Symmetry and crystal structure of Ti3SiC2

August 1994

Materials Letters 20 (1994) 319-324

Symmetry and crystal structure of Ti3SiC2 Sowmya Arunajatesan, A.H. Carim Department

of Materials Science and Engineering,

The Pennsylvania

’

State University, University Park, PA 16802, USA

Received 20 May 1994; accepted 23 May 1994

Abstract Convergent beam electron diffraction (CBED) and X-ray diffractometry (XRD) have been used to examine the symmetry, structure and lattice parameters of Ti,SiC,. The CBED analysis confirms that the material is hexagonal with a point group of 6/ mmm. The XRD diffraction peak positions and intensities are in good agreement with calculations based on previously proposed atomic positions.

1. Introduction

The Ti-Si-C system is of interest both for composites (Ti alloys reinforced with Sic) and for ceramic joining (brazing of SIC using Ti or Ti-containing foils) [ I-41. The interactions within the system have been widely investigated and the ternary phase diagram at elevated temperatures has been investigated f 5-71. While the formation of binary reaction products at Ti/SiC interfaces has been associated with a deterioration in the mechanical properties of the materials [8-l 11, the formation of the ternary compound, Ti3SiC2, appears to be desirable [ 12-141. Some of the properties of T&Sic2 and Ti,SiC,-containing materials have been reported; a possible melting point of 3000°C [ 151 combined with some suggestions of plastic behavior [ 16-l 8 ] give an indication of why Ti$iCz is not detrimental as an interfacial reaction product. A knowledge of the atomic and defect structure would clearly be valuable in understanding this unusual combination of properties. There is a limited literature available on the synthesis and characterization of Ti3SiC2 [ 15- 19 1. Our ’ Author to whom correspondence should be addressed.

studies on the synthesis of the phase have been reported elsewhere 1201. The only reported structure investigation of Ti3SiCz was conducted by Jeitschko and Nowotny [ 191 on single crystal samples using XRD techniques. A hexagonal structure was proposed and the lattice parameters of the hexagonal unit cell were found to be a= 3.06 8, and c= 17.66 A. The atomic positions of Ti correspond to the 2a (z,,=0.135), Si to the 2b, and C to the 4f (2,=0.567) Wyckoff positions of the space group D&,-P63/mmc (space group number 194) [21]. XRD data obtained by Nickl et al. (on single crystal Ti$i&) [ 161 and Goto and Hirai (on polycrystalline Ti3SiCz made by chemical vapor deposition) [ 17 ] indicate only slightly different unit cell parameters, as shown in Table 1. In previous studies, only small single crystals Table 1 Lattice parameters of TisSiC2 as determined by different authors Lattice parameters (A)

Reference

a= 3.068, c= 17.669

Jeitschko and Nowotny 1967 [ 191 Nick1 et al. 1972 [ 161 Goto and Hirai 1987 [ 17)

a= 3.066, c= 17.646 az3.064, c= f 7.65

0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0167-577x(94)00130-F

320

S. Arunajatesan,A.H. Carim /Materials Letters20 (I 994) 319-324

[ 16,191 or poiy~~stalline samples [ 17 ] have been available for analysis. Our recent work has allowed production of bulk Ti3SiCz samples and powders j20 1. This Letter first describes the symmetry analysis of this material by convergent beam electron diffraction (CBED) in the transmission electron microscope (TEM), which provides an independent method for verifying the information from X-ray measurements. Crystal structure is also evaluated by X-ray diffractometry (XRD) of powders, and the peak intensities are compared with those expected based on the proposed atomic structure. Defect analysis is currently in progress and will be described in a future report.

2. Experimental procedure The synthesis of bulk samples consisting of over 98 volI Ti3SiC2 has been discussed elsewhere [ 201. TEM samples were made by standard polishing and ion-beam thinning techniques. The microscopy was done on a Philips 420T instrument at the Materials Characterization Laboratory, The Pennsylvania State University, at an operating voltage of 120 kV. Simulated diffraction patterns were obtained using the commercial software package Diffract. Samples for diffractometry were prepared by crushing the bulk samples; XRD was performed on a Rigaku diffractometer using Cu Ku radiation.

