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High-temperature deformation-induced defects and Burgers vector determination of dislocations in the Al70Co15Ni15 decagonal quasicrystal
Journal of Non-Crystalline Solids 153&154 (1993) 103-107 North-Holland
~
] O U R N A L OF
U
High-temperature deformation-induced defects and Burgers vector determination of dislocations in the A170ColsNi15 decagonal quasicrystal R. W a n g
a,b
Y.F. Y a n a,b a n d K . H . K u o b
a Laboratory of Material Physics, Department of Physics, Wuhan University, 430072 Wuhan, China b Beijing Laboratory of Electron Microscopy, Academia Sinica, P,O. Box 2724, 100080 Beijing, China
By means of high-temperature deformation, stacking faults, dislocations and dislocation pairs have been induced abundantly in the A170ColsNi15 decagonal phase. Burgers vector b, their directions, their senses and their magnitudes, of dislocations were determined by large-angle convergent-beam electron diffraction (LACBED). Moreover, three kinds of small-angle grain boundaries (SAGBs) were observed in an annealed A170CotsNi15decagonal phase: (1) a tilt boundary consisting of an edge dislocation wall; (2) a twist boundary consisting of parallel screw dislocation pairs; and (3) a small-angle grain boundary consisting of a mixed dislocation network.
1. Introduction Studies of defects in quasicrystals (QCs) have drawn extensive attention. Zhang and his colleagues determined the directions of Burgers vectors of some dislocations in both an icosahedral Q C (IQC) [1] and a decagonal Q C ( D Q C ) [2]. Wang et al. [3] observed a small dislocation loop in an AI76Si4Mn20 IQC, and identified its Burgers vector to be parallel to one of the twofold axes of the IQC, Dai et al. [4] identified the displacement vector directions and habit planes of stacking faults in an A170ColsNi15 DQC. Discommensurations and domains in M n - S i - A I octagonal phase also have been studied by Jiang et al. [5]. However, all the defects observed in previous works were formed during the process of alloy preparation. It is useful to introduce defects in QCs by high-temperature deformation, not only to measure their properties and formation conditions, but also to explore the deformation mechanism of QCs and the effects of defects on physi-
Correspondence to: Dr R. Wang, Department of Physics,
Wuhan University, 430072 Wuhan, China. Tel: + 86-27 722712 ext208. Telefax: + 86~27 712661.
cal properties of QCs. Moreover, works on determination of the sense and magnitude of Burgers vectors of dislocations in QCs are still lacking. Wang and Chen [6] extended the dynamical theory of electron diffraction from the case of crystals to the case of QCs. Wang and Dai [7] studied the effect of dislocations in QCs on the shifting and splitting of higher-order reflections using the dynamical theory of electron diffraction. These studies show that the conventional contrast analysis and C B E D technique for the geometrical characteristic identification of stacking faults and dislocations may also be used in the case of QCs. In the present p a p e r we report our studies on defects introduced by high-temperature deformation and determination of the Burgers vectors including their directions, senses and magnitudes, of dislocations in the Al70COlsNit5 DQC.
2. Experimental The alloy Al70fOl5Nil5 was p r e p a r e d by melting the pure metals in an induction furnace in an A r atmosphere. After cooling to room tempera-
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
104
R. Wanget al. / Dislocations in the A17oColsNi15 decagonalquasicrystal
ture, the ingot was cut into slices. Some of the slices were annealed at 900 K for 24 h in an Ar atmosphere. Some of the slices were annealed at 1070 K for 4 h and compressed into pieces in situ,and then air-cooled. Electron diffraction patterns (EDPs) of these specimens reveal that there is ordering along the tenfold axis and the periodicity of the annealed A I - C o - N i D Q C increased from 0.4 to 0.8 nm in the as-cast specimens. In this paper we use the index system proposed by Yan et al. [8], where the first index corresponds to the periodic tenfold axis A10 with a period of 0.8 nm.
