F.M.Kgoete,A.P.I.Popool,O.S.I.Fyomi,b

aDepartment of Chemical,Metallurgical&Materials Engineering,Tshwane University of Technology,P.M.B X680,Pretoria,0001,South Africa

bDepartment of Mechanical Engineering,Covenant University,P.M.B X1034,Ota,Nigeria

Keywords:Spark plasma sintering Si3N4 Ti-6Al-4V Microstructure Corrosion

ABSTRACT Ti-6Al-4V-Si3N4composites were effectively fabricated by spark plasma sintering(SPS)technique.The addition of Si3N4on Ti-6Al-4V was varied from 5%to 15%(wt fraction).The effect of Si3N4addition on the densification,microstructure,and microhardness and corrosion behaviour of Ti-6Al-4V was investigated.An increase in microhardness value was recorded from 325.46 HV0.1to 585.73 HV0.1.X-ray diffraction(XRD)analysis showed that the intensity of diffraction peaks of Si3N4phase in the composites increased.The sintered Ti-6Al-4V reinforced with Si3N4compacts revealed the non-existence of intermediate phases,such as TiSi2(titanium silicide)which was expected.SEM analysis of the spark plasma sintered composites revealedαandβphase microstructures in Ti-6Al-4V with uniform distribution of Si3N4 particulates in the matrix.The corrosion resistance property of the material was improved by the addition of Si3N4from 0.986629mm/year to 0.030547mm/year.

1.Introduction

Titanium and its alloys are extensively employed in a variety of applications such as,aerospace,automotive,chemical and biomedical industries due to the greater combination of mechanical properties with low density and good corrosion resistance[1].The alloys contain a mixture of titanium and other chemical elements.They possess two phase structures that are broadly investigated for a variety of applications.The two phases are theα(alpha)andβ(beta)which are the possible phases that are suitable for high strength structural applications[2].The addition of reinforcements into the metallic matrix improves the stiffness,hardness,and specific strength,wear,creep and fatigue properties compared to conventional engineering materials[1,3].Numerous materials scientists and engineers have made different attempts at improving bulk properties of titanium based composites by reinforcing titanium alloys with metal nitrides or ceramic particulates.Among reinforcement materials,silicon nitride(Si3N4)particles can be counted in as they are of high strength over a wide temperature range,excellent strength at elevated temperatures,excellent oxidation resistance high hardness,and outstanding wear resistance[4].

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An extensive review work on corrosion behaviour of titanium and its alloys has been investigated by researchers.Wang et al.[5]reported on the corrosion behaviour of pure titanium,Ti-0.2Pd and Ti-0.3Mo-0.8Ni alloy in fluoride-containing sulfuric acid by electrochemical methods.The results showed that titanium alloys exhibited higher corrosion resistance than the pure titanium at fluoride concentrations lower than 0.002M owing to the higher acceleration effects of Pd and Ni on the cathodic process and the inhibition effect of Mo on anodic process.

Prakash et al.[6]investigated the corrosion resistance of Ti-6Al-4V reinforced with boron carbide(B4C)composite processed by powder metallurgy route.Reinforcement particles were mixed with the T6-6Al-4V alloy for the weight fractions ranging from 0%,5%and 10%so to analyse the effect of the reinforcements on the corrosion property.Ti-6Al-4V/B4C developed composite was found to possess enhanced corrosion resistance with a significant increase in B4C.The work showed that the corrosion resistance of the composites reinforced with B4C is higher than that of un-reinforced Ti-6Al-4V alloy.

Zhu et al.[7]studied the electrochemical corrosion behaviour of Ti3SiC in 3.5%NaCl solution by potentiodynamic polarization,potentiostatic polarization method.Commercially pure titanium(CP-Ti)was used for comparative and was cooperated with corrosion mechanism.The results showed that Ti3SiC exhibited excellent corrosion resistance compared to commercially pure titanium.In view of the above descriptions,an attempt has been made in this research to improve the corrosion resistance of Ti-6Al-4V alloy reinforced with Si3N4at varying weight fractions,such that it will be more relevant and appropriate for severe corrosive environments.

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(2)Ti-6Al-4V alloy reinforced with Si3N4exhibit better corrosion resistance than the unreinforced alloy.

