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Crystal structures and microstructures of (Ni, Cu)3Sn alloys
Materials Science and Engineering, A 130 (1990) 83-92
83
Crystal Structures and Microstructures of (Ni, Cu)3gn Alloys J. S. LEE PAK and K. M U K H E R J E E
Department of Metallurgy, Mechanic's and Materials Science, Michigan State Universio,, East Lansing, M148824 (U.S.A.) O. T. INAL
Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801 (U.S.A.) H. R. PAK
Department of Materials Science and Engineering, Universityof Illinois, Urbana, 1L 61801 (U.S.A.) (Received November 6, 1989; in revised form March 12, 1990)
Abstract A phase diagram for (Ni, Cu):Sn alloys containing 10-22 at. % Cu has been proposed in a previous paper. In this paper, the microstructure and crystal structure observations of as-quenched and furnace-cooled alloy specimens containing 14, 17.5 and 20 at. % Cu are made by transmission electron microscopy and electron diffraction. A high temperature phase having a DO3 structure transforms in part to a phase with a triclinic structure when furnace cooled from 1000 °C for these alloys. This triclinic phase is slightly distorted from a /3-Cu3Ti-type 2H structure (denoted by d2H) and appears in an acicular form having uniformly distributed internal faults. The acicular structure is surrounded by the 2H phase having faults on (101) and a low density of dislocations. The high temperature DO3 phase is found to be retained in alloys containing 16-22 at. % Cu when specimens are quenched from IO00°C into ice-water; on the contrary, a 2H martensite conraining (121) twins is formed in an alloy containing 14 at. % Cu. This is contradictory to a recent report claiming that a 2H martensite was formed in alloys containing 8-19 at.% Cu. A possible formation mechanism of the d2H phase is proposed. 1. Introduction A recently proposed phase diagram for the (Ni, Cu)3Sn system (Fig. 1) [1] shows that the furnace-cooled alloys of Ni-xat.%Cu-25at.%Sn (x is around 11-22 at.%) consist of three phases. The crystal structures are D03, ordered 2H (/3Cu3Ti type) and d2H. This d2H phase is a new 0921-5093/90/$3.50
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2H° DO3* d2H
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B Ni-xCu-25Sn
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Fig. 1. A phase diagram for (Ni, Cu)3Sn alloys.
phase and is investigated in detail in the present study. The identification and characterization of this phase have been performed by transmission electron microscopy and electron diffraction. Detailed microstructural observations of the phases formed under different conditions have also been carried out. A possible formation mechanism of the d2H phase is discussed. 2. Experimental procedures Pure nickel (99.999% purity), electrolytic copper (99.99% purity) and pure tin (99.999% purity) were encapsulated in evacuated quartz tubes and alloyed in a resistance furnace. Ni3Sn alloys containing copper contents ranging between 10 and 22 at.% were made; of them, the three Ni3Sn alloys with 14, 17.5 and 20 at.% Cu were extensively studied. These ingots were homogenized for 2 days at 1000°C (a fl D03 region) in encapsulated quartz tubes followed by air cooling. They were then sliced to approximately 0.5 mm thick. Some slices were annealed © Elsevier Sequoia/Printed in The Netherlands
84
at 1000°C for 1 h followed by furnace cooling. Others were quenched into ice-water after annealing at 1000°C for 1 h. Transmission electron microscopy observations were carried out with a Hitachi H-800 electron microscope, operated at 200 keV. A Hitachi HHS-2R scanning electron microscope operated at 30 keV was used for surface observations.
3. Experimental results and discussions
3.1. Optical and scanning microscopy Surface observations of the microstructures of the Ni-14at.%Cu-25at.%Sn alloy were carried out by optical and scanning electron microscopy. Figure 2(a) shows a typical microstructure of a Ni-14at.%Cu-25at.%Sn specimen, which was annealed at 1000 °C (in a high temperature fl D03 phase field) and then slowly cooled in a furnace. From the morphology, at least two phases are considered to be present in the furnace-cooled specimen. The cooling rate was estimated to be approximately 20 °C min-1 in the temperature range from 1000 to 400 °C. This suggests that all phases obtained are considered to be stable. As seen in Fig. 2(a), the most abundant phase has a rod-shaped microstructure, appearing as bright bands in the figure. The width of these bands varies from 10 to 30 ~m. This rod-shaped phase was identified as a 2H ordered structure by transmission electron diffraction, which will be described below. The less abundant phase is seen as dark areas in Fig. 2(a) between bands of the rod-shaped phase. Upon close examination by scanning electron microscopy, it was found that the dark areas are comprised of numerous very fine acicular structures. Such acicular structures (which are surrounded by 2H rod-shaped structures) appear to have some specific orientations to the parent phase, as seen in Fig. 3. This suggests that these acicular structures nucleate in the parent phase and grow to maintain a specific orientation relationship with their matrix, the D03 phase. Similar acicular structures were also observed in the alloys containing 17.5 and 20 at.% Cu (Figs. 2(b) and 2(c)), although no rod-shaped structures are seen but rather wider and longer plate shapes are seen in these cases. Transmission electron microscopy and diffraction have revealed that the three alloys containing 14, 17.5 and 20 at.% Cu show microstructures having 2H and d 2 H structures.
Fig. 2. Microstructures of (Ni, Cu)3Sn alloys, furnace cooled from 1000°C to room temperature: (a) Ni-14at.%Cu25at.%Sn; (b) Ni-17.5at.%Cu-25at.%Sn; (c) Ni-20at.%Cu25at.%Sn.
3.2. Microstructures of a furnace-cooled
Ni-14at. %Cu-25at. %Sn alloy Two microstructural features of the furnacecooled alloy are given in Fig. 4(a). One is seen in
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Fig. 3. Scanning electron micrograph of a furnace-cooled Ni- 14at .%Cu - 25 at.%Sn alloy.
