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On the structural determination of silicides in titanium alloys
Scripta M E T A L L U R G I C A
Vol. 21, pp. 1615-1617, 1987 Printed in the U.S.A.
Pergamon Journals, Ltd. All rights reserved
On The Structural Determination of Silicides Titanium Alloys
in
D.Banerjee Defence M e t a l l u r g i c a l Research Laboratory Hyderabad-500 258, INDIA (Received April 20, 1987) (Revised September 24, 1987) Introduction Woodfield and Loretto (I) recently reported the determination of the symmetry of the S o Sillcide (2) in an IMI829 alloy ~3,4) by convergent beam electron diffraction. They assign a space group P62m which differs from our earller conclusion (3) that the space group is either P6/mmm or P6./mcm (4). Woodfield and Loretto also point out that a single CBP along the [ i ~ 0 ] zone axis, as presented in our work (3), is inadequate to distinguish between these possibilities. While we agree with this latter statement, we present here additional evidence for the 6/mmm point group for the $2 silicide, and point out some inconsistencies in the Woodfield and Loretto analysis. Symmetry D e t e r m i n a t i o n We first present a clarification of our published work (3) in that the d e t e r m i n a t i o n of the space group for the $2 silicide was actually carried out on $2 silicides in a T i - 3 A 1 - 8 V - 6 C r - 4 z r - 4 M o alloy (4). This work is referred to in (3) as reference (12). Since the lattice parameters of the silicide in Ti-38644 (4) and IMI829 (3) were identical and Zr was found in both sil~cides, we assume that the space group derived in reference (4) would hold for the $2 siliclde in IMI829 as well. The distinction between the 62m point group and the 6/mmm can be made in the CBP along the [0001] zone axis as seen in Table-i which is abstracted from Buxton et a1.(5). There is no disagreement between (i) and (3) about the whole pattern symmetry of the [11~0] zone axis which is 2mm. However, we reproduce from (4) the CBP of the [0001] zone axis in Fig.la. The whole pattern symmetry is 6mm with a mirror perpendicular to both [1050] and [ii~0] in disagreement with woodfield and Loretto (i). The existence of a m i r r o r p e r p e n d i c u l a r to [ii~0], which is the point of contention, was confirmed in another zone axis containing the [11~0] plane as shown in Fig.l~ ~e conclude, therefore, that the point group of the $2 silicide in Ti-86444 is indeed 6/mmm. The space group P6/mmm for this point group was also confirmed in (4) and the details of this exercise are not given here. Discussion We have shown that the $2 Silicide in the Ti-38644 has a 6/mmm symmetry in disagreement with the results of Woodfield and Loretto (i) on the $2 Silicide in IMI829. Since the lattice parameters of the silicides in the two alloys are similar (a = 6.98; c = 3.65 in Ti-38644 and a = 7.14; c = 3.74 in IMI829), this disagreement is somewhat surprising. However, we point out possible sources of error in the symmetry determination by Woodfield and Loretto. Fig.2a ,b show the $2 silicide in Ti-38644 in 2 perpendicular orientations, parallel to [0001] and 1615 0036-9748/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals
Ltd.
1616
SILICIDES
IN Ti
Vol.
21 No.
12
[Ii~0],
respectively. Planar defects are seen to run across the silicide in the orientation. We found that if adequate care was not taken to avoid these faults, a projection diffraction symmetry of 3m or even just m could be obtained in the ZOLZ of [0001] patterns as shown in Fig.3. We suggest that the reduced symmetry of 3m observed by Woodfield and Loretto (i) in the [0001] zone axis ZOLZ could arise from such effects.
