目录结构

详情信息展示

参考文献

Rare Metals2017年第10期

Fabrication and characterization of Ti6A14V/TiB₂-TiC composites by powder metallurgy method

M.Anandajothi S.Ramanathan V.Ananthi P.Narayanasamy

Department of Manufacturing Engineering, Annamalai University

Physics Section,Faculty of Engineering and Technology,Annamalai University

Department of Mechanical Engineering,Kamaraj College of Engineering and Technology

收稿日期：23 November 2015

Fabrication and characterization of Ti6A14V/TiB₂-TiC composites by powder metallurgy method

M.Anandajothi S.Ramanathan V.Ananthi P.Narayanasamy

Department of Manufacturing Engineering, Annamalai University

Physics Section,Faculty of Engineering and Technology,Annamalai University

Department of Mechanical Engineering,Kamaraj College of Engineering and Technology

Abstract：

Titanium matrix(Ti6A14V) composites reinforced with TiB₂ and TiC were produced through powder metallurgy method. The effect of addition of both TiB₂ and TiC with different contents(2.5 wt% 5.0 wt% and7.5 wt%) on the density, microstructure and hardness properties of titanium matrix was investigated. The size distributions of the matrix alloy and reinforcement particles were measured by particle size analyzer. Microhardness of the sintered composites was evaluated using Vickers' s hardness tester with a normal load of 3 N and a dwell time of 10 s. Ti6A14V alloy and Ti6A14V/TiB₂-TiC composites were characterized through X-ray diffraction(XRD) and field emission scanning electron microscope(FESEM)equipped with energy-dispersive spectrometer(EDS). The addition of TiB₂ and TiC particles enriches the properties of Ti6A14V alloy. The sintered Ti6A14V/TiB₂-TiC composite features a dense and pore-free microstructure with varying TiB₂ and TiC particle distribution in the metal matrix. The results of this study show that the development of new phases plays a significant role in the properties of these composite materials.

Keyword：

Ti6A14V; TiB₂; TiC; Microstructure;

Author： M.Anandajothi,e-mail: manandajothi@gmail.com;

Received： 23 November 2015

1 Introduction

Titanium matrix composites (TMCs) are considered a candidate material for landing gear structures of the aircraft [ 1] .Ti6A14V alloys possess a unique combination of strength,density and good corrosion resistance,which make them very attractive for many structural applications.So they are adopted in structural components in aeroplanes and sports materials [ 2, 3] .

Ti-TiB₂ powder mixture has been used as precursor to obtain a dispersion of TiB needles in the Ti alloy matrix by means of an exothermic reaction between TiB₂ and Ti [ 4] .TiC and TiB₂ ceramic reinforcements show not only high hardness but also good chemical stability at high temperature.They also show the improved strength and melting point for long exposures at high sintering or hot-pressing temperatures to achieve a full density of its composites [ 5, 6] .All these advantages make it possible for the preparation of composites with high strength and extreme hardness [ 7] .The presence of TiB₂ prevents the development of B₄C grains,which decreases the sintering temperature,increases the mechanical properties and changes the microstructure of the resulting composites [ 8] .Conversely,the composites with TiB₂ and TiC having the highest fracture toughness are attributed to the reduction of thermal stress.In order to achieve more cluttered/cubic grains,the recommended ratio of TiB₂/TiC was 1:1 [ 9] .Titanium-based composite coatings reinforced by in situ manufactured TiB and TiC particles among titanium and B₄C were successfully fabricated on Ti6A14V by laser cladding [ 10] .

To the best of our knowledge,many articles focused on the synthesis of Ti alloy with TiB₂-TiC composites using different routes such as in situ process,stir-casting,squeeze casting,spray co-deposition and semi-solid powder densification.However,few reports were found on the fabrication of Ti6A14V/TiB₂-TiC composites by powder metallurgy technique.Production of metal matrix composites (MMC)by powder metallurgy route offers many advantages compared with stir-casting,ingot metallurgy and squeeze casting,since its low manufacturing temperature,which avoids strong interfacial reactions,minimizes the unwanted reactions among the matrix and the reinforcement.Uniform distribution of reinforcement in the matrix can be attained when fabricated through powder metallurgy route.

