[Research Paper] 대한금속･재료학회지 (Korean J. Met. Mater.), Vol. 55, No. 1 (2017), pp.10~15DOI: 10.3365/KJMM.2017.55.1.10

10

FeB와 TiH2 혼합분말의 유성볼밀 및 후속열처리에 의한 Fe-TiB2 나노복합분말 in situ 제조

Xuan-Khoa Huynh1･배선우2･김지순2,*

1하노이과학기술대학교 재료공학부2울산대학교 첨단소재공학부

in situ Fabrication of Fe-TiB2 Nanocomposite Powder by Planetary Ball Milling and Subsequent Heat-treatment of FeB and TiH2 Powder Mixture

Xuan-Khoa Huynh1, Sun-Woo Bae2, and Ji Soon Kim2,*

1School of Materials Science and Engineering, Hanoi Uneversity of Science and Technology, No 1, Dai Co Viet Street, Hanoi City, Vietnam

2School of Materials Science and Engineering, University of Ulsan, Ulsan 44610, Republic of Korea

Abstract: Fe-TiB2 powder was synthesized in-situ by the planetary ball milling and subsequent heat-treatment of an iron boride (FeB) and titanium hydride (TiH2) powder mixture. Mechanical activation of the (FeB+TiH2) powder mixtures was observed after a milling time of 3 hours at 700 rpm of rotation speed, but activation was not the same after 1 hour milling time. The particle size of the (FeB+ TiH2) powder mixture was reduced to the nanometer scale, and each constituent was homogeneously distributed. A sharp exothermic peak was observed at a lower temperature (749 ℃) on the DSC curves for the (FeB+TiH2) powder mixture milled for 3 hours, compared to the one milled for 1 hour (774 ℃). These peaks were confirmed to have resulted from the formation reaction of the TiB2 phase, from Ti and B elements in the FeB. The Fe-TiB2 composite powder fabricated in situ exhibited only two phases of Fe and TiB2 with homogeneous distribution. The size of the TiB2 particulates in the Fe matrix was less than 5 nm.

†(Received July 22, 2016; Accepted July 25, 2016)

Keywords: Fe-TiB2, nanocomposite, mechanical alloying/milling, in situ fabrication, powder processing, iron boride, titanium hydride

Ⅰ. INTRODUCTION

TiB2 is considered a superior material for reinforcing steels

because of its high melting point (2980 ℃), high hardness,

high elastic modulus, good corrosion resistance, and chemical

stability [1]. Hence, for the last several decades, Fe-TiB2

composites have attracted attention for many applications,

including tools, dies and wear-resistant parts.

The in situ fabrication method is known to be extremely

effective for producing a fine and homogeneous distribution

of particulate reinforcements in a matrix [2-7]. It usually

includes the synthesis of a dispersoid phase within a matrix

*Corresponding Author: Ji Soon Kim[Tel: +82-52-259-2244, E-mail: jskim@ulsan.ac.kr]Copyright ⓒ The Korean Institute of Metals and Materials

by a chemical reaction between the constituent starting

materials. The method has additional advantages, such as a

clean particulate-matrix interface, improved wettability and

high interfacial strength [8,9].

Mechanical activation by high-energy ball milling can

enhance the in situ reaction and be used to fabricate composite

powder materials. During the milling process a number of

effects occur. Structural and electronic defects are generated,

and the internal energy and specific surface area are increased,

and as a consequence, the materials become more chemically

reactive. The energy stored in the milled materials also ensures

that the subsequent solid state reaction occurs under more

favorable energetic conditions. Such enhancement can lead to

a decrease in reaction temperature and thereby associated

Xuan-Khoa Huynh･배선우･김지순 11

Table 1. Chemical composition of FeB starting powderElement Fe B Si C Al Mn Other

Weight, % 78 19 1.2 0.5 0.6 0.4 balance

Table 2. Results of particle size analysis of starting powders (volume distribution)

Powder D10(mm)

D50(mm)

D90(mm)

Specific Area(m2/g)

FeB 3.1 7.0 16.8 1.05TiH2 3.3 12.5 38.6 0.80

Fig. 1. Results of particle size analysis of starting powder mixture of FeB and TiH2 and powder mixtures after planetary ball milling at 700rpm for 0, 15, 60 and 180 min, respectively.

enthalpy value[10-12], the formation of nanostructured

powders[13], good sinterability[14], and etc.

In this work the in situ fabrication of nanoscale

TiB2-reinforced Fe matrix composite powder was

accomplished by planetary ball milling and subsequent heat

treatment, using iron boride (FeB) and titanium hydride

(TiH2) powders as the starting materials. FeB and TiH2

powder materials have significant advantages in both cost and

availability. In addition, because of their brittleness, they are

particularly suitable candidates, and can be successfully

exploited in the high-energy ball milling process. We expect

that this process can provide a novel method for rapid, simple

and cost-effective in situ fabrication.