Fig. 4. Microstructure of a furnace-cooled Ni-14at.%Cu25at.%Sn specimen: (a) bright field image of 2H and d2H phases; (b) diffraction pattern of the [234] zone axis taken from a 2H region (lower dislocated portion of the micrograph); (c) diffraction pattern taken from a two-phase region, showing the [23412H and [162]O2H zone axes.
the central portion of the micrograph containing bands (corresponding to acicular structures in Fig. 3) and dislocated regions surrounding them, and the other is seen in both the upper and the
lower portions of the micrograph containing primarily dislocations (corresponding to a rodshaped structure in Fig. 3). A densely packed internal structure is present in these bands and its contrast resembles those for either thin twins or stacking faults. It was found that dislocated regions surrounding bands have not only the same crystal structure but also the same crystal orientation as those for the upper and lower dislocated areas. As shown in Figs. 4(b) and 4(c), diffraction patterns taken from a lower dislocated region and from a central banded and dislocated region of the micrograph have the same orientation to each other, as framed by the full lines, and can be indexed as a 2H structure. In Fig. 4(c), two other diffraction patterns are also seen, which are indexed by a distorted 2H (i.e. d2H) structure. This is a new structure, which will be identified by electron diffraction. Figure 5 is a high magnification electron micrograph of another furnace-cooled Nil 4at.%Cu- 2 5 at.%Sn specimen. T he crystal structure in the upper right-hand side of the micrograph is the 2H structure. There is a small-angle boundary, consisting of dislocation network nmning from the top left-hand side to the bottom right-hand side of the micrograph. In the 2H area, dislocation loops and vacancy loops are seen. No d2H phase was observed to cut across smallangle boundaries. The lower part of Fig. 5 shows two distinct microstructural features. One is the band denoted by A; the other is composed of the microstructures denoted by B, C, D, E and E Band A shows similar characteristics to those for the bands with a d2H structure seen in Fig. 4. Close inspection of band A discloses that the d2H structure contains many planar faults, probably formed when such a band is nucleated in D0 3 regions. For a thin band such as that next to band A, contrasts become complicated owing to the overlapping of contrast from the top or bottom surfaces and the two side surfaces a and b of the d2H region. The microstructures denoted by B, C, D, E and F are considered to be nuclei of the d2H phase. Microstructures C and D are seen to contain striations oriented in two directions. Similar stuctural features have also been observed in nuclei of a martensite in an A u - C d alloy [2]. It should be noted that the striations in C and D are not parallel to the stacking faults in band A. Similar to Fig. 4, the 2H matrix area of the lower part and the 2H area of the upper part of Fig. 5 have the same
86
Fig. 5. Highmagnificationmicrographof a furnace-cooledNi-14at.%Cu-25at.%Snspecimen. crystal orientation to each other. It is of interest to envisage the origin of the microstructural difference in these two 2H areas. That microstructural difference may be associated with the following phase decomposition that may occur during furnace cooling of the (Ni, Cu)3Sn alloys: D03 ~
2H(h) + D03 2H(h) + {2H(1)+ d2H} + 003
Here, a portion of the D03 phase transformed to the 2H phase at high temperatures is denoted as 2H(h), and the other portion transformed later to the 2H phase at lower temperatures is denoted as 2H(I). The lattice parameters of these product phases differ slightly from each other. Therefore the dislocation network in Fig. 5 has been formed so as to accommodate lattice misfit between these two 2H areas. The small-angle boundaries could be original phase boundaries between 2H(h) and D03 areas. In Figs. 4 and 5, focus was placed on the microstructural features of the d2H phase appearing in the form of bands when Ni14at.%Cu-25at.%Sn specimens were furnace
Fig. 6. Electron micrograph of a 2H region taken from a furnace-cooled Ni-14at.%Cu-25at.%Sn specimen. The (101) planar faultsshouldbe noted.
cooled. It should be mentioned that a dislocated structure is predominant for the 2H phase (Fig. 6). This figure demonstrates dislocations and thin planar faults on the (101) plane, one of the two twinning planes in a 2H structure [2, 3].
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3.3. Morphology of a furnace-cooled Ni-2Oat. %Cu-25at. %Sn alloy
3.4. Identification of phases by electron diffraction 3.4.1. Crystal structure of dislocated regions
The alloy annealed at 1000°C followed by furnace cooling was found to have essentially the same microstructure as for furnace-cooled Ni-14at.%Cu-25at.%Sn alloy. Differences in microstructural features between these two alloys are summarized as follows: (1) the volume fraction of the 2H phase to the d2H phase is lower in the Ni-20at.%Cu-25at.%Sn alloy than in the Ni-14at.%Cu-25at.%Sn alloy, and (2) the 2H phase has a larger number of the (101) planar faults in the Ni-20at.%Cu-25at.%Sn alloy and has fewer faults in the Ni-14at.%Cu-25at.%Sn alloy, suggesting a relative change in lattice parameters of the 2H and d2H phases when the copper content is changed. A typical example of microstructures is shown in Fig. 7 obtained from a furnace-cooled alloy specimen containing 20 at.% Cu. A wide bandlike 2H region and narrow d2H bands contain internal faults, while narrow 2H regions do not contain any internal faults but contain many dislocations at or near low-angle boundaries. These dislocations may be transformation dislocations generated to accommodate a transformation strain when d2H is formed.
Figure 8 shows four zone axes of selected-area diffraction patterns taken from dislocated regions of Ni-14at.%Cu-25at.%Sn. All the diffraction patterns can be indexed by assigning the 2H structure (7-Cu3Ti type) having lattice parameters of a = 4.495/k, b = 5.380 A and c=4.285 A which were measured in X-ray experiments. The zone axes in Fig. 8 are [100], [2i0], [0il] and [012].
3.4.2. Crystal structure of banded regions (acicular structure) The crystal structure of the banded regions was also identified. Figure 9 shows three selected-area diffraction patterns taken from banded regions. These three diffraction patterns contain a, b and c axes; these three axes are not perpendicular to each other. From these diffraction patterns, the three angular lattice parameters can be measured, and they are a = 85 °, fl= 86 ° and 7 = 84 °. By adopting these angular lattice parameters and the unit-cell lengths estimated as a = 4 . 5 3 •, b=5.31 A and c = 4 . 3 4 / k , one can index all the diffraction patterns shown in Fig. 9 although the intensities of some diffraction spots
Fig. 7. Bright field image of a furnace-cooled Ni-20at.%Cu-25at.%Sn specimen. The dislocations and planar faults in 2H regions and planar faults in d2H regions should be noted.
