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Quasicrystals in rapidly solidified alloys of AlPt group metals-IV. Quasicrystals in rapidly solidified AlPd and AlPt alloys
Journal of the Less-Common
Metals, 163 (1990) 37-49
31
QUASICRYSTALS IN RAPIDLY SOLIDIFIED ALLOYS OF AI-Pt GROUP METALS-IV. QUASICRYSTALS IN RAPIDLY SOLIDIFIED Al-Pd AND Al-Pt ALLOYS L. MA*, R. WANG and K. H. KUO* Department of Materials Physics, University of Science and Technology, Beijing, loo083 Beijing (China)
(Received January 29, 1990; in revised form April 12, 1990)
Summary A decagonal quasicrystal has been found in a rapidly solidified A1,Pd alloy but not in an Al,Pt alloy. A metastable hexagonal phase isostructural with aAlFeSi found in Al& also exists in these two alloys. On heating to 600 “C, the Al-Pd decagonal quasicrystal transformed to an orthorhombic A1,Pd phase (a = 2.34, b = 1.67, and c = 1.23 mn) with vacancy-ordered long period CsCl-type structure.
1. Introduction The presence of a decagonal quasicrystal in Al-Pd has been well documented especially in connection with the discussion of diffraction peak shift and line broadening caused by the frozen phason strain in quasicrystals [5-71. In fact, the observation of a fivefold electron diffraction pattern and quasiperiodic distribution of spots in rapidly solidified Al-Pd alloys can be traced back to as early as 1978-1981 [8, 91, and the presence of a phase related to the icosahedral quasicrystal was noted in 1985 [lo]. On the other hand, the presence of an Al-Pt decagonal quasicrystal was mentioned only occasionally by Bancel and coworkers [l, 5, lo], and no details have been reported. Vapour deposition of the Al-Pd alloys resulted in the formation of a homogeneous supersaturated solid solution [ll]. Upon annealing, two intermetallic phases were precipitated: A1,Pd has an orthorhombic structure with a = 1.29, b = 1.68 and c= 2.35 nm, while A1,Pd has a hexagonal structure with a = 1.303 and c=O.958 nm [ll]. On heating to a temperature above 700 K, A1,Pd transformed to vacancy-ordered long period CsCl structure (the so-called z phases) while Al,Pd remained unchanged [ 121. In the rapidly solidified Al-Pd alloys containing 6-20 at.% Pd, however, only A1,Pd (a = 2.2505, b= 1.7729 and [l-7],
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c = 1.184 mn, originally designated as A1,Pd [9] but later changed to Al,Pd [12]) coexisted with aluminium solid solution, the volume fraction of Al,Pd increasing with palladium concentration. Sometimes a metastable Al,Pd phase with a CaF,type cubic structure was also detected and, according to Sastry and Suryanarayana [9], it has a strong crystallographic relationship with the Al,Pd. The electron diffraction pattern of the Al,Pd phase with a hexagon of strong diffraction spots [3] is similar or even identical with the twofold D pattern (nomenclature after Fung et al. [13]) of the decagonal phase. This led Sastry and Suryanarayana [3] to suggest that the A1,Pd they observed earlier [8, 9, 121 is in fact the decagonal quasicrystal. However, Thangaraj et al. [4] found that the crystalline phase coexisting with the decagonal phase is a rhombohedrally distorted cubic phase with a = 1.98 nm. Obviously, the question of the quasicrystalline and crystalline phases in rapidly solidified aluminium-rich Al-Pd and Al-Pt alloys is still open. In order to clarify this we undertook a systematic transmission electron microscopy (TEM) study of the phases existing in rapidly solidified A1,Pd and A1,Pt alloys and their transformation after heating to 600 “C for 0.5-16 h. A preliminary report has already been published [ 141. The Al,Pd and A1,Pt alloys were prepared by melting pure aluminium (99.97%) with palladium (99.9%) and platinum (99.9%) respectively in an arc furnace under an argon atmosphere. They were then remelted in quartz tubes in a high frequency induction furnace in argon, and injected onto the surface of a copper wheel, 340 mm in diameter, from a nozzle of diameter 1 mm at the tip of a quartz tube under a pressure of 0.1 MPa. Thin ribbons were then obtained by spinquenching and the cooling rate was about lo5 K s-l. The ribbons were cut into small pieces and thinned by ion-milling before TEM observation. In order to study the phase transformation process in these alloys, specimens were annealed at 600 “C for 0.5, 1,2,4,8 and 15 h. These foils were examined in a JEM- 1OOCXII electron microscope equipped with a large angle double-tilting goniometer stage. The composition analysis of various crystalline phases was made in a Philips EM420 electron microscope fitted with an EDAX 9 100 system for energy-dispersive X-ray spectroscopy (EDS).
