Composites by Mechanical Alloying and Hot-Pressing

Materials Transactions, Vol. 49, No. 7 (2008) pp. 1572 to 1578 #2008 The Japan Institute of Metals

Preparation of TiC-Ti3 SiC2 Composites by Mechanical Alloying and Hot-Pressing Hitoshi Hashimoto and Zheng Ming Sun Research Institute for Sustainable Materials, National Institute of Advanced Industrial Science and Technology, Nagoya 463-8560, Japan TiC-Ti3 SiC2 composites having the microstructure of fine Ti3 SiC2 grains dispersed in TiC matrix were synthesized by two processes, that is, mechanical alloying of a powder blend of Ti, Si and C followed by hot-pressing (MA-HP process) and hot-pressing of a powder blend of Ti, Si and TiC (MIX-HP process). The microstructure, damage tolerance and 4-point flexural strength of the composites were investigated. It was found that more uniform dispersion of Ti3 SiC2 grains in TiC matrix was achieved by MA-HP process than by MIX-HP process, while the flexural strength of the composites synthesized by MA-HP process was lower than that by MIX-HP process, which may be attributed to micropores formed in the composites synthesized by MA-HP process. Preferential orientation of Ti3 SiC2 (001) and TiC(111) occurred in the composites synthesized by MA-HP process. We proposed two mechanisms of the preferential orientation. We also found that the dispersion of Ti3 SiC2 in TiC matrix is effective in improving the damage tolerance of the brittle TiC, evidenced by the fact that no cracks emanated from the corners of Vickers indentations at a load of 9.8 N. [doi:10.2320/matertrans.MRA2008013] (Received January 8, 2008; Accepted April 7, 2008; Published May 28, 2008) Keywords: titanium silicon carbide, titanium carbide, mechanical alloying, damage tolerance

1.

Introduction

Titanium silicon carbide (Ti3 SiC2 ) is a damage-tolerant material having nano-layered crystal structure like other highly damage-tolerant materials such as mica and hexagonal boron nitride. A few energy absorbing mechanisms are believed to exist in Ti3 SiC2 such as delamination, crack deflection, buckling of individual crystal grains, kink bands and incipient kink bands formation and etc.1,2) These energy absorption mechanisms lead to stress relaxation and retard crack extension. The high damage tolerance of Ti3 SiC2 is usually illustrated by the fact that cracks do not emanate from the corners of Vickers indentations even at loads of 300 N.3) On the other hand, titanium carbide (TiC), a typical structural ceramic material, shows excellent hardness and strength in a wide temperature range while showing very low damage tolerance. In glass-bonded mica type machinable ceramics, fine mica grains are dispersed in glass matrix. This microstructure makes it difficult for cracks to propagate.4) Propagating cracks in the glass matrix are arrested by the damage-tolerant mica grains due to their energy absorbing mechanisms. It is, therefore, reasonable to presume that the damage tolerance of brittle TiC will be improved by designing a microstructure with fine dispersion of damage-tolerant Ti3 SiC2 . According to Ti-Si-C ternary phase diagram,5) there exists a wide equilibrium two-phase region between TiC and Ti3 SiC2 even at a temperature of 1500 K. It is, therefore, possible to synthesize TiC-Ti3 SiC2 composites which are stable in a wide temperature range. It is well known that microstructural uniformity and some mechanical properties of a composite including hardness, toughness and strength are improved by applying nonequilibrium processes such as mechanical alloying to the synthesis process.6,7) In this study, mechanical alloying followed by hot-pressing (abbreviated to MA-HP hereafter) was conducted on Ti/Si/C powder blends to synthesize TiC-Ti3 SiC2 composites having the microstructure with

fine Ti3 SiC2 grains dispersed in TiC matrix. The microstructure, damage tolerance and 4-point flexural strength of the composites were investigated and compared with those of TiC-Ti3 SiC2 composites synthesized by hot-pressing of Ti/Si/TiC powder blends (abbreviated to MIX-HP hereafter). The formation mechanisms of their microstructure were deduced from the result of investigation on the microstructure and phases. 2.