88
Fig. 8. Electrondiffractionpatterns taken from 2H areas of furnace-cooledNi-14at.%Cu-25at.%Sn specimensfor the following zone axes:(a)[100];(b)[2i 0]; (c)[0i 1]; (d)[0i2].
are not explained if the atomic arrangement of this phase is assumed to be the same as that of the 2H phase. It should be mentioned that the crystal structure of the banded regions identified here can consistently explain the unidentified X-ray peaks in Fig. 4 of ref. 1. The crystal structure is denoted as distorted 2H (i.e. d2H).
3.5. Microstructures and crystal structures of as-quenched DO3 (Ni, Cu)3Sn alloys Several (Ni, Cu)3Sn alloys were quenched from 1000 °C into ice-water in an attempt to retain both the high temperature D03 phase and its product phase. However, even with rapid quenching by crushing the encapsulated quartz tubes in ice-water, no dual phases could b e retained. For alloys containing 16-22 at.% Cu, the high temperature D03 phase was retained whereas, for an alloy with 14 at.% Cu, martensite was formed as a product phase of the D03 phase.
It should be emphasized that these observations except for the 14 at.% Cu alloy disagree with the data reported by Murakami and coworkers [4-6]. Figure 10 is an optical micrograph of the quenched Ni-14at.%Cu-25at.%Sn alloy, showing plate-like martensite which has fine striations, indicating fine planar structures. Bright and dark bands are martensite variants, and no parent D03 phase has been detected by electron microscopy. The microstructure of the martensite bears resemblance to that of the thermoelastic martensite in a Cu-Ni-Al alloy [7]. However, no thermoelastic characteristics have been observed for the martensite formed in the Ni-14at.%Cu25at.%Sn alloy. Figure 11 is an electron micrograph of a martensite containing a high density of microtwins. The crystal structure of the martensite was identified to be the 2H structure. Trace analysis of the micrograph showed that the twinning plane was the (121) plane of the 2H structure.
89
Fig. 9. Electron diffraction patterns taken from d2H areas of furnace-cooled Ni-14at.%Cu-25at.%Sn specimens for the following zone axes: (a) [I 00]; (b) [010]; (c) [214]; [00112..
4. Discussion of the crystal structures and formation mechanism
4.1. A formation mechanism of a distorted 2H phase
Fig. 10. Microstructure of an Ni- 14at.%Cu-25at.%Sn alloy, quenched from 1000 °C into ice-water.
Electron microscopy in the present study and differential thermal analyses of the specimens used in a previous study [1] have revealed that a D03 area was transformed to both 2H and d 2 H structures. T h e former is an ordered orthorhombic structure (fl-Cu3Ti type) and is a stable phase of (Ni, Cu)3Sn alloys with copper contents ranging between 7 and 22 at.%. T h e same crystal structure is also seen in a martensite (2H') transformed upon rapid quenching from the D03 phase of (Ni, Cu)3Sn alloys with copper contents up to 14 at.%. T h e reason for the formation of the
90
(a) ~ ~ ~
(O00I)DO19
(b) ~i(~~
(O01)2H fromDO3
(c) ~ ! Fig. 11. Electron micrograph of a 2H martensite (denoted as 2H') formed in an Ni-14at.%Cu-25at.%Sn specimen, quenched from 1000 °C into ice-water.
orthorhombic 2H structure from the D03 phase in these two cases has been explained by taking atom arrangements of their specific planes into account [3]. The d2H is an ordered triclinic structure, and a possible formation mechanism of the triclinic structure originating from the D 0 3 phase will be conceived here. A discussion will be given after describing the relationships of atom arrangements between the high temperature D 0 3 phase and its three products--the stable 2H and metastable 2H' phases and the D019 phase. 4.2. Relationship between DO3, 2H and D019 structures Figure 12 shows schematic atom arrangements of close-packed planes for D019 and 2H structures of the stoichiometric Ni3Sn alloy: Fig. 12(a) shows the (0001) basal plane of D019 structure and Fig. 12(b) the (001) plane of 2H structure. By comparing Fig. 12(b) with Fig. 13, one can see that the (001) plane of 2H martensite has an atom arrangement similar to that of the (110) plane of the D03 parent phase. This is because the transformation of the D03 phase to 2H martensite does not take place by diffusion of atoms but by shear motion of atoms. This transformation is known to be explained by the Burgers [8] relation. It should be noted from Fig. 12(b) that tin atoms are surrounded by six copper atoms holding two mirror planes, perpendicular to each other: (100) and (010). From this atom arrangement and because the third axis, the c axis, is perpendicular to the (001) plane, the 2H structure can have
(001) fromL21
@O@ Fig. 12. Atomic arrangements of closed-packed planes: (a) the (0001) plane of D0~9; (b) the (001) plane of 2H; (c) the (001) plane of d2H.
0 a-site 0 b-site c-site ~ d-site Ni3Sn(DO3) Ni:~O, Sn:O Cu2NiSn(L21) Cu:~, Ni:~,Sn:O Fig. 13. Unit cellof D03-andL21-typesuperlattices. orthorhombic symmetry. For a metastable 2H' martensite formed in the alloy with 14 at.% Cu, an atom arrangement similar to that shown in Fig. 12(b) is expected; in this case, tin atoms are surrounded by a solid solution of copper and nickel atoms randomly occupying the nickel sites in the stoichiometric alloy, resulting in an orthorhombic symmetry. For the stable 2H phase formed in both furnace-cooled and aged alloys with copper contents ranging between 7 and 20 at.%, a similar situation can be seen. In this case, however, because of diffusion of atoms associated in forming this 2H phase, the following situation is possible: the
91
arrangements of copper and nickel surrounding tin atoms are changed during the transformation from the D03 to the stable 2H phase but the orthorhombic symmetry remains unchanged because of the solid solution of nickel and copper around tin atoms. For the D0t9 structure, a similar explanation can be given. This phase is massively transformed in alloys with copper contents up to 6 at.%. By comparing an atom arrangement of the (000 l) plane shown in Fig. 12(a) with that of the (110) plane in Fig. 13, one can find that the basal plane can be formed by shear motion of atoms on every other (010)2 H plane by one atomic distance on the (001 )2H plane [3]. It should be noted that the tin atoms are surrounded by six nickel atoms with a sixfold symmetrical axis perpendicular to the basal plane. That is, the D0~9 structure has a hexagonal structure but can be taken as an orthorhombic structure. The a and b axes, when this orthorhombic lattice is considered, are shown in Fig. 12(a). Comparing the axial ratios of b/a for the 2H and D019 structures, one can see that the 2H structure is slightly distorted from the h.c.p, symmetry.
