the grain boundary interfacial energy per unit volume de-creases, therefore the driving force for grain growth lowers.

The grain size as a function of the time at 900 °C and 800 °Cis plotted logarithmically in Fig. 6 for Ti–4Mo and Ti–20Mo

Fig. 5 – Increase of the grain size in relation to solution timein β phase at 800 °C (Ti–20Mo) and 900 °C (Ti–4Mo).

Fig. 7 – Increase of the grain size in relation to solutiontemperature in β phase at 60 min.

109M A T E R I A L S C H A R A C T E R I Z A T I O N 8 4 ( 2 0 1 3 ) 1 0 5 – 1 1 1

alloys. Generally, the expression for grain growth can be givenby the following equation [14,15].

Dt−D0 ¼ Ktn ð1Þ

where Dt is the average grain size, D0 is the initial grain size,K is a temperature dependent constant, t is the time and n is thetime exponent obtained from the slope of the lnD vs. lnt plot.The grain growth time exponent n for Ti–4Mo (900 °C) and Ti–20Mo (800 °C) alloys is 0.42 and 0.26, respectively. It can beconcluded that the time exponent of the Ti–4Mo alloy is higherthan that of Ti–20Mo. The difference in growth time exponent ismainly caused by the different solution treatment temperatureand chemical composition [25]. Ti–20Mo alloy presents highalloying element content and very high dislocation densitywhich causes by high Mo content during solution treatmentprocess, and Ti–4Mo alloy possesses high purity and very lowdislocation density. For these reasons, the time exponent of Ti–20Mo alloy is lower than that of Ti–4Mo alloy. In general, thegrowth exponent in the kinetics of normal growth is below 0.5due to the role played by different grain growth parameterssuch as impurity-drag, free surface effect, texture, dislocation

Fig. 6 – Linear relationship between lnΔD and ln t for thegrain growth of Ti–4Mo and Ti–20Mo alloys.

substructure and heterogeneities [24 ~ 26]. It has been reportedthat the growth exponent of α-Ti increases with the increasingof solution temperature from0.30 at 700 °C to 0.45 at 800 °C, andthis can be explained by the crystal structure changing from α(hexagonal) to β (body centered cubic) [21].

The grain size as a function of solution temperature isplotted in Fig. 7 for Ti–4Mo and Ti–20Mo alloys. It is found thatthe grain size increases with the increasing of the solutiontreatment temperature, and the grain growth rate of Ti–4Moalloy is also much faster than that of Ti–20Mo alloy with thesame solution time. As expected, the graph showed very rapidgrain growth up to a heat treatment temperature of 50 °C abovethe β-transus. It is well known that the atomic diffusion acrossthe grain boundary is a simple thermal-activated process, theeffect of temperature on grain growth is actually the effect oftemperature on atomic diffusion crossing boundary. The graingrowth rate increases with the temperature, because theaverage migration rate of grain boundary is proportional toexp (-Q/RT). Generally, the grain growth rate under isothermalcondition can be expressed as [14,15]:

dD=dt ¼ K1D−1e−Q=RT ð2Þ

whereK1 is a constant,Q is the activation energyof grain growthobtained from the intercept of the In [(Dt

2-D02)/t] vs InT−1 plot, R is

the gas constant and T is the absolute temperature. As a naturallogarithm transformation of Eq. (2) is commonly written as;

In Dt2−D0

2� �=t

� � ¼ InK2−Q=RT ð3Þ

whereDt is the average grain size,D0 is the initial grain size and tis the soaking time. It can be seen thatQ obtained from the slope(S) of the In [(Dt

2-D02)/t] vs InT−1 plot, i.e. S = –Q/R.

The activation energy of the Ti–Mo alloys for grain growthare listed in Table 3 by fitting the experimental data in Fig. 8.

Table 3 – Activation energy of Ti–Mo alloys for graingrowth.

Alloy Ti–2Mo

Ti–4Mo

Ti–15Mo

Ti–20Mo

Activation energy (kJ/mol) 65.30 83.30 220.52 272.16

Fig. 8 – Plots of In [(Dt2-D0

2)/t] as a function of 1/T for solutiontime at 60 min of Ti–Mo alloys.