3. Results
3.1. High-temperature deformation-induced defects In most regions of the high-temperature deformed A I - C o - N i D Q C we observed either densely distributed stacking faults shown in fig. l(a) or densely distributed dislocations with larger screw component shown in fig. l(b). Figure l(a) is a center dark-field (CDF) image under the g = ( 0 0 0 0 1 0 ) two-beam condition, which shows a
fringe contrast typical of stacking faults. Contrast analysis and trace analysis reveal that the habit plane of these stacking faults is perpendicular to the A10 axis and their displacement vector R lies in the fault plane and parallel to the A2D axis. Fig. l(b) shows a g / 3 g weak-beam (WB) image of high-density dislocations introduced by high-temperature deformation in the A 1 - C o - N i DQC, where g = ( 4 0 0 0 0 0 ) is parallel to the A10 axis. Contrast analysis revealed that the Burgers vector of these dislocations is parallel to the A10 axis. Many dislocations are distributed pairwise (indicated A - D in fig. l(b)). It is found that the separation of dislocation pairs in the g / 3 g and - g / - 3g WB images do not change with a reversal of g. This invariance implies that they are dislocation pairs consisting of two adjacent dislocations with the same Burgers vector. Values of the separation r of the two constituent partial dislocations range from 10 to 50 nm. 3.2. Burgers vector determination of dislocations The insets in fig. 2 show an edge dislocation D I D 1 (fig. 2(a)) and a screw dislocation D2D 2 (fig. 2(b)) determined by diffraction contrast analyses. Figures 2(a)-(c) are L A C B E D patterns near
Fig. 1. High-temperature, deformation-induced,high-densitydefects:(a) stacking faults; (b) dislocations and dislocationpairs.
R. Wang et al. / Dislocations in the A17oCOlsNi15decagonal quasicrystal
105
Fig. 2. LACBED patterns near the A2P zone axis when the incident beam illuminates the positive edge dislocation D~D~ (a) and right-hand screw dislocations D~D~ (b) and D~D~ (c). Right-hand insets are contrast images of dislocations; those on the left are the corresponding simulated diffraction lines in the CBED patterns.
the A 2 P zone axis m a r k e d by a white star w h e n the incident b e a m is illuminated on the edge dislocation D1D1, screw dislocation D 2 D 2 and a n o t h e r screw dislocation, respectively. F o r these patterns, the crossover of the incident b e a m lies at the u p p e r side o f the foil. T h e shadow images of these dislocations in the L A C B E D patterns are D~D~, D~D~ and D~D~ respectively. T h e dislocation D~D~ in fig. 2(a) crosses the deficient lines ( 0 1 1 - 1 - 10) and ( 0 1 0 0 - 10), and lies n e a r the line ( 4 0 0 0 0 0 ) . T h e lines ( 0 1 1 - 1 - 10) neither split nor shift, but the line ( 4 0 0 0 0 0 ) splits into three lines forming two
nodes. These observations are consistent with the result of a contrast analysis that the Burgers vector b I o f the dislocation D I D 1 is parallel to the A10 axis: b l = [ b l 0 0 0 0 0 ] . H e n c e we have [ g(400000)"bl[ = 2 and b 1 = [ 1 / 2 0 0 0 0 0 ] . T h e dislocation D ~ D 2 induces the deficient line ( 4 0 0 0 0 0 ) to split into three nodes, so that b 2 = _+[ 3 / 4 0 0 0 0 0 ) . F r o m the twist sense of the deficient line ( 4 0 0 0 0 0 ) we d e t e r m i n e d the sense of the dislocation D~D~ to be the same as the dislocation line according to Niu et al. [9]. T h e r e fore, we have b z = [ 3 / 4 0 0 0 0 0 ] and a right-hand screw dislocation D z D 2. Similarly,the screw dislo-
Fig. 3. BF images of SAGBs, g HA10: (a) tilt boundary consisting of edge dislocations; (b) twist boundary consisting of screw dislocations; and (c) SAGB consisting of dislocation networks,
106
R. Wang et al. / Dislocations in the A170ColsNi15 decagonal quasicrystal
cation D~D~ in fig. 2(c) induces the splitting of the line ( - 4 0 0 0 0 0 ) into four nodes and we determined its Burgers vector to be b 3 = [ 1 0 0 0 0 0 ] , which is also a right-hand screw dislocation. We have simulated successfully the observed defocus CBED patterns when a dislocation crosses or lies near a higher-order deficient line. The left insets in figs. 2(a)-(c) show the respective simulated L A C B E D patterns. The good agreement between the experimental and simulated patterns supports our determination of the senses and magnitudes of the Burgers vectors of the dislocations in the A170COlsNi15 DQC.
