Materials Science & Engineering A 565 (2013) 326–332

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A new rapid route to in-situ synthesize TiB–Ti system functionally gradedmaterials using spark plasma sintering method

Zhaohui Zhang a,b,n, Xiangbo Shen a, Chao Zhang a, Sai Wei a, Shukui Lee a,b, Fuchi Wang a,b

a School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR Chinab National Key Laboratory of Science and Technology on Materials Under Shock and Impact, Beijing 100081, PR China

a r t i c l e i n f o

Article history:

Received 10 October 2012

Received in revised form

28 November 2012

Accepted 24 December 2012Available online 31 December 2012

Keywords:

Functionally graded material

Spark plasma sintering

Microstructure

Interface

Mechanical properties

93/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.msea.2012.12.060

esponding author at: School of Materials

Institute of Technology, Beijing 100081, PR Ch

6 10 6891 3951.

ail address: zhang@bit.edu.cn (Z. Zhang).

a b s t r a c t

A four-layer TiB–Ti system functionally graded material was rapidly synthesized by spark plasma

sintering method using a graphite die with an area-changing cross section. The composition and

microstructure of layers in the functionally graded material were characterized by X-ray diffraction and

scanning electron microscopy. The mechanical properties of each layer were evaluated using the three-

point bending method and the single-edge notched beam method. The results indicate that a stable

graded temperature field can be achieved during the SPS process. The in-situ synthesized TiB–Ti system

functionally graded material exhibits fine and dense microstructure with continuous and crack free

interfaces. The micro-hardness at the layer interfaces is relatively high, suggesting that the interfaces

between each layer in the material are well-bonded. The bending strength and the fracture toughness

of each layer in the TiB–Ti system functionally graded material synthesized using the pre-designed die

are much higher than those of the corresponding layer in the material synthesized using the common

cylindrical die.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

A functionally graded material (FGM) is an inhomogeneouscomposite material consisting of two or more phases in which thevolume fractions of the constituents changes continuously or step-wise as a function of position. The mechanical properties of the FGMsalso vary gradually with the variation of structure and composition[1–5]. FGMs have attracted considerable attention due to their uniqueperformance, sophisticated designation and great potential in engi-neering applications. Various production methods have been devel-oped to fabricate FGMs such as casting [6–9], deposition [10,11], lasercladding [12], friction stir processing [13], electromagnetic separation[14], combustion synthesis [15] and powder metallurgy [16–18].Among these methods, powder metallurgy is the most commonlyemployed techniques due to its good control capability on composi-tion and microstructure of the FGMs and the reliable shape formingcapability. However, the critical problem in the processing of FGMs iscracking and/or camber in the specimens, due to the residual stressescaused by mismatches in thermal expansion coefficients (CTEs)between the matrix and the reinforcement in successive layers.Therefore, the CTEs of the components in FGMs should be similar.Because titanium boride (TiB) and titanium (Ti) have different

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mechanical properties and similar CTEs [19–23], FGMs based on TiBand Ti systems should be attractive and have superior performances.In addition, a FGM with composition changing from pure Ti on oneside to TiB–Ti composite containing high volume fraction of TiB onthe other side has many potential application areas specifically indefense. TiB–Ti systems FGMs can be used in military applicationssuch as armor, armaments, and military vehicle structures due totheir excellent ballistic properties [15]. The interest in the Ti–TiBsystem FGMs is also because of the convenience of fabrication and thefact that TiB forms as whiskers which is effective in stiffening andstrengthening Ti matrix [24–26]. In recent years, TiB–Ti systemsFGMs have been fabricated by hot pressing and combustion synthesismethod, but crack is the main problem in the FGMs synthesized bythese processes [15].

Spark plasma sintering (SPS) is a recently developed consoli-dation method that allows the compacted powders to be sinteredat a low temperature with a short heating, holding and coolingtime [27]. These characteristics effectively prohibit the graingrowth of materials during the sintering process, and then theSPS process has been developed for fabricating ultrafine-grainedmaterials and nanocrystalline materials which are difficult insintering by common methods [28]. Moreover, in SPS process,joule heat generated by a direct current is the main heatingsource, based on which a die with an area-changing cross sectioncan be designed. Accordingly, an axial graded temperature fieldmay be produced in the die during the SPS process due to thedifferent heat-generation rates along the axis of the die [29]. Thus,

Z. Zhang et al. / Materials Science & Engineering A 565 (2013) 326–332 327

the SPS method is suitable to synthesize FGMs. Despite the factthat there is a large quantity of research on TiB–Ti compositesavailable in the literature, to the best of our knowledge thefabrication of TiB–Ti systems FGMs by SPS process using a diewith an area-changing cross section has not been reported yet.Hence, the aim of this study is to synthesize TiB–Ti systems FGMsby SPS method using a pre-designed graphite die, and to inves-tigate the microstructure and mechanical properties of the FGMs.

