available online at www.sciencedirect.com SCIENCE DIRECTO MATERIALS CHEMISTRY AND PHYSICS ELSEVIER Materials Chemistry and Physics 84(2004)243-246 www.elsevier.com/locate/matchemphys A study of the crystal structure of a commercial B-Sic whisker by high-resolution TEM L. Geng*, J. Zhang School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 433, Harbin 150001, PR China Received 7 January 2003; received in revised form 6 May 2003; accepted 28 May 2003 Abstract The crystal structure and defects in a commercial TWS-100 B-SiC whisker were observed and analyzed by means of high-resolution transmission electron microscopy. It was found that the SiC whiskers with triangular or hexagonal cross-sections both have a face-centere cubic structure, while their crystal defects are different. The defects in the triangular whisker are mainly stacking faults on the( 1) planes which are not perpendicular to the whisker axis, however, in the hexagonal whisker there are a great amount of micro-twins and stacking faults on the (111) planes perpendicular to the whisker axis The high-density defects account for the hexagonal close-packed (HCP) diffraction patter obtained in the HCP B-SiC whiskers, which is first pointed out by analyzing the crystal structure of the B-SiC whisker. 2003 Elsevier B. V. All rights reserved. Keywords: Ceramics; Composite materials, High-resolution transmission electron microscopy; Microstructure 1.Introduction TWS-100 B-SiC whiskers made by the vapor-liquid-solid method was observed and analyzed. B-SiC whiskers have been widely used as an effective The crystal structure and defects of the TWS-100 B-SiC reinforcement in metal matrix composites [1] and a tough- whiskers have been previously studied by an electron mi- ening phase in ceramic matrix composites [2]. For example crodiffraction technique [7], indicating that there are two because of the addition of high-performance SiC whiskers, kinds of B-SiC whiskers which have hexagonal and trian- SiCw/Al composites have shown a higher strength and mod gular shapes in transverse section. The triangular whisker known that the nature of the si is a single crystal with a face-centered cubic(FCC)crystal structure, while the hexagonal whisker consists of many ends on the e SiC/Al interface, which greatly thin hexagonal close-packed (HCP) and FCC crystal lay- Al com-ers which stack along the axis of the whisker [7]. In the present research, the crystal structure of the hexagonal ture of the SiC whiskers was examined and determined by means of the interfa of a JEM-4000EX high-resolution transmission electron microscope. New results which are meaningful for the un- microstructure erstanding of the crystal structure of the Sic whiskers although there have were obtaine B-SiC w ods. The vapor technique to produce B-Sic w 2. Experimental details ture of the SiC whiskers different. In the present paper, the crystal structure of the The TWS-100 B-SiC whiskers were made by the vapor-liquid-solid method and received from Tokai Car- bon, Japan. The diameter and length of the B-SiC whiskers *Corresponding author. Tel.: +86-451-641-8674; are.- 1.0 and 30- 100 respestively. Because it dif- fax:+86-451-641-3922 ficul t to prepare specimens of the SiC whiskers suitable for E-mail address: genglin @hit.edu.cn (L. Geng). observation by transmission electron microscopy (TEM). 0254-0584/S-see front matter 2003 Elsevier B. V. All rights reserved. doi:10.1016/s254-0584(03)00281-5