This homogeneity improves not only the structural properties,but also the mechanical strength.So,a scientific study is needed for obtaining enough knowledge in Ti6A14V/TiB₂-TiC composites fabricated through powder metallurgy technique.Hence,the objective of this study is to investigate the influence of TiB₂ and TiC addition on titanium alloy fabricated through powder metallurgy technique.

2 Experimental

Ti6A14V alloy reinforced with different TiB₂ and TiC contents (2.5 wt%,5.0 wt%and 7.5 wt%) was fabricated by powder metallurgy process route.Ti6A14V was used as the matrix material with mean particle size of 35μm.TiB₂and TiC powders were used as the reinforcements having a mean particle size of 15 Um with 99.1%purity and 25μm with 99.7%purity,respectively.Table 1 provides details about TiB₂ and TiC reinforcements.In the present study,three types of composites were prepared (Table 2).Particle size distribution is a property exclusive to powders and a significant physical property for defining behavior and nature of the powder.For this purpose,particle size distribution has to be measured during the use of powder.Particle sizes of the matrix alloy and reinforcement powders were analyzed by particle size analyzer (CILAS 920).

下载原图

Table 1 Details of reinforcements

2.1 Milling

Mixing of powder was performed in a high-energy ball mill(Fritsch pulverisette 6) at 50 r·min^-1 for 4 h.Stainless steel balls were used to mix the powder.Ball to powder weight ratio was 20:1 [ 11] .Toluene was used to prevent oxidation to obtain a homogenous mixture.Both TiB₂ and TiC reinforcements with different contents (2.5 wt%,5.0 wt%and 7.5 wt%) and Ti6A14V matrix alloy powder were mixed in the high-energy ball mill.

2.2 Compaction and sintering

The mixed powders were pressed uniaxially at a pressure of 950 MPa by hydraulic pressing machine with a suitable punch and die with a diameter of 10 mm and height of30 mm [ 12] .Before each run,die wall was lubricated with zinc stearate.The green compacts were sintered at 1250℃for 2 h in a high-temperature tubular furnace with argon atmosphere followed by cooling to room temperature in the furnace itself [ 13] .A similar procedure was adopted for the preparation of all composite specimens.

2.3 Measurement of physical and mechanical properties

The theoretical and experimental densities were measured by rule of mixture and Archimedes'principle,respectively.The microhardness of the sintered specimens was determined by Vickers's hardness testing apparatus (HMV-2T,Shimadzu) with an operating load of 3 N and a dwell time of 10 s.

2.4 Characterization of composites

Phase formation was recorded by X-ray diffractometer(XRD,EQUINOX 1000—Benchtop).The samples were cleaned with acetone and dried in air before measurement.The sintered composite samples were ground,polished mechanically and etched chemically using Kroll's reagent(a mixture of 10 ml HF,5 ml HNO₃ and 85 ml H₂O).Then the microstructures of the samples were observed using field electron scanning electron microscopy (FESEM,SEM-ZEISS SIGMA),and the chemical composition of elements was observed by energy-dispersive spectroscopy(EDS,EDS-BRUKER).

下载原图

Table 2 Physical and mechanical properties of Ti6A14V alloy and Ti6A14V/TiB₂-TiC composite

Fig.1 Particle size distribution of a Ti6A14V alloy,b TiB₂ powder and c TiC powder

3 Results and discussion

3.1 Particle analyzer

Particle size distribution was expressed in terms of accumulated distribution and frequency of particle size.The size distributions of matrix alloy and reinforcement particles were represented in the form of histogram and are shown in Fig.1.The average particle size of Ti6A14V matrix alloy is observed as 37.07μm and is shown in Fig.1a.Figure 1b shows that the average particle size of TiB₂ is 15μm,and Fig.1c shows that the average particle size of TiC is 21μm.