2.Experimental

The Ti-6Al-4V based composites were fabricated using Ti-6Al-4V powder(45-90μm particle spherical,from TLS Technik GmbH&Co.Spezialpulver KG.)and Si3N4(-90/+45μm,99.9%from Weartech(pty)LTO).Varying compositions of Silicon nitride(5,10 and 15,weight fraction,%)were added toTi-6Al-4V and mixed in a tubular mixer for 4 h to achieve homogeneity of the premixed powders.Subsequently,the admixed powders were consolidated by spark plasma sintering equipment(SPS FCT Systeme GmbH model)in a vacuum atmosphere.Prior to spark plasma sintering process,the admixed powders were transferred into a 30 mm graphite die with a 5 mm thickness and sinteredat a temperature of 1000oC at a constant heating rate of 100oC/min,applied pressure of 50 MPa and holding time of 6 min.Sandblasting was used to remove any graphite observed on the surface of the developed bulk composites.

Fig.6 shows the observed corrosion properties of spark plasma sintered(SPS)Ti-6Al-4V-Si3N4as investigated in 3.65NaCl containing 0.1M HCl environments with the aid of potentiodynamic polarization technique.Addition of reinforcements to titanium and its alloys improves not only the chemical,physical and mechanical properties;but improves the corrosion resistance of the alloy as well.Numerous titanium based metal matrixcomposites have been fabricated with various additions of hard ceramics such as TiC,TiB2,and SiC particles;where the studies revealed good permissible outcomes.A work done by Prakash et al.[6]showed an increase in bulk hardness and corrosion resistance of the novel developed composite with B4C addition.The polarization resistance of the developed compacts is shown in Table 1 and Fig.5.Observing Table 1,Ti-6Al-4V-10Si3N4(Sample 3)had 8976Ω.With reference to Figs.2 and 4,Ti-6Al-4V-10Si3N4showed improved hardness property with a higher relative density as compared to the other reinforced composites,thus high corrosion resistance of the alloy was expected with the least corrosion rate of 0.030547 mm/year.Furthermore,the developed sintered composites showed to possess better corrosion resistance properties with Si3N4addition in this order Ti-6Al-4V(Control),Ti-6Al-4V-5Si3N4(Sample 2),Ti-6Al-4V-15Si3N4(Sample 4),and Ti-6Al-4V-10Si3N4(Sample 3).It can be deducted that the addition of Si3N4on the Ti-6Al-4V significantly improves the corrosion resistance properties which affirms the work done by other researchers.

3.5.1.Corrosion behaviour

Fig.4(a)presents the distinctive microstructure of spark plasma sintered Ti-6Al-4V alloy which embraces a non-porous microstructure with two different phases[8,11].The microstructures of the Ti-6Al-4V-Si3N4binary composites are shown in Fig.4(c-d)and it is obvious that the images are different from the one observed in Fig.4(a)and they do not reveal any presence of pores.The binary composites confirm the presence of lamellar αandβstructure.All of the binary composites(5 wt%,10 wt%,15wt%)do not reveal any significance microstructural difference.The reinforcing particulate was uniformly distributed in the titanium matrix.

3.Results and discussion

3.1.As-received powder characterisation

The as-received Ti-6Al-4V and Si3N4powders were analysed using SEM-EDS.Fig.1(a and b)displays the SEM-EDS morphologies of the as-received powders.The microstructure of Ti-6Al-4V powder shown in Fig.1(a)is spherical and non-porous with some satellites attached to the bigger particles[8].Ozbilen[9]and Falodun et al.[8]showed that the presence of satellites inTi-6Al-4V indicates that the powder was produced by gas atomization.The EDS spectrum of Ti-6Al-4V on Fig.1 confirms that the titanium powder contains Ti,Al and V.Fig.1(b)shows that the silicon nitride(Si3N4)particles are uneven and the shape is hexagonal.The EDS analysis for silicon nitride on Fig.1(b)confirms the presence of Si,N and O.Oxygen is an impurity of titanium hence the high oxygen content on the EDS spectrum is attributed to the impurity content and the high affinity of titanium to oxygen containing environments.

