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Polytype structures in Zr-Cr-Fe laves phase
Journal of the Less-Common
Metals, 125 (1986)
POLYTYPE STRUCTURES IN Zr-Cr-Fe XIAN
YING
Department (Canada) (Received
MENG*
33
33 - 44
LAVES PHASE
and D. 0. NORTHWOOD
of Engineering
July 3, 1985;
Materials, University of Windsor, Windsor, Ontario N9B 3P4
in revised
form March
31, 1986)
Summary Hexagonal polytype structures (H) have been found in ZrCrz and Zr(CrFe), Laves phases in both bulk stoichiometric alloys and in precipitates in Zircaloy-4. Rhombohedral polytype structures (R) were found in both Zircaloy-4 and Zr-l.l5wt.%Cr-O.lwt.%Fe alloys where the Laves phase is in the form of a precipitate. Good agreement was obtained with respect to possible polytype structures between the diffraction patterns and lattice fringe images in electron microscopy. A comparison of observed and calculated relative diffraction intensities show the polytype structures to be of the 6H, 8H, 12H and 14H modifications.
1. Introduction Laves phase, with the general formula AB?, can have one of three related structures, namely the MgCuz, MgZq , or MgNiz structures based on magnesium. These three structures correspond to C15-cubic, C14-hexagonal and C36-hexagonal structures respectively. Out of 223 binary Laves phases 68% had the Cl5 MgCu&ype structure, 30% the Cl4 MgZn*-type structure and only 2% had the C36 MgNi,-type structure. In addition to the binary Laves phases many Laves phases have been reported to form in pseudobinaries or ternary systems [l]. The A or B atoms may come from any part of the Periodic Table, and the same element may play different roles in two Laves phases. The three structures, i.e. C14, Cl5 and C36, are closely related. The main reason for the existence of the three phases is considered to lie in the fact that the geometrical space of the lattices can be conveniently filled in more than one way when DA/D, = 1.225, where D represents the atomic diameter. The smaller B atoms are arranged in a tetrahedron. For MgZnz, the tetrahedra are arranged in rows and are joined alternately at bases and points. In MgCu, *On leave from 0022-5088/86/$3.50
Beijing
Center
of Physical
and Chemical
0 Elsevier
Analysis,
Sequoia/Printed
Beijing,
China.
in The Netherlands
34
the tetrahedra are joined at points. MgNiz is a mixture of the other two types. The larger atoms, A, are constructed from double layers with a hexagonal network, such that each A atom of the upper layer is directly above one in the lower layer. Thus the Cl5 type structure can be described as ABCABC.. ., and the Cl4 and C36 types as ABAB. .., and ABACABAC.. ., respectively [ 2,4 1. The phenomenon of polytypism was first discovered in Sic by Baumbauer in 1912 [5]. Successively, it has been found in ZnS, Cd&, Pb12, mica, AuMn and other alloys or minerals. So far more than one hundred kinds of polytype have been reported [ 6 - 111. Some polytypism has been reported in Laves phases such as Mg-Cu-Al, Mg-Cu-Ni and other m~nesium-based systems [12 - 16]. Invariably, several structures can coexist in a single specimen. With the development of electron microscopy, and particularly, high resolution electron microscopy which can give lattice or structure images, electron microscopy techniques have come to be preferred for the analysis of several micro-areas for evidence of poiytypism. The polytypism has also been found in binary ZrCr, or pseudobinary Zr(CrFe), Laves phases. In the present work, various stacking structures in binary ZrCrz or pseudobinary Zr(CrFe), Laves phases have been studied using lattice images in transmission electron microscopy (TEM) and analysis of the intensities of electron diffraction patterns. Evidence is presented for polytypism in both the binary and pseudobinary Laves phases. 2. Experimental: details Two basically different types of materials were inve~igat~, namely: bulk, stoichiometric Laves phases which are under consideration as hydrogen storage materials [ 17 - 191 and fine Laves phase intermetallic precipitates which are found in nuclear reactor full cladding materials [20 - 221. The bulk, stoichiometri~ ZrCr, and Zr(CrFe)z alloys were melted in an arc furnace four times under a high purity argon atmosphere. High purity elements were utilized, i.e. 99.999% pure zirconium, iron and chromium. Five kinds of stoichiometric alloys which are ZrCr,, Zr(Cro_lFeo&, and ZrFe, were studied as cast alloys. Zr(Cre.ssFee.&!, Zr(Cro.75Fe,.& Specimens suitable for TEM were prepared by suspending a powder of the alloys’in ethyl alcohol and then depositing a drop of this suspension on a perforated supporting film. Polytypes of the Zr(CrFe), pseudobinary Laves phase were also studied as the precipitates in Zircaloy-4 and Zr-l.l5wt.%Cr-O.lwt.%Fe (referred to hereafter as Zr-Cr-Fe). The chemical compositions and heat treatments for Zircaloy-4 and Zr-Q--Fe are given in Table 1. The thin foils of the two alloys were made by a twin-jet electropolishing technique using a 4:l perchloric acid:ethyl alcohol electrolyte. The stacking fault order was observed using lattice images in the transmission electron microscope which was operated at 100 kV. The distributional curves of relative intensity of the electron diffraction patterns for the polytype structures were measured
35 TABLE 1 Chemical composition and heat treatment of the zirconium alloys Specimen
Main alloying elements (wt.%)
Treatment
Cr
Fe
Sn
Zircaloy-4
0.10
0.21
1.45
1100 “C, 5 min; air coobd
Z&k-Fe
1.15
0.10
-
1000 OC,30 min; water quenched, 760 OC, 10.5 h
using a semiautomated device consisting of a microdensitometer and chart recorder. The integral area under curves was measured with a commercial graphic data image analyzer, Observed relative intensities were calculated using these integral areas. Calculated relative intensities based on structure factors were obtained by computer methods. 3. Results In agreement with previous observations, the ZrCrz and Zr(CrFe), Laves phases can have either of two structures, namely the Cl4 hexagonal or the Cl5 diamond cubic structures [20 - 221. For the close-packed hexagonal (C14) structure 2H, 4H, 8H, 12H, 14H and 20H polytype structures have been found in stoichiometric ZrCr, alloy, the 2H, 6H, 10H and 3R polytype structures in Zircaloy-4, and the 3R twinned structure in Zr-Cr-Fe alloys. The diffraction patterns for the 2H, 3R, 3R twinned, 4H, 6H, 8H, 12H, 14H and mixed structures are given in Fig. 1.
