Preparation of Directionally Solidified B4C TiB2 SiC Ternary Eutectic Composites by a Floating Zone Method and Their Properties

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Preparation of Directionally Solidified B

4

C–TiB

2

–SiC Ternary Eutectic

Composites by a Floating Zone Method and Their Properties

WenJun Li, Rong Tu and Takashi Goto

Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

Directionally solidified B4C–TiB2–SiC composites were prepared by a floating zone method using B4C, TiB2 and SiC as starting

materials. The B4C–TiB2–SiC system was ternary eutectic whose eutectic composition was 51.2B4C–8.1TiB2–40.7SiC (mol%). The ternary

eutectic composite showed a lamellar microstructure where B4C(104), TiB2(100) and SiC(111) planes were perpendicular to the growth

direction. The Vickers hardness of the B4C–TiB2–SiC ternary eutectic composite was 28 to 32 GPa. The electrical conductivity was 6 to

9104Sm1, and the thermal conductivity was 35 to 45 WK1m1in the temperature range from 298 to 1100 K.

(Received April 22, 2005; Accepted July 13, 2005; Published September 15, 2005)

Keywords: B4C–TiB2–SiC, eutectic, floating zone method, lamellar microstructure, electrical conductivity, thermal conductivity, hardness

1. Introduction

Boron carbide (B4C) has been used as abrasives, nuclear controlling rods and thermoelectrics due to its extremely high hardness, high neutron absorption cross-section and low thermal conductivity.1–9)Major limitation to the widespread

use of B4C could arise from its mechanical brittleness and poor oxidation resistance. Silicon carbide (SiC) is also a hard and wear resistant material having high oxidation resistance and excellent thermal shock resistance.10–12) We have prepared B4C–SiC eutectic composites by a floating zone (FZ) method, and reported significantly higher oxidation resistance than that of B4C.13)Titanium diboride (TiB2) can be a useful additive material to improve the performance of B4C because of its high electrical conductivity, thermal conductivity and hardness.14–20) We have reported the

characteristics and microstructure of B4C–TiB2 eutectic composites.18,21) Since the fracture toughness of B4C will

be improved by addition of TiB2, the B4C–TiB2 eutectic composite could be a candidate engineering material. There-fore, B4C–TiB2–SiC can be a potential structural material with several excellent combined properties of B4C–TiB2and B4C–SiC composites such as good ductility and oxidation resistance. However, no study on the B4C–TiB2–SiC ternary eutectic composites has been reported. In this paper, direc-tionally solidified B4C–TiB2–SiC ternary eutectic compo-sites were synthesized by a floating zone method, and their microstructure, hardness, electrical conductivity and thermal conductivity were investigated.

2. Experimental Procedure

B4C, TiB2 and SiC powders were used as starting materials. The powders were weighed and mixed in an agate mortar by adding a small amount of ethanol. The mixtures of powders were then isostatically pressed at 40 MPa in a latex tube with 10 mm in diameter. The pressed rods were sintered at 1873 K for 3.6 ks in Ar atmosphere. The rods were melted and directionally solidified by an FZ method using an Xe lamp heater in Ar atmosphere. The growth rate was fixed at 1:39105ms1.

The phases were identified by X-ray powder diffraction

(XRD). The growth direction and orientation of lamellar grains were investigated by pole figure X-ray diffraction. The microstructure was observed by scanning electron microsco-py (SEM). The Vickers hardness and fracture toughness of the B4C–TiB2–SiC composites were measured by a hardness tester (Akashi: MVK-E) under the indenter load from 0.98 to 9.8 N and calculated from the average value of 30 points in the composites indented at random. The calculating equa-tion22)of fracture toughness were:

KIc¼0:0719ðP=C3=2Þ ð1Þ

WherePis the indentation load (N) andCthe half of average cracks length (m).

The electrical conductivity was measured by a dc four-probed method for rectangular specimens (338mm). A laser flash method was employed to measure thermal conductivity using disk specimens (10 mm in diameter). The electrical and thermal conductivity measurements were conducted in vacuum in the temperature range between room temperature and 1073 K.