2. Decagonal quasicrystal Decagonal quasicrystals formed easily and extensively in the A1,Pd alloy during the melt-spin process. The electron diffraction patterns (EDPs) in the top rows of Figs. 1 and 2 belong to this phase. That in the top left corner of Figs. 1 and 2 is the tenfold EDP. The EDPs in the top row of Fig. 1 were obtained by tilting the decagonal quasicrystal under examination through various angles around the horizontal PP’ axis in the tenfold EDP (after Fung et al. [ 131). After rotating 90” the twofold D pattern [ 131 with six strong spots forming a hexagonal appeared. The tenfold axis, now in the vertical direction, is periodic and the periodicity is about 1.64 nm (the strong spot in the vertical direction in this EDP corresponds to
34’
0’
66O
66O
71°
36’
36’
36’
60’
60’
60’
Fig. 1. Corresponding electron diffraction patterns: top row, Al-W decagonal quasicrystal tilted 90” around PP’ from the tenfold EDP to the twofold D EDP; middle row, AI,Pd tilted 90” from [OlO] to [00 I]; bottor n row, A1,Pd tilted 90”from [OlO] to [308].
q
31°
0”
64’
64’
37O
36’
79O
79O
9o”
60’
Fig. 2. Corresponding electron diffraction patterns: top row, AI-W decagonal quasicrystal tilted 90” around DD’ from the tenfold EDP to the twofold P EDP; middle row, AI,Pd tilted 90”from [OlO] to [loo]; bottom row, AI,Pd tilted 90”from [OlO] to [507].
o”
41
0.205 nm and there are eight spots altogether between this strong spot and the central transmitted beam). However, the Al-Mn decagonal quasicrystal has six spots between the strong spot and the central beam, and it has a periodic@ of 1.23 nm [2]. This difference has been noted before not only in Al-Pd [2-4] but also in Al-Fe [13], Al-Rh and Al-Ir [15], Al-Co [16] and Al-Ni [17]. As a matter of fact, Sastry et al. [S] had already noticed such a difference in the EDPs of rapidly solidified Al-Mn-Ni and Al-Pd alloys, but they overlooked the quasiperiodical arrangement of spots in the direction perpendicular to the periodic tenfold axis. The EDPs in the top row in Fig. 2 were obtained by tilting the decagonal quasicrystal through various angles around the horizontal DD’ axis of the tenfold EDP (after rotating 18” around the tenfold axis from the tenfold EDP in Fig. 1). That in the top right earner is the twofold P pattern with the periodic tenfold axis also in the vertical direction. It is well known that there are only four strong spots between the strong spot and the central beam [2,13,15], indicating the presence of a screw axis [2]. The EDPs obtained after tilting 37”, 64” and 79” are the pseudotwofold (p2), pseudo-fivefold (~5) and pseudo-threefold (~3) patterns respectively. The presence of decagonal quasicrystal in rapidly solidified Al-Pt alloys has been mentioned by Bancel and coworkers [ 1, 5, lo], but they did not publish any details of their experiment. Since then no confirmation of this decagonal quasicrystal has been reported. We have made several attempts to produce this quasicrystal in Al,Pt by rapid solidification as we have done in other alloys of the Al-Pt group metals and we have performed careful TEM studies, but we failed to detect any trace of the decagonal quasicrystal in this system. This at least implies that the decagonal quasicrystal is difficult to form, if indeed it ever forms in the Al-Pt alloy system.
3. Crystalline phases 3.1. Metastable hexagonal phase The same metastable hexagonal phase isostructural with a-AlFeSi found in rapidly solidified Al& [ 14, 151 has also been found in the rapidly solidified A1,Pd and A1,Pt alloys. It occurs in small amounts in the former alloy but is the main constituent in the latter. Since this phase has already been described in detail in a paper dealing with Al-Ir [ 151, no further discussion will be given here. 3.2. Orthogonal Al, Pd This phase did not exist in the melt-spun A1,Pd but it formed gradually from the decagonal quasicrystal after heating at 600 “C for various times. However, even after 8 h at this temperature decagonal quasicrystals can still be found. This orthorhombic phase was first discovered by Koster et al. [ 1 l] and later Sastry and Suryanarayana [3] pointed out the resemblance of its [OOl] EDP to the D pattern of the decagonal quasicrystal (see Fig. 1). We have studied systematically the structural relationship between these two phases both from the experimental and theoretical points of view.
O0
S&3’
7 9:6’
9.0°
Fig. 3. The [OlOJ-[loo]-[OOl] quadrant of the EDPs of AI,Pd showing [OlO] as a pseudo-tenfold axis: the [OOl], [308] and [_503]EDPs are similar to the 2D EDP and the [loo], [507] and [2 0 111 EDPs are similar to the 2P EDP of the decagonal quasicrystal.