Materials Chemistry and Physics 84 (2004) 243–246 A study of the crystal structure of a commercial -SiC whisker by high-resolution TEM L. Geng∗, J. Zhang School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 433, Harbin 150001, PR China Received 7 January 2003; received in revised form 6 May 2003; accepted 28 May 2003 Abstract The crystal structure and defects in a commercial TWS-100 -SiC whisker were observed and analyzed by means of high-resolution transmission electron microscopy. It was found that the SiC whiskers with triangular or hexagonal cross-sections both have a face-centered cubic structure, while their crystal defects are different. The defects in the triangular whisker are mainly stacking faults on the (1 1 1) planes which are not perpendicular to the whisker axis, however, in the hexagonal whisker there are a great amount of micro-twins and stacking faults on the (1 1 1) planes perpendicular to the whisker axis. The high-density defects account for the hexagonal close-packed (HCP) diffraction patter obtained in the HCP -SiC whiskers, which is first pointed out by analyzing the crystal structure of the -SiC whisker. © 2003 Elsevier B.V. All rights reserved. Keywords: Ceramics; Composite materials; High-resolution transmission electron microscopy; Microstructure 1. Introduction -SiC whiskers have been widely used as an effective reinforcement in metal matrix composites [1] and a toughening phase in ceramic matrix composites [2]. For example, because of the addition of high-performance SiC whiskers, SiCw/Al composites have shown a higher strength and modulus compared with unreinforced aluminum alloys [3]. It is known that the nature of the SiC/Al interface, which greatly depends on the microstructure of the SiC whisker, plays the most important role in the properties of the SiCw/Al composites. Therefore, a further understanding of the microstructure of the SiC whisker will contribute to the improvement of the interface bonding and properties of the composites reinforced or toughened by SiC whiskers. However, the microstructure of the -SiC whiskers is not clear enough, although there have been some reports about them [4–7]. -SiC whiskers have been produced by numerous methods. The vapor–liquid–solid process is a widely adopted technique to produce -SiC whiskers [8]. The microstructure of the SiC whiskers made by various techniques is different. In the present paper, the crystal structure of the ∗ Corresponding author. Tel.: +86-451-641-8674; fax: +86-451-641-3922. E-mail address: genglin@hit.edu.cn (L. Geng). TWS-100 -SiC whiskers made by the vapor–liquid–solid method was observed and analyzed. The crystal structure and defects of the TWS-100 -SiC whiskers have been previously studied by an electron microdiffraction technique [7], indicating that there are two kinds of -SiC whiskers which have hexagonal and triangular shapes in transverse section. The triangular whisker is a single crystal with a face-centered cubic (FCC) crystal structure, while the hexagonal whisker consists of many thin hexagonal close-packed (HCP) and FCC crystal layers which stack along the axis of the whisker [7]. In the present research, the crystal structure of the hexagonal -SiC whiskers was examined and determined by means of a JEM-4000EX high-resolution transmission electron microscope. New results which are meaningful for the understanding of the crystal structure of the SiC whiskers were obtained. 2. Experimental details The TWS-100 -SiC whiskers were made by the vapor–liquid–solid method and received from Tokai Carbon, Japan. The diameter and length of the -SiC whiskers are 0.1–1.0 and 30–100m, respectively. Because it is dif- ficult to prepare specimens of the SiC whiskers suitable for observation by transmission electron microscopy (TEM), 0254-0584/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0254-0584(03)00281-5
L Geng, J. Zhang/ Materials Chemistry and Physics 84(2004)243-246 a SiCW/Al composite was fabricated by a squeeze casting According to the results from the analysis of the diffrac route. Thin lamellae of the composite were cut by spark tion patterns, it has been pointed out in Ref. [7] that the crys- machining and then were thinned by ion milling. In the tal structure of the hexagonal B-SiC whisker consists of both composite specimen, there are many SiC whiskers being FCC and HCP crystal structure. In order to further inves- oriented in various directions and suitable for tem obser tigate the crystal structure of the hexagonal B-SiC whisker vation. Therefore, the microstructure and crystal