3. Results and Discussion

3.1 Microstructure of B4C–TiB2–SiC composites

Figure 1 shows the XRD pattern of composites with the composition of 46.0B4C–17.5TiB2–36.5SiC, 43.1B4C– 6.9TiB2–50.0SiC and 58.3B4C–7.0TiB2–34.7SiC (mol%). The phases of the composites were B4C, TiB2 and SiC, and no other phases were detected. The relative intensities of B4C, TiB2 and SiC peaks were different depending on the content of B4C, TiB2 and SiC in the composites. Figure 2 shows several typical microstructures of the composites for the cross-section perpendicular to the growth direction. The microstructure at the composition of 55.0B4C–15.0TiB2– 30.0SiC (mol%) consisted of prismatic B4C (100mm in width) and needle-like TiB2 (several 100mm in length) dispersing in an eutectic texture [Fig. 2(a)]. It suggests that B4C and TiB2phases were first precipitated and the contents of B4C and TiB2 were higher than that of the eutectic composition. The microstructure of 46.0B4C–17.5TiB2– 36.5SiC (mol%) composite had needle rod-like TiB2 and eutectic texture [Fig. 2(b)], where the content of TiB2 was

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higher than that of the eutectic composition. The prismatic B4C (a few 100mm in length) was included at the composition of 58.3B4C–7.0TiB2–34.7SiC [Fig. 2(c)]; on the other hand, long rod-like SiC phases were observed at the composition of 43.1B4C–6.9TiB2–50.0SiC [Fig. 2(d)].

The B4C–TiB2–SiC eutectic composition was determined to be 51.2B4C–8.1TiB2–40.7SiC (mol%) where uniform lamellar structure was obtained. Figures 3(a) and (b) show the microstructures of the eutectic composites for the cross-section perpendicular and parallel to the growth direction, respectively. The eutectic lamellar microstructure was composed of black B4C matrix with the white TiB2 phase (1mmin thickness) and gray SiC phase (3mmin thickness). The lamellar TiB2and SiC grains, dispersed in the matrix of B4C, were perpendicular to the growth planes and paralleled to each other. The phase formation diagram of B4C–TiB2– SiC composites was presented in Fig. 4. It was reported that B4C–TiB2 and B4C–SiC were binary eutectic systems whose eutectic compositions were 75B4C–25TiB2,23) 80B4C–20TiB218) and 70B4C–30SiC to 47B4C–53SiC (mol%).10,13,24)We have found that TiB2–SiC was a binary eutectic system. The detailed results will be reported else-where. By the present study, the B4C–TiB2–SiC system was identified as a ternary system since no other phases than B4C, TiB2 and SiC were identified and a typical lamellar micro-structure was observed. The micromicro-structure of B4C–TiB2– SiC ternary composites generally included other end compo-nents and binary eutectic texture with ternary eutectic structure depending on composition and cooling rate. Since the cooling rate of directionally solidification by a floating zone method could be too fast to establish the thermody-namic equilibrium, phase boundaries or binary eutectic lines could not be demonstrated in the present study. However, the

10 20 30 40 50 60 70 80

TiB

2

(110)

TiB

2

(201)

TiB

2

(200)

TiB

2

(102)

TiB

2

(002)

TiB

2

(101)

TiB

2

(001)

TiB

2

(100)

SiC (311)

SiC (222)

SiC (220)

SiC (200)

SiC (111)

B4

C (220)

B4

C (018)

B4

C (125)

B4

C (303)

B4

C (205)

B4

C (021)

B4

C (104)

B4

C (012)

B4

C (110)

B4

C (003)

B4

C (101)

Intensity (a.u.)

2θ (CuKα) (a)

(b)

(c)

Fig. 1 XRD patterns of B4C–TiB2–SiC composites for the cross section

perpendicular to the growth direction at the composition of (a) 43.1B4C–

6.9TiB2–50.0SiC, (b) 58.3B4C–7.0TiB2–34.7SiC and (c) 46.0B4C–

17.5TiB2–36.5SiC (mol%).

50 µm

(b)

100 µm

(c)

100 µm

(d)

B4C

SiC

TiB2

100 µm

B4C

TiB2

(a)

Fig. 2 SEM photograph of B4C–TiB2–SiC composites for cross section perpendicular to growth direction at the compositions of

(a) 55.0B4C–15.0TiB2–30.0SiC, (b) 46.0B4C–17.5TiB2–36.5SiC, (c) 58.3B4C–7.0TiB2–34.7SiC and (d) 43.1B4C–6.9TiB2–50.0SiC

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ternary eutectic composition can be determined from the texture observation. Moreover, at the compositions on the lines between each end component as depicted by broken lines in Fig. 4, the microstructures should be mixtures of the ternary eutectic texture and the end component according to the rule of ternary phase diagram.25)Although the eutectic

lines between each binary eutectic point and the ternary eutectic point were not determined, the ternary eutectic point was exactly confirmed by preparing specimens with compo-sitions on the broken lines in Fig. 4 marked as closed symbols.