Experimental Procedure

Ti powder (99.9% pure and under 45 mm in particle size), Si powder (99.9% and average 10 mm), C powder (99.7% and average 5 mm) and TiC powder (99% and average 1.72 mm) were used as starting materials. SEM images of these powders are shown in Fig. 1. Ti, Si and C powders were blended at target compositions of TiC-40 and 60 vol%Ti3 SiC2 and mechanically alloyed in argon atmosphere. A laboratory vibratory ball mill (Chuo Kakouki MB-1) was used for mechanical alloying. A mill vessel was charged with steel balls of 4576 g which corresponds to 80 vol% of full charge in the vessel and the powder blend of 91.52 g (2% of the ball charge in weight) and then sealed in an argon glove box. Both oxygen and water contents in the glove box were kept under 10 ppm. Before mechanical alloying, 30.5-g Ti powder was milled in argon atmosphere to form Ti coating layer on ball surfaces and mill vessel walls, to avoid Fe-contamination. Thickness of the coating layer was estimated to be 20 mm from the weight of Ti powder and the total surface area of balls and vessel walls. Milling was interrupted at intervals to monitor the progress of mechanical alloying process and check the amount of free alloyed particles. (In earlier stages of mechanical alloying, unalloyed particles were mostly attached on ball surfaces and vessel walls. In later stages, alloyed particles were detached from ball surfaces and mill vessel walls.) Total milling time was 1080 ks for the powder blend with the target composition of TiC-40 vol%Ti3 SiC2 and 2340 ks for that with TiC-

Preparation of TiC-Ti3 SiC2 Composites by Mechanical Alloying and Hot-Pressing

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Fig. 1 SEM images of starting powders: (a) Ti, (b) Si, (c) C and (d) TiC.

60 vol%Ti3 SiC2 so as to obtain sufficient amounts of the free alloyed particles. After milling, the free alloyed particles were taken out from the vessel in the glove box. A cylindrical carbon mold having an inner diameter of 50 mm was charged with the alloyed powder and pressed by a press machine in the glove box because the alloyed powder was so active that it may burn in the air. To prevent reaction between the powder and carbon mold, a graphite sheet of 0.2 mm thick was inserted between the powder and the mold wall. The mold charged with the powder was taken out from the glove box and placed immediately in the vacuum chamber of a hot press (Sodic plasma activated sintering machine PAS-V) and sintered in vacuum. Sintering condition was 1773 K 50 MPa 900 s for TiC-40 vol%Ti3 SiC2 and 1823 K 50 MPa 900 s for TiC-60 vol%Ti3 SiC2 . For comparison, Ti, Si and TiC powders were blended at target compositions of TiC-35 and 60 vol%Ti3 SiC2 , and mixed for 86.4 ks by a powder mixer (WAB TURBULA shaker-mixer). These powder blends were hot-pressed with the cylindrical carbon mold in the same way as mentioned above except sintering temperature. Sintering condition was 1623 K 50 MPa 900 s for the pure Ti3 SiC2 and 1723 K 50 MPa 900 s for TiC-35 and 60 vol%Ti3 SiC2 . The abovementioned sintering temperatures were determined so as to obtain fully dense samples. X-ray diffraction analysis was made on the hot-pressed samples in disk shape and the mechanically alloyed powders by using Philips X’pert MPD with Cu K radiation to identify the phases. Before the analysis of hot-pressed disks, top and bottom surfaces of the disks were ground by a surface-grinding machine. Test pieces for 4-point bending (4 mm wide, 2 mm thick and 36 mm long) were cut from the hot-pressed disks by an electric discharge machine. The test pieces were polished with diamond abrasives and finished by buffing with 1 mm diamond suspension. 4-point bending test was performed according to JIS R1601 (bending tests at room temperature for fine ceramics, but the thickness of 2 mm was used instead of 3 mm as in the standard), with a test jig made of

silicon carbide by an Instron universal testing machine. As mentioned later, preferential orientation of Ti3 SiC2 grains were observed in the hot-pressed disks of mechanically alloyed powders. We presumed that the orientation of Ti3 SiC2 grains affects 4-point bending strength because Ti3 SiC2 crystals show the strength anisotropy due to the large difference in bonding strength of Ti-C and Ti-Si in Ti3 SiC2 crystal structure.8) The bonding of Ti-C is much stronger than that of Ti-Si, which results in high tensile strengths in the direction parallel to basal plane (c-plane) and low tensile strengths in the normal direction. In the bending test, the compressive stress is induced in a half part of the test pieces and the tensile stress is induced in the other half part. As the tensile stress breaks the test pieces, we performed the bending test so as to induce the tensile stress along to c-plane of the oriented Ti3 SiC2 crystal grains. We expected higher bending strengths by the preferential orientation. The damage tolerance of the hot-pressed samples was examined by observing the indentations formed by a Vickers hardness tester at a load of 9.8 N or 98 N for 15 s. The observation was made by using a scanning electron microscope (SEM) to check possible cracks emanating from the corners of the indentations. The hot-pressed samples polished and finished with 1 mm diamond suspension were chemically etched with an aqueous HF and HNO3 mix to observe its microstructure by SEM equipped with an EDX analyzer. Fracture surface of the bending test pieces was also observed by SEM. 3.

