Influence of Sintering Temperature on the Microstructureof TiB2 Sintered with Al3Ti Additive

Masashi Yoshida

Department of Mechanical Engineering, Ube National College of Technology, Ube 755-8555, Japan

Titanium diboride (TiB2) has been sintered using spark plasma sintering (SPS) with the addition of titanium tri-aluminide (Al3Ti) at thetemperature between 1273 and 1573K. By the X-ray diffraction measurements, it has been shown that TiB2­Al3Ti composites can be obtainedby the sintering at 1273K. Vickers hardness of the specimen increases as the amount of Al3Ti increases up to 30% by the sintering at 1273K,and TiB2­30mass% Al3Ti composite has the relative density of around 96% and Vickers hardness as high as 2100Hv. On the other hand, Al3Tievaporates during sintering above 1473K. TiB2 sintered with 10mass% Al3Ti at 1573K has the relative density of 94% and Vickers hardness2100Hv, while TiB2 sintered without Al3Ti has the relative density as low as 76%. This fact indicates that Al3Ti accelerates the diffusion of TiB2

causing densification of sintered TiB2 specimens in the sintering above 1473K. [doi:10.2320/matertrans.M2012113]

(Received March 23, 2012; Accepted June 6, 2012; Published July 19, 2012)

Keywords: spark plasma sintering, TiB2, Al3Ti, Vickers hardness

1. Introduction

Titanium diboride has many desirable properties such ashigh hardness (Hv = 3400), high melting point (3000K),low density, high electrical resistivity and good corrosionresistance.1) It is a promising candidate for cutting tools,wear proof parts, aircraft propulsion systems, space vehiclethermal protection and so on. The densification of mono-lithic TiB2 requires extremely high temperatures of up to2300K and long holding times due to the low diffusioncoefficient.2­7) Sintering with various additives has also beenexamined. However, to obtain highly dense TiB2 basedmaterials, high temperatures up to 2000K are needed.3­5)

Such extreme processing conditions result in exaggeratedgrain growth of the sintered materials leading to degradationof mechanical properties.5­7)

In the previous work, we have reported that the addition ofAl3Ti improves the sintering at 1273K of TiB2 powders.8) Ithas been shown that TiB2 and Al3Ti coexist stably at 1273Kand by the addition of Al3Ti up to 30mass%, the Vickershardness of the sintered materials increases up to as high as2100Hv.8) In the present study, in order to investigate thesintering behavior of TiB2 with the addition of Al3Ti, wehave sintered TiB2­Al3Ti mixed powders by using sparkplasma sintering method at the temperature between 1273and 1573K and investigated the microstructure and mechan-ical properties.

2. Experimental Method

The raw materials were commercially available TiB2

powders of 99.9% purity, Ti powders of 99.5% purity andAl powders of 99.9% purity (all powders by FuruuchiChemical Corp.). The diameter of TiB2 powder particles wasa few microns and those of Al and Ti powders were around10 microns. To obtain fine Al3Ti powders, Ti powders werehydrogenated by heating at 873K for 1 h in H2 environment.Then, powders of TiHx and Al were mixed, pulverized andheated at 973K for 1 h in Ar environment. By this procedure,fine powders of Al3Ti of several microns in diameter could beobtained.

The mass ratio of TiB2 and Al3Ti in the starting materialswere chosen as 100 : 0, 90 : 10, 80 : 20 and 70 : 30. Agraphite die, having an internal diameter of 20mm and awall thickness of 10mm was filled with the mixed powdersof 5 g weight, sealed by two graphite punches and mountedon the equipment, SPS-1050 fabricated by Sumitomo CoalMining Ltd. The mixed powders were heated to the definitetemperature between 1273 and 1573K with the rate of40K/min, kept for 10min, and then cooled to roomtemperature. The temperature was measured using an infra-red radiation thermometer IR-AHS0 fabricated by ChinoCorporation. The sintering was performed in a vacuum witha residual pressure of 50 Pa. A uniaxial pressure of 20MPawas applied during the sintering.

Disc shape samples of the radius of 20mm and thethickness of around 3­4mm were obtained. On the surface ofthe sample, TiC layer was formed by the reaction of Al3Tiwith graphite,8) which was removed by polishing. Then,weight and volume of the specimens were measured fromwhich the density was calculated. To examine the cross-section, the specimens were cut by using a diamond cutter.Scanning electron microprobe (SEM) observation wasperformed using Hitachi H-3300. In order to clarify thegrain boundaries, the specimens were etched in HNO3. X-raydiffraction spectra were measured by using Rigaku X-raydiffractometer with Cu-K¡ radiation source. Vickers hardnesswas measured by using Shimadzu HMV-2 micro hardnesstester with the load of 1.96N and pressing time of 15 s.