Hence, this kind of SAGB is a twist boundary consisting of parallel screw dislocations. Figure 3(c) shows the BF image of the third kind of SAGB under two-beam conditions. It can be seen that this kind of SAGB consists of dislocation networks. For the dislocations parallel to A A in fig. 3(c), the direction of the Burgers vector is along the A10 axis. These are mixed dislocations and arranged as dislocation pairs. The dislocation pairs constrict themselves at the nodes of the dislocation networks.
3.3. Small-angle grain boundaries (SAGBs)
4.1. Microscopic mechanism of high-temperature deformation
In addition to the random dislocations we found dislocation walls and networks in the annealed A 1 - C o - N i DQC, which form SAGBs. Figure 3(a) shows a bright-field (BF) image of the first kind of SAGB under g = [ 4 0 0 0 0 0 ] twobeam conditions. The contrast analysis using various diffraction vectors revealed the Burgers vector direction of the dislocations in this SAGB to be parallel to the A10 axis and perpendicular to the dislocation lines. These are obviously edge dislocations. Therefore, the first kind of S A G B is a tilt boundary consisting of an edge dislocation wall. Figure 3(b) shows the BF image of the second kind of SAGB under two-beam conditions. According to the diffraction contrast analysis, the Burgers vector direction of the dislocations in this SAGB is parallel to the A10 axis (g) and the dislocation lines. These are screw dislocations.
~9
4. D i s c u s s i o n
When the maximum resolved shear stress is T1, as shown in fig. 4(a), the slip plane will, be perpendicular to the A10 axis and is a quasiperiodic plane. Consequently the regions slipped by the moving dislocation become stacking fault regions, as shown in fig. l(a), with two peripheral dislocations. When the maximum resolved shear stress component is z 2 as shown in fig. 4(b), the slip plane will be parallel to the A10 axis and the slip direction is also parallel" to this periodic axis. In this case the plastic deformation may take place by the motion and multiplication of dislocations (denoted DD in fig. 4b) with the Burgers vector along the periodic axis as shown in fig, l(b) in the same way as in conventional crystals. The third case is shown in fig. 4(c), where the maximum resolved shear stress is ~'3. Therefore, the slip plane will be parallel to the A10 axis and the slip
Alo
AIO
J
Ta/ a
b
C
Fig. 4. Schematicdiagram illustrating the microscopicmechanism of high-temperaturedeformation in the decagonalphase.
R. Wanget al. / Dislocations in the A17oCo15Ni15 decagonal quasicrystal direction will be parallel to a twofold axis A2, which is a quasiperiodic direction. Similar to the case shown in fig. 4(a), the regions slipped by the moving dislocations become stacking fault regions with two peripheral dislocations. Indeed, Dai et al. [4] observed such stacking faults in A 1 - C o - N i D Q C with the habit plane perpendicular to the A2P axis and the displacement vector R lying in the fault plane and parallel to an A 2 D axis. In this case, high-temperature deformation may be caused by the phase transformation and thermal stresses induced by inhomogeneous solidification and cooling of the specimens. 4.2. Burgers vectors o f dislocations In this p a p e r we determined three Burgers vectors of dislocations in the AI70COlsNil5 D Q C for the first time. These are a positive edge dislocation with b 1 -- [ 1 / 2 0 0 0 0 0], and two right-hand screw dislocations with b = [ 3 / 4 0 0 0 0 0 ] and b 3 = [100000]. The fraction components ( 1 / 2 and 3 / 4 ) of the Burgers vectors along the A10 direction may be explained by the fact that the annealed AI70CoI5Ni15 D Q C is an ordered phase with a period of c = 0.8 nm along the A10 axis, which is twice of that of the disordered Al70COl5Nil5 DQC. Such an ordered D Q C may be described as stacked by atomic planes according to the sequence A B A ' B ' with a plane distance of 0.2 nm. Therefore, dislocations with Burgers vectors b 1 = [ 1 / 2 0 0 0 0 0 ] , b2--[3/ 4 0 0 0 0 0 ] and b 3 = [ 1 0 0 0 0 0 ] correspond to the displacements of 2, 3 and 4 atom layers respectively along the A10 axis between two sides of the dislocations. 4.3. Formation o f parallel screw dislocation wall Parallel screw dislocations repel each other and are unstable in conventional crystals. Such a
107
dislocation arrangement does exist in QCs (see fig. 3(b)), because the dislocations are very difficult to move.