Results and Discussion

The mechanically alloyed powder consists of particles mostly under 1 mm in size as shown by an SEM image in Fig. 2. In the XRD pattern, the powder shows large broadened peaks of TiC and small peaks of Ti as shown in Fig. 3. Diffraction peak of Si is not found, suggesting that Si solved totally in Ti. Crystallite size of TiC was calculated to be 5.6 nm by Scherrer equation from the peak width at half

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2µm Fig. 2 SEM image of mechanically alloyed powder with a target composition of TiC-60 vol%Ti3 SiC2 . Fig. 4 XRD patterns of the hot-pressed disks with a target composition of TiC-60 vol%Ti3 SiC2 synthesized by MA-HP process and MIX-HP process.

Fig. 3 XRD pattern of mechanically alloyed powder with a target composition of TiC-60 vol%Ti3 SiC2 .

height of TiC(200) peak after separating from Ti(101) peak by profile fitting technique. These figures indicate that the mechanically alloyed powder consisted of fine particles of nanocrystalline TiC and Ti-Si alloy.

(a)

XRD patterns of the hot-pressed disks with a target composition of TiC-60 vol%Ti3 SiC2 synthesized by the two processes, MA-HP and MIX-HP, are shown in Fig. 4. Both patterns show only sharp peaks from TiC and those from Ti3 SiC2 , which indicates TiC-Ti3 SiC2 composites were synthesized by both MA-HP and MIX-HP processes. The microstructure of hot-pressed composites synthesized by the two processes is shown by SEM images of etched surface of the disks. Figure 5(a) and (b) show the microstructure of the composite synthesized by MA-HP process, and (c) and (d) the composite synthesized by MIX-HP process. Dark regions correspond to Ti3 SiC2 and bright regions to TiC, which was confirmed by SEM-EDX analysis. It is obvious that the uniform dispersion of Ti3 SiC2 grains in TiC matrix was achieved by MA-HP process. In contrast, the dispersion of Ti3 SiC2 is evidently less uniform in the

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Fig. 5 SEM images of etched surface of (a), (b) the disk synthesized by MA-HP process and (c), (d) the disk synthesized by MIX-HP process.


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Fig. 6 SEM images of fracture surfaces of composites synthesized by (a) MA-HP process and (b) MIX-HP process. Hot pressing axis in vertical direction.

composite synthesized by MIX-HP process. Thus, the mechanical alloying is very effective to improve the microstructural uniformity. Referring to the XRD patterns shown in Fig. 4, it should be noted that the intensity of the peak from Ti3 SiC2 (008)plane (marked with the letter ‘‘B’’) is weaker than the main peak of Ti3 SiC2 from (104)-plane (marked with the letter ‘‘A’’) in the pattern of the composite synthesized by MIX-HP process. In contrast, B is much stronger than A in the pattern of the composite synthesized by MA-HP process. A ratio of integrated peak intensity of Ti3 SiC2 (008) to (104) is 2.83 for the MA-HP material, while it is 0.40 for the MIX-HP one. As this ratio is 0.19 according to JCPDS (Joint Committee on Powder Diffraction Standards) data, Ti3 SiC2 crystal grains in the MIX-HP composite having the ratio of 0.40 is supposed to be oriented nearly in a random manner like in powders. On the other hand, in the MA-HP disk showing the ratio of 2.83, (008)-plane of Ti3 SiC2 crystal grains is oriented preferentially in the direction parallel to the top surface of the disk on which the XRD measurement was performed. As the top surface of the disk was perpendicular to the loading axis of hot-press, the preferential orientation of c-plane in the direction perpendicular to the loading direction occurred during hot-pressing. This preferential orientation in microstructure is supported by SEM observation of the fracture surface of the disks. Figure 6 shows SEM images of the fracture surfaces of the composite synthesized by (a) MA-HP process and (b) MIXHP process. Plate-like Ti3 SiC2 grains and their long and thin cross sections are exposed on the fracture surface. The fracture surface is perpendicular to the top surface of the disk and the lateral direction of these images is parallel to the top surface. Therefore, a large part of the plate-like Ti3 SiC2 grains have a tendency to be parallel to the top surface in Fig. 6(a). On the other hand, Ti3 SiC2 grains appears to be randomly-oriented in Fig. 6(b). Referring to the XRD patterns shown in Fig. 4, again, reveals a more interesting fact. It should be noted that the intensity of TiC(111) (marked with the letter ‘‘D’’) is much stronger than the main peak of TiC(200) (marked with the letter ‘‘C’’) in the pattern of the MA-HP disk. The ratio of integrated peak intensity of TiC(111) to (200) is 0.93 according to JCPDS data while it is measured to be 4.00, which indicates (111)-plane of TiC crystal grains is oriented preferentially in the same direction to c-plane of Ti3 SiC2 .

