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FIM observation of GP zones in an Al-4%Cu alloy
,&rr mctdt. VoI. 33, No. 9, pp. 16314636, Printed in Great Britain. All rights Wrvad
1985
CopyrightQ
wo1-6160/85 $3.00 + 0.00 1985 Pcrgamon Press Ltd
FIM OBSERVATION OF GP ZONES IN AN Al--4%Cu ALLOY M. WADA, I-L KITA and T. MORI Department of Materials sdence and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan (Eeceived 23 February 1985)
Abstrset-GP
zones in an aged AH%Cu alloy have been examined by a field ion microscope. When the spacing of Cu atoms is large on the observing plane, as, for example, on {024}, { 113) or { 135) planes, which a GP[l] ZOI#intersects, the images of individual Cu atoms constituting the zone are clearly resolved. A change in image contrast upon mive field evaporation has shown that Cu atoms forming a GP[I] zone are brightly hnaged by prafarentially ionizing image gas atoms when those Cu atoms protrude from an otherwise smooth plane. When they do not protrude, a dark contrast appears because of the larger elazronegativity of Cu. All the Gfll] zones, observed and analyzed with respect to their structures, are identified to consist of a sh~gle Cu {2OOjplane. GP[2] zones with two Cu planes separated by three or four (200) matrix planes are observed. R&am&-Nous avons observb par microscopic d &mission de champ les zones GP dans un a&age Al+$u vieilli. L.orsque l’espacement des atomes de cuivre est grand dans les plans d’observation qui coupent tme zone GP [I& comme c’est le cas par exempk des ptans @24}, (113) ou {ISS), on peut
clairement r&sot&e Its images des atomes de cuim individuek qui constituent la zone. Un ebangement du contra& de l’image au tours d%vaporations sucussives a montrC gut les atomcs de euivre formant une zone GP [I] sent &g&s en clsir par ionisation pr&rentielle d’atomes de gaz, lorsque as atomes de cuivre d&assent d’une surf&e qui au&mat serait plane. Lorsqu’ils ne dipassent pas, il apparait un contraste sombre B cause de la plus grande &ctronCgativiti du Cu. Toutes Ies zones GP [I] dent nous avons observ& et anal* la structure coasistaient en un seul plan d’atomea de Cu (200). Nous avons &galementobservk des zones GP [2], awx deux plans de Cu &arts par trois ou quatre plans (200) de
mat&e. aufder b&&a&&~ Ebene (z.B. auf Feldionenm&ros&oP untemucbt.Wenn fy Abstaad der &-&me (0241, (113) oder f135)), wekha die GmmeMn~ton-Zonen, bier mit GP [IJ be&&et, &n&let, dann ‘~ilichdie~A~~a~Eine~imBildlcootrastnrchvenchisdmen f~~~ P~~f~~ weist darauf bin, da83die Cu-Atome dna GP [lE&te 4 ehler bevO~gten IonWlmg dea Bildgases bier hell ahgebikiet we&n, da die Cu-Atome aus da amst &men Ebenc herausngen. Ragan sie nicht heraus, dann entsteht wegen der hi%eren Elektronegativitiit des Cu ein dunkkr Kontrast. SImtliehe beobaehtcten GP [l&&men, die auf ihre Struktur hia aaalysieti
worden sind, butanden aus elner &zigen Cu+OO~-Ebene. GP I2l_zonen mit zwei Cu-Ebeaea, die durch drei oder vkr (200~Matdxebenen getmnnt waren, wurden au& beobachtct. 1. INTRODUCIlON GP zones in an aged Al-Cu alloy are Cu-rich layers
formed on (2OOf planes fl, 21. The zone structures have been extensively studied by X-ray diRraction and transmission electron microscopy [l-9]. It ap pears to have been genually accepted that a GP[l] zone is a plate-like single layer of Cu atoms and that a GPt2J zone consists of two layers of Cu atoms, separated by three Al layers (Gerald’s model). However, several groups have recently reexamined GP zone structures in Al-Cu alloys and reported the existence of GP zones having structuras different from the above mentioned simple ones. For example, from measurement of diffuse X-ray scattering, together with simulation of atom arrangements, Auvray, Georgopoulos and Cohen have concluded that some 20-“/,of GP[I] zones have multi-layered Cu atoms [?J. GPflf zones with multi-layered Cu atoms have also been observed on a high resolution electron
microscope by Sato, Kojima and Takahashi and Sato er al. [g. 91. In addition, using field ion microscopy (FIM), Abe er al. have constructed a structure of a GP[l] zone which is described by multi-layered Cu atoms [lo], more complicated than those reported by Auvray ef at. and Sato et a!. [7-91. Silty, GP[2] zone structures, different from the Gerold model, have been observed by transmission electron microscopy [S, 1I ] and constructed to account for observed X-ray diffuse scattering by Matsubara and Cohen [12]. In short, the structure of GP zones in AI-Q alloys has not been resolved yet. More observations are needed to achieve a better understanding of this subject. In thii paper, we will report FIM observations of GP[I] and GP[2] zones in an aged AI-4”/,Cu alloy. Although some FIM studies have been made on GP zones in Al-01 alloys [IO, 13,141, additional observations complement the previous ones, because the number of 20nes observed is naturatly limited in FIM
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WADA ef cd.: FIM OBSERVATION OF GP ZONES