5. Conclusions In the A170ColsNi15 D Q C densely distributed stacking faults, dislocations and dislocation pairs have been induced by means of high-temperature compression. One positive edge dislocation with Burgers vector b~ = [ 1 / 2 0 0 0 0 0 ] and two righthand screw dislocations with b: = [ 3 / 4 0 0 0 0 0 ] and b 3 = [ 1 0 0 0 0 0 ] have been identified using the L A C B E D technique. Moreover, tilt and twist SAGBs consisting of edge dislocation wall, parallel screw dislocation pairs and a mixed dislocation network have been observed. This project has been supported by the National Natural Science Foundation of China.
References [1] Z. Zhang and K. Urban, Phil. Mag. Lett. 60 (1989) 97. [2] Z. Zhang, M. Wollgarten and K. Urban, Phil. Mag. Lett, 61 (1990) 125. [3l Z.G. Wang, R. Wang and W.F, Deng, Phys. Rev. Lett. 66 (1991) 2124. [4] M.X. Dai, R. Wang, J.N. Gui and Y.F. Yan, Phil. Mag, Lett. 64 (1991) 21. [5] J.C. Jiang, N. Wang, K.K. Fung and K.H. Kuo, Phys, Rev. Lett. 67 (1991) 1302. [6] R. Wang and Y. Cheng, Mater. Sci. Forum 22-24 (1987) 409. [7] R. Wang and M.X. Dai, in Proc. China-Japan Seminar on Quasicrystals, ed. K.H. Kuo and T. Ninomiya (World Scientific, Singapore, 1991) p. 182. [8l Y.F. Yan, R. Wang, J.N. Gui and M.X. Dai, Acta Crystallogr. (1992). [9] F. Niu, G. Lu and R. Wang, Acta Crystallogr. A47 (1991) 36.
~
] O U R N A L OF
U
High-temperature deformation-induced defects and Burgers vector determination of dislocations in the A170ColsNi15 decagonal quasicrystal R. W a n g
a,b
Y.F. Y a n a,b a n d K . H . K u o b
a Laboratory of Material Physics, Department of Physics, Wuhan University, 430072 Wuhan, China b Beijing Laboratory of Electron Microscopy, Academia Sinica, P,O. Box 2724, 100080 Beijing, China
By means of high-temperature deformation, stacking faults, dislocations and dislocation pairs have been induced abundantly in the A170ColsNi15 decagonal phase. Burgers vector b, their directions, their senses and their magnitudes, of dislocations were determined by large-angle convergent-beam electron diffraction (LACBED). Moreover, three kinds of small-angle grain boundaries (SAGBs) were observed in an annealed A170CotsNi15decagonal phase: (1) a tilt boundary consisting of an edge dislocation wall; (2) a twist boundary consisting of parallel screw dislocation pairs; and (3) a small-angle grain boundary consisting of a mixed dislocation network.
1. Introduction Studies of defects in quasicrystals (QCs) have drawn extensive attention. Zhang and his colleagues determined the directions of Burgers vectors of some dislocations in both an icosahedral Q C (IQC) [1] and a decagonal Q C ( D Q C ) [2]. Wang et al. [3] observed a small dislocation loop in an AI76Si4Mn20 IQC, and identified its Burgers vector to be parallel to one of the twofold axes of the IQC, Dai et al. [4] identified the displacement vector directions and habit planes of stacking faults in an A170ColsNi15 DQC. Discommensurations and domains in M n - S i - A I octagonal phase also have been studied by Jiang et al. [5]. However, all the defects observed in previous works were formed during the process of alloy preparation. It is useful to introduce defects in QCs by high-temperature deformation, not only to measure their properties and formation conditions, but also to explore the deformation mechanism of QCs and the effects of defects on physi-
Correspondence to: Dr R. Wang, Department of Physics,
Wuhan University, 430072 Wuhan, China. Tel: + 86-27 722712 ext208. Telefax: + 86~27 712661.