Concerning the reason why the preferential orientation of both Ti3 SiC2 and TiC crystal grains occurred in the composites synthesized by MA-HP process, we can propose several explanations. According to Tang et al.9) who investigated the orientation relationship between Ti3 SiC2 and TiC grains in composites synthesized by hot-pressing of Ti/Si/C powder blend, the interface between Ti3 SiC2 and TiC grains is Ti3 SiC2 (001) and TiC(111) coherent interface. They explained that the interface of Ti3 SiC2 and TiC grains preferentially became Ti3 SiC2 (001) and TiC(111) interface because its interfacial energy is the lowest when Ti3 SiC2 grains nucleated and grew in TiC matrix during the synthesis reaction of Ti3 SiC2 from the Ti/Si/C powder blend. This mechanism may account for our results. As the mechanically alloyed powder consisted of nanocrystalline TiC and Ti-Si alloy as mentioned above, it is presumed that Ti3 SiC2 crystal grains were synthesized by the reaction between the nanocrystalline TiC and Ti-Si alloy and they grew in TiC matrix and had Ti3 SiC2 (001) and TiC(111) coherent interface because of the lowest interfacial energy. The synthesis reaction of Ti3 SiC2 grains presumably occurred at lower temperatures than usual because the reaction temperature may have been lowered due to a huge amount of interfacial area10) between the nanocrystalline TiC and Ti-Si alloy and mechanical activation.11) After the growth of the plate-like Ti3 SiC2 grains, sintering shrinkage started and the plate-like grains rotated together with TiC grains around them and aligned parallel to the disk top surface by the load of hotpress as schematically illustrated in Fig. 7. On the other hand, the synthesis reaction temperature of Ti3 SiC2 grains was as high as the sintering shrinkage temperature in MIX-HP process. Therefore, the synthesis of Ti3 SiC2 grains and the sintering shrinkage occurred simultaneously, which resulted in the formation of TiC with the dispersion of randomlyoriented Ti3 SiC2 grains.

TiC(111)// Ti3SiC2(001)

Load Load TiC

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TiC matrix Plate-like Ti3SiC2

Fig. 7 Schematic illustration of the occurrence of preferential orientation of Ti3 SiC2 and TiC crystal grains.