Figure 5 shows the XRD pattern of the B4C–TiB2–SiC ternary eutectic composite for the cross-section perpendicu-lar to the growth direction. The planes perpendicuperpendicu-lar to the growth direction (growth plane) were B4C(104) (hexagonal), TiB2(100) (hexagonal) and SiC(111) (cubic, type) in the B4C–TiB2–SiC ternary eutectic composite, respectively. Figure 6 shows the XRD pole figures of the B4C–TiB2–SiC eutectic composites for the cross-section perpendicular to the growth direction. The B4C(104) plane appeared in the center

of the pole figure and had an angle of 13with the B4C(205) plane indicating that B4C phase was single crystal in the eutectic composite. The TiB2(100) plane in the center of the pole figure, had an angle of 30 with the TiB2(110) plane tilted 60to TiB2(1110) plane. This result was consistent with the relative orientation between {100} and {110} in the crystal structure of TiB2. The SiC(111), appearing in the center of pole figure, had an angle of 35 with the SiC(220) plane inclined at 60.5 to (202) plane. This was consistent with the relative orientation between {220} and {111} in the crystal structure of SiC. Above results indicated that the growth planes of the eutectic composite were B4C(104), TiB2(100) and SiC(111), and all TiB2or SiC lamellar grains arranged in the same orientation in the B4C matrix, respectively. The lattice distances of B4C(104), TiB2(100) and SiC(111) are 0.2565, 0.2625 and 0.2514 nm respectively indicating that B4C, TiB2 and SiC grains had a good crystal lattice match along the growth direction. Therefore, the preferable orientation of B4C(104), TiB2(100) and SiC(111) maybe associate with the similar d-values along the growth direction.

10 µm

Growth direct

ion

Surface

(a)

(b)

10 µm

Fig. 3 Microstructures of the B4C–TiB2–SiC eutectic composites (51.2B4C–8.1TiB2–40.7SiC) for (a) the cross-section perpendicular to

and (b) parallel to the growth direction.

SiC TiB2

B4C

B4C+T

Binary eutectic point[13]

Ternary eutectic composition

TiB2+T

Binary eutectic point[21]

Binary eutectic point SiC+T

B4C+SiC+T

B4C+TiB2+T

Not melted

T ternary eutectic texture

Fig. 4 Phase formation diagram of B4C–TiB2–SiC composites in an FZ

method.

10 20 30 40 50 60 70 80

SiC (111)

B4

C (104)

TiB

2

(100)

Intensity (a.u.)

2θ / (CuKα)

Fig. 5 XRD pattern of B4C–TiB2–SiC eutectic composites for the cross

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3.2 Characterizations of B4C–TiB2–SiC composites

Figure 7 shows the indenter load dependence of Vickers micro hardness of the B4C–TiB2–SiC composites for the ternary eutectic composition (51.2B4C–8.1TiB2–40.7SiC (mol%)), and B4C, TiB2 and SiC excess compositions. The hardness of B4C–TiB2–SiC composites increased with decreasing the indenter load. The hardness of B4C–TiB2– SiC composites increased with increasing of B4C content because of the higher hardness than TiB2 and SiC. The hardness of B4C–TiB2–SiC eutectic composite was about 28 to 32 GPa. Figure 8 shows the indentations of B4C–TiB2– SiC composites at the composition of 46.0B4C–17.5TiB2– 36.5SiC, 51.2B4C–8.1TiB2–40.7SiC and 43.1B4C–6.9TiB2–

50.0SiC (mol%). The crack was propagated in the form of transgranular mode. The fracture toughness of eutectic composite was about 4 MPa m1=2, which was greater than 2.1 MPa m1=2 of 46.0B4C–17.5TiB2–36.5SiC composite and 3.6 MPa m1=2of 43.1B4C–6.9TiB2–50.0SiC composite.

Figure 9 shows the temperature dependence of electrical conductivity for the B4C–TiB2–SiC composites and com-pared with the reference values of B4C, TiB2 and SiC together with the B4C–TiB2 and B4C–SiC eutectic compo-sites.13,21) The electrical conductivity of B4C–TiB2–SiC composite slightly decreased with increasing temperature. The electrical conductivity of B4C–TiB2–SiC composite was much higher than that of B4C, SiC and B4C–SiC eutectic composite, and increased with the increase of the content of TiB2because the higher electrical conductivity of TiB2than those of B4C and SiC. The electrical conductivity of B4C– TiB2–SiC eutectic composite was about 6 to9104Sm1in the range of 298 to 1073 K, which was slightly smaller than the calculated values (cal) from the mixing law of parallel