(a)
(b)
(d)
(e)
(f)
(confinued)
Fig. 1. Diffraction patterns showing polytypism for the ZrCrz and Zr(CrFe)z Laves phases: (a) 2H; (b) 3R, Zircaloy4; (c) 3R twin, Zr-Cr-Fe alloy;(d) 4H; (e) 6H, Zircaloy4; (f, g) SH; (h) 12H and 14H; (i) 20H.
Using lattice images, the one-dimensional stacking structures could be confirmed. Figure 2 gives the lattice images for the 2H structure, 4H and 8H, 12H and 14H mixed structures. The lattice images show that dislocations exist in the Laves phase. Figure 3 is a micrograph for the 3R twinned
(b)
Cd) Fig. 2. Lattice images of polytype structures for ZrCrz Laves phases: (a) 2H, ZrCr2 and Zr(CrFe)z; (b, c) 4H and 8H; (d) 12H and 14H.
lb) Fig. 3. The micrographs for 3R structure in: (a) Zircaloy-4 G-Fe alloy.
and (b) 3R twinning in Zr-
Using the microdensitometer the intensities of the electron diffraction patterns have been measured and the intensity distributions along (IOL) for 8H and 3R twinned structures are shown in Fig. 4.
4. Discussion 4.1. Structure determination of polytypes The f.c.c. and h.c.p. structures are regarded as being assembled from the close-packed atomic layers which have the same triangular or hexagonal net. The sequence of the atomic layers is considered to be ABC for the f.c.c. lattice and AB for the h.c.p. lattice. According to the distribution of the large atoms the Laves phases are considered to be cubic or hexagonal. The structure of Laves phase, when discussed as the distribution of both A and B atoms, is composed of two kinds of sheets: one is a denser layer forming a net of triangles and hexagons, the 0 layers as shown in Fig. 5. The other is found between these denser layers and consists of three triangular nets of A or B atoms stacked together in a close-packed manner [12, 23, 24]. If the distance from the denser layer to second denser layer is taken as a unit, the distance from a denser layer to each triangular net can be indicated by fractions of the distance between two denser layers. There are two kinds of such stacking possible, called the A and A’ layers respectively, by A and B atoms of the Laves phase. If the A and A’ layers are shifted f or $ in the [llO] direction of the hexagonal cell the B, C, B’, C’ layers are obtained. The structure of the polytypes of the Laves phases are considered to be due to the different stacking sequences of these layers. The structure factors of the hkl reflections for these layer types can be calculated from the atomic scattering amplitudes of the A and B atoms and the phase shift due to each layer. The diffraction intensity is in proportion to the square of the structure factor. Since any one layer can be put
020
iaf
Oil
ot3
012
014
Of5
013
HKL
120s
ap
loo-
?
0 010 (b)
011
012
013
014
015
016
517
018
HKL
Fig. 4. The distribution of relative intensities for OlL reflection (b) 8H Laves phases. 0, Observed relative intensity; X, calculated.
about: (a) 3R twinning;
over the other in two different ways, the possible number of arranged ways is 2N-1, where N is the layer number of the polytype structure. A summary of the polytype structures observed in the zirconium alloys in this
Fig. 5. The fundamental stacking structures bers show the distance between the layers.