2. Experimental procedures

2.1. Starting powders

Commercially available Ti powders (99.8% pure, MengtaiPowder Business Department, Beijing, China) and TiB2 powders(99.6% pure, Ningxia Machinery Research Institute, Ningxia,China) were used as raw materials. The average particle size ofTi and TiB2 powders is about 30 mm and 4.5 mm, respectively. TiBwas in-situ synthesized by chemical reaction of Ti with TiB2

during the SPS process (TiþTiB2-2TiB). Powder mixtures withtarget volume fractions of 20%, 40%, 60%, and 80% TiB wereblended by planetary ball milling for 30 min at milling rotationspeed of 300 rpm, using ethanol and agate balls as a millingmedium. The weight ratio of balls to powder was fixed to 5:1.The resultant slurry was dried in vacuum evaporator.

2.2. Sintering process

DR.SINTER type SPS-3.20 equipment (Sojitz Machinery Cor-poration, Tokyo, Japan) was used in this procedure. Fig. 1 sche-matically shows the pre-designed die used in the investigation.The die should produce a temperature gradient, i.e., the

Fig. 1. Schematic diagram of the

temperature is lower at the bottom of the die than that at thetop, due to the graded current density distribution along the axialdirection of the die. The mixtures were stacked layer by layer inthe graphite die. The specimen was sintered in a 0.5 Pa vacuumchamber. Because the temperatures at different points along theaxial direction on the exterior surface of the die are different, thetemperature at Point C on the exterior surface of the die wasmeasured as the average sintering temperature by an infraredthermometer for convenience purpose. The specimen was sin-tered at 1250 1C with a heating rate of 100 1C/min and a holdingtime of 5 min. The applied compressive pressure level was50 MPa. Uniaxial pressure was gradually applied up to 50 MPawithin the first minute and maintained at 50 MPa during theremaining sintering process. The sintered FGM specimens consistof four layers with a diameter of 30 mm and a height of 16 mm.

2.3. Characterization tests

Archimedes method was used to measure the bulk density.Phase identification was performed using X-ray diffraction ana-lysis (X’Pert PRO MPD, PANalytical B.V., Netherlands). The speci-mens were polished and then etched with a solution of 5 ml HF,10 ml HNO3 and 85 ml H2O. Microstructure of the sintered FGMswas investigated using scanning electron microscopy (HitachiS-4800, Hitachi, Tokyo, Japan). The bending strength of the eachlayer of the FGMs was evaluated using the three-point bendingmethod on an Instron instrument using 2�4�22 mm specimens.Fracture toughness was studied by the single-edge notched beam(SENB) method on notched 3�4�15 mm specimens using thesame Instron equipment. At least four specimens were tested foreach sintering temperature.

pre-designed graphite die.

Fig. 2. X-ray diffraction patterns of the each layer in TiB–Ti system FGMs, the inset

shows the layer structure of the FGMs.

Fig. 3. Scanning electron microscopy observations of the FGMs synthesized by the SP

using a pre-designed die; and (b) image of Sample 2 sintered using common cylindric

Z. Zhang et al. / Materials Science & Engineering A 565 (2013) 326–332328

3. Results and discussion

3.1. X-ray diffraction analysis

The samples were cut through the interface boundaries forphase identification. Typical X-ray diffraction patterns of individuallayers of TiB–Ti system FGMs are shown in Fig. 2. The inset showsthe layer structure of the FGMs. Ti and TiB diffraction peaks arepresented in all of the diffraction patterns, and no diffraction peaksof TiB2 are observed for each layer in FGMs, which indicate that thechemical reactions between Ti and TiB2 are completed, and eachlayer in FGMs consists of Ti and TiB phases. In addition, theintensity of TiB peaks increases from Layer 1 to Layer 4, indicatingthat the volume fraction of TiB increase. The relative volumefractions of the composition phases can be computed from theintegrated intensities of the selected peaks in the X-ray diffractionpattern by the direct comparison method [30]. The calculationresults reveal that the volume fractions of TiB in layers 1–4 inFGMs sintered at 1250 1C were 18.3%, 37.2%, 56.6%, and 78.1%,respectively, close to the target ones, also indicating that thechemical reaction in each layer is fully completed during the SPSprocess.