defects of and explain the hcp diffraction pattern shown in Fig. 2b, he p-SiC whiskers could be examined by means of TEM. the hexagonal B-SiC whiskers were examined in detail by a high-resolution TEM, and the results are shown in Fig. 3, in which it should be mentioned that the closely spaced Si-C 3. Results and discussion pairs are not resolved. From Fig 3a it can be seen that in all the perfect crystal areas, the crystal structure is FCC in which The morphology and microstructure of a triangular B-Sic the(111) plane stack in a sequence of ABCABCABC whisker are shown in Fig. 1. Both images(Fig. la and c)and Fig 3a also shows clearly the micro-twins(marked by T) the diffraction pattern(Fig. 1b) indicate that the triangular and stacking faults(marked by S)in the hexagonal B-Sic whisker is a single crystal with FCC structure. TEM image whisker. In fact, in most of the area of the whisker the den (Fig. la)and high-resolution TEM image( Fig. Ic)also show sity of micro-twins and stacking faults is very high as shown stacking faults in the(11 1)planes, which is consistent with in Fig. 3b. Therefore, it can be concluded that the hexago the previous results [7]. Fig. 2 shows the morphology and nal B-SiC whisker is also a single crystal of FCC structure microstructure of a hexagonal B-SiC whisker. The indexing being characterized by a high density of twins and stacking of the diffraction pattern(Fig. 2b) shows that there are extra faults on(11 1) planes which is perpendicular to the whisker spots in addition to the spots of the [1 1 llFcc zone axis. axis([11 1 direction) It has been proved in Ref. [7 that the diffraction pattern From the high-resolution TEM results, a stacking ig. 2b is consistent with the diffraction pattern of an quence of the close-packed I 1) plane in the hexagonal HCP crystal with the electron beam direction of [0001]. B-SiC whisker can be schematically shown in Fig 4. It can The difference in diffraction patterns between triangular and be seen that in the perfect area, the stacking sequence of the hexagonal whiskers made it easy to distinguish the two kinds (111) plane is ABCABCABC., which will result in the of whiskers in the longitudinal direction by checking thei FCC diffraction pattern. However, in the defect areas, the diffraction patterns in a TEM images stacking sequence becomes ABAB, BCBC, CACA, ACA 02 000 isker a IOnm Fig. 1. Microstructure of the triangular whisker: (a)TEM image of the whisker in transverse section;(b) diffraction pattern of the whisker in(a);( high-resolution TEM image of the whisker in longitudinal section
244 L. Geng, J. Zhang / Materials Chemistry and Physics 84 (2004) 243–246 a SiCw/Al composite was fabricated by a squeeze casting route. Thin lamellae of the composite were cut by spark machining and then were thinned by ion milling. In the composite specimen, there are many SiC whiskers being oriented in various directions and suitable for TEM observation. Therefore, the microstructure and crystal defects of the -SiC whiskers could be examined by means of TEM. 3. Results and discussion The morphology and microstructure of a triangular -SiC whisker are shown in Fig. 1. Both images (Fig. 1a and c) and the diffraction pattern (Fig. 1b) indicate that the triangular whisker is a single crystal with FCC structure. TEM image (Fig. 1a) and high-resolution TEM image (Fig. 1c) also show stacking faults in the (1 1 1) planes, which is consistent with the previous results [7]. Fig. 2 shows the morphology and microstructure of a hexagonal -SiC whisker. The indexing of the diffraction pattern (Fig. 2b) shows that there are extra spots in addition to the spots of the [1 1 1]FCC zone axis. It has been proved in Ref. [7] that the diffraction pattern of Fig. 2b is consistent with the diffraction pattern of an HCP crystal with the electron beam direction of [0 0 0 1]. The difference in diffraction patterns between triangular and hexagonal whiskers made it easy to distinguish the two kinds of whiskers in the longitudinal direction