3. Results and discussion Fig. la is a CBED pattern obtained from a Ti3SiC2 grain along the [ 000 1 ] zone axis. If we consider only the low-order reflections which appear as discs around the central transmitted beam, there is a 6-fold axis of rotational symmetry as well as two independent and mutually perpendicular mirror planes (each of which is reproduced every 60” by the action of the 6-fold axis). The symmetry of the zero-order Laue zone is thus seen to be 6mm. The whole pattern symmetry is more evident in Fig. lb, obtained from a thicker sample region, and is also 6mm. The distinct mirror planes are indicated. Fig. lc, which has been recorded at a longer camera length, shows the higher order Laue zone (HOLZ) lines within the transmit-

ted disc. In this figure, it is evident that the crystal orientation is slightly off the exact zone axis direction. Nevertheless, the symmet~ of the HOLZ lines is also clearly 6mm. Fig. Id is the computer-simulated HOLZ line pattern based on the structure proposed by Jeitschko and Nowotny [ 191; it is discussed below. From the relationships established by Buxton et al. [ 221, this combination of the three symmetries corresponds to one of two possible diffraction groups 6mm or 6mmln. From Table 3 in Buxton et al.‘s report [ 22 1, the diffraction group 6mm corresponds to the crystal point group 6mm and the diffraction group 6mmlR corresponds to the point group 6/mmm. To uniquely determine the point group of Ti3SiCz, it is necessary to eliminate one of these two possibilities. For this purpose, it is instructive to examine diffraction along the other primary zone axis in this hexagonal material, [ 2 1lo]. Fig. 2a is a CBED pattern recorded along the [ 2 1lo] zone axis. The corresponding selected area diffraction pattern (SADP) is shown in Fig. 2b. Note the extremely short distance between the diffraction spots corresponding to a large d-spacing (dW, x 17.7 A) along one direction. As a consequence of such a large d-spacing, any practical condenser aperture size produces overlapping discs in CBED patterns recorded along this direction (Fig. 2a). Hence, no clear conclusions can be drawn about the symmetry of the zero-order zone or the HOLZ lines from the CBED information in Fig. 2a. However, the symmetry of the zero-order zone is clearly seen in the SADP and is 2mm. A zero-order symmetry of 2mm corresponds to any of four diffraction groups: 2mRmR, 2mm, 2,mm,, or 2mm lR. From Buxton et al’s tables [ 22 1, it is clear that of the two possible crystal point groups established from the diffraction patterns in Fig. 1, 6mm is not consistent with any of these two-fold diffraction groups. Hence, the crystal point group of Ti,SiC, is uniquely determined to be 6/mmm. The complete analysis is summarized in Table 2. As mentioned earlier, Jeitschko and Nowotny [ 19 1, based on their XRD studies, proposed that the space group of T&Sic2 was D&-P6,/mmc and suggested specific atomic positions for Ti, Si, and C. The point group of Ti3SiCz determined in this work, 6/mmm, is consistent with the P6,/mmc space group. Further space group info~ation from CBED relies on the

S. Arunajatesan, A.H. Carim /Materials Letters 20 (1994) 319-324

321

m

I

m-

-m

Fig. 1.co‘nvi:rgent beam ele:ctron diffraction patterns of Ti3SiC2 along the [OOOl ] zone axis showing (a) the sjrrnmetry in the!Z era,-order zone, (b) theI whole pattern symmetry, (c) the HOLZ line symmetry and (d) the HOLZ line pattern simulate1d using Dif frac:t,ba!ied on latticr: par-ameters proposed I by Jeitschko and Nowotny [ 191.