model as given by eq. (2).17)

cal ðparallelÞ ¼VAAþVBBþVCC ð2Þ

where V is volume fraction, and subscribes (A, B and C) indicate the component of the composite. The electrical conductivity of B4C–TiB2–SiC composites is much higher than the 100 Sm1 required in the electric discharge machining of the work material, indicating that these composites can be machined by electric discharge machin-ing. Figure 9 suggests that the electrical conductivity of the eutectic composite is mainly dominated by the TiB2 phase because of the same temperature dependence of TiB2. Figure 10 shows the temperature dependence of thermal conductivity for B4C–TiB2–SiC composite and compared with the reference values of B4C, TiB2and SiC together with the B4C–TiB2 and B4C–SiC eutectic composites.13,21) The thermal conductivity of B4C–TiB2–SiC eutectic composite was 35 to 45 WK1m1 in the range of 298 to 1073 K and slightly decreased with increasing temperature, which was almost in agreement with calculated values (cal) from the

mixing law of parallel model as given by eq. (3).17)

B4C(205)

B4C(104)

13 o

β

= 270

o

β = 180o

β = 0o

SiC (202)

60o

SiC (111)

SiC (220) 35o

TiB2(110) TiB2(110)

TiB2(100)

60o

30o

30 20 10 10 20 30

30

20

10

10

20

30

β

= 90

o

β = 0o

β

= 90

o 50 40 30 20 10 20 30 40 50

50

40

30

20

10

10

20

30

40

50

10

β

= 270

o

β = 180o

β = 0o

50 40 30 20 10

50

40

30

20

10

10 20 30 40 50

20

30

40

50

β

= 90

o

β

= 270

o

β = 180o

Fig. 6 Pole figure of (a) B4C(104), (b) TiB2(100) (c) SiC(111) in the B4C–

TiB2–SiC eutectic composite for the cross section perpendicular to the

growth direction.

0 2 4 6 8 10

26 28 30 32 34 36 38

Vickers micro hardness, Hv / GPa

Load, P/N

51.2B4C-8.1TiB2-40.7SiC (eutectic)

43.1B4C-6.9TiB2-50.0SiC

58.3B4C-7.0TiB2-34.7SiC 46.0B4C-17.5TiB2-36.5SiC

24

Fig. 7 Indenter load dependence of Vickers microhardness of the B4C–

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cal ðparallelÞ ¼VAAþVBBþVCC ð3Þ

where V is volume fraction, and subscribes (A, B and C) indicate the component of the composite. This value showed greater than that of B4C–SiC and B4C–TiB2 binary eutectic composites, suggesting more significant configuration of each phase in parallel to the growth direction.

4. Conclusion

B4C–TiB2–SiC was a ternary eutectic system. The eutectic composition was 51.2B4C–8.1TiB2–40.7SiC (mol%), and typical lamellar textures were observed. The growth plane of lamellar grains was B4C(104), TiB2(100) and SiC(111), respectively. The hardness of B4C–TiB2–SiC composites increased with the increase of B4C content. The hardness of B4C–TiB2–SiC eutectic composite was 28 to 32 GPa. The electrical conductivity of B4C–TiB2–SiC composites in-creased with the increase of TiB2 content. The thermal conductivity of eutectic composite was 35 to 45 WK1m1, and the electrical conductivity of B4C–TiB2–SiC eutectic composite was 6 to9104Sm1in the temperature range of

5

µ

m

(b)

(a)

5

µ

m

(c)

5

µ

m

Fig. 8 SEM photograph of Vickers microhardness indention of the B4C–

TiB2–SiC eutectic composite at the loads of (a) 1.96 N, (b) 4.9 N, (c) 9.8 N.

1.0 1.5 2.0 2.5 3.0 -1

0 1 2 3 4 5 6 7 8

log(Electrical conductivity,

σ

/Sm

T-1 / 10-3K-1 2

-1 )

SiC TiB

B4C

1000 500 300

Temperature, T / K

3.5

46.0B4C-17.5TiB2-36.5SiC

51.2B4C-8.1TiB2-40.7SiC (eutectic)

58.3B4C-7.0TiB2-34.7SiC

70.0B4C-5.0TiB2-25.0SiC σB4C-TiB2

σ

σ

σ

calculated values σB4C-SiC

Fig. 9 Temperature dependence of the electrical conductivity for the B4C–

TiB2–SiC composites.