3.2. Microstructure characteristics

An overview with scanning electron microscopy of the crosssection of the TiB–Ti system FGMs synthesized using the graphitedie with different structures is depicted in Fig. 3. The inset showsa magnified image of the squared area at layer interface. Typicalmulti-layered structures are presented in the micrograph, and thethickness of each layer in FGMs is about 4 mm. A continuouschange in composition was examined from the viewpoint of themicrostructure at the interfaces. The FGM prepared using the pre-designed die exhibits a uniform and dense layered structure. Nosignificant residual pores and interlayer cracks were detected inthe cross section, and the adjacent layers of the FGM were firmlybonded to each other, as shown in Fig. 3(a). However, a macro-crack was observed between Layer 3 and Layer 4 in the crosssection of the FGM prepared using the conventional cylindricaldie. A lot of pores were detected in the microstructure near theinterface between Layer 2 and Layer 3, as shown in Fig. 3(b). Theimages indicate that a well-bonded composites structure can be

S process using the dies with different structures. (a) Image of Sample 1 sintered

al die.

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obtained in the TiB–Ti system FGM synthesized by SPS processusing the pre-designed die, while the FGM synthesized using thecylindrical die has many defects, especially at the interfaces. InFGMs, interfaces always initiate failure. Therefore, the interface isan important factor controlling the properties of FGMs. The TiB–Tisystem FGM prepared using the pre-designed die should havegood mechanical properties due to the well-bonded interfaces.

Because the volume fraction of TiB increases from 18.3% in thefirst layer to 78.1% in the fourth layer, the sintering temperaturesshould be theoretically decreased from top to the bottom of thespecimen. Fig. 4 presents the temperatures at Points A, B, C, D, and Eon the surface of the die shown in Fig. 1. The temperatures at the five

Fig. 4. Temperatures at Points A, B, C, D, and E obtained at different sintering times.

Fig. 5. Microstructures of the etched surface of the each layer in TiB–Ti system F

points initially increased rapidly. Subsequently, the increasing rategradually slowed until the final sintering temperature. The heatingstage was terminated as the final sintering temperature was reached.During the holding stage, a dynamic equilibrium between the heatgeneration and loss was established. Then, the temperatures at thefive points remained nearly constant during the whole holding stage.The heating rates at the five points are different due to the differentcross section area along the axis of the die, leading a temperaturedifference between these points. The measuring results reveal thatthe final sintering temperatures at Points A, B, C, D, and E are 1335 1C,1291 1C, 1252 1C, 1189 1C, and 1116 1C, respectively, which indicatethat a graded temperature distribution can be achieved in the die

Fig. 6. Scanning electron microscopy image of the agglomerated TiB.

GMs. (a)–(d) indicate the microstructure of Layer 1 to Layer 4, respectively.

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with an area-changing cross section compared with a uniformtemperature distribution in the cylindrical die.

Fig. 5 shows the microstructure of the etched surface of eachlayer in the TiB–Ti system FGM. Few pores and cracks wereobserved in the microstructure of each layer, indicating that theFGM has a high density. The measuring result reveals that therelative density of the FGM is approximately 99.3% (the theore-tical density of the TiB–Ti system FGM is defined as 4.5 g/cm3 dueto the similar density of about 4.5 g/cm3 for TiB whiskers and Timatrix), indicating that each layer in FGM synthesized using thepre-designed die is well sintered with a dense microstructure. TiBhas a B27 crystal structure characterized by zig-zag chains ofboron atoms parallel to the [010] direction, with each B atomlying at the center of a trigonal prism of six Ti atoms. Then TiBshould exhibit much faster growth along [010] direction anddevelop a needle-shaped or rod-like morphology [30,31]. Fig. 5indicates that the in-situ synthesized TiB has three morphologies,i.e. needle-like, rod-like, and agglomerate shape in the structureof each layer, and the agglomerated TiB consist of many inter-connected rod-like whiskers, as shown in Fig. 6. From Layer 1 toLayer 4, the volume fraction of the agglomerated TiB whiskersincreases and the size of rod shaped TiB decreases due tocontinuous increase in the TiB volume content. This phenomenoncan be explained as follows: with increasing content of TiB2

particles in initial mixtures, the nucleating rate of TiB increasesand the growth of TiB are restricted, which promotes the forma-tion of agglomerated TiB.