3.2 Study of physical and mechanical properties

The theoretical density of the composite was calculated using the rule of mixture.The experimental density of the sintered composite was determined by Archimedes'principle [ 14] according to the ASTM B962-13.Porosity (P)was calculated for all the compositions by using the following equation.

where w_sat is saturated weight,W_d is dry weight,and w_sus is suspended immersed weight.

From Table 2,it is inferred that the density increases after the addition of both TiB₂ and TiC reinforcements in Ti6A14V alloy.This can be attributed to the addition of higher-density reinforcements.It can be also understood from Table 2 that the porosity of the sintered composites increases with the increase in contents of both TiB₂ and TiC reinforcements.Porosity of Ti6A14V/7.5TiB₂-7.5TiC shows the high porosity because of agglomeration.Pore nucleation at the matrix and reinforcement interface is also observed due to the addition of hard reinforcement in the matrix.The porosity is the main factor that affects the microhardness of the composites (Fig.2).Pores not only reduce the hardness or loading cross-sectional area,but also lead to stress concentration [ 15] .So they can reduce the strength of composites.

Fig.2 Microhardness of prepared samples

Microhardness test was conducted on the polished specimens of Ti6A14V alloy [ 16] and Ti6A14V/TiB₂-TiC composite,and the result is shown in Fig.2.Test was carried out at three different positions to avoid the potential result of indenter resting on the hard reinforcement particle.The averages of the six readings were reported.Figure 2 shows that hardness of composite increases with the content of reinforcements increasing up to 5.0 wt%inpidually,which could be attributed to the existence of TiB₂and TiC hard particles acting as obstacles to the motion of dislocation of the composite density [ 17] .The high hardness is observed in Ti6A14V/5.0TiB₂-5.0TiC composite.

Fig.3 XRD patterns of sintered Ti6A14V matrix alloy and Ti6A14V/TiB₂-TiC composite

The further increase of TiB₂ and TiC contents in Ti6A14V matrix alloy results in a decrease in hardness.The result also shows that a large scattering of hardness is observed at each indention.The Vickers hardness is known to be sensible to residual and internal stress [ 18] .Variations in residual internal stress of composite with different contents of reinforcement materials result from the thermal expansion mismatch between the matrix (Ti6Al4V) and reinforcements (TiB₂-TiC).

3.3 Phase identification analysis

Figure 3 shows XRD patterns of sintered alloy and composite specimens with different contents of TiB₂ and TiC.Theα-Ti phase is observed as a major phase in matrix alloy and composites for all the spectra nearly at 2θ=40.19°(101).The same phase is also observed at 2θ=53.093°(102) reflection in Ti6A14V composite only.The observed values well coincide with the standard JCPDS card No.89-5009.Theβ-Ti phase is observed at 2θ=38.759°(110)in Ti6A14V composite specimen (JCPDS card No.89-4913).

Theα-Ti phase at 2θ=40.l9°(101) is coexistent with a new peak observed at 20=27.514°(001) and 20=36.13°(111) formed in Ti6A14V/2.5TiB₂-2.5TiC and Ti6A14V/5.0 TiB₂-5.0TiC composites,respectively.The XRD patterns of TiB₂ and TiC reinforcements are in good agreement with JCPDS card No.89-3923.Similarly,the trends are observed in Ti6Al4V/7.5TiB₂-7.5TiC composite samples.The high intensity of TiB₂-TiC is formed with the new addition peak TiB₂ at 20=44.319°(101) and TiC at2θ=41.735°(200).The diffraction intensity of TiB₂ and TiC peaks increases significantly as reinforcement content increases [ 19] .Therefore,the presence of Ti6A14V,TiB₂and TiC is confirmed by XRD analysis.

3.4 Microstructure analysis

Figure 4a-d shows the microstructures of the sintered Ti6A14V matrix alloy and Ti6A14V/TiB₂-TiC composites.In Fig.4a,it can be seen that Ti6A14V alloy has distinctive Widmanstatten structure.In the Widmanstatten structure,theα-phase begins to appear in the form of plates as the specimen was slowly cooled near theβ-transus temperature(～850℃) [ 20] .Someβ-phase (thin dark region) is observed betweenα-phases.The majority ofα-platelets are observed in the monolithic alloy,which is in good agreement with Ref. [ 21] in which approximately 95 vol%α-phase and only 5 vol%β-phase are observed for furnacecooled Ti6A14V specimen.