κ

400 600 800 1000 1200

4 2

4 4

200 0 20 40 60 80 100 120 140

Thermal conductivity, k / WK

-1 m

-1

Temperature, T / K

SiC

TiB2

B C-TiB

B C B C-SiC

51.2B4C - 8.1TiB2 - 40.7SiC

κ

κ

κ κ

calculated values

Fig. 10 Temperature dependence of the thermal conductivity for B4C–

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298 to 1073 K. Both electrical and thermal conductivity of the eutectic composite was mainly understood by a parallel model of each component.

Acknowledgment

The research was mainly conducted by using facilities of Laboratory for Advanced Materials, Institute for Materials Research.

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Figure

Fig. 1XRD patterns of B4C–TiB2–SiC composites for the cross sectionperpendicular to the growth direction at the composition of (a) 43.1B4C–6.9TiB2–50.0SiC, (b) 58.3B4C–7.0TiB2–34.7SiC and (c) 46.0B4C–17.5TiB2–36.5SiC (mol%).

Fig 1XR.

D patterns of B4C TiB2 SiC composites for the cross sectionperpendicular to the growth direction at the composition of a 43 1B4C 6 9TiB2 50 0SiC b 58 3B4C 7 0TiB2 34 7SiC and c 46 0B4C 17 5TiB2 36 5SiC mol . View in document p.2
Fig. 2SEM photograph of B4C–TiB2–SiC composites for cross section perpendicular to growth direction at the compositions of(a) 55.0B4C–15.0TiB2–30.0SiC, (b) 46.0B4C–17.5TiB2–36.5SiC, (c) 58.3B4C–7.0TiB2–34.7SiC and (d) 43.1B4C–6.9TiB2–50.0SiC(mol%).

Fig 2SE.

M photograph of B4C TiB2 SiC composites for cross section perpendicular to growth direction at the compositions of a 55 0B4C 15 0TiB2 30 0SiC b 46 0B4C 17 5TiB2 36 5SiC c 58 3B4C 7 0TiB2 34 7SiC and d 43 1B4C 6 9TiB2 50 0SiC mol . View in document p.2
Fig. 5XRD pattern of B4C–TiB2–SiC eutectic composites for the crosssection perpendicular to the growth direction.

Fig 5XR.

D pattern of B4C TiB2 SiC eutectic composites for the crosssection perpendicular to the growth direction . View in document p.3
Fig. 3Microstructures of the B4C–TiB2–SiC eutectic composites (51.2B4C–8.1TiB2–40.7SiC) for (a) the cross-section perpendicular toand (b) parallel to the growth direction.

Fig 3.

Microstructures of the B4C TiB2 SiC eutectic composites 51 2B4C 8 1TiB2 40 7SiC for a the cross section perpendicular toand b parallel to the growth direction . View in document p.3
Figure 5 shows the XRD pattern of the B4BTiBFigure 6 shows the XRD pole figures of the Bternary eutectic composite for the cross-section perpendicu-lar to the growth direction
Figure 5 shows the XRD pattern of the B4BTiBFigure 6 shows the XRD pole gures of the Bternary eutectic composite for the cross section perpendicu lar to the growth direction. View in document p.3
Fig. 6Pole figure of (a) B4C(104), (b) TiB2(100) (c) SiC(111) in the B4C–TiB2–SiC eutectic composite for the cross section perpendicular to thegrowth direction.

Fig 6.

Pole gure of a B4C 104 b TiB2 100 c SiC 111 in the B4C TiB2 SiC eutectic composite for the cross section perpendicular to thegrowth direction . View in document p.4
Fig. 7Indenter load dependence of Vickers microhardness of the B4C–TiB2–SiC eutectic composite.

Fig 7.

Indenter load dependence of Vickers microhardness of the B4C TiB2 SiC eutectic composite . View in document p.4
Fig. 9Temperature dependence of the electrical conductivity for the B4C–TiB2–SiC composites.

Fig 9.

Temperature dependence of the electrical conductivity for the B4C TiB2 SiC composites . View in document p.5
Fig. 10Temperature dependence of the thermal conductivity for B4C–TiB2–SiC composites.

Fig 10.

Temperature dependence of the thermal conductivity for B4C TiB2 SiC composites . View in document p.5
Fig. 8SEM photograph of Vickers microhardness indention of the B4C–TiB2–SiC eutectic composite at the loads of (a) 1.96 N, (b) 4.9 N, (c) 9.8 N.

Fig 8SE.

M photograph of Vickers microhardness indention of the B4C TiB2 SiC eutectic composite at the loads of a 1 96 N b 4 9 N c 9 8 N . View in document p.5

References

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