Powder Technology 264 (2014) 536–540

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Densification and microstructural studies of titanium–boron carbide(B4C) powder mixture during spark plasma sintering

V.S. Balaji, S. Kumaran ⁎Department of Metallurgical and Materials Engineering, National Institute of Technology, Tiruchirappalli 620015, Tamil Nadu, India

⁎ Corresponding author. Tel.: +91 9944434705; fax: +E-mail address: kumara_rec@yahoo.co.in (S. Kumaran

http://dx.doi.org/10.1016/j.powtec.2014.05.0500032-5910/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 22 July 2013Received in revised form 30 April 2014Accepted 31 May 2014Available online 11 June 2014

Keywords:TitaniumDensificationIn-situ compositeSpark plasma sintering

The present study investigates the densification behavior of Ti–B4C powder mixture based on the analysis oflinear shrinkage and current data obtained during spark plasma sintering. Pure titanium attains fasterdensification than Ti–B4C powder mixture. Ti–B4C powder mixture takes higher temperatures to achievemaximum densification as B4C percentage increases. The phase evolution of TiB and TiC according to the in-situ reactions was analyzed by X-ray diffraction (XRD) technique. Scanning electron microscopy (SEM) imagesprovide evidence that the massive spherical particles transform into equiaxed and needle like structures overincreasing sintering time. Energy dispersive spectroscopy (EDS) analysis reveals the presence of TiB and TiCreinforcements as needle like and equiaxed structures respectively.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Among several in-situ ceramic particles or whiskers reinforced Tita-nium Matrix Composites (TMCs), Ti/(TiB + TiC) composites have gen-erated considerable research interest over recent years. TiB and TiCparticulates are preferred reinforcements due to their excellent thermo-dynamic stability, strong interfacial bonding and similar thermal expan-sion co-efficient with titaniummatrix [1]. TiB and TiC also possess somedesirable mechanical and electrical properties such as high hardness,low density, good corrosion resistance and excellent electrical conduc-tivity [2–4]. Recent years of research have yielded results to suggestthat the morphology of TiB and TiC can significantly affect the mechan-ical properties of Ti/(TiB + TiC) composites. During tensile loading, TiBwhiskers can undertakemore load than TiC. However, TiC spherical par-ticles prevent crack nucleation and crack propagation is restrictedmoreeffectively than TiB [5]. Due to their unique microstructural evolution,the incorporation of TiB and TiC reinforcements is expected to improvethe mechanical and tribological properties of titanium and its alloys.TMCs are commonly fabricated by P/M method because of its ease offabrication and simple processing technique. Also, P/M method facili-tates near net shape formability and offers the fabricator a good controlover determining the final properties of the TMCs. However, conven-tional P/M processing methods of TMCs have their own limitations inthe form of residual micro-porosity, uneven distribution of reinforce-ments and control of matrix reinforcement interface [6]. To overcomethese limitations, advanced processing methods like in-situ processing

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techniques have been developed to fabricate TMCs. In-situ processingtechniques have several advantages over conventional processingtechniques such as good thermodynamic stability of reinforcements,uniform distribution of reinforcements, clean and improved matrix re-inforcements interface [7,8]. Spark plasma sintering has emerged as anefficient sintering technique to consolidate the powders to achieve den-sity almost equal to theoretical density [9]. This paper investigates thedensification behavior and microstructural evolution of Ti/B4C powdermixture by spark plasma sintering. Densification behavior is expectedto be a function of linear shrinkage which in turn depends on thesintering temperature, pressure, time and current intensity. However,

Fig. 1. Relative density of titanium and Ti/(TiB + TiC) composites sintered at 30 min.

Fig. 2. Densification curves of titanium and Ti/B4C powder mixture sintered at 30 min.

Fig. 3. Current curves of titanium and Ti/B4C powder mixture sintered at 30 min.

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in SPS method, sintering temperature and current intensity are signifi-cant variables that help to understand the densification behavior [10,11]. Therefore understanding the individual effects of sintering para-meters such as sintering temperature, time and current density on thedensification behavior and the final microstructure of Ti/B4C powdermixture is important.