4.3. Formation of d2H from DQ~via L21 The afore-mentioned examples show that the orthorhombic symmetry of the 2H and D0t9 structures can be derived from the atom arrangements of the (110) plane of the D03 structure. This suggests that the parent phase of the d2H cannot be the D03 phase. Distortion from the 2H lattice, when the d2H lattice is formed, can be explained by assuming some compositional changes that occur in the vicinity of D03 regions wherein the stable 2H phase is formed. As reported in a previous paper [1], it is possible for an L2~ phase to be transformed from the high temperature DO t phase by the following processes: (1) spinodal decomposition of the D03 phase, and (2) a diffusion process associated with the formation of a nickel-rich 2H phase. Figure 14 shows schematic illustrations describing the formation process of a region of an L21 structure. As shown in a proposed phase diagram [1], the D03 structure in a specimen is stable and the L2~ structure bears great resemblance to the D03. When the temperature is further lowered below 470 °C, a d2H phase is formed in the L2~ region and can be distorted owing to an atom arrangement of the (001) plane of the phase, which is shown in Fig. 12(c). Comparing Figs. 12(a), 12(b) and 12(c) one can see
D03
high
temperature
////
//:/71--L21
I "-Ni-rich 2H formed from D03 low "'-Cu-rieh distorted 2H formed from L21 Fig. 14. Schematic illustration describing a formation process of d2H from D03 via L2~.
that the crystal symmetry of the basal plane becomes the highest for the D0j,) structure and the lowest for the d2H structure. Since the twofold symmetry of the (001)2 H plane still remains unaltered in the (001) plane of the d2H phase, further distortion from the 2H lattice needs to take place in Fig. 12(c). It is known that some alloys such as V-Ir and M n - A u [9] have a tetragonality from a CsCl-type structure even though they have high lattice symmetry as long as the rigid atom model is concerned. Therefore it is likely that the crystal structure from the L21 phase can undergo further lattice distortion, resulting in a triclinic structure of the d2H phase. Detection of L2~ regions next to 2H regions in a D03 matrix is important, although it is hard to do this because of the structural similarity between D03 and L2j. Discrimination between D03 and L2~ can be made by a recent electron diffraction technique called the Alchemi method [10]. The observation of the D03 phase by electron microscopy and diffraction is also important to support the mechanism proposed. For some reason, we could not observe the D03 phase in this study, although a previous X-ray study [1] showed that this phase was present in bulk samples of the furnace-cooled alloys containing 14, 17.5 and 22 at.% Cu. One reason for no observation of the D03 phase in this study may be that retained DO 3 regions may have transformed to martensite owing to a thin film effect. Such an effect was observed in Cu-A1-Ni martensitic alloys [11] and was attributed to stress relaxation, leading to increase in M~ (the temperature at which transformation of austenite to martensite starts during cooling) [11]. The 2H regions with many planar faults in Fig. 7 may be martensite
92
formed in a thin film. In order to clarify this, thick samples need to be observed using a high voltage electron microscope. 5. Conclusions
(1) Alloys with copper contents of 14, 17.5 and 20 at.% were furnace cooled from 1000 °C and it was found that they showed the presence of two phases, which is contradictory to a previous report [5]. One phase is an abundant phase exhibiting rod-shaped microstructures, and the other is the less abundant phase with very fine acicular structures. The former phase has an ordered 2H (fl-Cu3Ti type) orthorhombic structure, containing dislocations and ( 101 ) faults. The latter phase has a triclinic structure, slightly distorted from the 2H structure, and is denoted as d2H. The acicular structures of this d2H phase contain a large number of uniformly distributed internal planar faults. The lattice parameters of the d2H phase determined by electron diffraction are a = 4.53 A, b = 5 . 3 1 A , c = 4 . 3 4 A , a = 8 5 °, f l = 8 6 ° and 7=84°. (2) (Ni, Cu)3Sn alloys containing copper ranging from 14 to 22 at.% were quenched from 1000 °C into ice-water. A high temperature D03 phase can be retained for as-quenched specimens containing copper ranging from 16 to 22 at.%. This observation disagrees with the data obtained
by Murakami and coworkers [4-6] who reported that the high temperature phase was transformed to a 2H martensite. In this study, such 2H martensite containing (121) twins could only be observed in specimens with a copper content of 14 at.%. (3) It is proposed that the d2H phase is formed at about 460 °C from an L21 phase which is considered to be transformed in residual D03 regions neighboring 2H areas. References 1 J. S. Lee Pak, K. Mukherjee, O. T. Inal and H. R. Pak, Mater. Sci. Eng. A, 117 (1989) 167. 2 P. L. Ferraglio and K. Mukherjee, Acta Metall., 22 (1974) 835. 3 H. R. Pak, T. Saburi and S. Nenno, Trans. Jpn. Inst. Met., 37(1973) 1128. 4 Y. Yatanabe, Y. Murakami and S. Kachi, Trans. Jpn. Inst. Met., 6 (1981) 551. 5 Y. Murakami and S. Kachi, Trans. Jpn. Inst. Met., 24 (1983) 9. 6 Y. Murakami and S. Kachi, Trans. Jpn. Inst. Met., 24 (1983) 747. 7 K. Ostuka and K. Shimizu, Jpn. J. Appl. Phys., 8 (1969) 1196. 8 W. G. Burgers, Physica, 1 (1934) 561. 9 A. E. Dwight, in J. H. Westbrook (ed.), lntermetallic Compounds, Wiley, New York, 1967. 10 J.C.H. Spence and J. Tafto, J. Microsc., 130 (1983) 147. 11 K. Ostuka and K. Shimizu, Trans. Jpn. Inst. Met., 15 (1974) 200.
