BY DENJIRO WATANABE AND KIICHI TAKASHIMA. Department of Physics, Tohoku University, Sendai, Japan

Transcription

1 598 J. Appl. Cryst. (1975). 8, 598 Periodic Antiphase Domain Structure in the Off-Stoiehiometrie CuAu II Phase BY DENJIRO WATANABE AND KIICHI TAKASHIMA Department of Physics, Tohoku University, Sendai, Japan (Received 7 April 1975" accepted 27 May 1975) The ordered structures of Cu-Au alloys containing 56 to 65 at.% Au have been investigated by electron diffraction and electron microscopy, using thin foils prepared from bulk specimens. Single-crystal diffraction patterns taken from specimens in the CuAu II phase region are interpreted basically in terms of the CuAu II-type structure. In the composition range of at.% Au, however, the splitting direction of superlattice reflexions at 110 and equivalent positions is not parallel to the [100] direction, but deviates slightly. To explain the deviation of the splitting direction, regular steps have to be introduced into the periodic antiphase domain boundaries of the CuAu II-type. The high-resolution electron microscope study has confirmed the existence of the boundary steps. 1. Introduction The copper-gold alloy system has been the object of studies on the order-disorder transformation by many investigators, and it is well known that the ordered phase CuAu II is stable in the narrow temperature range immediately below the ordering temperature over a wide range of composition from about 36 to 65 at.% Au. The crystal structure of CuAu II was first analysed from X-ray powder diffraction patterns by Johansson & Linde (1936), and then was confirmed with a single-crystal electron diffraction study (Ogawa & Watanabe, 1954). The structure is called a 'periodic antiphase domain structure', since it is constructed by shifting periodically the CuAu I lattice, which has a face-centred tetragonal structure consisting of alternately stacked Cu-atom layers and Au-atom layers, by (0,b/2,e/2) at intervals of five unit cells along the a axis. Details of the structure, e.g. lattice parameters, antiphase domain size, lattice modulation associated with the antiphase structure, have been studied extensively over the wide composition range by X-ray diffraction with single crystals as well as with powder specimens (e.g. Jehanno & Perio, 1962, 1964; Jehanno, 1965; Okamura, Iwasaki & Ogawa, 1968). However, the only single-crystal diffraction study which has been carried out in the composition range from 56 to 65 at.% Au, is one on the 60 at.% Au alloy by Jehanno (1965). In the present experiment, the ordered structures of alloys containing 56 to 65 at.% Au were investigated mainly by electron diffraction and electron microscopy, thin foils prepared from bulk specimens by electropolishing being used. Single-crystal diffraction patterns taken from specimens in the CuAu II phase region are interpreted basically in terms of the CuAu II-type structure. In the composition range of at. ~ Au, however, an anomaly in the splitting of the superlattice reflexions is observed and a modification is required for the structure model of the CuAu II-type. 2. Experimental Specimens with different compositions were prepared by melting 99"999~o oxygen-free Cu and 99"99% pure Au in a plasma-jet melting furnace. The buttons were remelted several times and annealed in vacuum at C for 7-10 days to homogenize them and then rolled into the sheets mm thick. The sheets were then sealed in evacuated silica tubes, annealed at C for 20 h and finally quenched by dropping them into iced water. The quenched specimens were annealed at various temperatures within the CuAu II phase region for h to establish the ordered state with periodic antiphase domain structure and again quenched into iced water. After the ordering heat treatment, thin foils suitable for transmission electron microscopy were obtained by electropolishing in a solution containing 133 ml acetic acid, 25 g chromic acid and 7 ml water. A 100 kv electron microscope equipped with a specimen-tilting device was operated. Lattice parameters of the specimens quenched from 800 C were measured with a Debye-Scherrer X-ray camera of mm diameter and Cu Ka radiation, and a Nelson-Riley plot was used. Specimen compositions were determined with an accuracy of _+ 0.2 at.vo Au by referring to the lattice parameter vs. composition relation given in the literature (Pearson, 1958). 3. Results and interpretation Fig. 1 shows an electron diffraction pattern obtained from the 56"6 at.~o Au alloy which was annealed at 345 C for 100 h. At 110 and equivalent positions, split superlattice spots characteristic of the periodic antiphase domain structure are observed and all the fine spots are explained in terms of the CuAu II structure of domain size of (5-07_+0.02)a. The c/a value of the fundamental tetragonal cell of the CuAu I-type and