A thorough experimental study of the EDPs of this orthorhombic phase at different orientations has been conducted. Figure 3 shows the mutually orthogonal [loo], [OlO] and [OOl] zone patterns and the important intermediate EDPs. Firstly, the [OlO], [loo] and [OOl] EDPs look strikingly similar to the respective tenfold, twofold P and twofold D EDPs of the decagonal quasicrystal (see Figs. 1 and 2 and C$ also Fig. 2 in ref. 15). Secondly, the [OOl], [308] and [503] EDPs with about 36” intervals look very much alike, as do the [ 1001, [507] and [2 0 1 l] EDPs. These two sets of EDPs resemble respectively the twofold D and P EDPs of the decagonal quasicrystal. This once again shows the pseudo-tenfold symmetry of the [0 lo] zone pattern. In order to study the resemblance of the EDPs of these two phases in more detail, EDPs of this orthorhombic phase tilted around some relevant directions are presented in the second and third rows of Figs. 1 and 2 to be compared
43
directly with the corresponding EDPs of the decagonal quasicrystal in the first row. In Fig. 1 the tilting axis for the second row is [loo] and that for the third row [803], and the corresponding axes in Fig. 2 are [OOl] and [‘705] respectively. The distribution of strong spots as well as the angle of tilt of the corresponding pair of EDPs are rather similar. This proves beyond any doubt the structural similarity of these two phases. Zhang and Kuo [18f have recently made a theoretical study of the structural transformation of the decagonal quasicrystal to a relevant crystalline phase. As is well known, the Penrose pattern [ 191 of a two-dimensional quasilattice is made of two kinds of rhombus, a thin one of 36” and a thick one of 72”, arranged aperioditally according to some exact matching rules. A special matching mistake is called a phason, and this will introduce local translational order or periodic tiling of two or even more rhombi of the same kind. Increasing the phason strain in two orthogonal directions in the quasiperiodic plane of the decagonal quasicrystal, for example in one P and one D direction ~rpendicular to it, will eventually introduce long-range periodicity in these two directions. Together with the third periodicity originally existing along the tenfold axis, an orthorhombic crystalline phase will result. The lattice parameters of Al,Pd agree well with those predicted [18]. In other words, Al,Pd is a rational approximation of the irrational Penrose tiling. In reciprocal space, this corresponds to a consistent shift of diffraction spots towards their periodic positions. Several spots may migrate to the same position and become one spot of the crystalline phase. Figures 4(a)-4(d) show the simulated EDPs of the continuous transformation from the decagonal quasicrystal (tenfold EDP in Fig. 4(a)) to the orthorhombic Al,Pd phase ([OlO] EDP in Fig. 4(d)). The latter shows a rectangular array of periodically arranged spots, but the strong spots with more or less tenfold symmetry and an aperiodic arrangement still resemble those of the decagonal quasic~stal. The circle on which the ten spots are evenly located in Fig. 4(a) has become an ellipse in Fig. 4(b), this effect increasing for weaker spots. This is a somewhat distorted tenfold EDP of the decagonal quasicrystal caused by the presence of some phason strain in it. The EDPs corresponding to Figs. 4(c) and 4(d) are shown in Figs. 4(e) and 4(f ). There are more spots in Fig. 4(e) than in Fig, 4(f) depicting the perfect Al&l phase, but they are grouped together showing an average rectangular array not unlike Fig. 4(f). The weak spots have a zigzag appearance, as do the simulated weak spots in Fig. 4(c) with a large phason strain. This can be visualized as the distorted [Olo] EDP of the Al,Pd crystalline phase caused by the presence of some tiling mistakes so that local quasiperiodicity exists. Evidently, we have the follo~g o~entation relations~p~
AI,Pd
[ 100]//2P
decagonal phase
fOOIlN2D Since there are 10 sets of mutually orthogonal twofold P and D directions, the crystalline Al,Pd phase can form along any of these 10 directions. Obviously, there should be 10 possible orientations for the Al,Pd formed from the decagonal phase.
Fig. 4. Simulated tenfold EDP of decagonal quasicrystal (a), which changes gradually through (b) and (c) with increasing phason strain, and finally to the [OlO] EDP of AI,Pd (d). The somewhat distorted [OlO] EDP (c)is shown for comparisonpith (c). The [OlO] EDP of AI,Pd is shown in(f).
45
other words, tenfold rotational twins of 36” around the [0 lo] axis should exist, as in the case reported earlier for NiZr [20] and Al,,Fe, [21]. Figure 5 is a composite EDP of to such twins with two sets of parallel layer lines (outlined in Fig. 5). As discussed above, A1,Pd can be treated as a Penrose tiling approximate of the decagonal quasicrystal. By introducing linear phason strain successively to the two-dimensional Penrose pattern, we obtained finally the tiling model of A1,Pd projected on (010) (see Fig. 6). It consists of the same two kinds of rhombus tiles but now they are arranged periodically. This is a structural model derived from the two-dimensional quasilattice and it accounts well for the diffraction patterns of Al,Pd and its orientation relationship with the decagonal quasicrystal. However, it is too early to predict whether this model is correct or not, since neither the structure of the Al,Pd nor that of the decagonal quasicrystal is known.
In
Fig. 5. A composite [OlO] EDP of three twin variants of AI,Pd (two sets of layer lines are marked).
3.3. Tphases Lu and Chang [22] were the first to make a detailed study of the vacancyordered CsCl phases in Al-Ni-Cu alloys. Vacancies on (111) are arranged periodically in the [ 11 l] direction giving polytypical series of 3, 5, 8, 13, . . . layers which are called t3, t5, rs, r13, . . . phases. The existence of these phases in Al-Pd alloys after annealing at high temperature has been reported by Sastry et al. [ 121. Recently, Chattopadhyay et al. [23] pointed out the possible relationship between this series of t phases and the one-dimensional quasicrystals with periodicities following the
Fig. 6. A tiling model of Al,Pd projected onto (010).