Fig. 7 presents the fracture surfaces of each layer. Three types offracture modes were observed in the fracture surfaces of the FGM:cleavage of the TiB whiskers, pull-out of the TiB whiskers, and quasi-cleavage of the Ti metal. However, the volume fractions of pull-out ofthe TiB whiskers in each layer are all relatively low, indicating that

Fig. 7. Fracture surfaces of the each layer in TiB–Ti system FGMs. (a)–(

the in-situ synthesized TiB whiskers have a high bonding strengthwith Ti matrix. In addition, Fig. 7 reveals that almost no pores aredetected in the fracture surface of each layer in the FGM, which alsoindicate that the present TiB–Ti system FGM has a dense micro-structure. Fig. 8 presents the fracture surfaces of Layer 1 and Layer4 at high magnification. Obviously, the quasi-cleavage of the Ti matrixis the main fracture modes in Layer 1, while cleavage of the TiBwhiskers becomes the dominant fracture modes in Layer 4. Based onthe ductile fracture and the brittle fracture for Ti matrix and TiBwhiskers, respectively, it is revealed that the fracture toughness ofLayer 1 is higher than that of Layer 4.

3.3. Mechanical properties

Table 1 presents the mechanical properties of each layer inTi–TiB FGMs synthesized by SPS process using the die withdifferent structures. Sample 1 was sintered using the die withan area-changing cross section, and Sample 2 was sintered usingthe common cylindrical die. Each layer in Sample 1 has a highrelative density, while the relative density of the third and fourthlayer in Sample 2 are only 91.8% and 97.6%, respectively. Corre-spondingly, the bending strength and the fracture toughness ofLayer 3 and Layer 4 in the TiB–Ti system FGM synthesized usingthe common cylindrical die are much lower than those of thethese layers in the FGM synthesized using the pre-designed die.From Layer 1 to Layer 4 in Sample 1, the micro-hardness increasecontinuously, but the bending strength and fracture toughnessdecrease gradually due to an increase in volume fraction of theTiB whiskers. In addition, the micro-hardness at the layer inter-faces is relatively high, suggesting that the interfaces betweeneach layer in the FGMs are well-bonded.

d) indicate the fracture surface of Layer 1 to Layer 4, respectively.

Fig.8. Fracture surfaces of Layer 1 and Layer 4 in TiB–Ti system FGMs observed at high magnification. (a) Fracture surface of Layer 1 and (b) fracture surface of Layer 4.

Table 1Mechanical properties of each layer in Ti–TiB FGMs synthesized by SPS process using the die with different structures (Sample 1 was sintered using the die with an

area-changing cross section, and Sample 2 was sintered using the common cylindrical die).

Layer code Layer 1 Interface Layer 2 Interface Layer 3 Interface Layer 4

Volume fraction of TiB (%) Sample 1 18.2 27.5 37.2 48.2 56.6 71.3 78.1

Sample 2 18.1 27.2 36.9 47.6 52.5 66.8 72.1

Micro-hardness (GPa) Sample 1 5.9 6.4 7.8 9.3 12.9 13.9 16.8

Sample 2 5.8 6.2 7.7 8.9 11.3 – 11.6

Relative density (%) Sample 1 99.6 – 99.2 – 99.5 – 99.1

Sample 2 99.3 – 99.1 – 97.6 – 91.8

Bending strength (MPa) Sample 1 1221 – 952 – 786 – 655

Sample 2 1195 – 936 – 692 – 503

Fracture toughness (MPa m1/2) Sample 1 13.55 – 11.86 – 9.55 – 7.37

Sample 2 13.06 – 11.55 – 8.26 – 5.11

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4. Conclusions

A four-layer TiB–Ti system FGMs were synthesized by SPSmethod using the graphite die with an area-changing crosssection. The results indicate that the pre-designed die provideda stable graded temperature field during the SPS process. Thechemical reaction between TiB2 and Ti was completed in eachlayer of the FGMs. The in-situ synthesized TiB–Ti system FGMshave fine and dense microstructures and the interfaces betweeneach layer in the FGMs are well-bonded. From top to the bottomof the FGMs, the micro-hardness decrease, but the bendingstrength and fracture toughness increase gradually due to con-tinuous decrease in volume fraction of the TiB whiskers. In theTiB–Ti system FGMs, the synthesized TiB were needle shape, rodshape and short agglomerated whiskers, and the volume fractionof agglomerated TiB whiskers increase and the size of the rodshape TiB decrease with increasing target TiB content. Thebending strength and the fracture toughness of each layer in theTiB–Ti system FGMs synthesized using the pre-designed die aremuch higher than those of the corresponding layer in the FGMssynthesized using the common cylindrical die. In conclusion, SPSis an effective method to fabricate the TiB–Ti system FGMs atrelatively low sintering temperature within a short dwelling time.

Acknowledgments

The authors wish to thank Prof. Hongnian Cai and Dr. LindaWang for their contributions to the investigation. The study was

supported by the Program for Peking Excellent Talents in theUniversity under Grant number 20121D0503200316, and theNational Defense Pre-Research Foundation of China under Grantnumber of 9140A12050209BQ0137.

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