3. Results and Discussion

Figure 1 shows the X-ray diffraction pattern from thecross-section of the TiB2­30% Al3Ti sample sintered at 1273,1473 and 1573K, where the amount of Al3Ti is given as themass-ratio of Al3Ti powders to the total mass of startingpowders. In all the diffraction patterns shown in Fig. 1, peaksfrom hexagonal TiB2 of the AlB2 type lattice structure areobserved at 2ª = 27.6, 34.1, 44.8 and 56.9 degrees. In thediffraction pattern from the specimen sintered at 1273K,peaks from tetragonal Al3Ti were observed in addition to thepeaks from TiB2 showing that TiB2 and Al3Ti co-exist in

Materials Transactions, Vol. 53, No. 9 (2012) pp. 1648 to 1651©2012 The Japan Institute of Metals

equilibrium at 1273K. Diffraction peaks from Al3Ti becomeweak when the sample is sintered at 1473K and almostdisappear when sintered at 1573K. This fact indicates thatevaporation of Al3Ti proceeds considerably above 1473K.

Figure 2 shows the variation of the density of the TiB2

samples sintered at the temperature between 1273 and 1573Kwith and without Al3Ti additive. The density of TiB2 sinteredat 1273K without Al3Ti is as low as 2200 kg/m3, whichis nearly a half of the theoretical density of TiB2 (4470kg/m3).9) The density increases with the amount of Al3Tiwhen sintered at 1273K because Al3Ti fills the spacebetween TiB2 grains (see Fig. 4). The density of TiB2

sintered without Al3Ti increases up to 3380 kg/m3 as thesintering temperature increases to 1573K but is still smallerthan the theoretical density of TiB2. The density of TiB2

sintered at 1573K with Al3Ti is larger than that of TiB2

sintered without Al3Ti even though almost all Al3Tievaporates at 1573K as confirmed by the X-ray diffractionmeasurements (see Fig. 1). This fact indicates that Al3Tihas the effect to accelerate the diffusion of TiB2 causingdensification of sintered TiB2 specimens. The density of theTiB2­10% and 20% Al3Ti samples increases as the sinteringtemperature increases from 1273 to 1573K. On the otherhand, the density of the TiB2­30%Al3Ti sample is nearly thesame when sintered at 1273 and 1573K. This is because thedecrease of the density brought about by the evaporation ofAl3Ti and the increase of that brought by the densification ofTiB2 nearly cancel out in this sample.

The amount of Al3Ti in the sintered samples has beenestimated by the intensity of the X-ray diffraction peaks at

2ª = 39.1 (diffraction peak from Al3Ti) and 44.8 (diffractionpeak from TiB2). Figure 3 shows the dependence of therelative density of the sintered TiB2­Al3Ti samples on thesintering temperature. The relative density of TiB2 sinteredwithout Al3Ti increases from 49 to 76% as the sinteringtemperature increases from 1273 to 1573K. On the otherhand, the relative density of the sample with the startingcomposition TiB2­30%Al3Ti decreases from 96 to 88%. Therelative density of the sample with the starting compositionTiB2­10%Al3Ti is 94% which is the highest among the 4samples sintered at 1573K.

Figure 4 shows the SEM images of the cross-section of(a) TiB2 (b) TiB2­10%Al3Ti and (c) TiB2­30%Al3Ti sinteredat 1273K. The specimen sintered without Al3Ti is granularas shown in Fig. 4(a). The size of TiB2 grains is a fewmicrons which is the same as that of the starting TiB2

powders. The link between grains seems to be loose. Thesample sintered with 10% Al3Ti is also granular as shownin Fig. 4(b). On the other hand, the TiB2­30%Al3Ti samplesintered at 1273K has a flat and dense surface composed oftwo phases. Since by the X-ray diffraction measurements, ithas been confirmed that TiB2 and Al3Ti exist in this sample,the matrix phase in Fig. 4(c) is Al3Ti and grains distributedin the matrix are TiB2. In this sample Al3Ti fills the spacebetween TiB2 grains. It should be noticed that a flat surfacecan be obtained by cutting using a diamond cutter only whenTiB2 particles are fixed strongly in the matrix so that eachTiB2 particle is cut by the diamond cutter, which will beconfirmed by the Vickers hardness measurements shownlater (see Fig. 6).