S. A~~aj~t~s~~, A.H. Carim I ~ateriu~s Letters 20 (1994) 319-324

322

d

Fig. I. Continued.

observation of dynamic absences in the patterns that indicate the presence of glide planes and/or screw axes. The patterns recorded along [ 00011 and [i? 1lo] in Ti3SiC2 did not show any dynamic absences, although overlapping discs in the latter case make inte~retation difficult. Based on the structure proposed by Jeitschko and Nowotny, the commircially available software Diffract was used to generate HOLZ line patterns along [ 000 1] within the transmitted disc, The actual recorded pattern and the simulation are shown in Figs. lc and Id and it is seen that they are in excellent agreement, thus providing support for the previously suggested atomic positions [191* The higher order Laue zone rings seen in the CBED patterns contain information about the crystal dimension parallel to the electron beam; this can also be used to further characterize the structure. It has been shown [ 231 that the reciprocal layer spacing H, in units of distance in real space, can be calculated

Fig. 2. [? IlO] diffraction patterns from Ti,SiCz recorded in (a) convergent beam and (b ) selected area modes.

from the first order Laue zone (FOLZ) ring radius using the equation I!?, =2/AG2, where I&, is the measured lattice layer spacing, A is the wavelength of the electron beam and G is the measured radius of the FOLZ ring in reciprocal units. This m~surement gives the value of H without any prior knowledge of the crystal structure or lattice parameters. For a hexagonal system, H can then be related to the lattice parameters by [ 231:

S. Arunajatesan.A.H. Carim /Materials Letters 20 (1994) 319-324

323

Table 2 Determination of the point group of Ti3SiC2 from convergent beam and selected area electron diffraction patterns Zone axis

Symmetry of zero-order zone

Whole pattern symmetry

HOLZ line symmetry

Possible diffraction groups

Possible point groups

(0001)

6mm

6mm

6mm

6mm, 6mm1,

6mm, 6/mmm

(2110)

2mm

not clear

not clear

cannot be 6mm

must be 6/mmm

Table 3 Comparison of experimental and calculated XRD data. The calculated values are based on the atomic positions proposed by Jeitschko and Nowotny [ 191. Note that measured peak positions are to the nearest increment of 0.05” in 28, since the data were collected in increments of this size. Peak intensities indicated by “ t” may include contributions from the small amount of secondary phase present in the sample Indices

0006 loii ioi3 lOT4 0008 lOT5 lOi lOi 1120 ioiio ioii2 2024 1128

28

Peak intensity

Lattice spacing

measured

calculated

measured

calculated

measured

calculated

30.30 34.10 37.10 39.55 40.80 42.50 53.90 58.40 60.30 63.15 73.40 74.45 75.30

30.33 34.10 37.09 39.55 40.82 42.53 53.94 58.40 60.28 63.14 73.47 74.53 75.38

2.941 2.623 2.421 2.217 2.210 2.123 1.700 1.579 1.534 1.471 1.289 1.270 1.261

2.945 2.627 2.422 2.217 2.209 2.124 1.698 1.579 1.534 1.471 1.288 1.272 1.262

7.4 20.4 5.1 100.0 64.1* 43.8 5.6 19.3 16.3 4.2 14.4’ 6.9 13.8

3.1 30.6 6.8 100.0 19.3 40.2 4.4 13.4 26.1 3.0 8.6 10.5 13.5

where H,is the calculated lattice layer spacing, ( UVW) is the zone axis of CBED pattern and a, c are the lattice parameters. The two H values have been estimated for Ti3SiC2 and are in good agreement; the calculated value of H, (along [0001 ] ) based on the lattice parameters measured by Jeitschko and Nowotny [ 19 ] is 17.66 8, and the measured value H,,,, from a CBED pattern recorded along the [0001 ] zone axis, is 17.8 12 f 0.175 A. Table 3 compares the measured 28 values, d-spacings, and peak intensities for XRD scans of the Ti3SiC2 specimens with the corresponding calculated values based on the crystal structure and lattice parameters proposed by Jeitschko and Nowotny [ 19 1.