Fibonacci sequence (F, + 1= F,, + F,, _ I), such as the quasicrystals found recently in some Al-Cu-M alloys [ 241. The presence of r3, r5 and rs twin in the annealed A1,Pd ahoy has been confirmed in the present study. Figure 7(a) shows the [I lo] EDP of the simple CsCl structure and Fig. 7(b) shows a corresponding EDP of the r3 phase, in which there are two spots between the 111 spot of the CsCl structure and the central beam dividing the reciprocal g,,, into three parts. According to the same reasoning, Fig. 7(c) might be considered as an EDP of the r, phase, but this is in fact not the case. The distance between two spots in the [ 11 l] direction is now not the same. Moreover, these spots do not lie exactly on a straight line. These anomalies are certainly due to some modulation of the z5 structure. The arrangement of spots in the [ 11 i] direction in Fig. 7(c) is reminiscent of the aperiodic arrangement of spots in quasicrystals. The close relationship between the r phases and the decagonal quasicrystal has already been pointed out [ 251. Since Al,Pd is also closely related to the decagonal quasicrystal, it is natural that a definite orientation relationship exists between A1,Pd and the CsCl structure. Figure 8 shows the composite EDPs of some main zones of these two phases: the dense array of spots belongs to the orthorhombic Al,Pd whereas the few marked strong spots belong to the CsCl structure. The orientation relationship between these two phases, as shown in Figs. 8(a), (b) and (c), is the following:
Fig. 7. EDPs of (a) the CsCl structure, (b) t) and (c) modulated rS.
Fig. 8. Composite EDPs of A&W (dense array of spots) and CsCl structure (marked strong spots): ia) [0101,,,,//[1 iol,; (b)[1001,,,,//[0011,; (4 [OO~l,,,d/[l~Ol,~ (4 [3081AI,W//[1111,.
A1,Pd
[lOO]//[OOl]
CsCl
The weak spots in these EDPs are the results of multiple diffractions. Figure 8(d) is the composite EDP of the [308] EDP of A1,Pd and the [ 11 l] EDP of the cubic CsCl structure. The interzonal angle between [308] and [OOl] is 35.5”, which compares favourably with the 35.3” between [ 1 lo] and [ 11 l] of the CsCl structure. Sastry and Suryanaryana [9] have interpreted the strong spot pattern in Fig. 8(c) as that of A1,Pd with the CaF, structure. However, from our systematic study of the r phases based on the CsCl structure it seems more probable that it is not A1,Pd but rather is the CsCl structure in coexistence with Al,Pd and the decagonal quasicrystal after annealing at high temperatures.
4. Conclusions ( 1) Decagonal quasicrystal with a periodicity of about 1.6 mn has been found in rapidly solidified Al,Pd but not in Al,Pt. (2) The metastable hexagonal phase isostructural with a-AlFeSi was present in these two alloys in the as-melt-spun state and was the main constituent in Al,Pt. (3) The Al-Pd decagonal quasicrystal transformed partially to the orthorhombic Al,Pd and CsCl-based r phases after heating to 600 “C. These three phases are structurally related and their orientation relationships are 1ODeca//[O lOi,,,//]
1 i Oicsa
~~~~,,//~~~~l*,~,//~~~~l,s,~ ~~,,,,//~~~~1~1,~//~~1Olcsc~ (4) The orthorhombic Al,Pd with a = 2.34, b = 1.67 and c = 1.23 mn is a rational Penrose tiling approximate of the irrational decagonal quasicrystal. Electron diffraction patterns of the continuous transformation of the latter to the former have been simulated and a tiling model of A1,Pd projected on (010) has been proposed. (5) Tenfold rotational twins of Al,Pd around the [OlO] axis have been observed and these are supposed to form along the ten sets of D-P twofold directions of the decagonal quasicrystal. (6) The previously reported A1,Pd with a CaF, structure and a cubic Al-Pd phase with a = 1.98 mn have not been verified.
Acknowledgments The authors thank sincerely Dr. Hong Zhang for many interesting discussions and Drs. Y. Q. Gao and Z. M. Wang for the supply of samples. This work is
49
supported China.
by a special grant from the National Natural Science Foundation
of
References 1 P. A. Bancel and P. A. Heiney, Phys. Rev. B, 33 (1986) 7917. 2 L. Bendersky, J. Phys. (Paris), Colloq., C3,47 (1986) 457. 3 G. V. S. Sastry and C. Suryanarayana, Ser. Metall., 20 ( 1986) 1359. 4 N. Thangaraj, K. Chattopadhyay, S. Ranganathan, C. Small and H. A. Davies, in T. Imura, S. Maruse and T. Suzuki (eds.), Proc. Znt. Congr. on Electron Microscopy, Kyoto, Japan, 1986, Vol. 2, 1986, p. 1525. 5 P. A. Bancel and P. A. Heiney, J. Phys. (Paris), Colloq., C3,47 (1986) 341. 6 S. Idziak, P. A. Heiney and P. A. Bancel, Mater. Sci. Forum, 22-24 (1987) 353. 7 S. Idziak and P. A. Heiney, preprint. 8 G. V. S. Sastry, C. Suryanarayana, M. Van Sande and G. Van Tandeloo, Mater. Rex Bull., 13 (1978) 1065. 9 G. V. S. Sastry and C. Suryanarayana,
Mater. Sci. Eng., 47 (198 1) 193. 10 P. A. Bancel, P. A. Heiney, P. W. Stephens, A. L. Goldman and P. M. Horn, Phys. Rev. Lett., 54 (1985) 2422.