2 DENJIRO WATANABE AND KIICHI TAKASHIMA 599 the domain size Ma increase with increasing Au content. Table 1 shows the values of c/a and M measured from electron diffraction patterns; these agree well with those measured by Jehanno (1965) by X-ray diffraction. Fig. 2 shows the diffraction pattern taken from the 60-9 at.~ Au alloy annealed at 290 C for 100 h. Although the split spots are seen again at 110 and equivalent positions, there are some differences in this pattern as compared with that in Fig. 1. Firstly, the intensities of the higher-order splits are very weak, and secondly, the direction of splitting is not parallel to the [-100] direction, but deviates slightly, as schematically illustrated in Fig. 3. These phenomena were observed in the composition range of at.~o Au, and the angle of deviation 6 changes with composition, the maximum angle being 4 at about 61 at.~o Au. The deviation of the splitting direction from [100] cannot be explained by the structure model of the well known CuAu II-type, but requires a modification. One of the possible structure models which can explain the observed diffraction pattern is illustrated in Fig. 4, where the crystallographic axes refer to the basic CuAu I lattice and the atomic arrangement on the (001) plane is shown. The antiphase boundaries are on the bc plane at intervals of M unit cells, as in the CuAu II structure, and every m unit cells along the b direction these boundaries shift by half the unit cell length along the a direction, forming a stepped configuration of boundaries, as depicted by the dotted lines in Fig. 4. A large unit cell is newly defined as outlined by the thick lines. Macroscopically, the average antiphase boundaries are parallel to the (2m, l,0) plane. The intensity of the superlattice reflexions l(hkl) is given by the equation l(hkl)oc(fau-fc,) , o "O" "O' Fig. 2. Electron diffraction pattern of the Cu-60.9 at.% Au alloy annealed at 290 C..:..; o..:...:. l 0... Fig. 1. Electron diffraction pattern of the Cu-56"6 at.% Au alloy annealed at 345 C Fig. 3. Schematic illustration of the diffraction pattern shown in Fig. 2. Composition (at.~ Au) 56"6 c/a M 5"07 Table 1. Experimental values of c/a and M Error 58"7 60"9 62"4 63"5 65"2 65"4 +0"2 0"943 0"955 0"959 0"971 0"980 0"985 +0"002 5"42 5"75 5"80 6"30 6"63 7"03* +0"02 +0"05*

3 600 PERIODIC ANTIPHASE DOMAIN STRUCTURE IN CuAu II where I1 = sin 2 2 (h+2mk) for h,k=odd, 7~ = cos 2~(h+2rnk) for h,k=even, sin 2 7tMh sin / rt(2m)nlh I2 = sin 2 zrmh sin2 7rh sin 2 rc(2m)h ' sin 2 n(h + 2ink)N2 I3 = sin 2 n(h+2mk) ' sin 2 nmk 14 = sin 2 nk ' sin 2 ~zn31 I5- sin 27zl ' Fig. 4. Structure model with boundary steps of one atom plane width _--_ - -=-.~ ~ ~ _.._1_ k 2m,_ 3- (1 ~oi --'--- --I 2M 2M ~ h =_~-... =~ Fig. 5. Explanation of the splitting of the superlattice reflexion around the 110 position. Maxima of the terms 12 and 13 in l(hkl) are indicated by thick full lines and dash-dot lines respectively. The term I,~ has a main maximum when k =0. fau and fcu are the atomic scattering factors of Au and Cu, N 1, N 2 and N3 the numbers of large unit cells along the [100], [1,2m,0] and [001] directions, respectively. The splitting of the superlattice reflexions is determined by the three terms, I2, I3 and I4. The roles of these terms around the 110 position are illustrated in Fig. 5, where the origin of the h and k coordinates is taken at 110. I 2 gives the maxima at h--+ 1/2M, +3/2M... and I3 the maxima along the lines of k=-h/2m+~/2m (~=0,+1,+2... ), and thus I(hkl) has maxima at l+(n/2m), 1-(n/4mM), 0 (n= + 1, +_3,...) around the 110 position, in good agreement with observation. The reason why the intensities of the higher-order splits are very weak when the splitting direction deviates from [100] is found in the role of the term 14, which has main maxima when k is integral. The angle of deviation 6 is determined by the domain size along the b axis, i.e. tan 6 = 1/2m. In the schematic drawing of Fig. 4, M and m are assumed to be 5 and 4 respectively. However, the observed value of 6 is small and the angle of 4 corresponds to 7 for the m value (domain size of about 28 A). In the structure model shown in Fig. 4, the steps of the antiphase boundaries have a width of half the unit cell constant of the fundamental tetragonal lattice of CuAu I-type, i.e. the width of one atom plane. However, models with boundary steps of different widths, e.g. two atom planes, three atom planes, etc. can also explain the observed diffraction pattern. Of course, the domain size along the b direction varies with the width of the steps for the same value of ~. For example, the m value for the steps of two atom planes is twice that for the steps of one atom plane. Since these structure models give the same effect on the diffraction pattern, it is impossible to determine the step width from the diffraction experiment alone. However, if the lattice image of this phase is taken with a highresolution electron microscope, it will be possible to observe these domain boundary steps on the image. An example of the preliminary results of the electron microscope study is shown in Fig. 6, which was taken from the 63 at.~o Au alloy annealed at 250 C