cal properties of QCs. Moreover, works on determination of the sense and magnitude of Burgers vectors of dislocations in QCs are still lacking. Wang and Chen [6] extended the dynamical theory of electron diffraction from the case of crystals to the case of QCs. Wang and Dai [7] studied the effect of dislocations in QCs on the shifting and splitting of higher-order reflections using the dynamical theory of electron diffraction. These studies show that the conventional contrast analysis and C B E D technique for the geometrical characteristic identification of stacking faults and dislocations may also be used in the case of QCs. In the present p a p e r we report our studies on defects introduced by high-temperature deformation and determination of the Burgers vectors including their directions, senses and magnitudes, of dislocations in the Al70COlsNit5 DQC.
2. Experimental The alloy Al70fOl5Nil5 was p r e p a r e d by melting the pure metals in an induction furnace in an A r atmosphere. After cooling to room tempera-
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
104
R. Wanget al. / Dislocations in the A17oColsNi15 decagonalquasicrystal
ture, the ingot was cut into slices. Some of the slices were annealed at 900 K for 24 h in an Ar atmosphere. Some of the slices were annealed at 1070 K for 4 h and compressed into pieces in situ,and then air-cooled. Electron diffraction patterns (EDPs) of these specimens reveal that there is ordering along the tenfold axis and the periodicity of the annealed A I - C o - N i D Q C increased from 0.4 to 0.8 nm in the as-cast specimens. In this paper we use the index system proposed by Yan et al. [8], where the first index corresponds to the periodic tenfold axis A10 with a period of 0.8 nm.
3. Results
3.1. High-temperature deformation-induced defects In most regions of the high-temperature deformed A I - C o - N i D Q C we observed either densely distributed stacking faults shown in fig. l(a) or densely distributed dislocations with larger screw component shown in fig. l(b). Figure l(a) is a center dark-field (CDF) image under the g = ( 0 0 0 0 1 0 ) two-beam condition, which shows a
fringe contrast typical of stacking faults. Contrast analysis and trace analysis reveal that the habit plane of these stacking faults is perpendicular to the A10 axis and their displacement vector R lies in the fault plane and parallel to the A2D axis. Fig. l(b) shows a g / 3 g weak-beam (WB) image of high-density dislocations introduced by high-temperature deformation in the A 1 - C o - N i DQC, where g = ( 4 0 0 0 0 0 ) is parallel to the A10 axis. Contrast analysis revealed that the Burgers vector of these dislocations is parallel to the A10 axis. Many dislocations are distributed pairwise (indicated A - D in fig. l(b)). It is found that the separation of dislocation pairs in the g / 3 g and - g / - 3g WB images do not change with a reversal of g. This invariance implies that they are dislocation pairs consisting of two adjacent dislocations with the same Burgers vector. Values of the separation r of the two constituent partial dislocations range from 10 to 50 nm. 3.2. Burgers vector determination of dislocations The insets in fig. 2 show an edge dislocation D I D 1 (fig. 2(a)) and a screw dislocation D2D 2 (fig. 2(b)) determined by diffraction contrast analyses. Figures 2(a)-(c) are L A C B E D patterns near
Fig. 1. High-temperature, deformation-induced,high-densitydefects:(a) stacking faults; (b) dislocations and dislocationpairs.
R. Wang et al. / Dislocations in the A17oCOlsNi15decagonal quasicrystal
105
Fig. 2. LACBED patterns near the A2P zone axis when the incident beam illuminates the positive edge dislocation D~D~ (a) and right-hand screw dislocations D~D~ (b) and D~D~ (c). Right-hand insets are contrast images of dislocations; those on the left are the corresponding simulated diffraction lines in the CBED patterns.