11 U. Koster, P. S. Ho and M. Ron, Thin Solid Films, 67 (1980) 35. 12 G. V. S. Sastry, C. Suryanarayana and G. Van Tendeloo, Phys. Status Solidi A, 73 ( 1982) 267. 13 K. K. Fung, C. Y. Yang, Y. Q. Zhou, J. G. Zhoa, W. S. Zhan and B. G. Shen, Phys. Rev. Lett., .56 (1986) 2060. 14 L. Ma, R. Wang and K. H. Kuo, Ser. Metall., 22 (1989) 1791. 15 R. Wang, L, Ma and K. H. Kuo, J. Less-Common Met., 163 (1990) 27. 16 C.Dong,G.B.LiandK.H.Kuo,;l.Phys.F,17(1987)1571. 17 X. Z. Li and K. H. Kuo, Philos. Mag. Lett., 58 (1988) 167.
18 H. Zhang and K. H. Kuo, private communication. 19 R. Penrose, Bull. Inst. Math. Appl., IO ( 1974) 266. 20 W. J. Jiang, Z. K. Hei, Y. X. Guo and K. H. Kuo, Philos. Mag. A, 52 (1985) L53. 21 X. D. Zou, K. K. Fung and K. H. Kuo, Phys. Rev. B, 35 (1987) 4526. 22 S. S. Lu and T. Chang, Acta Phys. Sinica, 13 (1957) 150 (in Chinese). 23 K. Chattopadhyay, S. Lele, N. Thangaraj and S. Ranganathan, Acta Metall., 35 (1987) 727. 24 L. X. He, X. Z. Li, Z. Zhang and K. H. Kuo, Phys. Rev. Lett., 61 (1988) 1116. 25 H. Zhang and K. H. Kuo, Ser. Metall., 23 (1989) 355.
Metals, 163 (1990) 37-49
31
QUASICRYSTALS IN RAPIDLY SOLIDIFIED ALLOYS OF AI-Pt GROUP METALS-IV. QUASICRYSTALS IN RAPIDLY SOLIDIFIED Al-Pd AND Al-Pt ALLOYS L. MA*, R. WANG and K. H. KUO* Department of Materials Physics, University of Science and Technology, Beijing, loo083 Beijing (China)
(Received January 29, 1990; in revised form April 12, 1990)
Summary A decagonal quasicrystal has been found in a rapidly solidified A1,Pd alloy but not in an Al,Pt alloy. A metastable hexagonal phase isostructural with aAlFeSi found in Al& also exists in these two alloys. On heating to 600 “C, the Al-Pd decagonal quasicrystal transformed to an orthorhombic A1,Pd phase (a = 2.34, b = 1.67, and c = 1.23 mn) with vacancy-ordered long period CsCl-type structure.
1. Introduction The presence of a decagonal quasicrystal in Al-Pd has been well documented especially in connection with the discussion of diffraction peak shift and line broadening caused by the frozen phason strain in quasicrystals [5-71. In fact, the observation of a fivefold electron diffraction pattern and quasiperiodic distribution of spots in rapidly solidified Al-Pd alloys can be traced back to as early as 1978-1981 [8, 91, and the presence of a phase related to the icosahedral quasicrystal was noted in 1985 [lo]. On the other hand, the presence of an Al-Pt decagonal quasicrystal was mentioned only occasionally by Bancel and coworkers [l, 5, lo], and no details have been reported. Vapour deposition of the Al-Pd alloys resulted in the formation of a homogeneous supersaturated solid solution [ll]. Upon annealing, two intermetallic phases were precipitated: A1,Pd has an orthorhombic structure with a = 1.29, b = 1.68 and c= 2.35 nm, while A1,Pd has a hexagonal structure with a = 1.303 and c=O.958 nm [ll]. On heating to a temperature above 700 K, A1,Pd transformed to vacancy-ordered long period CsCl structure (the so-called z phases) while Al,Pd remained unchanged [ 121. In the rapidly solidified Al-Pd alloys containing 6-20 at.% Pd, however, only A1,Pd (a = 2.2505, b= 1.7729 and [l-7],
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of Electron Microscopy,
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c = 1.184 mn, originally designated as A1,Pd [9] but later changed to Al,Pd [12]) coexisted with aluminium solid solution, the volume fraction of Al,Pd increasing with palladium concentration. Sometimes a metastable Al,Pd phase with a CaF,type cubic structure was also detected and, according to Sastry and Suryanarayana [9], it has a strong crystallographic relationship with the Al,Pd. The electron diffraction pattern of the Al,Pd phase with a hexagon of strong diffraction spots [3] is similar or even identical with the twofold D pattern (nomenclature after Fung et al. [13]) of the decagonal phase. This led Sastry and Suryanarayana [3] to suggest that the A1,Pd they observed earlier [8, 9, 121 is in fact the decagonal quasicrystal. However, Thangaraj et al. [4] found that the crystalline phase coexisting with the decagonal phase is a rhombohedrally distorted cubic phase with a = 1.98 nm. Obviously, the question of the quasicrystalline and crystalline phases in rapidly solidified aluminium-rich Al-Pd and Al-Pt alloys is still open. In order to clarify this we undertook a systematic transmission electron microscopy (TEM) study of the phases existing in rapidly solidified A1,Pd and A1,Pt alloys and their transformation after heating to 600 “C for 0.5-16 h. A preliminary report has already been published [ 141. The Al,Pd and A1,Pt alloys were prepared by melting pure aluminium (99.97%) with palladium (99.9%) and platinum (99.9%) respectively in an arc furnace under an argon atmosphere. They were then remelted in quartz tubes in a high frequency induction furnace in argon, and injected onto the surface of a copper wheel, 340 mm in diameter, from a nozzle of diameter 1 mm at the tip of a quartz tube under a pressure of 0.1 MPa. Thin ribbons were then obtained by spinquenching and the cooling rate was about lo5 K s-l. The ribbons were cut into small pieces and thinned by ion-milling before TEM observation. In order to study the phase transformation process in these alloys, specimens were annealed at 600 “C for 0.5, 1,2,4,8 and 15 h. These foils were examined in a JEM- 1OOCXII electron microscope equipped with a large angle double-tilting goniometer stage. The composition analysis of various crystalline phases was made in a Philips EM420 electron microscope fitted with an EDAX 9 100 system for energy-dispersive X-ray spectroscopy (EDS).