of A and B atoms
in Laves phase. The num-
study is given in Table 2. A comparison of the calculated and observed relative intensities, is given in Table 3, for the 8H, 12H and 14H structures, and in Table 4 for 3R twinned structure. Good agreement was obtained between the calculated and observed intensities for the 8H structure. Although the agreement was not so good for the 12H and 14H structures, this was considered to be due to the dense pattern of diffraction spots with TABLE
2
Polytypes Polytypea 2H 3R 3R twin 4H 6H 8H 10H 12H 14H
of Zr( CrFe)z
Laves phases identified
in the present
investigation
Structureb
Alloy
2: AB
ZrCrz Zr(CrFe);? Zr( CrFe)l C. Zr( CrFe)2 d ZrCr* Zr( CrFe)z ’ ZrCr2 Zr( CrFe)z ’ ZrCr? ZrCrz
3: ABC 3 : ABC and A’C’B’ 121: AB’A’C 2211: ABC’B’AB’ 111122: AB’AB’ABC’B’ 11121121: AB’AB’A’CA’CAB’ 22221111: ABC’B’ABC’B’AB’AB’ 22222211: ABC’B’ABC’B’ABC’B’AB’
aH, Hexagonal lattice; R, rhombohedral; the numbers n before repeat period along c axis. bThe numbers represent the layer numbers in a unit cell. CIn Zircaloy-4. din Zr-Cr-Fe alloy.
H or R indicates
n layered
40 TABLE 3 Comparison of the measured and calculated relative intensities of the 8H, 12H and 14H polytypes of the ZrCr2 Laves phases hkl
010 011 012 013 014 015 016 017 018 019 0110 0111 0112 0113 0114
8H (AB’AB’ABC’B’)
12H (ABC’B’ABC’B’AB’AB’)
14H (ABC’B’ABC’B’AtiC’B’AB’)
I( talc.)
Z(obs.)
Z(ealc. )
Z(obs.)
Z(talc. )
Z(obs.)
62.6 12.3 2.2 5.0 100.0 19.8 30.2 32.2 62.5
68.8 18.9 23.1 16.0 100.0 28.8 26.2 20.2 60.7
33.6 5.0 2.1 1.5 0.7 1.2 100.0 6.6 9.6 47.1 12.7 12.0 39.7
32.7 5.3 4.4 64.1 22.2 28.1 100.0 24.6 25.3 43.6 10.5 6.5 34.4
27.5 1.3 1.4 3.7 0.2 0.1 0.4 100.0 1.4 4.3 41.9 47.2 6.1 2.8 31.6
30.0 4.9 6.2 45.5 17.0 22.5 11.2 100.0 27.8 27.2 63.1 11.4 2.7 2.7 38.5
TABLE 4 Comparison of the measured and calculated relative intensities of the 3R twinned structure hkl
010 011 012 013 014 015 016
A’C’B’
ABC Z(calc.)
Z(obs.)
Z(calc.)
Z(obs.)
0.0 0.0 100.0 0.0 0.0 56.4 0.0
0.0 0.0 100.0 0.0 0.0 61.5 0.0
0.0 56.4 0.0 0.0 100.0 0.0 0.0
0.0 71.9 0.0 0.0 89.7 0.0 0.0
some degree of overlapping, such that the intensities of neighbouring diffraction spots gave high background intensities for the measured diffraction spot. Double diffraction of electrons can also play an important role in altering the observed relative diffraction intensities. In ZrCr-Fe alloy, the 3R twinned structure was found. This 3R twinned structure can be considered to be composed of ABC and A’C’B’ respectively. In Fig. 5, it is demonstrated that the stacking A’C’B’ is equivalent to the twinning of ABC. The relative intensities of the 3R twinned structure also show good agreement between calculated and observed values. The possible structures for the 6H, 8H, 12H and 14H Laves phases are given in Figs. 6(a)-(d).
(d)
(cl Fig. 6. The structures Lams phase.
of:
(a) 6H; (b) 8H;
(c)
12H;
and (d) 14H
polytypes
of the ZrCrz
42
4.2. Images of polytypes and defect structures Figure 2 gives the lattice images of the different stacking structures. The 2H structure lattice images in Fig. 2(a) correspond with the diffraction pattern in Fig. l(a). As indicated in Table 2, the stacking sequence for the 2H structure is AB. The structure of A or B layers is given in Fig. 5. One period of the fringe is 0.82 nm. This is one period of AB stacking. The 2H structures for both ZrCr, and Zr(CrFe), Laves phase alloys show only one type of fringe image. Stacking faults have not been found in 2H structures. The diffraction patterns in Figs. l(f)-l(i) show 8H, 12H, 14H and 20H stacking structures. The lattice images are given in Figs. 2(b)-2(d). In Fig. 2(b), the 4H and 8H structures are labelled. The long period areas show 40 layer stacking which is integral times the 4H and 8H structures. The lattice images of 12H and 14H structures are shown in Fig. 2(d). The long period structures are produced which are an integral number times the 4H, 8H, 12H or 14H structures. The period of contrast variation in all of the lattice fringe images show the same behaviour as indicated in the diffraction patterns. The formation of polytype structures in ZrCrz Laves phases is considered to be based on the 4H structure. Comparing the diffraction patterns for the 8H, 12H and 20H structures it