studies. Our observations will also fill the vacuum that has existed in the above mentioned subject between different studies. For example, by X-ray and transmission electron microscopy, most of the GP[l] zones have been identified to be single layered (200) Cu planes [7,8], while the structural analysis by Abe ef al. with FIM was limited to a multi-layered zone, Moreover, GP[I] zone images reported in past FIM studies were not clearly resolved. However, as shown previously [ 151,under a certain circumstance, individual Cu atoms can be resolved on an FIM image. Together with additional observations, we will show the existence of single layered GP[l] zones. We will also report some observed GP[2] zone structures. An analysis of the structure of a GP zone with FIM involves discussion of the image formation. The principle of FIM image formation is that imaginggas atoms, usually He or Ne, are preferentially field-ionized on protruding surface atoms above which the local electric field is high, and the ionized gas atoms are accelerated toward a screen and form an image [16]. In an alloy, there is an additional problem of selective field ionization of the imaging gas on surface atoms of particular species of the alloy [ 17. Therefore, when one studies GP zones structures by FIM, one has to discuss at the same time why Cu atoms are discernible in the Al matrix. We will also conduct a brief discussion of this subject, when we analyze the structure of GP zones. 2. EXPERIMENTAL An Al-&tQZu single crystal was grown by the Bridgman method and homogenized at 793 K for cut 1.7 x 106s. The was ilIt9 crystal 10 x 0.6 x 0.6 mm square rod samples with the longer axis parallel to [OOl]. After solution treatment at 793 K for 7.2 x I@ s and subsequent quenching into ice water. the samples were aged at 403 K for 6 x 104s to produce mainly GP[l] zones. Some samples were further aged at 443 K for 1.8 x 10 s. With this second aging most GP zones should have been converted to GP[2] zones. FIM specimens were prepared by elcctropolishing in a solution of HCl (1), HN03 (1) and HZ0 (2) (N 5 V d.c.) or HNO, (1) and HZ0 (1) (N 5 V a.c.). The imaging gas was 10e3 Pa of Ne and the background pressure was about lo-’ Pa. The specimen temperature during FIM observation was 20-30 K. 3. RESULTS AND DlSCUSSION 3.1. GP[I] On FIM images of [OOI] oriented specimens, regions where the Al matrix is clearly imaged are restricted approximately to triangular areas cornered by, for example, {022), (I 13) and (T13) poles. There are four such regions around the (002) pole. On other areas, the matrix is quite dark and a FlM image can not be easily analyzed in detail.
Fig. 1. (a) A GP(l] zone, parallel to (200), as observed on the (022) plane as a single bright line. Imaged at 8.8 kY. (b) At -a slightiy higher voltage, edge atoms of the (022) planes are removed and the size of the top (022) is reduced. (c) ARer the complete evaporation of this (022) plane, a dark line contrast appears. (d) After the top (022) plane in (c) is removed, the bright line again appears. Figure 1 shows a series of FIM images as surface atoms around a GP[l] zone are removed by field evaporation. The bright line in the center of the (022) pole, such as in Fig. 1 (a), (b) and (d), and the dotted images along this line at the concentric steps of the (022) planes undoubtediy originated from the edge of a GP[l] zone on the surface. For the reason given below, we think that this zone consists of a single layer of Cu atoms. The zone is parallel to the (200) plane. The spacing between the Cu atoms on the (022) plane is too small for the Cu atoms to be resolved into isoiated bright spots. As the applied field is increased, edge atoms of the (022) planes field-evaporate and the concentric rings due to the edge atoms move toward the (022) pole as shown in Fig. 1 (b). When the top Al plane is reduced to a smali critical size, both the top Al plane and the row of Cu atoms on the plane field-evaporate almost simultaneously, resulting in the image shown in Fig. 1 (cc).It is interesting to note that the removal of the protruding Cu atoms, which we will discuss later, is always accompanied by the removal of the top matrix plane on the (022) surface. In Fig. 1 (c) a dark line is observed at the position where a bright line was observed before the evaporation, Fig. 1 (a). As the top plane in Fig. 1 (c) is removed, a bright line appears again as shown in Fig. 1 (d). This evaporation sequence is schematically illustrated in Fig. 2 which shows a cross-section of the (022) plane. During a short time period between Fig. 2 (b) and Fig. 2 (c), the remaining top At plane is removed together with the row of Cu atom protruding from the plane, resulting in a flat (022) plane as shown in
WADA Ed al.:
FIM OBSERVATION OF GP ZONES
t0111
L,
Cl001
Fig. 2. Field evaporation sequence of (022) layers intersected by a (200) GP[ I] wne. The plane of the paper is (02) and atoms marked by a horizontal bar are located at (G/4)0 above and below the paper plane (a: lattice constant). Solid circles an Cu atoms. The most probable
sequence of the evaportion is (a)-*(b)+(c)+(d). The (022) layers are numbered from the top layer in (a).