Ⅱ. EXPERIMENTAL PROCEDURES

Commercial iron boride powder with 19 wt% boron (Table

1), and TiH2 powder (laboratory type, 99.8%), were used as

starting materials. Table 2 shows the results from the particle

size analysis, where the mean particle size of TiH2 and FeB

are 12.5 and 7.0 mm, respectively. 67.4 wt% FeB and 32.6

wt% TiH2 powder were weighed out with an atomic-percent

ratio of 2:1.1 (in excess of 10 at% TiH2) to give a composition

of Fe-40 wt% TiB2 (55 vol%), corresponding to the following

reaction equation:

2FeB(s) + TiH2(s) → 2Fe(s) + TiB2(s) + H2(g) ↑ (1)

The FeB and TiH2 powders were mixed for 2h in a tubular

mixer and then high-energy milled in a planetary ball mill

(AGO-2). SKD11-steel vials and WC balls (5 mm in

diameter) were used in the mill. The ball-to-powder weight

ratio was kept at 10:1 for all experiments. The vials were

evacuated and filled with 0.3 MPa pure argon gas to prevent

oxidation during the milling. The powder mixtures were

milled at a disk-revolution speed of 700 rpm, with milling

times ranging from 15 to 180 minutes. For a given milling

time, 0.2 g powder was loaded out for characteristics analysis.

To synthesize TiB2 particulates in a Fe matrix, the

as-milled powders were heat-treated in a tube furnace

(ThermVac., South Korea). A small alumina boat, 20 × 20 ×

30 mm in height x width x length, was filled with the

as-milled powder. The synthesis temperature was determined

from thermal analysis results, and was determined to be 800

℃ and 850 ℃ for the powder mixtures milled under the

conditions of 700 rpm/180 min and 700 rpm/60 min,

respectively. The holding time at the synthesis temperature

was fixed at 2 hours. The heating rate was 10 ℃/ min, and all

samples were synthesized in flowing ultra-high-purity Ar gas

for all experiments.

The particle size distribution was measured using a laser

scattering particle size analyzer, Mastersizer 2000 (Malvern).

Phase identification was performed using an Ultima IV X-ray

diffractometer (Rigaku) and Cu Kα radiation. The

microstructure and chemical elements of the powders were

observed and analyzed by field-emission scanning electron

microscope (FE-SEM, JEOL JSM-6500F) equipped with

12 대한금속･재료학회지 제55권 제1호 (2017년 1월)

Fig. 2. SEM images and EDS maps for Fe- and Ti-element in (a) [FeB+TiH2] powder mixture after turbular mixing, (b) after planetary ball milling for 15 min. and (c) 60 min. at 700 rpm

Fig. 3. XRD pattern of FeB-TiH2 powder mixture after planetary ball milling at 700rpm for 15, 60 and 180 min, respectively.

energy-dispersive spectroscopy (EDS). Transmission electron

microscopy (TEM, JEOL JEM-2100F) was used to observe

the microstructure and the particulate size. Thermal behaviors

during the heating process were investigated with use of a

Differential Scanning Calorimeter (DSC 404 F1 Pegasus,

Netzsch). The analysis was performed with a heating rate of

300/min, 50 mg powder/time, in extra pure Ar gas flow (50

ml/min).

Ⅲ. Results and Discussion

Ⅲ.1. High-energy ball-milling of (FeB, TiH2) powder

mixture

Figure 1 shows the results of particle size analysis of the

starting powder mixture and the powder mixtures milled at

700 rpm for 15, 60, 180 min. It is evident that the size of

powder is drastically reduced only after the milling for 15

min at 700rpm. The size distribution curve is shifted to the

smaller diameter range but still broad similar to the curve of

the powder mixture. In case of 180 min milling time, the

average particle size of powder mixture becomes larger, but

the size distribution is narrower. This seems to be resulted

from the agglomeration of fine TiH2 or ductile Ti particles

formed during milling. From SEM images and EDS maps

Fig. 2 it is revealed that TiH2 is very fine while FeB powder

remains still coarse particle even after milling for 15min.

According to other reports[15,16], brittle TiH2 can be easily

milled to nanoscale by high-energy milling for a very short

period of time and can be decomposed into Ti during milling,

while also brittle but tougher FeB than TiH2 may retain its

particle size and act as secondary milling medium beside WC

balls to enhance the decrease of the TiH2 particle size, but the

particle size of FeB changed slowly. However, the EDS maps

for Fe- and Ti-element indicate that each of initial starting

powders are homogeneously mixed after milling for 60 min

(Fig. 2(c)).