can be seen that there are four stronger diffraction spots in (100) to (108), (100) to (10.12) or (100) to (10.20). As mentioned previously, the 4H stacking is mixed structure of the 2H and 3R polytypes. The images of 3R and 3R twinned structures are given in Fig. 3. The precipitate of Zr(CrFe), Laves phase in Zircaloy-4 looks like it is formed by the stacking of (111) crystal planes. Between the sheets of (111) crystal planes, whose size is about hundreds of tigstroms, parallel or twist subboundaries appear. It is considered that these are an array of parallel edge dislocations or a mix of more than one direction edge dislocation. Figure 3(b) gives a partial lattice fringe image of partial (111) twinned planes in an orientation giving a sharp projected fringe image of the (111) planes. Dislocations were also found in 3R twinned and ZrCrz stoichiometric bulk Laves phase alloys, and examples are shown in Figs. 2(c) and 3(b). 4.3. Factors affecting the formation of polytypism in zirconium alloys Five different kinds of stoichiometric Zr-Cr-Fe alloys were studied with TEM. According to the electron diffraction patterns, the ZrFez alloy showed only the 3R cubic structure, whereas all of the ZrCrz and Zr(CrFe), stoichiometric alloys had hexagonal structures. However, only ZrCrz exhibited the polytypism phenomenon, having the 2H, 4H, 8H, 12H, 14H and 20H structures. All of the Zr(CrFe)p cast alloys showed the 2H structures only. In Zircaloy-4 and Zr-&-Fe alloys, the main structure of Zr(CrFe)2type Laves phase precipitates was the 3R rhombohedral structure, i.e. the cubic structure. A twinning structure was found in the Zr-G-Fe alloy which had been heat treated at 1000 “C for 30 min, water quenched, and
43
then annealed at 760 “C for 10.5 h. In the Zircaloy-4 some Zr(CrFe)z-type Laves precipitates have the simple 3R cubic structure, whereas other polytypes had the 6H or a higher period structure. The morphology of the Laves phase showed a layer structure along the [OOl] direction, e.g. Fig. 3(a). Several different explanations for formation of polytypism have been advanced [ 10,111, including Frank’s screw dislocation theory, Jagodzinski’s one-dimensional disorder explanation based on thermodynamic consideration, and others involving high-order phase transformations. From the present work, the polytype structures of the Laves phases in zirconium alloys show one-dimensional periodic stacking faults. The formation of the different polytypes is affected both by the composition of the alloys and the heat treatment schedules to which they had been exposed. Further experiments are planned where the bulk Laves phases are subjected to varying heat treatment schedules.
5. Conclusions From studies involving electron diffraction and high resolution electron microscopy, it is shown that polytypism exists in Laves phases in zirconium alloys. Comparison of the calculated and observed relative intensities for electron diffraction indicated the following polytypes of the Laves phase to be present. (1) In a cast bulk stoichiometric ZrFe? alloy, the 3R type cubic structure is the only polytype found. However, bulk Zr(Cr,.sFeO.&, Zr(Cr,_s5Fe,.,,),, Zr(Cr0.25Fe0.75)2 and ZrCr, alloys have the 2H type hexagonal structures. (2) The 2H, 4H, 8H, 12H, 14H and 20H polytypism structures have been found in the cast ZrCr, stoichiometric Laves phase alloy. All the polytype structures are based on the 4H structure. (3) Two kinds of Zr(CrFe)* type Laves phase precipitates have been found in Zircaloy-4 alloys: a cubic Laves phase with 3R structure and a hexagonal Laves phase precipitate with 2H, 6H and 10H polytype structures. Longer period R-type stacking structures were also found. (4) 3R type Zr(CrFe), Laves phase precipitates were found in the Zr-l.l5wt.%Cr-O.lwt.%Fe alloy. Also found was a 3R twinned structure, with the two parts of the twin being formed by ABC and A’C’B’ stacking respectively. No other polytypes have been found in this alloy. (5) The chemical composition and heat treatment states are considered to be important factors for formation of different polytype structures.
References 1 C. Barrett and T. B. Massalaski, Structure 2 G. V. Raynor, hog. Met. Phys., 1 (1949)
ofMet&, 1.
Pergamon,
Oxford,
1980,
p. 256.