the A 2 P zone axis m a r k e d by a white star w h e n the incident b e a m is illuminated on the edge dislocation D1D1, screw dislocation D 2 D 2 and a n o t h e r screw dislocation, respectively. F o r these patterns, the crossover of the incident b e a m lies at the u p p e r side o f the foil. T h e shadow images of these dislocations in the L A C B E D patterns are D~D~, D~D~ and D~D~ respectively. T h e dislocation D~D~ in fig. 2(a) crosses the deficient lines ( 0 1 1 - 1 - 10) and ( 0 1 0 0 - 10), and lies n e a r the line ( 4 0 0 0 0 0 ) . T h e lines ( 0 1 1 - 1 - 10) neither split nor shift, but the line ( 4 0 0 0 0 0 ) splits into three lines forming two
nodes. These observations are consistent with the result of a contrast analysis that the Burgers vector b I o f the dislocation D I D 1 is parallel to the A10 axis: b l = [ b l 0 0 0 0 0 ] . H e n c e we have [ g(400000)"bl[ = 2 and b 1 = [ 1 / 2 0 0 0 0 0 ] . T h e dislocation D ~ D 2 induces the deficient line ( 4 0 0 0 0 0 ) to split into three nodes, so that b 2 = _+[ 3 / 4 0 0 0 0 0 ) . F r o m the twist sense of the deficient line ( 4 0 0 0 0 0 ) we d e t e r m i n e d the sense of the dislocation D~D~ to be the same as the dislocation line according to Niu et al. [9]. T h e r e fore, we have b z = [ 3 / 4 0 0 0 0 0 ] and a right-hand screw dislocation D z D 2. Similarly,the screw dislo-
Fig. 3. BF images of SAGBs, g HA10: (a) tilt boundary consisting of edge dislocations; (b) twist boundary consisting of screw dislocations; and (c) SAGB consisting of dislocation networks,
106
R. Wang et al. / Dislocations in the A170ColsNi15 decagonal quasicrystal
cation D~D~ in fig. 2(c) induces the splitting of the line ( - 4 0 0 0 0 0 ) into four nodes and we determined its Burgers vector to be b 3 = [ 1 0 0 0 0 0 ] , which is also a right-hand screw dislocation. We have simulated successfully the observed defocus CBED patterns when a dislocation crosses or lies near a higher-order deficient line. The left insets in figs. 2(a)-(c) show the respective simulated L A C B E D patterns. The good agreement between the experimental and simulated patterns supports our determination of the senses and magnitudes of the Burgers vectors of the dislocations in the A170COlsNi15 DQC.
Hence, this kind of SAGB is a twist boundary consisting of parallel screw dislocations. Figure 3(c) shows the BF image of the third kind of SAGB under two-beam conditions. It can be seen that this kind of SAGB consists of dislocation networks. For the dislocations parallel to A A in fig. 3(c), the direction of the Burgers vector is along the A10 axis. These are mixed dislocations and arranged as dislocation pairs. The dislocation pairs constrict themselves at the nodes of the dislocation networks.
3.3. Small-angle grain boundaries (SAGBs)
4.1. Microscopic mechanism of high-temperature deformation
In addition to the random dislocations we found dislocation walls and networks in the annealed A 1 - C o - N i DQC, which form SAGBs. Figure 3(a) shows a bright-field (BF) image of the first kind of SAGB under g = [ 4 0 0 0 0 0 ] twobeam conditions. The contrast analysis using various diffraction vectors revealed the Burgers vector direction of the dislocations in this SAGB to be parallel to the A10 axis and perpendicular to the dislocation lines. These are obviously edge dislocations. Therefore, the first kind of S A G B is a tilt boundary consisting of an edge dislocation wall. Figure 3(b) shows the BF image of the second kind of SAGB under two-beam conditions. According to the diffraction contrast analysis, the Burgers vector direction of the dislocations in this SAGB is parallel to the A10 axis (g) and the dislocation lines. These are screw dislocations.
~9
4. D i s c u s s i o n
When the maximum resolved shear stress is T1, as shown in fig. 4(a), the slip plane will, be perpendicular to the A10 axis and is a quasiperiodic plane. Consequently the regions slipped by the moving dislocation become stacking fault regions, as shown in fig. l(a), with two peripheral dislocations. When the maximum resolved shear stress component is z 2 as shown in fig. 4(b), the slip plane will be parallel to the A10 axis and the slip direction is also parallel" to this periodic axis. In this case the plastic deformation may take place by the motion and multiplication of dislocations (denoted DD in fig. 4b) with the Burgers vector along the periodic axis as shown in fig, l(b) in the same way as in conventional crystals. The third case is shown in fig. 4(c), where the maximum resolved shear stress is ~'3. Therefore, the slip plane will be parallel to the A10 axis and the slip
Alo
AIO
J
Ta/ a
b
C
Fig. 4. Schematicdiagram illustrating the microscopicmechanism of high-temperaturedeformation in the decagonalphase.