by checking their diffraction patterns in a TEM images. Fig. 1. Microstructure of the triangular whisker: (a) TEM image of the whisker in transverse section; (b) diffraction pattern of the whisker in (a); (c) high-resolution TEM image of the whisker in longitudinal section. According to the results from the analysis of the diffraction patterns, it has been pointed out in Ref. [7] that the crystal structure of the hexagonal -SiC whisker consists of both FCC and HCP crystal structure. In order to further investigate the crystal structure of the hexagonal -SiC whisker and explain the HCP diffraction pattern shown in Fig. 2b, the hexagonal -SiC whiskers were examined in detail by a high-resolution TEM, and the results are shown in Fig. 3, in which it should be mentioned that the closely spaced Si–C pairs are not resolved. From Fig. 3a it can be seen that in all the perfect crystal areas, the crystal structure is FCC in which the (1 1 1) plane stack in a sequence of ABCABCABC... Fig. 3a also shows clearly the micro-twins (marked by T) and stacking faults (marked by S) in the hexagonal -SiC whisker. In fact, in most of the area of the whisker the density of micro-twins and stacking faults is very high as shown in Fig. 3b. Therefore, it can be concluded that the hexagonal -SiC whisker is also a single crystal of FCC structure, being characterized by a high density of twins and stacking faults on (1 1 1) planes which is perpendicular to the whisker axis ([1 1 1] direction). From the high-resolution TEM results, a stacking sequence of the close-packed (1 1 1) plane in the hexagonal -SiC whisker can be schematically shown in Fig. 4. It can be seen that in the perfect area, the stacking sequence of the (1 1 1) plane is ABCABCABC... , which will result in the FCC diffraction pattern. However, in the defect areas, the stacking sequence becomes ABAB, BCBC, CACA, ACA
L Geng, J. Zhang/Materials Chemistry and Physics 84(2004)243-240 E02 Fig. 2. Microstructure of a hexagonal B-SiC whisker: (a) TEM image of the whisker in transverse section; (b) diffraction pattem of the whisker in(a) Whisker axis Whisker axi l 11111 Fig. 3. High-resolution images of the whiskers in the longitudinal direction showing micro-twins and stacking faults (a) and a high density of defects b)(the specimen orientation both is [01 1]). BAB. CBC. ABA. BCB or CAC. which is formed in the con dition shown in Table 1. It can be found that all the stacking sequences shown in Table 1 correspond to the HCP struc Whisker axis ture. Although each of these areas is too thin( three or four atom layers)to form enough diffraction strength, because they have same structure and same crystal orientation, the overlapping of each diffraction will lead to an obvious HCP diffraction patterns as shown in Fig. 2b. Therefore, it is con- cluded that the HCP diffraction patterns shown in Fig. 2b dose not mean that the hexagonal B-siC whisker contains HCP crystal layers, but that they are due to the large amount of micro-twins and stacking faults in the(111) plane of the whisker Stacking sequence of the atoms in the hexagonal B-Sic whisker Perfect area tacking faults ABCABC ABA ABAB Fig 4. A model of stacking sequence of the close-packed (1 1 the hexagonal B-Sic whisker (each dot corresponds to a pair of atoms CAC CACA
L. Geng, J. Zhang / Materials Chemistry and Physics 84 (2004) 243–246 245 Fig. 2. Microstructure of a hexagonal -SiC whisker: (a) TEM image of the whisker in transverse section; (b) diffraction pattern of the whisker in (a). Fig. 3. High-resolution images of the whiskers in the longitudinal direction showing micro-twins and stacking faults (a) and a high density of defects (b) (the specimen orientation both is [0 1 1]). ¯ A B A B T S A B C A B A B C A B C A B C B A C B A B C B Whisker axis [111] Fig. 4. A model of stacking sequence of the close-packed (1 1 1) plane in the hexagonal -SiC whisker (each dot corresponds to a pair of atoms). BAB, CBC, ABA, BCB or CAC, which is formed in the condition shown in Table 