Both peak positions and intensities are in good agreement, providing further corroboration of the atomic positions proposed earlier. Least-squares analysis of the present experimental data based on the indexing given in Table 3 produces very similar lattice parameters of a=3.078,and c= 17.69 8, for a hexagonal unit cell, with an average angular deviation of less than 0.023 in the 28 values. The complex Ti$iC2 structure proposed earlier and confirmed here does not suggest any easy mechanisms for slip on planes other than (000 1)) making it difficult to explain the plasticity associated with this compound. Although Burgers vectors lying outside the basal plane are prohibitively long for unit dislocations, it may be possible for partial dislocations to move readily in the crystal if stacking fault or domain boundary energies are sufficiently low. Initial exam-

324

S. Arunajatesan, A.H. Carim /Materials Letters 20 (1994) 319-324

inations have shown some evidence of faulting and possible polytypism; further efforts to clarify the situation are currently underway.

Acknowledgement This work was supported by the Department of the Navy, Office of the Chief of Naval Research, Young Investigator program under grant #NO0014-9 I-J405 1. The information contained herein does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.

References [l]P.R.SmithandF.H.Froes,J.Met. 19 (1984) 19. [2] A.J. Reeves, H. Dunlop and T.W. Clyne, Metall. Trans. 23 A (1992) 917. [3] A.J. Reeves, H. Dunlop and T.W. Clyne, Mater. Sci. Eng. A 141 (1991) 129. [4] G. Das, Metall. Trans. 21 A (1990) 1.571. [5] C.E. Brukl, AFMLTR-65-2 Part II, Vol. 7, Air Force Materials Laboratory, Wright Patterson Air Force Base, OH (1966). [ 61 J.L. Ratliff and G.W. Powell, AFML Tech. Rep. 70-42, US. Dept. of Comm. NTIS, VA (1970).

[ 71 S. Sambasivan and W.T. Petuskey, J. Mater. Res. 7 ( 1992) 1473. [S] J.A. Snide, F.A. Ashdown and J.R. Meyers, Fiber Sci. Technol. 5 (1972) 61. [ 91 H.J. Dudek, R. Leucht and G. Ziegler, in: Proc. 15th Intern. Conf. on Ti, Vol. 3 (DGM, Munich, 1984) p. 1773. [lo] H.J. Dudek, L.A. Larson and R. Browning, Surf Interface Anal. 6 (1984) 274. [ 111 E.P. Zironi and H. Poppa, J. Mater. Sci. 16 ( 198 1) 3 115. [ 12 ] E. Gyarmati, W. Kestemich and R. Forthmann, Ceram. Forum Intern. 66 (1989) 292. [ 131 S. Morozumi, M. Endo, M. Kikuchi and K. Hamajima, J. Mater. Sci. 20 (1985) 3976. [ 141 T. Nishino, S. Urai and M. Naka, Eng. Fract. Mech. 40 (1991) 829. [ 151 R. Pampuch, J. Lis, L. Stobierski and M. Tymkiewicz, J. Eur. Ceram. Sot. 5 (1989) 283. [ 161 J.J. Nickl, K.K. Schweitzer and P. Luxenberg, J. LessCommon Met. 26 (1972) 335. [ 171 T. Goto and T. Hirai, Mater. Res. Sot. Bull. 22 ( 1987) 1195. [ 181 R. Pampuch, J. Lis, J. Piekarczyk and L. Stobierski, J. Mat. Syn. Process. 1 (1993) 93. [ 191 W. Jeitschko and H. Nowotny, Monatsh. Chem. 98 (1967) 329. [20] S. Arunajatesan and A.H. Carim, J. Am. Ceram. Sot., submitted for publication. [21] T. Hahn, ed., International tables for crystallography, 3rd Ed. (Kluwer, Dordrecht, 1992). [22] B.F. Buxton, J.A. Eades, J.W. Steeds and G.M. Rackham, Phil. Trans. Roy. Sot. A 281 (1976) 171. [ 231 M. Raghavan, S.C. Scanlon and J.W. Steeds, Metall. Trans. 15A(1984) 1299.