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Crystal Structures and Microstructures of (Ni, Cu)3gn Alloys J. S. LEE PAK and K. M U K H E R J E E
Department of Metallurgy, Mechanic's and Materials Science, Michigan State Universio,, East Lansing, M148824 (U.S.A.) O. T. INAL
Department of Materials and Metallurgical Engineering, New Mexico Institute of Mining and Technology, Socorro, NM 87801 (U.S.A.) H. R. PAK
Department of Materials Science and Engineering, Universityof Illinois, Urbana, 1L 61801 (U.S.A.) (Received November 6, 1989; in revised form March 12, 1990)
Abstract A phase diagram for (Ni, Cu):Sn alloys containing 10-22 at. % Cu has been proposed in a previous paper. In this paper, the microstructure and crystal structure observations of as-quenched and furnace-cooled alloy specimens containing 14, 17.5 and 20 at. % Cu are made by transmission electron microscopy and electron diffraction. A high temperature phase having a DO3 structure transforms in part to a phase with a triclinic structure when furnace cooled from 1000 °C for these alloys. This triclinic phase is slightly distorted from a /3-Cu3Ti-type 2H structure (denoted by d2H) and appears in an acicular form having uniformly distributed internal faults. The acicular structure is surrounded by the 2H phase having faults on (101) and a low density of dislocations. The high temperature DO3 phase is found to be retained in alloys containing 16-22 at. % Cu when specimens are quenched from IO00°C into ice-water; on the contrary, a 2H martensite conraining (121) twins is formed in an alloy containing 14 at. % Cu. This is contradictory to a recent report claiming that a 2H martensite was formed in alloys containing 8-19 at.% Cu. A possible formation mechanism of the d2H phase is proposed. 1. Introduction A recently proposed phase diagram for the (Ni, Cu)3Sn system (Fig. 1) [1] shows that the furnace-cooled alloys of Ni-xat.%Cu-25at.%Sn (x is around 11-22 at.%) consist of three phases. The crystal structures are D03, ordered 2H (/3Cu3Ti type) and d2H. This d2H phase is a new 0921-5093/90/$3.50
,ooc
-:~!-=
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r~ 400
t-c~-200
IL II
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2H \ ~ - - -
D0 9+ 2 I
1
4
.
.
2H° DO3* d2H
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0
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Ii
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B Ni-xCu-25Sn
t
[ 16
I"
l ' i 1 20
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Fig. 1. A phase diagram for (Ni, Cu)3Sn alloys.
phase and is investigated in detail in the present study. The identification and characterization of this phase have been performed by transmission electron microscopy and electron diffraction. Detailed microstructural observations of the phases formed under different conditions have also been carried out. A possible formation mechanism of the d2H phase is discussed. 2. Experimental procedures Pure nickel (99.999% purity), electrolytic copper (99.99% purity) and pure tin (99.999% purity) were encapsulated in evacuated quartz tubes and alloyed in a resistance furnace. Ni3Sn alloys containing copper contents ranging between 10 and 22 at.% were made; of them, the three Ni3Sn alloys with 14, 17.5 and 20 at.% Cu were extensively studied. These ingots were homogenized for 2 days at 1000°C (a fl D03 region) in encapsulated quartz tubes followed by air cooling. They were then sliced to approximately 0.5 mm thick. Some slices were annealed © Elsevier Sequoia/Printed in The Netherlands
84
at 1000°C for 1 h followed by furnace cooling. Others were quenched into ice-water after annealing at 1000°C for 1 h. Transmission electron microscopy observations were carried out with a Hitachi H-800 electron microscope, operated at 200 keV. A Hitachi HHS-2R scanning electron microscope operated at 30 keV was used for surface observations.
3. Experimental results and discussions
3.1. Optical and scanning microscopy Surface observations of the microstructures of the Ni-14at.%Cu-25at.%Sn alloy were carried out by optical and scanning electron microscopy. Figure 2(a) shows a typical microstructure of a Ni-14at.%Cu-25at.%Sn specimen, which was annealed at 1000 °C (in a high temperature fl D03 phase field) and then slowly cooled in a furnace. From the morphology, at least two phases are considered to be present in the furnace-cooled specimen. The cooling rate was estimated to be approximately 20 °C min-1 in the temperature range from 1000 to 400 °C. This suggests that all phases obtained are considered to be stable. As seen in Fig. 2(a), the most abundant phase has a rod-shaped microstructure, appearing as bright bands in the figure. The width of these bands varies from 10 to 30 ~m. This rod-shaped phase was identified as a 2H ordered structure by transmission electron diffraction, which will be described below. The less abundant phase is seen as dark areas in Fig. 2(a) between bands of the rod-shaped phase. Upon close examination by scanning electron microscopy, it was found that the dark areas are comprised of numerous very fine acicular structures. Such acicular structures (which are surrounded by 2H rod-shaped structures) appear to have some specific orientations to the parent phase, as seen in Fig. 3. This suggests that these acicular structures nucleate in the parent phase and grow to maintain a specific orientation relationship with their matrix, the D03 phase. Similar acicular structures were also observed in the alloys containing 17.5 and 20 at.% Cu (Figs. 2(b) and 2(c)), although no rod-shaped structures are seen but rather wider and longer plate shapes are seen in these cases. Transmission electron microscopy and diffraction have revealed that the three alloys containing 14, 17.5 and 20 at.% Cu show microstructures having 2H and d 2 H structures.
Fig. 2. Microstructures of (Ni, Cu)3Sn alloys, furnace cooled from 1000°C to room temperature: (a) Ni-14at.%Cu25at.%Sn; (b) Ni-17.5at.%Cu-25at.%Sn; (c) Ni-20at.%Cu25at.%Sn.
3.2. Microstructures of a furnace-cooled
Ni-14at. %Cu-25at. %Sn alloy Two microstructural features of the furnacecooled alloy are given in Fig. 4(a). One is seen in
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Fig. 3. Scanning electron micrograph of a furnace-cooled Ni- 14at .%Cu - 25 at.%Sn alloy.