1. It can be found that all the stacking sequences shown in Table 1 correspond to the HCP structure. Although each of these areas is too thin (three or four atom layers) to form enough diffraction strength, because they have same structure and same crystal orientation, the overlapping of each diffraction will lead to an obvious HCP diffraction patterns as shown in Fig. 2b. Therefore, it is concluded that the HCP diffraction patterns shown in Fig. 2b dose not mean that the hexagonal -SiC whisker contains HCP crystal layers, but that they are due to the large amount of micro-twins and stacking faults in the (1 1 1) plane of the whisker. Table 1 Stacking sequence of the atoms in the hexagonal -SiC whisker Perfect area Twins Stacking faults Intrinsic Extrinsic ABCABC... ABA ABAB ACA BCB BCBC BAB CAC CACA CBC
L Geng, J. Zhang/ Materials Chemistry and Physics 84(2004)243-246 Based on the explanation of the formation of the HCP Acknowledgements diffraction patterns in a hexagonal p-SiC whisker, it is initially pointed out that the hexagonal TWS-100 B-SiC The authors acknowledge the financial supports from the whisker is also a single crystal with FCC structure. This National Nature Science Foundation of China under grant conclusion is very important for correctly analyzing the number 50071018 side surface structure of the whisker and understanding the mechanism of the interface bonding in a metal matrix com- osite reinforced by the SiC whisker and a ceramic matrix References composite toughened by the SiC whisker 1]. Geng, S Ochiai, J Q. Hu, C K. Yao, Mater. Sci. Eng. A 246(1998) 2]F. Ye, T.Q. Lei, Y. Zhou, Mater. Sci. Eng. A 281(2000)305 4. Conclusions 3]ZY. Ma, Y.X. Lu, M. Luo, J. Bi, J Mater. Sci. TechnoL. 11(1995) that the hexagonal TWS-100 B-SiC whisker is a single [5]SR Nutt lom Ceram Soc. 71(1988)149 By means of high-resolution TEM observation, it is clear m Ceram. Soc. 67(1984)428 crystal with FCC structure. There are many micro-twins 6Y. Yan, J. Chen, L. Wang, Q. Li, D. Feng, L Cao, C. Yao, Mater. Le.8(1989)305 and stacking faults in the(1 1 1)plane perpendicular to the [7]Q. Liu, K. Wu, L Geng, C.K. Yao, Mater. Sci Eng. A 130(1990) whisker axis. The high density of defects account for the HCP diffraction patterns obtained in the hexagonal B-Sic [8]Jv Milewski, F.D. Gac, J.J. Petrovic, S.R. Skaggs, J Mater. Sci. 20 whiskers (1985)1160
246 L. Geng, J. Zhang / Materials Chemistry and Physics 84 (2004) 243–246 Based on the explanation of the formation of the HCP diffraction patterns in a hexagonal -SiC whisker, it is initially pointed out that the hexagonal TWS-100 -SiC whisker is also a single crystal with FCC structure. This conclusion is very important for correctly analyzing the side surface structure of the whisker and understanding the mechanism of the interface bonding in a metal matrix composite reinforced by the SiC whisker and a ceramic matrix composite toughened by the SiC whisker. 4. Conclusions By means of high-resolution TEM observation, it is clear that the hexagonal TWS-100 -SiC whisker is a single crystal with FCC structure. There are many micro-twins and stacking faults in the (1 1 1) plane perpendicular to the whisker axis. The high density of defects account for the HCP diffraction patterns obtained in the hexagonal -SiC whiskers. Acknowledgements The authors acknowledge the financial supports from the National Nature Science Foundation of China under grant number 50071018. References [1] L. Geng, S. Ochiai, J.Q. Hu, C.K. Yao, Mater. Sci. Eng. A 246 (1998) 302. [2] F. Ye, T.Q. Lei, Y. Zhou, Mater. Sci. Eng. A 281 (2000) 305. [3] Z.Y. Ma, Y.X. Lu, M. Luo, J. Bi, J. Mater. Sci. Technol. 11 (1995) 291. [4] S.R. Nutt, J. Am. Ceram. Soc. 71 (1988) 149. [5] S.R. Nutt, J. Am. Ceram. Soc. 67 (1984) 428. [6] Y. Yan, J. Chen, L. Wang, Q. Li, D. Feng, L. Cao, C. Yao, Mater. Lett. 8 (1989) 305. [7] Q. Liu, K. Wu, L. Geng, C.K. Yao, Mater. Sci. Eng. A 130 (1990) 113. [8] J.V. Milewski, F.D. Gac, J.J. Petrovic, S.R. Skaggs, J. Mater. Sci. 20 (1985) 1160