To confirm phase transformation induced by planetary ball

milling, X-ray diffraction analysis was carried out (Fig. 3).

The result shows that there is β-Ti phase together with the

phases of FeB and TiH2 even after milling for 15 min at 700

rpm. It seems to be formed by the decomposition of TiH2

during high-energy ball milling. The intensity of main X-ray

peak decreases and the width become broader with an

increase of milling time. It seems to be resulted from the

decrease in particle/crystallite size and the increase in lattice

strain.

Ⅲ.2. DSC analysis of (FeB, TiH2) powder mixture

after planetary ball milling

The DSC analysis was carried out to determine the reaction

Xuan-Khoa Huynh･배선우･김지순 13

Fig. 4. DSC curves of the as-milled powders at 700 rpm for (a) 1 hour and (b) 3 hours

Fig. 5. XRD-pattern of the Fe-TiB2 composite powder after reaction synthesis by heat-treatment

Table 3. Results of particle size analysis of Fe-TiB2 composite powders (volume distribution)

Sample ProcessingConditions

D10

(μm)D50

(μm)D90

(μm)Mean(μm)

P1 700rpm/60min. +850 ℃/120min. 2.0 4.6 8.4 4.9

P2 700rpm/180min.+800 ℃/120min. 2.1 5.0 10.3 5.6

Fig. 6. SEM backscattered-electron images of Fe-TiB2 composite powder after reaction synthesis by heat-treatment: (a) P1 and (b) P2 powder (Left: x10,000, Right: x30,000)

temperature of FeB and TiH2 powders after planetary ball

milling at 700 rpm for 1 and 3 hours, respectively (Fig. 4).

The exothermic peaks are confirmed at 749 ℃ and 774 ℃ for the powder mixtures milled for 3 hours and 1 hour,

respectively. These temperatures are in good agreement with

the results reported by K.-T. Lu et al. on the reaction

temperature of Ti-B system [17]. It can be concluded that

these thermal behaviours are resulted from the formation

reaction between Ti and B in FeB to form TiB2 phase. It

should be noted that the powder milled with the longer

milling time shows the sharper peak at lower temperature,

which means that an enhanced formation reaction occur due

to homogeneous distribution of FeB and TiH2 with finer

particle size.

Ⅲ.3. Reaction synthesis of milled powders by heat

treatment

The powder mixtures of FeB and TiH2 after milling at 700

rpm for 1h and 3 h were heat-treated at 800 and 850 ℃ for 2

hours, respectively, to synthesize the TiB2 particulates in Fe

matrix. Figure 5 shows the results of XRD patterns for the

Fe-TiB2 composite powders after reaction synthesis. The

composite powders revealed only two phases, Fe and TiB2, as

expected. It is noticed that there is no intermediate or

uncompleted reaction.

Table 3 shows the results of the particle size analysis of the

14 대한금속･재료학회지 제55권 제1호 (2017년 1월)

Fig. 7. TEM images of (a) the Fe-TiB2 composite powder fabricated by planetary ball-milling at 700 rpm for 3 hours and subsequently heat-treated at 800 ℃ for 2 hours and (b) the TiB2particulates after acid-leaching of Fe-matrix phase of this composite powder

Table 4. Results of EDS analysis on the points marked in Fig. 6

PointsComposition (at%)

Possible phasesFe B Ti① 90.2 9.8 Fe-rich② 35.3 42.5 22.2 (Fe, TiB2) layer③ 34.7 36.7 28.6 Fe, TiB2, (Ti)④ 40.0 39.4 20.6 Fe, TiB2

⑤ 42.0 38.6 19.4 Fe, TiB2

⑥ 41.1 38.1 20.8 Fe, TiB2, (Ti)

Fe-TiB2 composite powders after the reaction synthesis. The

(FeB, TiH2) powder mixture milled at a longer milling time

(P2) shows a stronger tendency to agglomeration, even

though the temperature for the reaction synthesis is lower.

This is because the initial particle size is finer for the powder

mixture milled for a longer milling time.

Figure 6 shows SEM-images of the Fe-TiB2 composite

powders, synthesized as above. The difference in the size and

distribution of the FeB in the as-milled powders results in a

different microstructure. In the x10,000 magnification (left

photos), the P1 powder has many bright spots several hundred

nanometers in size, which seem to be relatively large Fe

particulates from the FeB starting powder. With longer

milling time the P2 powder shows a more homogeneous

microstructure than the P1 powder, with fewer Fe-rich spots,

and the white spots were determined to be WC from the

milling media.