Fig. 4. Microstructure of a furnace-cooled Ni-14at.%Cu25at.%Sn specimen: (a) bright field image of 2H and d2H phases; (b) diffraction pattern of the [234] zone axis taken from a 2H region (lower dislocated portion of the micrograph); (c) diffraction pattern taken from a two-phase region, showing the [23412H and [162]O2H zone axes.
the central portion of the micrograph containing bands (corresponding to acicular structures in Fig. 3) and dislocated regions surrounding them, and the other is seen in both the upper and the
lower portions of the micrograph containing primarily dislocations (corresponding to a rodshaped structure in Fig. 3). A densely packed internal structure is present in these bands and its contrast resembles those for either thin twins or stacking faults. It was found that dislocated regions surrounding bands have not only the same crystal structure but also the same crystal orientation as those for the upper and lower dislocated areas. As shown in Figs. 4(b) and 4(c), diffraction patterns taken from a lower dislocated region and from a central banded and dislocated region of the micrograph have the same orientation to each other, as framed by the full lines, and can be indexed as a 2H structure. In Fig. 4(c), two other diffraction patterns are also seen, which are indexed by a distorted 2H (i.e. d2H) structure. This is a new structure, which will be identified by electron diffraction. Figure 5 is a high magnification electron micrograph of another furnace-cooled Nil 4at.%Cu- 2 5 at.%Sn specimen. T he crystal structure in the upper right-hand side of the micrograph is the 2H structure. There is a small-angle boundary, consisting of dislocation network nmning from the top left-hand side to the bottom right-hand side of the micrograph. In the 2H area, dislocation loops and vacancy loops are seen. No d2H phase was observed to cut across smallangle boundaries. The lower part of Fig. 5 shows two distinct microstructural features. One is the band denoted by A; the other is composed of the microstructures denoted by B, C, D, E and E Band A shows similar characteristics to those for the bands with a d2H structure seen in Fig. 4. Close inspection of band A discloses that the d2H structure contains many planar faults, probably formed when such a band is nucleated in D0 3 regions. For a thin band such as that next to band A, contrasts become complicated owing to the overlapping of contrast from the top or bottom surfaces and the two side surfaces a and b of the d2H region. The microstructures denoted by B, C, D, E and F are considered to be nuclei of the d2H phase. Microstructures C and D are seen to contain striations oriented in two directions. Similar stuctural features have also been observed in nuclei of a martensite in an A u - C d alloy [2]. It should be noted that the striations in C and D are not parallel to the stacking faults in band A. Similar to Fig. 4, the 2H matrix area of the lower part and the 2H area of the upper part of Fig. 5 have the same
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Fig. 5. Highmagnificationmicrographof a furnace-cooledNi-14at.%Cu-25at.%Snspecimen. crystal orientation to each other. It is of interest to envisage the origin of the microstructural difference in these two 2H areas. That microstructural difference may be associated with the following phase decomposition that may occur during furnace cooling of the (Ni, Cu)3Sn alloys: D03 ~
2H(h) + D03 2H(h) + {2H(1)+ d2H} + 003
Here, a portion of the D03 phase transformed to the 2H phase at high temperatures is denoted as 2H(h), and the other portion transformed later to the 2H phase at lower temperatures is denoted as 2H(I). The lattice parameters of these product phases differ slightly from each other. Therefore the dislocation network in Fig. 5 has been formed so as to accommodate lattice misfit between these two 2H areas. The small-angle boundaries could be original phase boundaries between 2H(h) and D03 areas. In Figs. 4 and 5, focus was placed on the microstructural features of the d2H phase appearing in the form of bands when Ni14at.%Cu-25at.%Sn specimens were furnace
Fig. 6. Electron micrograph of a 2H region taken from a furnace-cooled Ni-14at.%Cu-25at.%Sn specimen. The (101) planar faultsshouldbe noted.
cooled. It should be mentioned that a dislocated structure is predominant for the 2H phase (Fig. 6). This figure demonstrates dislocations and thin planar faults on the (101) plane, one of the two twinning planes in a 2H structure [2, 3].
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3.3. Morphology of a furnace-cooled Ni-2Oat. %Cu-25at. %Sn alloy
3.4. Identification of phases by electron diffraction 3.4.1. Crystal structure of dislocated regions
The alloy annealed at 1000°C followed by furnace cooling was found to have essentially the same microstructure as for furnace-cooled Ni-14at.%Cu-25at.%Sn alloy. Differences in microstructural features between these two alloys are summarized as follows: (1) the volume fraction of the 2H phase to the d2H phase is lower in the Ni-20at.%Cu-25at.%Sn alloy than in the Ni-14at.%Cu-25at.%Sn alloy, and (2) the 2H phase has a larger number of the (101) planar faults in the Ni-20at.%Cu-25at.%Sn alloy and has fewer faults in the Ni-14at.%Cu-25at.%Sn alloy, suggesting a relative change in lattice parameters of the 2H and d2H phases when the copper content is changed. A typical example of microstructures is shown in Fig. 7 obtained from a furnace-cooled alloy specimen containing 20 at.% Cu. A wide bandlike 2H region and narrow d2H bands contain internal faults, while narrow 2H regions do not contain any internal faults but contain many dislocations at or near low-angle boundaries. These dislocations may be transformation dislocations generated to accommodate a transformation strain when d2H is formed.
Figure 8 shows four zone axes of selected-area diffraction patterns taken from dislocated regions of Ni-14at.%Cu-25at.%Sn. All the diffraction patterns can be indexed by assigning the 2H structure (7-Cu3Ti type) having lattice parameters of a = 4.495/k, b = 5.380 A and c=4.285 A which were measured in X-ray experiments. The zone axes in Fig. 8 are [100], [2i0], [0il] and [012].
3.4.2. Crystal structure of banded regions (acicular structure) The crystal structure of the banded regions was also identified. Figure 9 shows three selected-area diffraction patterns taken from banded regions. These three diffraction patterns contain a, b and c axes; these three axes are not perpendicular to each other. From these diffraction patterns, the three angular lattice parameters can be measured, and they are a = 85 °, fl= 86 ° and 7 = 84 °. By adopting these angular lattice parameters and the unit-cell lengths estimated as a = 4 . 5 3 •, b=5.31 A and c = 4 . 3 4 / k , one can index all the diffraction patterns shown in Fig. 9 although the intensities of some diffraction spots
Fig. 7. Bright field image of a furnace-cooled Ni-20at.%Cu-25at.%Sn specimen. The dislocations and planar faults in 2H regions and planar faults in d2H regions should be noted.