4 DENJIRO WATANABE AND KIICHI TAKASHIMA 601 for 100 h. In this case, the lattice image was taken with the diffraction spots of 000, 110, 1i0 and 200, using the tilted-illumination technique. Fringes of 2.8 A spacing perpendicular to the [110] direction and those of about 24 A spacing corresponding to the periodic antiphase domain boundaries are observed in the micrograph. The former fringes are shifted by a half period at each antiphase boundary. These features of fringes have been interpreted by Mihama (1971) in terms of the interference among the diffracted waves concerned. In Fig. 6 the fringes of the antiphase domain boundaries are not straight and their average directions deviate from [010] by 3-4, in agreement with the deviation angle 6 of the splitting direction of the superlattice spots. It is to be noted, however, that boundary steps of about 4 A, which correspond to the width of two atom planes, are observed in the right half of the image and the fringes are parallel to the [010] direction between two neighbouring steps. Thick arrows in Fig. 6 indicate the positions of some of these steps. Although such steps are not seen in the left half of the image, steps of about 2 A corresponding to one atom plane width can be recognized by tracing the shift of (110) fringes at the boundaries. Thin arrows in Fig. 6 indicate some of the steps identified in this way. It is concluded, therefore, that a structure model with boundary steps of two atom planes as well as that with one atom plane steps express the real structure quite well. 4. Discussion As already mentioned in 1, the structure of the CuAu II phase in the composition range from 56 to 65 at.9/o Au has been studied mainly with polycrystalline specimens and a thorough investigation with single crystals has not been performed. This is the reason why the antiphase boundary steps have not been revealed hitherto. The question may arise whether the periodic antiphase domain structure with boundary steps observed in the present study is a stable feature of the equilibrium state or not, since it is considered that a long annealing time is required to obtain the equilibrium ordered state because of the low transition temperatures (less than about 300 C) and the consequent lower isothermal annealing temperatures. In the present experiment, however, all the specimens were quenched from temperatures of C which are 50 C below their melting temperatures, prior to the ordering treatment. This procedure leads to the acceleration of Fig. 6. Lattice image of the ordered phase of the Cu-63 at.% Au alloy taken at 100 kv. Thick arrows indicate the positions of some of the regular boundary steps ofabout 4 A width and thin arrows those of about 2 A width.

5 602 PERIODIC ANTIPHASE DOMAIN STRUCTURE IN CuAu II the ordering kinetics by the elimination of quenched-in vacancies in the course of annealing, as demonstrated by Gratias, Condat & Fayard (1972) in the study of ordering phenomena in Au3Cu alloys. In fact, the specimens annealed for 20 h gave essentially the same diffraction pattern as shown in Fig. 2, and no subsequent change was observed after annealing up to 100 h. Therefore it can be safely said that the antiphase domain structure with boundary steps exists in the stable equilibrium ordered state. The agreement between the values of domain size M and c/a measured in the present experiment (Table 1) and those of Jehanno (1965) also supports this conclusion. It is interesting to examine if the regular antiphase boundary steps exist also in the copper-rich off-stoichiometric CuAu II phase. A preliminary experiment was made on the 39.3 at.% Au alloy in the present study. However, the superlattice reflexions did not show any anomaly in the splitting, indicating that the boundary steps do not exist. On the basis of a single-crystal X-ray diffraction experiment, Jehanno (1965) has reported that the excess gold atoms are distributed randomly in the antiphase domain structure in the 60 at.~ Au alloy, whereas the excess copper atoms are distributed preferentially at the domain boundaries in the 40 at. o Au alloy. However, a thorough X-ray investigation will be required for the gold-rich off-stoichiometric CuAu II alloys with different compositions to obtain the detailed knowledge of the distribution of excess gold atoms. Although the deviation angle fi of the splitting direction of the superlattice reflexion is small and hence the experimental error is large, it will also be interesting to obtain the fi-vs.-composition relation experimentally. Such information will be indispensable for the understanding of the origin of the antiphase boundary steps and attempts along these lines are in progress. The high-resolution lattice images were taken with a HU-12A electron microscope in the Hitachi Central Research Laboratory. The authors wish to thank Dr T. Komoda and his group for the provision of laboratory facilities and their technical help. References GRATIAS, D., CONDAT, M. & FAYARD, M. (1972). Phys. Star. Sol. (a), 14, JEHANNO, G. (1965). Th~se, Facult6 Sci., Univ. Paris, pp Jm-IaNNO, G. & PERIO, P. (1962). J. Phys. Radium, 23, JZHANNO, G. & PERIO, P. (1964). J. Phys. 25, JOHANSSON, C. H. & LrNDE, J. O. (1936). Ann. Phys. 25, MtHAMA, K. (1971). J. Phys. Soc. Japan, 31, OGAWA, S. & WATANABE, D. (1954). J. Phys. Soc. Japan, 9, OKAMURA, K., IWASAKI, H. & OGAWA, S. (1968). J. Phys. Soc. Japan, 24, PEARSON, W. B. (1958). A Handbook of Lattice Spacings and Structures of Metals and Alloys. Oxford: Pergamon Press. i ~ : ~ ~S :'..'.i,-. ~ :~:,: - :i~,~ ~ '..~ ~= ~ "~