The EDS point analysis for ①-⑥ (Table 4) reveals that the

microstructure is mainly composed of Fe and TiB2. It should

be noted here that the FeB particles have retained a relatively

large size, even after intensive planetary ball milling, and

have an inhomogeneous microstructure, with an ‘Fe-rich

core’ phase with a thin (Fe+TiB2) outer layer.

Figure 7 shows TEM images of the Fe-TiB2 composite

powder (P2). Nanoscale TiB2 particulates are homogeneously

distributed in the Fe matrix (Fig. 7(a)). To observe these TiB2

particulates more clearly, this Fe-TiB2 composite powder was

acid-leached to remove the Fe matrix. The resulting TEM

images are shown in Fig. 7(b). The size of the TiB2

particulate is approximately 5 nm.

Ⅳ. CONCLUSIONS

Fe-TiB2 nanocomposite powders were successfully

synthesized in-situ from FeB and TiH2 powders, by planetary

ball milling at 700 rpm for 1 and 3 hours and subsequent heat

treatment at 850 and 800 ℃ for 2 hours.

(1) TiB2 was not formed during the high-energy ball milling,

but the TiH2 was decomposed to pure Ti. When the milling time

was increased from 1 hour to 3 hours at 700 rpm, the activation

of the (FeB, TiH2) powder mixtures was significantly enhanced,

since the resulting fine particle size and the homogeneous

distribution increased the reactivity of the constituents.

(2) The increase in milling time from 1 hour to 3 hours at

700 rpm resulted in a sharper exothermic peak at a lower

temperature (774 ℃ vs. 749 ℃) on the DSC curves, due to

the homogeneous distribution of the FeB and TiH2 and finer

particle size. Both peaks were characterized as the formation

reaction of the TiB2 phase from Ti and B in FeB.

(3) Fe-TiB2 composite powders which were synthesized at

800 ℃ for 2 hours, after planetary ball milling at 700 rpm for

3 hours, showed only two phases, Fe and TiB2, with a

homogeneous microstructure. FeB particles with relatively

large size had an inhomogeneous microstructure of a Fe-rich

core phase with a thin (Fe+TiB2) outer layer. The TiB2

particulates in the Fe matrix were less than 5 nm in size.

ACKNOWLEDGEMENTS

This work was supported by the 2014 Research Fund of

University of Ulsan.

Xuan-Khoa Huynh･배선우･김지순 15

REFERENCES

1. H. O. Pierson, Handbook of Refractory Carbides and Nitrides, William Andrew Publishing, Noyes (1996).

2. Y. H. Lee, X. K. Huynh, and J. S. Kim, J. Korean Powder Metall. Inst. 21, 382 (2014).

3. B. H. Lee, K. B. Ahn, S. W. Bae, S. W. Bae, H. X. Khoa, B. K. Kim, and J. S. Kim, J. Korean Powder Metall. Inst. 22, 208 (2015).

4. H. S. Kang, J. M. Doh, J. K. Yoon, and I. J. Shon, Korean J. Met. Mater. 52, 777 (2014).

5. S. M. Kwon, N. R. Park, J. W. Shin, S. H. Oh, B. S. Kim, and I. J. Shon, Korean J. Met. Mater. 53, 555 (2015).

6. S. A. Jung, H. J. Kwon, K. M. Roh, C. Y. Suh, and W. B. Kim, Met. Mater. Int. 21, 923 (2015).

7. T. Ai, N. Yu, X. Feng, N. Xie, W. Li, and P. Xia, Met. Mater. Int. 21, 179 (2015).

8. R. M. Aikin, JOM 49, 35 (1997). 9. R. Casati and M. Vedani, Metals 4, 65 (2014).10. V. Berbenni, A. Marini, and G. Bruni, J. Alloys Compd.

329, 230 (2001).11. S. Singh, M. M. Godkhindi, R. V. Krishnarao, and B. S.

Murty, Mater. Sci. Eng. A 382, 321 (2004).12. P. Pourghahramani and E. Forssberg, Int. J. Miner.

Process. 82, 96 (2007).13. C. Gras, D. Vrel, E. Gaffet, and F. Bernard, J. Alloys

Compd. 314, 240 (2001).14. S. E. Lee, J. M. Xue, D. M. Won, and J. Wang, Acta

Mater. 47, 2633 (1999).15. V. Bhosle, E. G. Baburaj, M. Miranoja, and K. Salama,

Mater. Sci. Eng. A 356, 190 (2003)16. C. J. Lu and Z. Q. Li, J. Alloys Compd. 448, 198 (2008).17. K. T. Lu, Y. C. Wang, T. F. Yeh, and C. W. Wu,

Combust. Flame 156, 1677 (2009).