2. Experimental work

The raw materials used for fabricating the Ti/(TiB + TiC) in-situcomposite are primarily commercially pure titanium powder (Purity:99.6%) and B4C powder (Purity of 99%). The average particle size of Tiand B4C powders was 100 μm and 50 μm, respectively. Three differentweight fractions of B4C powder were thoroughly mixed with titaniumpowder using centrifugal ball mill for 10 h. The powders were sinteredat 1400 °C by Dr. SINTER SPS equipment, SPS SYNTEX, JAPAN with aheating rate of 150 °C/min up to 600 °C followed by 100 °C/min up to1200 °C and then 50 °C/min up to 1400 °C. Sintering time was variedfrom 5 min to 30 min with constant pressure of 40 MPa. During thesintering process, the response of current and Z-axis movement weremonitored. Bulk density of the sintered samples was measured by Ar-chimedes principle. X-ray diffraction analysis was done with a scanrate of 0.02°/4 s using Rigagu ultima III, Japan with Cu-Kα radiation tostudy the phase evolution of TiB and TiC during sintering. Phase quanti-fication analysis was done from theXRDpatternswith the help of X'pertHighScore Plus. SEM studies were done to understand the in-situ phaseevolution and morphological changes with respect to sintering time.EDS analysis was also done to confirm the chemistry of the particulatesevolved during sintering.

Table 1Temperature range, densification and densification rate.

Composition Temperaturerange

Densification(%)

Densificationrate (%/min)

Ti Up to 573 °C 45 11573 °C to 850 °C 53 24850 °C to 1400 °C 2 b0.5

Ti–1.2%B4C Up to 573 °C 37 9573 °C to 900 °C 62 18900 °C to 1400 °C 1 b0.5

Ti–2.3%B4C Up to 573 °C 35 9573 °C to 950 °C 64 17950 °C to 1400 °C 1 b0.5

Ti–3.4%B4C Up to 573 °C 29 7573 °C to 1150 °C 70 151150 °C to 1400 °C 1 b0.5

3. Results and discussion

3.1. Densification behavior

Fig. 1 shows the relative density values of Ti and Ti/(TiB+ TiC) com-posites sintered at 1400 °C with a sintering time of 30 min. The relativedensity of pure titanium is found to be close to the theoretical densityi.e. 99.74 ± 0.234%. However, it was observed that there is a slight de-crease in the relative density of compositeswith increasing B4C percent-age. This is due to the presence of refractory phase B4C and its reactionwith titanium,which hinders the diffusion process leading to a decreasein the final relative density during sintering [12]. The measured relativedensity of Ti–5%(TiB + TiC), Ti–10%(TiB + TiC) and Ti–15%(TiB + TiC)composites is 99.06± 0.224%, 98.46 ± 0.223%, and 98.04 ± 0.236% re-spectively. Fig. 2 shows the densification curves of pure Ti and Ti/B4Cpowder mixture sintered at 1400 °C for 30 min. Densification of pureTi and Ti/B4C powder mixture takes place in several stages at differentdensification rates. During the initial stages of sintering, pure Ti showsa significant increase in densification than Ti/B4C powder mixture.Pure titaniumwas found to attain 45%densification at 573 °C. Spark dis-charge and particle sliding to the close-packed arrangement take placeduring the initial stages of sintering [13–16]. Spark discharge betweenthe particles removes the surface oxide because of the voltage breakdown effect followed by the initiation of neck formation of powder par-ticles [17]. It is noted that, pure titanium reaches 98% densification at atemperature of 850 °C. During this intermediate stage, densification isaccelerated and rapid densification takes place due to the grain bound-ary diffusion leading into a complete neck formation between theparticles [18]. However, after 850 °C, densification of pure titanium de-creases and it reaches almost negligible percentage. Ti–1.2%B4C and Ti–2.3%B4C powdermixtures show37% and 35%densification at 573 °C. Ti–1.2%B4C takes 900 °C to achieve 99% densification and for Ti–2.3%B4C, ittakes 950 °C to achieve 99% densification. Ti–3.4%B4C powder mixtureshows 29% densification at 573 °C and the densification slowly acceler-ates and reaches 99% densification at 1150 °C. It is observed that theinitial densification decreases with an increasing B4C percentage. Con-sequently, it takes higher temperatures 850 °C, 900 °C, 950 °C and1150 °C to achieve full densification for Ti, Ti–1.2%B4C, Ti–2.3%B4C andTi–3.4%B4C powder mixtures respectively. During the final stages of

Table 2Quantitative analysis of Ti/(TiB + TiC) composites sintered at 30 min.

Composite TiB (wt %) TiC (wt %) Ti (wt %)

Ti–5%(TiB + TiC) 3.40 2.61 BalanceTi–10%(TiB + TiC) 6.57 5.97 BalanceTi–15%(TiB + TiC) 10.18 6.54 Balance

Fig. 4.X-ray diffraction line profiles of Ti/B4C powdermixture and Ti/(TiB+TiC) composites.a) Ti–B4C powder b) Ti–5%(TiB + TiC) c) Ti–10%(TiB + TiC) d) Ti–15%(TiB + TiC).