44 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
18 19
20 21 22 23 24 25
D. Shaltiel, J. Less-Common Met., 62 (1978) 40’7. R. I. Berry and G. V. Raynor, Acta Crystollogr., 6 (1953) 178. R. S. Mitchell, 2. Kristallogr., 111 (1959) 372. W. I. Bragg, Philos. Mug., 39 (1920) 647. R. M. Bozorth, J. Am. Chem. Sot., 44 (1922) 2232. H. Baumhauer, 2. Kristallogr., 50 (1912) 33. M. Ross, H. Takeda and D. R. Wones, Science, 151 (1966) 191. G. C. Trigunayat and G. K. Chadha, Phys. Status Solidi A, 4 (1971) 9. A. R. Verma and P. Krishna, Polymorphism and Polytypism in Crystals, Wiley, New York, 1966, p. 61. Y. Komura, Acta Crystallogr., 15 (1962) 770. Y. Komura, A. Nakaue and M. Mitarai, Acta Crystnllogr., Sect. B, 28 (1972) 727. Y. Komura and Y. Kitano, Acta Crystallogr., Sect. B, 33 (1977) 2496. Y. Komura and K. Tokunaga, Actu Crystallogr., Sect. B, 36 (1980) 1548. Y. Kitano, Y. Komura and H. Kajiwara, Trans. Jpn. Inst. Met., 18 (1977) 39. D. G. Ivey and D. 0. Northwood, Hydrogen absorption-desorption characteristics of Zr(FexCrl_&, in T. N. Veziroglu and J. B. Taylor (eds.), Proc. 5th World Hydrogen Energy Conf, Toronto, 15 - 20 July, 1984, Pergamon, Oxford, 1984, p. 395. D. G. Ivey and D. 0. Northwood, J. Mater. Energy Syst., 4 (1983) 222. D. 0. Northwood and D. G. Ivey, Storing hydrogen as a metal hydride: a review of potential materials with special emphasis on zirconium intermetallics, in 0. G. Hancock and K. G. Sheinkopf (eds.), Proc. 2nd Znt. Symp. on Hydrogen Produced From Renewable Energy, Cocoa Beach, Florida, 22 - 24 October, 1985, Florida Solar Energy Center, 1985, p. 199. Xian Ying Meng and D. 0. Northwood, J. Nucl. Mater., 132 (1985) 80. Xian Ying Meng and D. 0. Northwood, J. Nucl. Mater., 136 (1985) 83. Xian Ying Meng and D. 0. Northwood, J. Nucl. Mater., 137 (1986) 217. F. C. Frank and J. S. Kasper, Acta Crystallogr., 11 (1958) 184. F. C. Frank and J. S. Kasper, Acta Crystallogr., 12 (1959) 483. D. Van Dyck, Direct structure imaging in electron microscopy, in S. Amelinckx, R. Gevers and J. van Landuyt (eds.), Diffraction and Imaging Techniques in Material Science, North-Holland, 1978, p. 368.
Metals, 125 (1986)
POLYTYPE STRUCTURES IN Zr-Cr-Fe XIAN
YING
Department (Canada) (Received
MENG*
33
33 - 44
LAVES PHASE
and D. 0. NORTHWOOD
of Engineering
July 3, 1985;
Materials, University of Windsor, Windsor, Ontario N9B 3P4
in revised
form March
31, 1986)
Summary Hexagonal polytype structures (H) have been found in ZrCrz and Zr(CrFe), Laves phases in both bulk stoichiometric alloys and in precipitates in Zircaloy-4. Rhombohedral polytype structures (R) were found in both Zircaloy-4 and Zr-l.l5wt.%Cr-O.lwt.%Fe alloys where the Laves phase is in the form of a precipitate. Good agreement was obtained with respect to possible polytype structures between the diffraction patterns and lattice fringe images in electron microscopy. A comparison of observed and calculated relative diffraction intensities show the polytype structures to be of the 6H, 8H, 12H and 14H modifications.
1. Introduction Laves phase, with the general formula AB?, can have one of three related structures, namely the MgCuz, MgZq , or MgNiz structures based on magnesium. These three structures correspond to C15-cubic, C14-hexagonal and C36-hexagonal structures respectively. Out of 223 binary Laves phases 68% had the Cl5 MgCu&ype structure, 30% the Cl4 MgZn*-type structure and only 2% had the C36 MgNi,-type structure. In addition to the binary Laves phases many Laves phases have been reported to form in pseudobinaries or ternary systems [l]. The A or B atoms may come from any part of the Periodic Table, and the same element may play different roles in two Laves phases. The three structures, i.e. C14, Cl5 and C36, are closely related. The main reason for the existence of the three phases is considered to lie in the fact that the geometrical space of the lattices can be conveniently filled in more than one way when DA/D, = 1.225, where D represents the atomic diameter. The smaller B atoms are arranged in a tetrahedron. For MgZnz, the tetrahedra are arranged in rows and are joined alternately at bases and points. In MgCu, *On leave from 0022-5088/86/$3.50
Beijing
Center
of Physical
and Chemical
0 Elsevier
Analysis,
Sequoia/Printed
Beijing,
China.