R. Wanget al. / Dislocations in the A17oCo15Ni15 decagonal quasicrystal direction will be parallel to a twofold axis A2, which is a quasiperiodic direction. Similar to the case shown in fig. 4(a), the regions slipped by the moving dislocations become stacking fault regions with two peripheral dislocations. Indeed, Dai et al. [4] observed such stacking faults in A 1 - C o - N i D Q C with the habit plane perpendicular to the A2P axis and the displacement vector R lying in the fault plane and parallel to an A 2 D axis. In this case, high-temperature deformation may be caused by the phase transformation and thermal stresses induced by inhomogeneous solidification and cooling of the specimens. 4.2. Burgers vectors o f dislocations In this p a p e r we determined three Burgers vectors of dislocations in the AI70COlsNil5 D Q C for the first time. These are a positive edge dislocation with b 1 -- [ 1 / 2 0 0 0 0 0], and two right-hand screw dislocations with b = [ 3 / 4 0 0 0 0 0 ] and b 3 = [100000]. The fraction components ( 1 / 2 and 3 / 4 ) of the Burgers vectors along the A10 direction may be explained by the fact that the annealed AI70CoI5Ni15 D Q C is an ordered phase with a period of c = 0.8 nm along the A10 axis, which is twice of that of the disordered Al70COl5Nil5 DQC. Such an ordered D Q C may be described as stacked by atomic planes according to the sequence A B A ' B ' with a plane distance of 0.2 nm. Therefore, dislocations with Burgers vectors b 1 = [ 1 / 2 0 0 0 0 0 ] , b2--[3/ 4 0 0 0 0 0 ] and b 3 = [ 1 0 0 0 0 0 ] correspond to the displacements of 2, 3 and 4 atom layers respectively along the A10 axis between two sides of the dislocations. 4.3. Formation o f parallel screw dislocation wall Parallel screw dislocations repel each other and are unstable in conventional crystals. Such a
107
dislocation arrangement does exist in QCs (see fig. 3(b)), because the dislocations are very difficult to move.
5. Conclusions In the A170ColsNi15 D Q C densely distributed stacking faults, dislocations and dislocation pairs have been induced by means of high-temperature compression. One positive edge dislocation with Burgers vector b~ = [ 1 / 2 0 0 0 0 0 ] and two righthand screw dislocations with b: = [ 3 / 4 0 0 0 0 0 ] and b 3 = [ 1 0 0 0 0 0 ] have been identified using the L A C B E D technique. Moreover, tilt and twist SAGBs consisting of edge dislocation wall, parallel screw dislocation pairs and a mixed dislocation network have been observed. This project has been supported by the National Natural Science Foundation of China.
References [1] Z. Zhang and K. Urban, Phil. Mag. Lett. 60 (1989) 97. [2] Z. Zhang, M. Wollgarten and K. Urban, Phil. Mag. Lett, 61 (1990) 125. [3l Z.G. Wang, R. Wang and W.F, Deng, Phys. Rev. Lett. 66 (1991) 2124. [4] M.X. Dai, R. Wang, J.N. Gui and Y.F. Yan, Phil. Mag, Lett. 64 (1991) 21. [5] J.C. Jiang, N. Wang, K.K. Fung and K.H. Kuo, Phys, Rev. Lett. 67 (1991) 1302. [6] R. Wang and Y. Cheng, Mater. Sci. Forum 22-24 (1987) 409. [7] R. Wang and M.X. Dai, in Proc. China-Japan Seminar on Quasicrystals, ed. K.H. Kuo and T. Ninomiya (World Scientific, Singapore, 1991) p. 182. [8l Y.F. Yan, R. Wang, J.N. Gui and M.X. Dai, Acta Crystallogr. (1992). [9] F. Niu, G. Lu and R. Wang, Acta Crystallogr. A47 (1991) 36.