3.2.Relative density

The microhardness of the sintered compacts with and without varying weight fraction of Si3N4is illustrated on Fig.5.It is observed that the reinforcement of Ti-6Al-4V with silicon nitride particulates had an impact on microhardness property of the Ti-6Al-4V alloy.The microhardness value of the Ti-6Al-4V was 325.46 HV whereas microhardness values of reinforced Ti-6Al-4V with 5,10 and 15%Si3N4additions ranged from 597.58 HV,668.56 HV and 585.73 HV respectively.The increasing trend in microhardness is very evident with the increase of Si3N4content.However at 15wt%Si3N4a drop in the microhardness value is observed.It can be deducted that increasing the Si3N4addition above 10wt%may result in a drop in microhardness property which will affect the mechanical properties of the materials.

3.3.XRD analysis and microstructural properties

The XRD pattern of developed Ti-6Al-4V-Si3N4sintered compacts at sintering temperature of 1000oC,pressure of 50 MPa,heating rate of 100oC/min with a holding time of 6mins are shown in Fig.3.The XRD patterns confirms the metallic materials used in the study.The presence of Titanium,aluminium and Silicon nitride spectra was seen.Silicon nitride and aluminium are characterized by low intensities of diffraction peaks while vanadium peaks could not be firmly established.This may be as a result of the small vanadium amount added to Ti-6Al-4V making it undetectable.The sintered Ti-6Al-4V reinforced with Si3N4compacts revealed the non-existence of intermediate phase,such as TiSi2)Titanium silicide which was expected.

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Emco Test Dura scan Micro-hardness tester was used to determine the hardness property of the developed samples at room temperature.Indentations were made across the sintered surface at three locations using a load of 1000gf for 10sec and the average hardness was used as a final micro-hardness of the materials.The corrosion studies were carried out to study the electrochemical behaviour of sintered Ti-6Al-4V and developed Ti-6Al-4V-xSi3N4in 3.65wt%NaCl containing 0.1M HCl solution using Autolab PGSTAT 101 potentiostat/galvanostat with a conventional three electrode cell,consisting of saturated calomel(SCE),graphite,and spark plasma sintered sample as reference,auxiliary,and working electrode respectively.The corrosion rate(CR),current density and polarization resistance were observed after the test.

3.4.Microhardness studies

Fig.2 shows the relative densities of spark plasma sintered Ti-6Al-4V alloy with and without varying Si3N4content.A material with a higher relative density possess good mechanical properties which result in improved performance.Teber et al.[10]indicated that a higher relative density is as a result of a decrease in the level of porosities,sintering temperature and time.It is for this reason that composites must possess high relative densities.It can be observed from the figure that,the relative density of Ti-6Al-4V decreased from 98.6%to 96.2%when 15%of Si3N4was added.This may be attributed to the increase in porosity when the reinforcement was increased leading to a less dense material.It can therefore be concluded from this that further addition of Si3N4from 15%could result in the presence of pores within the developed matrix.

3.5.Electrochemical studies

The microstructure of the developed sintered composites,was evaluated by scanning electron microscopy attached with energy dispersive spectroscopy(SEM-EDS).X-ray diffractometer analysis was performed to identify the phases formed on the composites.

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To discover the impact of Si3N4particulates on Ti-6Al-4V alloy,density measurements were carried out using Archimedes' principle.The principle was employed to explore the densities of the sintered compacts with deionized water as the immersion liquid and in air.The densitometer automatically calculated the density of each compact using Archimedes' principle.The density recorded was an average of five measurements obtained on each sample and the relative density was then calculated as a function of both the theoretical and measured density of the sintered composites.

Table 1 Linear polarization Tafel data.

4.Conclusion

From the above results and discussions,the following conclusions are made:

(1)Silicon nitride reinforced Ti-6Al-4V composite was successfully fabricated through spark plasma sintering technique and its microstructural evolution,mechanical and corrosion properties were evaluated.

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(3)Corrosion rate decreased with silicon nitride addition.

(4)It was observed that Si3N4addition was the major contributing factor that influenced the corrosion resistance of the composite.

(5)A significant improvement in corrosion resistance and drastic reduction in the corrosion rate was achieved as evidenced in the potentiodynamic polarization curve.

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Acknowledgements

The authors gratefully acknowledge the financial support from National Research Foundation(NRF),Pretoria,South Africa.

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