[1170]
Finally, there are inconsistencies in the Woodfield and Loretto paper (i) between text, tables and figures which make for confusing reading. Thus, in the text and Fig.3b, the whole pattern symmetry of the [1010] zone axis is shown as 2mm while the table indicates it as m. Again the text suggests that the whole pattern symmetry of the [0001] zone axis is 6mm while the table indicates it as 3m. These points require clarification. Acknowledgements The author is grateful to Director, DMRL for permission to publish this work. Partial support of the work by RMI, USA, while the author was st Carnegie Mellon University, is gratefully acknowledged. References l.A.P.Woodfield and M.H.Loretto, Scripts Met., 21, 229 (1987). 2.F.Barbier, C.Servant, C.Quesne and M.P.Lacombe, J.Mic.Spec. Elec., 6, 299 (1981). 3.D.Banerjee, J.E.Allison, F.H.Froes and J.C.Willlams, 'Titanium, Science and Technology', Ed. G.Lutjering, U.Zwicker and W.Bunk, Deutsche Gesslschaft Metal1., Oberursel, 1519 (1985). 4.D.Banerjee, S.Ankem and J.C.Williams, unpublished research, Carnegie Mellon University, USA, (1984). 5.B.F.Buxton, J.A.Eades, J.W.Steeds and G.M.Rackham, Phil. Trans. Roy. Soc., A281, 171 (1976). Table-1 Zone axis
Point group
: CBP S~anmetries
Diffraction group
Whole pattern symmetry
~2m
3m 1 R
3m
6/mmm
6mm i R
6mm
~2m
2mm
2mm
6/mmm
2mm 1 R
2mm
[0001]
[112o]
* The table is altered from Buxton et al (5) in that the CBP s~mmetries shown are for the ~2m point group rather than 6m2 as given in (5).
Vol.
21 No.
12
SILICIDES IN Ti
1617
F i g . l : a)[[ 0001] CBDP f r o m $2 Silicide in Ti-t86#0, b)[ 1~O2] CBDP with the zero order p a t t e r n correctly oriented as inset. Note the single mirror perpendicular to [ 1150]
a
L Fig.2 : $2 silicide in a) [ 0001] zone axis and b) [ 11~0] zone axis
Fig.3 - z o L z in[ 0002] zone axis a) 3m symmetry b) m symmetry
Vol. 21, pp. 1615-1617, 1987 Printed in the U.S.A.
Pergamon Journals, Ltd. All rights reserved
On The Structural Determination of Silicides Titanium Alloys
in
D.Banerjee Defence M e t a l l u r g i c a l Research Laboratory Hyderabad-500 258, INDIA (Received April 20, 1987) (Revised September 24, 1987) Introduction Woodfield and Loretto (I) recently reported the determination of the symmetry of the S o Sillcide (2) in an IMI829 alloy ~3,4) by convergent beam electron diffraction. They assign a space group P62m which differs from our earller conclusion (3) that the space group is either P6/mmm or P6./mcm (4). Woodfield and Loretto also point out that a single CBP along the [ i ~ 0 ] zone axis, as presented in our work (3), is inadequate to distinguish between these possibilities. While we agree with this latter statement, we present here additional evidence for the 6/mmm point group for the $2 silicide, and point out some inconsistencies in the Woodfield and Loretto analysis. Symmetry D e t e r m i n a t i o n We first present a clarification of our published work (3) in that the d e t e r m i n a t i o n of the space group for the $2 silicide was actually carried out on $2 silicides in a T i - 3 A 1 - 8 V - 6 C r - 4 z r - 4 M o alloy (4). This work is referred to in (3) as reference (12). Since the lattice parameters of the silicide in Ti-38644 (4) and IMI829 (3) were identical and Zr was found in both sil~cides, we assume that the space group derived in reference (4) would hold for the $2 siliclde in IMI829 as well. The distinction between the 62m point group and the 6/mmm can be made in the CBP along the [0001] zone axis as seen in Table-i which is abstracted from Buxton et a1.(5). There is no disagreement between (i) and (3) about the whole pattern symmetry of the [11~0] zone axis which is 2mm. However, we reproduce from (4) the CBP of the [0001] zone axis in Fig.la. The whole pattern symmetry is 6mm with a mirror perpendicular to both [1050] and [ii~0] in disagreement with woodfield and Loretto (i). The existence of a m i r r o r p e r p e n d i c u l a r to [ii~0], which is the point of contention, was confirmed in another zone axis containing the [11~0] plane as shown in Fig.l~ ~e conclude, therefore, that the point group of the $2 silicide in Ti-86444 is indeed 6/mmm. The space group P6/mmm for this point group was also confirmed in (4) and the details of this exercise are not given here. Discussion We have shown that the $2 Silicide in the Ti-38644 has a 6/mmm symmetry in disagreement with the results of Woodfield and Loretto (i) on the $2 Silicide in IMI829. Since the lattice parameters of the silicides in the two alloys are similar (a = 6.98; c = 3.65 in Ti-38644 and a = 7.14; c = 3.74 in IMI829), this disagreement is somewhat surprising. However, we point out possible sources of error in the symmetry determination by Woodfield and Loretto. Fig.2a ,b show the $2 silicide in Ti-38644 in 2 perpendicular orientations, parallel to [0001] and 1615 0036-9748/87 $3.00 + .00 Copyright (c) 1987 Pergamon Journals
Ltd.