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Fig. 8. Electrondiffractionpatterns taken from 2H areas of furnace-cooledNi-14at.%Cu-25at.%Sn specimensfor the following zone axes:(a)[100];(b)[2i 0]; (c)[0i 1]; (d)[0i2].
are not explained if the atomic arrangement of this phase is assumed to be the same as that of the 2H phase. It should be mentioned that the crystal structure of the banded regions identified here can consistently explain the unidentified X-ray peaks in Fig. 4 of ref. 1. The crystal structure is denoted as distorted 2H (i.e. d2H).
3.5. Microstructures and crystal structures of as-quenched DO3 (Ni, Cu)3Sn alloys Several (Ni, Cu)3Sn alloys were quenched from 1000 °C into ice-water in an attempt to retain both the high temperature D03 phase and its product phase. However, even with rapid quenching by crushing the encapsulated quartz tubes in ice-water, no dual phases could b e retained. For alloys containing 16-22 at.% Cu, the high temperature D03 phase was retained whereas, for an alloy with 14 at.% Cu, martensite was formed as a product phase of the D03 phase.
It should be emphasized that these observations except for the 14 at.% Cu alloy disagree with the data reported by Murakami and coworkers [4-6]. Figure 10 is an optical micrograph of the quenched Ni-14at.%Cu-25at.%Sn alloy, showing plate-like martensite which has fine striations, indicating fine planar structures. Bright and dark bands are martensite variants, and no parent D03 phase has been detected by electron microscopy. The microstructure of the martensite bears resemblance to that of the thermoelastic martensite in a Cu-Ni-Al alloy [7]. However, no thermoelastic characteristics have been observed for the martensite formed in the Ni-14at.%Cu25at.%Sn alloy. Figure 11 is an electron micrograph of a martensite containing a high density of microtwins. The crystal structure of the martensite was identified to be the 2H structure. Trace analysis of the micrograph showed that the twinning plane was the (121) plane of the 2H structure.
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Fig. 9. Electron diffraction patterns taken from d2H areas of furnace-cooled Ni-14at.%Cu-25at.%Sn specimens for the following zone axes: (a) [I 00]; (b) [010]; (c) [214]; [00112..
4. Discussion of the crystal structures and formation mechanism
4.1. A formation mechanism of a distorted 2H phase
Fig. 10. Microstructure of an Ni- 14at.%Cu-25at.%Sn alloy, quenched from 1000 °C into ice-water.
Electron microscopy in the present study and differential thermal analyses of the specimens used in a previous study [1] have revealed that a D03 area was transformed to both 2H and d 2 H structures. T h e former is an ordered orthorhombic structure (fl-Cu3Ti type) and is a stable phase of (Ni, Cu)3Sn alloys with copper contents ranging between 7 and 22 at.%. T h e same crystal structure is also seen in a martensite (2H') transformed upon rapid quenching from the D03 phase of (Ni, Cu)3Sn alloys with copper contents up to 14 at.%. T h e reason for the formation of the
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(a) ~ ~ ~
(O00I)DO19
(b) ~i(~~
(O01)2H fromDO3
(c) ~ ! Fig. 11. Electron micrograph of a 2H martensite (denoted as 2H') formed in an Ni-14at.%Cu-25at.%Sn specimen, quenched from 1000 °C into ice-water.
orthorhombic 2H structure from the D03 phase in these two cases has been explained by taking atom arrangements of their specific planes into account [3]. The d2H is an ordered triclinic structure, and a possible formation mechanism of the triclinic structure originating from the D 0 3 phase will be conceived here. A discussion will be given after describing the relationships of atom arrangements between the high temperature D 0 3 phase and its three products--the stable 2H and metastable 2H' phases and the D019 phase. 4.2. Relationship between DO3, 2H and D019 structures Figure 12 shows schematic atom arrangements of close-packed planes for D019 and 2H structures of the stoichiometric Ni3Sn alloy: Fig. 12(a) shows the (0001) basal plane of D019 structure and Fig. 12(b) the (001) plane of 2H structure. By comparing Fig. 12(b) with Fig. 13, one can see that the (001) plane of 2H martensite has an atom arrangement similar to that of the (110) plane of the D03 parent phase. This is because the transformation of the D03 phase to 2H martensite does not take place by diffusion of atoms but by shear motion of atoms. This transformation is known to be explained by the Burgers [8] relation. It should be noted from Fig. 12(b) that tin atoms are surrounded by six copper atoms holding two mirror planes, perpendicular to each other: (100) and (010). From this atom arrangement and because the third axis, the c axis, is perpendicular to the (001) plane, the 2H structure can have
(001) fromL21
@O@ Fig. 12. Atomic arrangements of closed-packed planes: (a) the (0001) plane of D0~9; (b) the (001) plane of 2H; (c) the (001) plane of d2H.
0 a-site 0 b-site c-site ~ d-site Ni3Sn(DO3) Ni:~O, Sn:O Cu2NiSn(L21) Cu:~, Ni:~,Sn:O Fig. 13. Unit cellof D03-andL21-typesuperlattices. orthorhombic symmetry. For a metastable 2H' martensite formed in the alloy with 14 at.% Cu, an atom arrangement similar to that shown in Fig. 12(b) is expected; in this case, tin atoms are surrounded by a solid solution of copper and nickel atoms randomly occupying the nickel sites in the stoichiometric alloy, resulting in an orthorhombic symmetry. For the stable 2H phase formed in both furnace-cooled and aged alloys with copper contents ranging between 7 and 20 at.%, a similar situation can be seen. In this case, however, because of diffusion of atoms associated in forming this 2H phase, the following situation is possible: the
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arrangements of copper and nickel surrounding tin atoms are changed during the transformation from the D03 to the stable 2H phase but the orthorhombic symmetry remains unchanged because of the solid solution of nickel and copper around tin atoms. For the D0t9 structure, a similar explanation can be given. This phase is massively transformed in alloys with copper contents up to 6 at.%. By comparing an atom arrangement of the (000 l) plane shown in Fig. 12(a) with that of the (110) plane in Fig. 13, one can find that the basal plane can be formed by shear motion of atoms on every other (010)2 H plane by one atomic distance on the (001 )2H plane [3]. It should be noted that the tin atoms are surrounded by six nickel atoms with a sixfold symmetrical axis perpendicular to the basal plane. That is, the D0~9 structure has a hexagonal structure but can be taken as an orthorhombic structure. The a and b axes, when this orthorhombic lattice is considered, are shown in Fig. 12(a). Comparing the axial ratios of b/a for the 2H and D019 structures, one can see that the 2H structure is slightly distorted from the h.c.p, symmetry.