in The Netherlands
34
the tetrahedra are joined at points. MgNiz is a mixture of the other two types. The larger atoms, A, are constructed from double layers with a hexagonal network, such that each A atom of the upper layer is directly above one in the lower layer. Thus the Cl5 type structure can be described as ABCABC.. ., and the Cl4 and C36 types as ABAB. .., and ABACABAC.. ., respectively [ 2,4 1. The phenomenon of polytypism was first discovered in Sic by Baumbauer in 1912 [5]. Successively, it has been found in ZnS, Cd&, Pb12, mica, AuMn and other alloys or minerals. So far more than one hundred kinds of polytype have been reported [ 6 - 111. Some polytypism has been reported in Laves phases such as Mg-Cu-Al, Mg-Cu-Ni and other m~nesium-based systems [12 - 16]. Invariably, several structures can coexist in a single specimen. With the development of electron microscopy, and particularly, high resolution electron microscopy which can give lattice or structure images, electron microscopy techniques have come to be preferred for the analysis of several micro-areas for evidence of poiytypism. The polytypism has also been found in binary ZrCr, or pseudobinary Zr(CrFe), Laves phases. In the present work, various stacking structures in binary ZrCrz or pseudobinary Zr(CrFe), Laves phases have been studied using lattice images in transmission electron microscopy (TEM) and analysis of the intensities of electron diffraction patterns. Evidence is presented for polytypism in both the binary and pseudobinary Laves phases. 2. Experimental: details Two basically different types of materials were inve~igat~, namely: bulk, stoichiometric Laves phases which are under consideration as hydrogen storage materials [ 17 - 191 and fine Laves phase intermetallic precipitates which are found in nuclear reactor full cladding materials [20 - 221. The bulk, stoichiometri~ ZrCr, and Zr(CrFe)z alloys were melted in an arc furnace four times under a high purity argon atmosphere. High purity elements were utilized, i.e. 99.999% pure zirconium, iron and chromium. Five kinds of stoichiometric alloys which are ZrCr,, Zr(Cro_lFeo&, and ZrFe, were studied as cast alloys. Zr(Cre.ssFee.&!, Zr(Cro.75Fe,.& Specimens suitable for TEM were prepared by suspending a powder of the alloys’in ethyl alcohol and then depositing a drop of this suspension on a perforated supporting film. Polytypes of the Zr(CrFe), pseudobinary Laves phase were also studied as the precipitates in Zircaloy-4 and Zr-l.l5wt.%Cr-O.lwt.%Fe (referred to hereafter as Zr-Cr-Fe). The chemical compositions and heat treatments for Zircaloy-4 and Zr-Q--Fe are given in Table 1. The thin foils of the two alloys were made by a twin-jet electropolishing technique using a 4:l perchloric acid:ethyl alcohol electrolyte. The stacking fault order was observed using lattice images in the transmission electron microscope which was operated at 100 kV. The distributional curves of relative intensity of the electron diffraction patterns for the polytype structures were measured
35 TABLE 1 Chemical composition and heat treatment of the zirconium alloys Specimen
Main alloying elements (wt.%)
Treatment
Cr
Fe
Sn
Zircaloy-4
0.10
0.21
1.45
1100 “C, 5 min; air coobd
Z&k-Fe
1.15
0.10
-
1000 OC,30 min; water quenched, 760 OC, 10.5 h
using a semiautomated device consisting of a microdensitometer and chart recorder. The integral area under curves was measured with a commercial graphic data image analyzer, Observed relative intensities were calculated using these integral areas. Calculated relative intensities based on structure factors were obtained by computer methods. 3. Results In agreement with previous observations, the ZrCrz and Zr(CrFe), Laves phases can have either of two structures, namely the Cl4 hexagonal or the Cl5 diamond cubic structures [20 - 221. For the close-packed hexagonal (C14) structure 2H, 4H, 8H, 12H, 14H and 20H polytype structures have been found in stoichiometric ZrCr, alloy, the 2H, 6H, 10H and 3R polytype structures in Zircaloy-4, and the 3R twinned structure in Zr-Cr-Fe alloys. The diffraction patterns for the 2H, 3R, 3R twinned, 4H, 6H, 8H, 12H, 14H and mixed structures are given in Fig. 1.
(a)
(b)
(d)
(e)
(f)
(confinued)
Fig. 1. Diffraction patterns showing polytypism for the ZrCrz and Zr(CrFe)z Laves phases: (a) 2H; (b) 3R, Zircaloy4; (c) 3R twin, Zr-Cr-Fe alloy;(d) 4H; (e) 6H, Zircaloy4; (f, g) SH; (h) 12H and 14H; (i) 20H.
Using lattice images, the one-dimensional stacking structures could be confirmed. Figure 2 gives the lattice images for the 2H structure, 4H and 8H, 12H and 14H mixed structures. The lattice images show that dislocations exist in the Laves phase. Figure 3 is a micrograph for the 3R twinned
(b)
Cd) Fig. 2. Lattice images of polytype structures for ZrCrz Laves phases: (a) 2H, ZrCr2 and Zr(CrFe)z; (b, c) 4H and 8H; (d) 12H and 14H.
lb) Fig. 3. The micrographs for 3R structure in: (a) Zircaloy-4 G-Fe alloy.
and (b) 3R twinning in Zr-
Using the microdensitometer the intensities of the electron diffraction patterns have been measured and the intensity distributions along (IOL) for 8H and 3R twinned structures are shown in Fig. 4.
4. Discussion 4.1. Structure determination of polytypes The f.c.c. and h.c.p. structures are regarded as being assembled from the close-packed atomic layers which have the same triangular or hexagonal net. The sequence of the atomic layers is considered to be ABC for the f.c.c. lattice and AB for the h.c.p. lattice. According to the distribution of the large atoms the Laves phases are considered to be cubic or hexagonal. The structure of Laves phase, when discussed as the distribution of both A and B atoms, is composed of two kinds of sheets: one is a denser layer forming a net of triangles and hexagons, the 0 layers as shown in Fig. 5. The other is found between these denser layers and consists of three triangular nets of A or B atoms stacked together in a close-packed manner [12, 23, 24]. If the distance from the denser layer to second denser layer is taken as a unit, the distance from a denser layer to each triangular net can be indicated by fractions of the distance between two denser layers. There are two kinds of such stacking possible, called the A and A’ layers respectively, by A and B atoms of the Laves phase. If the A and A’ layers are shifted f or $ in the [llO] direction of the hexagonal cell the B, C, B’, C’ layers are obtained. The structure of the polytypes of the Laves phases are considered to be due to the different stacking sequences of these layers. The structure factors of the hkl reflections for these layer types can be calculated from the atomic scattering amplitudes of the A and B atoms and the phase shift due to each layer. The diffraction intensity is in proportion to the square of the structure factor. Since any one layer can be put
020
iaf
Oil
ot3
012
014
Of5
013
HKL
120s
ap
loo-
?