1616
SILICIDES
IN Ti
Vol.
21 No.
12
[Ii~0],
respectively. Planar defects are seen to run across the silicide in the orientation. We found that if adequate care was not taken to avoid these faults, a projection diffraction symmetry of 3m or even just m could be obtained in the ZOLZ of [0001] patterns as shown in Fig.3. We suggest that the reduced symmetry of 3m observed by Woodfield and Loretto (i) in the [0001] zone axis ZOLZ could arise from such effects.
[1170]
Finally, there are inconsistencies in the Woodfield and Loretto paper (i) between text, tables and figures which make for confusing reading. Thus, in the text and Fig.3b, the whole pattern symmetry of the [1010] zone axis is shown as 2mm while the table indicates it as m. Again the text suggests that the whole pattern symmetry of the [0001] zone axis is 6mm while the table indicates it as 3m. These points require clarification. Acknowledgements The author is grateful to Director, DMRL for permission to publish this work. Partial support of the work by RMI, USA, while the author was st Carnegie Mellon University, is gratefully acknowledged. References l.A.P.Woodfield and M.H.Loretto, Scripts Met., 21, 229 (1987). 2.F.Barbier, C.Servant, C.Quesne and M.P.Lacombe, J.Mic.Spec. Elec., 6, 299 (1981). 3.D.Banerjee, J.E.Allison, F.H.Froes and J.C.Willlams, 'Titanium, Science and Technology', Ed. G.Lutjering, U.Zwicker and W.Bunk, Deutsche Gesslschaft Metal1., Oberursel, 1519 (1985). 4.D.Banerjee, S.Ankem and J.C.Williams, unpublished research, Carnegie Mellon University, USA, (1984). 5.B.F.Buxton, J.A.Eades, J.W.Steeds and G.M.Rackham, Phil. Trans. Roy. Soc., A281, 171 (1976). Table-1 Zone axis
Point group
: CBP S~anmetries
Diffraction group
Whole pattern symmetry
~2m
3m 1 R
3m
6/mmm
6mm i R
6mm
~2m
2mm
2mm
6/mmm
2mm 1 R
2mm
[0001]
[112o]
* The table is altered from Buxton et al (5) in that the CBP s~mmetries shown are for the ~2m point group rather than 6m2 as given in (5).
Vol.
21 No.
12
SILICIDES IN Ti
1617
F i g . l : a)[[ 0001] CBDP f r o m $2 Silicide in Ti-t86#0, b)[ 1~O2] CBDP with the zero order p a t t e r n correctly oriented as inset. Note the single mirror perpendicular to [ 1150]
a
L Fig.2 : $2 silicide in a) [ 0001] zone axis and b) [ 11~0] zone axis
Fig.3 - z o L z in[ 0002] zone axis a) 3m symmetry b) m symmetry