Figure 4b-d shows the microstructures of sintered Ti6A14V/2.5TiB₂-2.5TiC,Ti6A14V/5.0TiB₂-5.0TiC,Ti6A14V/7.5TiB₂-7.5TiC composites.The matrix microstructure along with distributed TiB₂ and TiC particles in the composite is shown in Fig.4b,c.A micrograph of Ti6A14V/TiB₂-TiC-reinforced composite (Fig.4b,c)clearly depicts good interface bonding between Ti6A14V matrix and TiB₂ and TiC.TiB₂ and TiC are distributed uniformly in the entire Ti6A14V matrix.Among them,Ti6A14V/5.0 TiB₂-5.0TiC composite is relatively more homogenous along with the matrix compared with other composites,as shown in Fig.4c.

The reinforcements add a strong interface and good wettability in Ti6A14V matrix and show no macroporosity particularly at the grain boundaries of interface [ 22] .There is no perceptibility in bonded or delamination interface detected between the matrix and reinforcement particles.In addition,the absence of pores and cracks can also be experienced in the micrographs.It clearly shows that5.0 wt%addition of both TiC and TiB₂ in Ti6A14V matrix alloy is a good choice [ 23] .The microstructure of the composite with 7.5 wt%addition of both TiB₂ and TiC is shown in Fig.4d.It leads to the occurrence of non-homogeneous distribution,agglomeration of particles and certain degree of porosity.A weak interfacial bond is identified between matrix alloy and reinforcements when more than 5.0 wt%of both TiB₂ and TiC is added to the matrix,and leads to an embrittlement of composite due to cluster formation.The outcome in composites is associated with a decrease in microhardness.

However,clusters appearing at the high content of both TiB₂ and TiC are due to not occupying an ambiguous surface of Ti6A14V.The interfaces distributed among the particles cannot be eliminated completely,and a few pores are entrapped in the composite,as shown in Fig.4d.The residual pores result in stress concentration and weaken the grain boundaries [ 24] .

Fig.4 FESEM images of sintered composites:a Ti6A14V alloy,b Ti6A14V/2.5TiB₂-2.5TiC,c Ti6A14V/5.0TiB₂-5.0TiC,and d Ti6A14V/7.5TiB₂-7.5TiC

Fig.5 EDS results of sintered composites:a Ti6A14V alloy,b Ti6A14V/2.5TiB₂-2.5TiC,c Ti6A14V/5.0TiB₂-5.0TiC,and d Ti6A14V/7.5TiB₂-7.5TiC

3.5 EDS analysis

An EDS analysis was performed to identify the chemical composition of elements present.Figure 5a shows the EDS spectrum of Ti6A14V matrix alloy with Ti,Al and V peaks.Figure 5b-d shows the EDS spectra of Ti6A14V/TiB₂-TiC composites with peaks of Ti,Al,and V,B and C.B and C peaks are observed in the composites alone but not in alloy.The intensity values of B and C increase with the increase of TiB₂ and TiC reinforcement contents.This confirms the presence of TiB₂ and TiC reinforcements in Ti6A14V/TiB₂-TiC composites.

4 Conclusion

The density of the composites increases while increasing the TiB₂ and TiC contents in Ti6A14V alloy.Ti6A14V/5.0TiB₂-5.0TiC composites show significant increase in microhardness when compared to alloy as well as other composites.The TiB₂ and TiC particles are noted to be uniformly distributed within the Ti6A14V matrix alloy,and also they improve the microstructure of the composite.However,when the reinforcement contents of TiB₂ and TiC increase to 7.5 wt%,it weakens the interface by the pre-existing force.The brittle-interfaced reaction product phases are formed,resulting in non-beneficial effect on the strength above or below 5.0 wt%TiB₂ and TiC.Major peaks of titanium and minor peaks of others such as Al,V,B and C are found in Ti6A14V/TiB₂-TiC composites in sintered condition.