2. Decagonal quasicrystal Decagonal quasicrystals formed easily and extensively in the A1,Pd alloy during the melt-spin process. The electron diffraction patterns (EDPs) in the top rows of Figs. 1 and 2 belong to this phase. That in the top left corner of Figs. 1 and 2 is the tenfold EDP. The EDPs in the top row of Fig. 1 were obtained by tilting the decagonal quasicrystal under examination through various angles around the horizontal PP’ axis in the tenfold EDP (after Fung et al. [ 131). After rotating 90” the twofold D pattern [ 131 with six strong spots forming a hexagonal appeared. The tenfold axis, now in the vertical direction, is periodic and the periodicity is about 1.64 nm (the strong spot in the vertical direction in this EDP corresponds to
34’
0’
66O
66O
71°
36’
36’
36’
60’
60’
60’
Fig. 1. Corresponding electron diffraction patterns: top row, Al-W decagonal quasicrystal tilted 90” around PP’ from the tenfold EDP to the twofold D EDP; middle row, AI,Pd tilted 90” from [OlO] to [00 I]; bottor n row, A1,Pd tilted 90”from [OlO] to [308].
q
31°
0”
64’
64’
37O
36’
79O
79O
9o”
60’
Fig. 2. Corresponding electron diffraction patterns: top row, AI-W decagonal quasicrystal tilted 90” around DD’ from the tenfold EDP to the twofold P EDP; middle row, AI,Pd tilted 90”from [OlO] to [loo]; bottom row, AI,Pd tilted 90”from [OlO] to [507].
o”
41
0.205 nm and there are eight spots altogether between this strong spot and the central transmitted beam). However, the Al-Mn decagonal quasicrystal has six spots between the strong spot and the central beam, and it has a periodic@ of 1.23 nm [2]. This difference has been noted before not only in Al-Pd [2-4] but also in Al-Fe [13], Al-Rh and Al-Ir [15], Al-Co [16] and Al-Ni [17]. As a matter of fact, Sastry et al. [S] had already noticed such a difference in the EDPs of rapidly solidified Al-Mn-Ni and Al-Pd alloys, but they overlooked the quasiperiodical arrangement of spots in the direction perpendicular to the periodic tenfold axis. The EDPs in the top row in Fig. 2 were obtained by tilting the decagonal quasicrystal through various angles around the horizontal DD’ axis of the tenfold EDP (after rotating 18” around the tenfold axis from the tenfold EDP in Fig. 1). That in the top right earner is the twofold P pattern with the periodic tenfold axis also in the vertical direction. It is well known that there are only four strong spots between the strong spot and the central beam [2,13,15], indicating the presence of a screw axis [2]. The EDPs obtained after tilting 37”, 64” and 79” are the pseudotwofold (p2), pseudo-fivefold (~5) and pseudo-threefold (~3) patterns respectively. The presence of decagonal quasicrystal in rapidly solidified Al-Pt alloys has been mentioned by Bancel and coworkers [ 1, 5, lo], but they did not publish any details of their experiment. Since then no confirmation of this decagonal quasicrystal has been reported. We have made several attempts to produce this quasicrystal in Al,Pt by rapid solidification as we have done in other alloys of the Al-Pt group metals and we have performed careful TEM studies, but we failed to detect any trace of the decagonal quasicrystal in this system. This at least implies that the decagonal quasicrystal is difficult to form, if indeed it ever forms in the Al-Pt alloy system.
3. Crystalline phases 3.1. Metastable hexagonal phase The same metastable hexagonal phase isostructural with a-AlFeSi found in rapidly solidified Al& [ 14, 151 has also been found in the rapidly solidified A1,Pd and A1,Pt alloys. It occurs in small amounts in the former alloy but is the main constituent in the latter. Since this phase has already been described in detail in a paper dealing with Al-Ir [ 151, no further discussion will be given here. 3.2. Orthogonal Al, Pd This phase did not exist in the melt-spun A1,Pd but it formed gradually from the decagonal quasicrystal after heating at 600 “C for various times. However, even after 8 h at this temperature decagonal quasicrystals can still be found. This orthorhombic phase was first discovered by Koster et al. [ 1 l] and later Sastry and Suryanarayana [3] pointed out the resemblance of its [OOl] EDP to the D pattern of the decagonal quasicrystal (see Fig. 1). We have studied systematically the structural relationship between these two phases both from the experimental and theoretical points of view.