0 010 (b)
011
012
013
014
015
016
517
018
HKL
Fig. 4. The distribution of relative intensities for OlL reflection (b) 8H Laves phases. 0, Observed relative intensity; X, calculated.
about: (a) 3R twinning;
over the other in two different ways, the possible number of arranged ways is 2N-1, where N is the layer number of the polytype structure. A summary of the polytype structures observed in the zirconium alloys in this
Fig. 5. The fundamental stacking structures bers show the distance between the layers.
of A and B atoms
in Laves phase. The num-
study is given in Table 2. A comparison of the calculated and observed relative intensities, is given in Table 3, for the 8H, 12H and 14H structures, and in Table 4 for 3R twinned structure. Good agreement was obtained between the calculated and observed intensities for the 8H structure. Although the agreement was not so good for the 12H and 14H structures, this was considered to be due to the dense pattern of diffraction spots with TABLE
2
Polytypes Polytypea 2H 3R 3R twin 4H 6H 8H 10H 12H 14H
of Zr( CrFe)z
Laves phases identified
in the present
investigation
Structureb
Alloy
2: AB
ZrCrz Zr(CrFe);? Zr( CrFe)l C. Zr( CrFe)2 d ZrCr* Zr( CrFe)z ’ ZrCr2 Zr( CrFe)z ’ ZrCr? ZrCrz
3: ABC 3 : ABC and A’C’B’ 121: AB’A’C 2211: ABC’B’AB’ 111122: AB’AB’ABC’B’ 11121121: AB’AB’A’CA’CAB’ 22221111: ABC’B’ABC’B’AB’AB’ 22222211: ABC’B’ABC’B’ABC’B’AB’
aH, Hexagonal lattice; R, rhombohedral; the numbers n before repeat period along c axis. bThe numbers represent the layer numbers in a unit cell. CIn Zircaloy-4. din Zr-Cr-Fe alloy.
H or R indicates
n layered
40 TABLE 3 Comparison of the measured and calculated relative intensities of the 8H, 12H and 14H polytypes of the ZrCr2 Laves phases hkl
010 011 012 013 014 015 016 017 018 019 0110 0111 0112 0113 0114
8H (AB’AB’ABC’B’)
12H (ABC’B’ABC’B’AB’AB’)
14H (ABC’B’ABC’B’AtiC’B’AB’)
I( talc.)
Z(obs.)
Z(ealc. )
Z(obs.)
Z(talc. )
Z(obs.)
62.6 12.3 2.2 5.0 100.0 19.8 30.2 32.2 62.5
68.8 18.9 23.1 16.0 100.0 28.8 26.2 20.2 60.7
33.6 5.0 2.1 1.5 0.7 1.2 100.0 6.6 9.6 47.1 12.7 12.0 39.7
32.7 5.3 4.4 64.1 22.2 28.1 100.0 24.6 25.3 43.6 10.5 6.5 34.4
27.5 1.3 1.4 3.7 0.2 0.1 0.4 100.0 1.4 4.3 41.9 47.2 6.1 2.8 31.6
30.0 4.9 6.2 45.5 17.0 22.5 11.2 100.0 27.8 27.2 63.1 11.4 2.7 2.7 38.5
TABLE 4 Comparison of the measured and calculated relative intensities of the 3R twinned structure hkl
010 011 012 013 014 015 016
A’C’B’
ABC Z(calc.)
Z(obs.)
Z(calc.)
Z(obs.)
0.0 0.0 100.0 0.0 0.0 56.4 0.0
0.0 0.0 100.0 0.0 0.0 61.5 0.0
0.0 56.4 0.0 0.0 100.0 0.0 0.0
0.0 71.9 0.0 0.0 89.7 0.0 0.0
some degree of overlapping, such that the intensities of neighbouring diffraction spots gave high background intensities for the measured diffraction spot. Double diffraction of electrons can also play an important role in altering the observed relative diffraction intensities. In ZrCr-Fe alloy, the 3R twinned structure was found. This 3R twinned structure can be considered to be composed of ABC and A’C’B’ respectively. In Fig. 5, it is demonstrated that the stacking A’C’B’ is equivalent to the twinning of ABC. The relative intensities of the 3R twinned structure also show good agreement between calculated and observed values. The possible structures for the 6H, 8H, 12H and 14H Laves phases are given in Figs. 6(a)-(d).