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However, the authors propose another possible mechanism for the above-mentioned preferential orientation phenomenon. The authors12) formerly synthesized a Ti3 SiC2 powder by heating of a Ti/Si/TiC powder blend in vacuum at 1673 K. The synthesized powder was ball-milled to reduce its particle size and make each particle consists of a single crystal grain of Ti3 SiC2 . The milled power was hot-pressed at 1623 K to form a disk, and the microstructure and the phase of the hotpressed disk were examined by SEM observation and XRD measurement. This hot-pressing temperature, 1623 K, is 50 K lower than the synthetic temperature of the Ti3 SiC2 powder and the same as the temperature which the authors have used for the synthesis of Ti3 SiC2 by hot-pressing of a Ti/Si/TiC powder blend. From the XRD measurement, it was found that the sintered material consisted of Ti3 SiC2 and a certain amount of TiC, while XRD measurement did not indicate any existence of TiC in the milled powder. It is, therefore, natural to conclude that TiC was formed by decomposition of Ti3 SiC2 during hot-pressing. The XRD measurement on the hot-pressed disk also revealed that c-plane of Ti3 SiC2 and TiC(111) plane were parallel to the top surface of the disk, which is consistent with the SEM observation on the fracture surface. Since the milled powder consists of Ti3 SiC2 flaky or plate-like particles, which was confirmed by SEM observation, it is presumed that these particles were aligned parallel to the top surface of the disk during hot-pressing. Since Ti3 SiC2 has the crystal structure with planar Si layers linked together by TiC octahedra of which (111) plane is parallel to c-plane of Ti3 SiC2 as shown in Fig. 8, (111)-plane of TiC formed by the decomposition of Ti3 SiC2 should be parallel to c-plane of Ti3 SiC2 . Consequently, we can explain our results as follows; (1) Ti3 SiC2 was synthesized from the mechanically alloyed powder at lower temperatures than from the powder blend of Ti, Si and TiC due to the activation effect of

TiC(111) plane // Ti3SiC2 c -plane TiC Octahedra

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Si

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Unit cell of Ti3 SiC2 crystal.

mechanical alloying. (2) The synthesized Ti3 SiC2 crystal grains became plate-like or flaky in shape because of the anisotropy of crystal growth rate. (3) These plate-like grains synthesized at lower temperatures were rotated and aligned preferentially in the direction perpendicular to the loading direction of hot-pressing during sintering shrinkage at higher temperatures. (4) The synthesized Ti3 SiC2 decomposed partly to TiC and titanium silicides at higher temperatures. TiC formed by the decomposition of Ti3 SiC2 should have (111) plane parallel to c-plane of Ti3 SiC2 . Although we could not confirm titanium silicides formed by the decomposition by XRD, we believe that the above-mentioned explanation is one of the possible mechanisms. SEM images of indentations formed by the Vickers hardness tester on the composites at loads of 9.8 N and 98 N are shown in Fig. 9. Figures 9(a) and 9(b) are respectively the indentations on MA-HP compact at load of 9.8 N and 98 N. Figures 9(c) and 9(d) are those on MIX-HP compact at load of 9.8 N and 98 N, respectively. Fracture toughness of brittle ceramic materials can be estimated by the length of cracks running from the corners of Vickers indentation. The test load should be selected so as to obtain enough crack length to measure. For instance, the load of 9.8 N was selected for Al2 O3 -TiC ceramic composites13) and 6 N was selected for TiC0:96 single crystals.14) Therefore, the load of 9.8 N is enough to induce cracks from the corners of Vickers indentation for the brittle ceramic materials. Whereas cracks are running from the corners of indentations formed at the load of 98 N, we cannot see any crack running from the corners of indentations formed at the load of 9.8 N. This means the dispersion of Ti3 SiC2 crystal grains in TiC matrix is effective to give the damage tolerance to the brittle TiC. 4-point flexural strength of the composites is plotted against Ti3 SiC2 content in Fig. 10. The content of Ti3 SiC2 in the figure was calculated by the density of composites by assuming the composites were fully dense. The composites synthesized by MIX-HP process show higher strength than those synthesized by MA-HP process. This result is beyond our expectation because the composites synthesized by MAHP process have more uniform microstructure than those synthesized by MIX-HP process and showed the preferential orientation of Ti3 SiC2 crystal grains. In general, composites having more uniform microstructure show higher strength. It should be noted that the bending test of the MA-HP composites was performed so as to show higher strengths as mentioned above in the experimental procedure. SEM images shown in Fig. 11 may give an answer to the question, which shows the fracture surfaces of (a) composite synthesized by MA-HP process and (b) composite synthesized by MIX-HP process. The composite synthesized by MA-HP process shows many micropores indicated by arrows which may be responsible for the lower strength. These pores seem to exist at the interface between TiC grains. So, a possible formation mechanism of the pores is that a huge amount of vacancies introduced into TiC during mechanical alloying diffused and gathered during hot-pressing. From Fig. 10, you might have found that the Ti3 SiC2 content calculated by the density is larger than the target one. For example, MIX-HP composites show Ti3 SiC2 content of


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Fig. 9 SEM images of indentations formed by the Vickers hardness tester on MA-HP compact at load of (a) 9.8 N and (b) 98 N, and those on MIX-HP compact at load of (c) 9.8 N and (d) 98 N.