O0
S&3’
7 9:6’
9.0°
Fig. 3. The [OlOJ-[loo]-[OOl] quadrant of the EDPs of AI,Pd showing [OlO] as a pseudo-tenfold axis: the [OOl], [308] and [_503]EDPs are similar to the 2D EDP and the [loo], [507] and [2 0 111 EDPs are similar to the 2P EDP of the decagonal quasicrystal.
A thorough experimental study of the EDPs of this orthorhombic phase at different orientations has been conducted. Figure 3 shows the mutually orthogonal [loo], [OlO] and [OOl] zone patterns and the important intermediate EDPs. Firstly, the [OlO], [loo] and [OOl] EDPs look strikingly similar to the respective tenfold, twofold P and twofold D EDPs of the decagonal quasicrystal (see Figs. 1 and 2 and C$ also Fig. 2 in ref. 15). Secondly, the [OOl], [308] and [503] EDPs with about 36” intervals look very much alike, as do the [ 1001, [507] and [2 0 1 l] EDPs. These two sets of EDPs resemble respectively the twofold D and P EDPs of the decagonal quasicrystal. This once again shows the pseudo-tenfold symmetry of the [0 lo] zone pattern. In order to study the resemblance of the EDPs of these two phases in more detail, EDPs of this orthorhombic phase tilted around some relevant directions are presented in the second and third rows of Figs. 1 and 2 to be compared
43
directly with the corresponding EDPs of the decagonal quasicrystal in the first row. In Fig. 1 the tilting axis for the second row is [loo] and that for the third row [803], and the corresponding axes in Fig. 2 are [OOl] and [‘705] respectively. The distribution of strong spots as well as the angle of tilt of the corresponding pair of EDPs are rather similar. This proves beyond any doubt the structural similarity of these two phases. Zhang and Kuo [18f have recently made a theoretical study of the structural transformation of the decagonal quasicrystal to a relevant crystalline phase. As is well known, the Penrose pattern [ 191 of a two-dimensional quasilattice is made of two kinds of rhombus, a thin one of 36” and a thick one of 72”, arranged aperioditally according to some exact matching rules. A special matching mistake is called a phason, and this will introduce local translational order or periodic tiling of two or even more rhombi of the same kind. Increasing the phason strain in two orthogonal directions in the quasiperiodic plane of the decagonal quasicrystal, for example in one P and one D direction ~rpendicular to it, will eventually introduce long-range periodicity in these two directions. Together with the third periodicity originally existing along the tenfold axis, an orthorhombic crystalline phase will result. The lattice parameters of Al,Pd agree well with those predicted [18]. In other words, Al,Pd is a rational approximation of the irrational Penrose tiling. In reciprocal space, this corresponds to a consistent shift of diffraction spots towards their periodic positions. Several spots may migrate to the same position and become one spot of the crystalline phase. Figures 4(a)-4(d) show the simulated EDPs of the continuous transformation from the decagonal quasicrystal (tenfold EDP in Fig. 4(a)) to the orthorhombic Al,Pd phase ([OlO] EDP in Fig. 4(d)). The latter shows a rectangular array of periodically arranged spots, but the strong spots with more or less tenfold symmetry and an aperiodic arrangement still resemble those of the decagonal quasic~stal. The circle on which the ten spots are evenly located in Fig. 4(a) has become an ellipse in Fig. 4(b), this effect increasing for weaker spots. This is a somewhat distorted tenfold EDP of the decagonal quasicrystal caused by the presence of some phason strain in it. The EDPs corresponding to Figs. 4(c) and 4(d) are shown in Figs. 4(e) and 4(f ). There are more spots in Fig. 4(e) than in Fig, 4(f) depicting the perfect Al&l phase, but they are grouped together showing an average rectangular array not unlike Fig. 4(f). The weak spots have a zigzag appearance, as do the simulated weak spots in Fig. 4(c) with a large phason strain. This can be visualized as the distorted [Olo] EDP of the Al,Pd crystalline phase caused by the presence of some tiling mistakes so that local quasiperiodicity exists. Evidently, we have the follo~g o~entation relations~p~
AI,Pd
[ 100]//2P
decagonal phase
fOOIlN2D Since there are 10 sets of mutually orthogonal twofold P and D directions, the crystalline Al,Pd phase can form along any of these 10 directions. Obviously, there should be 10 possible orientations for the Al,Pd formed from the decagonal phase.
Fig. 4. Simulated tenfold EDP of decagonal quasicrystal (a), which changes gradually through (b) and (c) with increasing phason strain, and finally to the [OlO] EDP of AI,Pd (d). The somewhat distorted [OlO] EDP (c)is shown for comparisonpith (c). The [OlO] EDP of AI,Pd is shown in(f).
45
other words, tenfold rotational twins of 36” around the [0 lo] axis should exist, as in the case reported earlier for NiZr [20] and Al,,Fe, [21]. Figure 5 is a composite EDP of to such twins with two sets of parallel layer lines (outlined in Fig. 5). As discussed above, A1,Pd can be treated as a Penrose tiling approximate of the decagonal quasicrystal. By introducing linear phason strain successively to the two-dimensional Penrose pattern, we obtained finally the tiling model of A1,Pd projected on (010) (see Fig. 6). It consists of the same two kinds of rhombus tiles but now they are arranged periodically. This is a structural model derived from the two-dimensional quasilattice and it accounts well for the diffraction patterns of Al,Pd and its orientation relationship with the decagonal quasicrystal. However, it is too early to predict whether this model is correct or not, since neither the structure of the Al,Pd nor that of the decagonal quasicrystal is known.