4.3. Formation of d2H from DQ~via L21 The afore-mentioned examples show that the orthorhombic symmetry of the 2H and D0t9 structures can be derived from the atom arrangements of the (110) plane of the D03 structure. This suggests that the parent phase of the d2H cannot be the D03 phase. Distortion from the 2H lattice, when the d2H lattice is formed, can be explained by assuming some compositional changes that occur in the vicinity of D03 regions wherein the stable 2H phase is formed. As reported in a previous paper [1], it is possible for an L2~ phase to be transformed from the high temperature DO t phase by the following processes: (1) spinodal decomposition of the D03 phase, and (2) a diffusion process associated with the formation of a nickel-rich 2H phase. Figure 14 shows schematic illustrations describing the formation process of a region of an L21 structure. As shown in a proposed phase diagram [1], the D03 structure in a specimen is stable and the L2~ structure bears great resemblance to the D03. When the temperature is further lowered below 470 °C, a d2H phase is formed in the L2~ region and can be distorted owing to an atom arrangement of the (001) plane of the phase, which is shown in Fig. 12(c). Comparing Figs. 12(a), 12(b) and 12(c) one can see
D03
high
temperature
////
//:/71--L21
I "-Ni-rich 2H formed from D03 low "'-Cu-rieh distorted 2H formed from L21 Fig. 14. Schematic illustration describing a formation process of d2H from D03 via L2~.
that the crystal symmetry of the basal plane becomes the highest for the D0j,) structure and the lowest for the d2H structure. Since the twofold symmetry of the (001)2 H plane still remains unaltered in the (001) plane of the d2H phase, further distortion from the 2H lattice needs to take place in Fig. 12(c). It is known that some alloys such as V-Ir and M n - A u [9] have a tetragonality from a CsCl-type structure even though they have high lattice symmetry as long as the rigid atom model is concerned. Therefore it is likely that the crystal structure from the L21 phase can undergo further lattice distortion, resulting in a triclinic structure of the d2H phase. Detection of L2~ regions next to 2H regions in a D03 matrix is important, although it is hard to do this because of the structural similarity between D03 and L2j. Discrimination between D03 and L2~ can be made by a recent electron diffraction technique called the Alchemi method [10]. The observation of the D03 phase by electron microscopy and diffraction is also important to support the mechanism proposed. For some reason, we could not observe the D03 phase in this study, although a previous X-ray study [1] showed that this phase was present in bulk samples of the furnace-cooled alloys containing 14, 17.5 and 22 at.% Cu. One reason for no observation of the D03 phase in this study may be that retained DO 3 regions may have transformed to martensite owing to a thin film effect. Such an effect was observed in Cu-A1-Ni martensitic alloys [11] and was attributed to stress relaxation, leading to increase in M~ (the temperature at which transformation of austenite to martensite starts during cooling) [11]. The 2H regions with many planar faults in Fig. 7 may be martensite
92
formed in a thin film. In order to clarify this, thick samples need to be observed using a high voltage electron microscope. 5. Conclusions
(1) Alloys with copper contents of 14, 17.5 and 20 at.% were furnace cooled from 1000 °C and it was found that they showed the presence of two phases, which is contradictory to a previous report [5]. One phase is an abundant phase exhibiting rod-shaped microstructures, and the other is the less abundant phase with very fine acicular structures. The former phase has an ordered 2H (fl-Cu3Ti type) orthorhombic structure, containing dislocations and ( 101 ) faults. The latter phase has a triclinic structure, slightly distorted from the 2H structure, and is denoted as d2H. The acicular structures of this d2H phase contain a large number of uniformly distributed internal planar faults. The lattice parameters of the d2H phase determined by electron diffraction are a = 4.53 A, b = 5 . 3 1 A , c = 4 . 3 4 A , a = 8 5 °, f l = 8 6 ° and 7=84°. (2) (Ni, Cu)3Sn alloys containing copper ranging from 14 to 22 at.% were quenched from 1000 °C into ice-water. A high temperature D03 phase can be retained for as-quenched specimens containing copper ranging from 16 to 22 at.%. This observation disagrees with the data obtained
by Murakami and coworkers [4-6] who reported that the high temperature phase was transformed to a 2H martensite. In this study, such 2H martensite containing (121) twins could only be observed in specimens with a copper content of 14 at.%. (3) It is proposed that the d2H phase is formed at about 460 °C from an L21 phase which is considered to be transformed in residual D03 regions neighboring 2H areas. References 1 J. S. Lee Pak, K. Mukherjee, O. T. Inal and H. R. Pak, Mater. Sci. Eng. A, 117 (1989) 167. 2 P. L. Ferraglio and K. Mukherjee, Acta Metall., 22 (1974) 835. 3 H. R. Pak, T. Saburi and S. Nenno, Trans. Jpn. Inst. Met., 37(1973) 1128. 4 Y. Yatanabe, Y. Murakami and S. Kachi, Trans. Jpn. Inst. Met., 6 (1981) 551. 5 Y. Murakami and S. Kachi, Trans. Jpn. Inst. Met., 24 (1983) 9. 6 Y. Murakami and S. Kachi, Trans. Jpn. Inst. Met., 24 (1983) 747. 7 K. Ostuka and K. Shimizu, Jpn. J. Appl. Phys., 8 (1969) 1196. 8 W. G. Burgers, Physica, 1 (1934) 561. 9 A. E. Dwight, in J. H. Westbrook (ed.), lntermetallic Compounds, Wiley, New York, 1967. 10 J.C.H. Spence and J. Tafto, J. Microsc., 130 (1983) 147. 11 K. Ostuka and K. Shimizu, Trans. Jpn. Inst. Met., 15 (1974) 200.