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Ti3SiC2 content [vol%] Fig. 10 Flexural strength (4-point bending) of the composites plotted against Ti3 SiC2 content.

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45.4% and 70.6% for the target content of 35% and 60%, respectively. MA-HP composites also show 70.3% and 78.2% for the target content of 40% and 60%, respectively. In the strict sense, the microstructure of these MA-HP and MIX-HP composites should be compared at the same composition. Namely, the microstructure of the MA-HP TiC-40%Ti3 SiC2 composite having 70.3%Ti3 SiC2 content should be compared with that of the MIX-HP TiC60%Ti3 SiC2 composite having 70.6%Ti3 SiC2 content. However, we found that both the microstructural uniformity and the crystal grain orientation were affected very little by the composition in our experimental range of composition. We achieved the same conclusions on the microstructure as mentioned above when we compared the microstructure of the MA-HP TiC-40%Ti3 SiC2 composite with that of the MIX-HP TiC-60%Ti3 SiC2 composite.

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Fig. 11 SEM images of fracture surface of the composites synthesized by (a) MA-HP process and (b) MIX-HP process. Micropores indicated with arrows.

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Conclusions

TiC-Ti3 SiC2 composites having the microstructure of fine Ti3 SiC2 grains dispersed in TiC matrix were synthesized by two processes, that is, mechanical alloying of the powder blend of Ti, Si and C followed by hot-pressing (MA-HP process) and hot-pressing of the powder blend of Ti, Si and TiC (MIX-HP process). The microstructure, damage tolerance and flexural strength of the composites were investigated. The following conclusions are drawn based on the experimental results; (1) More uniform dispersion of Ti3 SiC2 grains in TiC matrix is achieved by MA-HP process than by MIX-HP process. (2) Preferential orientation of both c-plane of Ti3 SiC2 crystal grains and (111)-plane of TiC crystal grains occurs in the composites synthesized by MA-HP process. (3) The dispersion of Ti3 SiC2 crystal grains in TiC matrix is effective to give the damage tolerance to the brittle TiC. (4) The flexural strength of the composites synthesized by MA-HP process is lower than that by MIX-HP process in spite of the more uniform microstructure, which might be due to large quantity of micropores formed in the MA-HP composites.

REFERENCES 1) B. J. Kooi, R. J. Poppen, N. J. M. Carvalho, J. Th. M. De Hosson and M. W. Barsoum: Acta Mater. 51 (2003) 2859–2872. 2) M. W. Barsoum, T. Zhen, S. R. Kalidindi, M. Radovic and A. Murugaiah: Nature Materials 2 (2003) 107–111. 3) T. El-Raghy, A. Zavaliangos, M. W. Barsoum and S. R. Kalidindi: J. Am. Ceram. Soc. 80 (1997) 513–516. 4) D. Mog: Vacuum 26 (1976) 25–29. 5) P. Villars, S. Prince and H. Okamoto: Handbook of Ternary Alloy Phase Diagrams on CD-ROM, Ver. 1.0, ASM International, 1997. 6) M. Krasnowski and T. Kulik: Scr. Mater. 48 (2003) 1489–1494. 7) C. J. Zhu, X. F. Ma, W. Zhao, H. G. Tang, J. M. Yan and S. G. Cai: Scr. Mater. 51 (2004) 993–997. 8) M. W. Barsoum, T. El-Raghy and M. Radovic: Interceram 49 (2000) 226–233. 9) K. Tang, C. Wang, Y. Huang and L. Wu: Mater. Lett. 57 (2002) 106–109. 10) F. Karimzadeh, M. H. Enayati and M. Tavoosi: Mater. Sci. Eng. A 486 (2008) 45–48. 11) Y. Chen, T. Hwang, M. Marsh and J. S. Williams: Mater. Sci. Eng. A226–228 (1997) 95–98. 12) H. Hashimoto, Z. M. Sun, S. Tada and Y. Zou: Mater. Trans. 48 (2007) 2427–2431. 13) K. F. Cai, D. S. McLachlan, N. Axen and R. Manyatsa: Ceram. Intern. 28 (2002) 217–222. 14) C. Maerky, M.-O. Guillou, J. L. Henshall and R. M. Hooper: Mater. Sci. Eng. A209 (1996) 329–336.