110 M A T E R I A L S C H A R A C T E R I Z A T I O N 8 4 ( 2 0 1 3 ) 1 0 5 – 1 1 1

The activation energy increaseswith increasingMo content. Theactivation energy for grain growth of Ti–4Mo alloy is 83.30 kJ/mol, which is much lower than 272.16 kJ/mol for Ti–20Mo alloy.Many researches have been done to investigate the activationenergy for grain growth of titanium alloys. The differences inactivation energy for grain growth is found to be ascribed totemperature or solute additions mainly [26,27]. The increase ofthe temperature and the reduction of the solute addition pro-mote the diffusion of atoms. Because of high solute content ofTi–20Mo alloy, the diffusion of Ti–20Mo alloy across grainboundary is more difficult than that of Ti–4Mo alloy. Therefore,Ti–20Mo alloy needs higher activation energy for grain growth. Anumber of studies [14,26,27] have been conducted to study theeffect of solution time on activation energy of titanium alloy andthe activation energy for grain growth of titanium alloy in-creased with extending the solution time. This increasing of theactivation energy can be explained by “solute drag effect”. At theearly stage of solution, the migration velocity of grain boundaryis faster than the diffusion of intracrystalline solute atoms, andthe solute atoms do not aggregate in the boundaries. As a result,the grain boundaries are free from the “solute drag effect” andhave lower activation energy. The driving force of grain growthdecreases with the solution time extending, and then themigration velocity of grain boundary decreased. So the amountof solute atom which aggregated at boundaries increased withdecreasing the migration velocity of grain boundary [28].Therefore, enrichment of solute atoms in the boundaries

Table 4 –Mechanical properties of Ti–Mo alloys.

Alloy Solutioncondition

Averageβ-grain size (mm)

Yielstren(MP

Ti–4Mo 900 °C/1 h AC 0.0868 628900 °C/2 h AC 0.1405 620900 °C/4 h AC 0.1808 617900 °C/6 h AC 0.2163 614

Ti–20Mo 800 °C/1 h AC 0.0351 871800 °C/2 h AC 0.0525 866800 °C/4 h AC 0.0711 860800 °C//6 h AC 0.0845 848

induces the drag effect, and the activation energy for graingrowth increases with extending the solution time.

3.3. Influence of Grain Size on the Mechanical Properties

Table 4 shows the variation of mechanical properties of thealloys with the average β-grain size. The room-temperaturestrength and plasticity of Ti–Mo alloys increase with theincreasing of Mo content and decreasing of grain size. It can beobviously seen that themechanical property of Ti–20Mo alloy isbetter than that of Ti–4Mo alloy, the enhancement in mechan-ical property ismainly due to the refinement of grain size whichis caused by the addition of Mo. On the other hand, the strengthand plasticity for Ti–20Mo alloy increase obviously with in-creasing the grain size, the same trend is also found in Ti–4Moalloy. This phenomenon is mainly due to the Hall-Petchmechanism and dislocation density variation, large increasesin the strength are ascribed to the decreasing of the grain size,this is consistent with the research showing that both thetensile and yield strength of B-modified Ti–6Al–4 V alloysincreased with decreasing the β-grain size [29]. However, someother strengthening methods also exist in the present alloys,such as solution strengthening. But in Ti–4Mo alloy, the grainrefinement effect for the improvement ofmechanical propertiesis not evidence, the crystal structure/phase and solutionstrengtheningmay be themain reason. As a result, the increaseof Mo content in titanium alloy leads to grain refinement andimprovement of plasticity, both the room-temperature strengthand plasticity of Ti–Mo alloys increase with increasing Mocontent and decreasing the β-grain size.

4. Conclusions

The microstructure and the β grain growth kinetics of Ti–Mobinary alloys are studied and the following conclusions aredrawn from this work:

(1) Ti–1Mo alloy mainly contains hexagonal α phase, whileTi–4Mo alloy has some acicular α′ grains precipitated inβ grains. When Mo content increases to 10 wt% orhigher, the retained β phase becomes the only phase.

(2) The grain growth kinetics of the alloys studied follows anormal growth. The grain size of Ti–Mo alloys increaseswith the increasing of solution treatment time, and thetime exponent of Ti–4Mo alloy (0.42) is higher than that ofTi–20Mo alloy (0.26) due to the low dislocation density.

dgtha)

Ultimate tensilestrength (MPa)

Elongation(%)

Areareduction

(%)

799 18.5 58787 17.0 55784 16.5 49786 15.5 52898 21.5 74893 20.0 76890 18.5 73877 19.5 73

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(3) The grain growth rate of Ti–4Mo alloy with temperatureis much faster than that of Ti–20Mo alloy in the samesolution-treated time. Because of being free from the“solute drag effect”, the activation energy for graingrowth of Ti–4Mo alloy (83.30 kJ/mol) is also lower thanthat of Ti–20Mo alloy (272.16 kJ/mol).

(4) The room-temperature strength and plasticity of Ti–Moalloys increase with the increasing of Mo content anddecreasing of grain size. It coincides with the Hall-Petchmechanism.

Acknowledgements

This work is performed partially under the support of theNational Basic Research Development Program of China(Grant Nos. 2007CB613805).

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