Figure 5 shows the SEM images of the cross-section of(a) TiB2 (b) TiB2­10%Al3Ti and (c) TiB2­30%Al3Ti sinteredat 1573K. The specimen sintered at 1573K without Al3Tiis granular as well as the TiB2 sample sintered at 1273Kshown in Fig. 4(a). On the other hand, flat and dence surfacesare obtained for the TiB2­10%Al3Ti and TiB2­30%Al3Tisamples shown in Figs. 5(b) and 5(c), respectively. It shouldbe noticed that, when TiB2­30%Al3Ti sample is sintered at1573K, only one phase is observed by the SEM observationas shown in Fig. 5(c) being different from the case sinteredat 1273K shown in Fig. 4(c). This is not in contradiction tothe fact that only peaks from TiB2 is observed in the X-raydiffraction measurements of TiB2­30%Al3Ti sintered at1573K (see Fig. 1). The fact that dense material is obtainedwhen TiB2 is sintered with Al3Ti additive implies that the

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Influence of Sintering Temperature on the Microstructure of TiB2 Sintered with Al3Ti Additive 1649

diffusion of TiB2 is accelerated by the presence of Al3Ti tofill the space between TiB2 grains even though Al3Ti itselfevapolates.

It is found in Fig. 5 that the grain size of TiB2 in thespecimens sintered at 1573K is a few microns which isnearly the same as that of the starting powders. Thus, graingrowth rarely proceeds at 1573K. Wang et al. reportedthat average grain size of TiB2 increases gradually by thesintering at 1973K by the hot pressing sintering method.6)

The grain size of their specimens sintered at 1973K is around4 microns which is twice of their starting powders (the sizeof their starting powders is 2 microns).6) It has been reported

that an exaggerated grain growth brought about by hightemperature sintering causes strong internal stress becauseof anisotropy effects and severe micro cracking occurs oncooling.10) The grain growth can be suppressed by the presentmethod using Al3Ti as an sintering aid.

Figure 6 shows the dependence of the Vickers hardness(Hv) on the amount of the Al3Ti additive in the startingpowders. The Hv value increases rapidly with the amount ofAl3Ti up to 30% when sintered at 1273K. The Hv value ofthe specimen containing 10% Al3Ti is as high as 1400 in spiteof its granular structure shown in Fig. 4(b). This fact suggeststhat link between grains is improved remarkably by the

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Fig. 5 SEM images of the cross-section of (a) TiB2 (b) TiB2­10%Al3Ti and(c) TiB2­30%Al3Ti sintered at 1573K.

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M. Yoshida1650

addition of Al3Ti. The Hv of TiB2­30% Al3Ti sintered at1273K is as high as 2100. These results have been reportedin Ref. 8). The increase of the composition of Al3Ti above30% leads to the decrease of Hv because the Hv of Al3Ti issmaller than that of TiB2.8)

The Hv of TiB2 sintered without Al3Ti increases as thesintering temperature increases. However, the Hv value ofTiB2 sintered at 1573K is 800 which is considerably lowerthan the reported Hv value, 3400, of TiB2 sintered at 2073Kby hot pressing.7) This implies that sintering proceedsscarcely at 1573K, probably because the sintering temper-ature is too low in comparison with the melting temperatureof TiB2. The Hv of TiB2 sintered with 10 and 20% Al3Tiincreases as the sintering temperature increases. Since, by thesintering at 1573K, almost all Al3Ti evaporates as shownin Figs. 1 and 5, the increase of Hv is caused by theimprovement of link between TiB2 grains. This hypothesiscan be proved by the SEM observation that dense samples areobtained and the contact area between TiB2 grains becomeslarger when sintered at 1573K as shown in Fig. 5. The Hv ofthe nominal TiB2­30%Al3Ti sample decreases when sinteredat 1473K and increases again when sintered at 1573K. Thedecrease of Hv when sintered at 1473K is caused by theevaporation of Al3Ti and the increase of the Hv value sinteredat 1573K is caused by the improvement of direct linkbetween TiB2 grains.

4. Conclusions

In the present study it has been shown that Al3Ti has tworoles in the sintering of TiB2. When sintered at 1273K, Al3Tifills in the spaces between TiB2 grains and TiB2­Al3Ticomposite samples are obtained. The TiB2­30%Al3Ti samplesintered at 1273K has the relative density of 96% andHv = 2100. When sintered at above 1473K, evaporation ofAl3Ti proceeds. However, even in that case, it has beenconfirmed that Al3Ti accelerates the diffusion of TiB2 causingthe formation of dense TiB2 sintered materials. By thesintering at 1573K of TiB2 with 10% Al3Ti, monolithic TiB2

samples with the relative density of 94% and Hv = 2100 areobtained.

Acknowledgement

This work is supported by the Grant-in-Aid for ScienceResearch from Japan Society for the Promotion of Science(Kakenhi 21560106).

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Fig. 6 Dependence of Vickers hardness (Hv) of TiB2 on the amount ofAl3Ti additive in the starting powders.

Influence of Sintering Temperature on the Microstructure of TiB2 Sintered with Al3Ti Additive 1651