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sintering, lattice diffusion and annihilation of closed pores take place atthe grain and grain boundaries [19]. However, no significant densifica-tion takes place after 1200 °C.

a

c

Fig. 5. Scanning electron micrographs of Ti–10%(TiB + TiC) composites at

Densification rates of Ti and Ti/B4C powder mixtures for varioustemperature ranges were determined from densification curves asshown in Table 1. The different temperature ranges were classifiedbased on the densification rate. From the densification values, it isclearly observed that pure titanium shows higher densification whencompared to Ti/B4C powder mixtures. During the initial stages, densifi-cation of Ti/B4C powder mixtures decreases with increasing B4C con-tent. In the intermediate stages, densification increases as the B4Ccontent increases. Hence, a gradual increase in the upper limit of inter-mediate temperature range was observed for Ti/B4C powder mixtures.This serves as evidence to prove that with an increasing B4C contentthe Ti/B4C powder mixtures require higher temperature to attain fulldensification. During the final stages of sintering, densification slowlycomes to an end and the densification rate becomes almost negligible.

The effect of current on the densification behavior of Ti and Ti/B4Cpowder mixture is explained in Fig. 3. It is observed that the rate of cur-rent varies with the heating rate during sintering. Pure titanium showshigher current consumption as compared to Ti/B4C powdermixture. Ti/B4C powder mixtures were found to consume lesser current as the B4Ccontent increases due to their poor electrical conductivity.

3.2. Phase and microstructural analysis

X-ray diffraction line profiles of Ti/B4C powder mixture and Ti/(TiB+ TiC) composites are shown in Fig. 4. X-ray diffraction peaks showthe presence of TiB and TiC particulates and the absence of B4C particles.The absence of B4C peaks confirms the complete reaction between Tiand B4C at 1400 °C. The intensity of TiB and TiC peaks is lesser for Ti–5%(TiB + TiC) composite. However, there is a gradual increase in thepeak intensity of the evolved particulates (TiC and TiB) with increasingB4C content.

TTiBBTiiC

b

d

different sintering time. a) 5 min b) 10 min c) 20 min and d) 30 min.

Fig. 6. Energy dispersive spectroscopy imaging. a) TiB and b) TiC.

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Phase quantification of Ti/(TiB + TiC) in-situ composites from theXRD patterns is presented in Table 2. The amount of in-situ reinforce-ments TiB and TiC increases with the increase in B4C content. It was re-ported that at lower temperatures, TiC formation takes place before theTiB formation since the atomic diffusion of carbon is faster than that forboron [20,21]. However, due to the lesser diffusive nature of boron,promising possibilities are there for the formation of TiB2 and Ti3B4

phases at the lower temperatures [22].Fig. 5 shows the SEMmicrographs of the Ti–10%(TiB+ TiC) compos-

ites at different sintering time. It is observed that there is a clear indica-tion in the formation of in-situ reinforcements in two differentmorphologies of equiaxed and needle like structures during sintering.The reinforcements are homogenously distributed in the matrix. Thereis a gradual transformation of massive spherical particles into equiaxedand needle like structures with increasing sintering time from 5 min to30 min. The amount of needles and equiaxed structures increases withthe increase in sintering time. The change in morphology of reinforce-ments is attributed to the function of sintering time and temperature.The EDS analysis (Fig. 6) shows the chemistry of the elements presentin the phases as shown in the SEM images. The results of EDS analysisreveal that the reinforcement with needle structure is TiB and whereas,the reinforcement with equiaxed structure is TiC.

4. Conclusion

• Titanium/B4C powdermixture is consolidated during spark plasmasintering and the consolidated mixture is found to possess densityclose to the theoretical density. The relative density decreaseswith the increase in reinforcement percentage due to matrix dis-continuity and in-situ reaction between titanium and B4C.

• Titanium/B4C powder mixture exhibits lesser densification atinitial stages of sintering. During the intermediate stages ofsintering, densification rate is accelerated due to the propagationof grain boundary diffusion.

• There is a gradual increase in the upper limit of intermediatetemperature which implies the requirement of higher sinteringtemperature in order to attain the complete densification withincreasing B4C percentage.

• Ti–B4C powder mixture formed in uniform distribution of TiBand TiC particulates in the Ti matrix. TiB and TiC particulates

grow in needle-like and equiaxed shapes respectively. Sinteringtemperature and time play a significant role in obtaining in-situTiB and TiC particulates. XRD and EDS studies confirm the forma-tion of in-situ particulates during spark plasma sintering.

Acknowledgement

The authors would like to thank The Director, National Institute ofTechnology—Tiruchirappalli, India for providing the needful facilitiesto conduct the experiments. The authors also thankDr. RameshChandraMallik, Assistant Professor, Department of Physics, Indian Institute ofScience, Bangalore for SEM studies.

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