(d)
(cl Fig. 6. The structures Lams phase.
of:
(a) 6H; (b) 8H;
(c)
12H;
and (d) 14H
polytypes
of the ZrCrz
42
4.2. Images of polytypes and defect structures Figure 2 gives the lattice images of the different stacking structures. The 2H structure lattice images in Fig. 2(a) correspond with the diffraction pattern in Fig. l(a). As indicated in Table 2, the stacking sequence for the 2H structure is AB. The structure of A or B layers is given in Fig. 5. One period of the fringe is 0.82 nm. This is one period of AB stacking. The 2H structures for both ZrCr, and Zr(CrFe), Laves phase alloys show only one type of fringe image. Stacking faults have not been found in 2H structures. The diffraction patterns in Figs. l(f)-l(i) show 8H, 12H, 14H and 20H stacking structures. The lattice images are given in Figs. 2(b)-2(d). In Fig. 2(b), the 4H and 8H structures are labelled. The long period areas show 40 layer stacking which is integral times the 4H and 8H structures. The lattice images of 12H and 14H structures are shown in Fig. 2(d). The long period structures are produced which are an integral number times the 4H, 8H, 12H or 14H structures. The period of contrast variation in all of the lattice fringe images show the same behaviour as indicated in the diffraction patterns. The formation of polytype structures in ZrCrz Laves phases is considered to be based on the 4H structure. Comparing the diffraction patterns for the 8H, 12H and 20H structures it can be seen that there are four stronger diffraction spots in (100) to (108), (100) to (10.12) or (100) to (10.20). As mentioned previously, the 4H stacking is mixed structure of the 2H and 3R polytypes. The images of 3R and 3R twinned structures are given in Fig. 3. The precipitate of Zr(CrFe), Laves phase in Zircaloy-4 looks like it is formed by the stacking of (111) crystal planes. Between the sheets of (111) crystal planes, whose size is about hundreds of tigstroms, parallel or twist subboundaries appear. It is considered that these are an array of parallel edge dislocations or a mix of more than one direction edge dislocation. Figure 3(b) gives a partial lattice fringe image of partial (111) twinned planes in an orientation giving a sharp projected fringe image of the (111) planes. Dislocations were also found in 3R twinned and ZrCrz stoichiometric bulk Laves phase alloys, and examples are shown in Figs. 2(c) and 3(b). 4.3. Factors affecting the formation of polytypism in zirconium alloys Five different kinds of stoichiometric Zr-Cr-Fe alloys were studied with TEM. According to the electron diffraction patterns, the ZrFez alloy showed only the 3R cubic structure, whereas all of the ZrCrz and Zr(CrFe), stoichiometric alloys had hexagonal structures. However, only ZrCrz exhibited the polytypism phenomenon, having the 2H, 4H, 8H, 12H, 14H and 20H structures. All of the Zr(CrFe)p cast alloys showed the 2H structures only. In Zircaloy-4 and Zr-&-Fe alloys, the main structure of Zr(CrFe)2type Laves phase precipitates was the 3R rhombohedral structure, i.e. the cubic structure. A twinning structure was found in the Zr-G-Fe alloy which had been heat treated at 1000 “C for 30 min, water quenched, and
43
then annealed at 760 “C for 10.5 h. In the Zircaloy-4 some Zr(CrFe)z-type Laves precipitates have the simple 3R cubic structure, whereas other polytypes had the 6H or a higher period structure. The morphology of the Laves phase showed a layer structure along the [OOl] direction, e.g. Fig. 3(a). Several different explanations for formation of polytypism have been advanced [ 10,111, including Frank’s screw dislocation theory, Jagodzinski’s one-dimensional disorder explanation based on thermodynamic consideration, and others involving high-order phase transformations. From the present work, the polytype structures of the Laves phases in zirconium alloys show one-dimensional periodic stacking faults. The formation of the different polytypes is affected both by the composition of the alloys and the heat treatment schedules to which they had been exposed. Further experiments are planned where the bulk Laves phases are subjected to varying heat treatment schedules.
5. Conclusions From studies involving electron diffraction and high resolution electron microscopy, it is shown that polytypism exists in Laves phases in zirconium alloys. Comparison of the calculated and observed relative intensities for electron diffraction indicated the following polytypes of the Laves phase to be present. (1) In a cast bulk stoichiometric ZrFe? alloy, the 3R type cubic structure is the only polytype found. However, bulk Zr(Cr,.sFeO.&, Zr(Cr,_s5Fe,.,,),, Zr(Cr0.25Fe0.75)2 and ZrCr, alloys have the 2H type hexagonal structures. (2) The 2H, 4H, 8H, 12H, 14H and 20H polytypism structures have been found in the cast ZrCr, stoichiometric Laves phase alloy. All the polytype structures are based on the 4H structure. (3) Two kinds of Zr(CrFe)* type Laves phase precipitates have been found in Zircaloy-4 alloys: a cubic Laves phase with 3R structure and a hexagonal Laves phase precipitate with 2H, 6H and 10H polytype structures. Longer period R-type stacking structures were also found. (4) 3R type Zr(CrFe), Laves phase precipitates were found in the Zr-l.l5wt.%Cr-O.lwt.%Fe alloy. Also found was a 3R twinned structure, with the two parts of the twin being formed by ABC and A’C’B’ stacking respectively. No other polytypes have been found in this alloy. (5) The chemical composition and heat treatment states are considered to be important factors for formation of different polytype structures.
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