In
Fig. 5. A composite [OlO] EDP of three twin variants of AI,Pd (two sets of layer lines are marked).
3.3. Tphases Lu and Chang [22] were the first to make a detailed study of the vacancyordered CsCl phases in Al-Ni-Cu alloys. Vacancies on (111) are arranged periodically in the [ 11 l] direction giving polytypical series of 3, 5, 8, 13, . . . layers which are called t3, t5, rs, r13, . . . phases. The existence of these phases in Al-Pd alloys after annealing at high temperature has been reported by Sastry et al. [ 121. Recently, Chattopadhyay et al. [23] pointed out the possible relationship between this series of t phases and the one-dimensional quasicrystals with periodicities following the
Fig. 6. A tiling model of Al,Pd projected onto (010).
Fibonacci sequence (F, + 1= F,, + F,, _ I), such as the quasicrystals found recently in some Al-Cu-M alloys [ 241. The presence of r3, r5 and rs twin in the annealed A1,Pd ahoy has been confirmed in the present study. Figure 7(a) shows the [I lo] EDP of the simple CsCl structure and Fig. 7(b) shows a corresponding EDP of the r3 phase, in which there are two spots between the 111 spot of the CsCl structure and the central beam dividing the reciprocal g,,, into three parts. According to the same reasoning, Fig. 7(c) might be considered as an EDP of the r, phase, but this is in fact not the case. The distance between two spots in the [ 11 l] direction is now not the same. Moreover, these spots do not lie exactly on a straight line. These anomalies are certainly due to some modulation of the z5 structure. The arrangement of spots in the [ 11 i] direction in Fig. 7(c) is reminiscent of the aperiodic arrangement of spots in quasicrystals. The close relationship between the r phases and the decagonal quasicrystal has already been pointed out [ 251. Since Al,Pd is also closely related to the decagonal quasicrystal, it is natural that a definite orientation relationship exists between A1,Pd and the CsCl structure. Figure 8 shows the composite EDPs of some main zones of these two phases: the dense array of spots belongs to the orthorhombic Al,Pd whereas the few marked strong spots belong to the CsCl structure. The orientation relationship between these two phases, as shown in Figs. 8(a), (b) and (c), is the following:
Fig. 7. EDPs of (a) the CsCl structure, (b) t) and (c) modulated rS.
Fig. 8. Composite EDPs of A&W (dense array of spots) and CsCl structure (marked strong spots): ia) [0101,,,,//[1 iol,; (b)[1001,,,,//[0011,; (4 [OO~l,,,d/[l~Ol,~ (4 [3081AI,W//[1111,.
A1,Pd
[lOO]//[OOl]
CsCl
The weak spots in these EDPs are the results of multiple diffractions. Figure 8(d) is the composite EDP of the [308] EDP of A1,Pd and the [ 11 l] EDP of the cubic CsCl structure. The interzonal angle between [308] and [OOl] is 35.5”, which compares favourably with the 35.3” between [ 1 lo] and [ 11 l] of the CsCl structure. Sastry and Suryanaryana [9] have interpreted the strong spot pattern in Fig. 8(c) as that of A1,Pd with the CaF, structure. However, from our systematic study of the r phases based on the CsCl structure it seems more probable that it is not A1,Pd but rather is the CsCl structure in coexistence with Al,Pd and the decagonal quasicrystal after annealing at high temperatures.
4. Conclusions ( 1) Decagonal quasicrystal with a periodicity of about 1.6 mn has been found in rapidly solidified Al,Pd but not in Al,Pt. (2) The metastable hexagonal phase isostructural with a-AlFeSi was present in these two alloys in the as-melt-spun state and was the main constituent in Al,Pt. (3) The Al-Pd decagonal quasicrystal transformed partially to the orthorhombic Al,Pd and CsCl-based r phases after heating to 600 “C. These three phases are structurally related and their orientation relationships are 1ODeca//[O lOi,,,//]
1 i Oicsa
~~~~,,//~~~~l*,~,//~~~~l,s,~ ~~,,,,//~~~~1~1,~//~~1Olcsc~ (4) The orthorhombic Al,Pd with a = 2.34, b = 1.67 and c = 1.23 mn is a rational Penrose tiling approximate of the irrational decagonal quasicrystal. Electron diffraction patterns of the continuous transformation of the latter to the former have been simulated and a tiling model of A1,Pd projected on (010) has been proposed. (5) Tenfold rotational twins of Al,Pd around the [OlO] axis have been observed and these are supposed to form along the ten sets of D-P twofold directions of the decagonal quasicrystal. (6) The previously reported A1,Pd with a CaF, structure and a cubic Al-Pd phase with a = 1.98 mn have not been verified.
Acknowledgments The authors thank sincerely Dr. Hong Zhang for many interesting discussions and Drs. Y. Q. Gao and Z. M. Wang for the supply of samples. This work is
49
supported China.
by a special grant from the National Natural Science Foundation
of
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