Fig. 2 (c). Figure 1 (c) corresponds to Fig. 2 (c) and this indicates that the ionization probability of NC on a flat (022) plane must be lower for Cu atoms than for the Al matrix. The origin of the dark line contrast in Fig. 1 (c) is quite probably due to the higher electronegativity of Cu atoms. Electrons are redistributed to the Cu atoms of the GP zone and the ionization probability of Ne gas on the surrounding Al atoms is increased (selective field ionization). If a
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bright line image such as in Fig. 1 (a) were not produced by protrudin$,,Cu atoms but instead by those forming a smooth plane such as shown in Fig. 2 (c). then Fig. I (c) must have been the result of the atomic arrangement in Fig. 2 (a’). The top layer in Fig. 2 (a’) consists only of Al atoms. It is highly unlikely that this configuration results in the image seen in Fig. 1 (c). Thus, we conclude that Cu atoms of a GP zone are brightly imaged only when they protrude from the Al matrix. The protrusion is the result of a stronger binding between Cu atoms and preferential field evaporation of surrounding Al atoms. (This discussion is slightly different from our previous one [18], although the conclusion is essentially the same.) When the spacing of protruding Cu atoms of a GP[ l] zone is sufficiently large, each Cu atom can be resolved into an isolated spot as shown in Fig. 3. Here, GP[l] zones formed on the (200), (020) and (020) and observed on the (024), (113) and (TX), respectively, are shown. The spacings between Cu atoms are 0.91, 0.64 and 1.03 nm on the (024), (113) and (TX) planes, respectively. The spacings between the bright spots in Fig. 3 compare very favorably with these calculated atom spacings. (For magnification, refer to the discussion of GP[2] zones.) It must be emphasized here that clear GP[ I] images such as in Fig. 3 can be obtained only when zones
Fig. 3. GP[I] wnes obsarsd on &t&matplanes. (a) The zone is on the (200) plane and is observed on the (024) surf= at 10.4 kV. (b) The wne is on the (020) phme and is observed 011the (113) surface at 7.0 kV. (c) The zone is on the (020) plane and is observedon the 035) surfaceat 10.5kV. Cu atoms an resolved on each surface.
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WADA el al.:
FIM OBSERVATION OF GP ZONES
appear near the center of atomically smooth and reasonably large low index top planes. In these planes, the extent of the protrusion of a GP zone is large enough to produce a discernible cuntrast, On the other hand, if a GP zone appears on a higher index plane or if a zone image extends over many atomic steps, the protrusion may not be large enough for the imaging of only Cu atoms and the evaporation at the zone may become more complicated because of the presence of atomic steps of the matrix. Therefore, the analysis of images on such regions is diffcult and may not be reliable even if attempted. Abe et of. have constructed a detailed structure of B GP[I] zone by a sequence of images obtained by controlled field evaporation [IO]. It appears that the image of the zone analyzed was, however, off the center of 8 flat top plane. For these reasons, we think that the GP[l] zone structure constructed by Abe ef al. requires ~nsider~tion before un~~rno~ acceptance. For similar reasons, we restricted ourselves to GP zones which show relatively clear images around the centers of smooth and large planes. These GP[l] zones can be identified to consist essentially of single layered (200) Cu atoms. If the GP[I] in Fig. 1 should contain two adjacent (200) Cu planes and were supposed tu be represented by the solid circled plane in Fig. 2 and either ad¢ open circled plane, an image such as Fig. 1 (c) would never have been observed. Also consider the GP[l] zones seen in Fig. 3 (a). If it were a two-layered zone, the protruding Cu atoms would be rep-ted by the two rows of solid and hatched circIes in Fig. 4. Then, there would he no reason for the bright spots being resolved with B spacing of N 0.91 nm, The spacing of the brig& mfi would become -WSntn, which seem9 Wow the resolution, or a continuous broad line contrast would be produced. That the GP[l] zone observed in Fig. 3 (a) consists of a single row of Cu atoms enables one to see the resolved spots of the individual Cu atoms. A similar discussion can be applied to the zones in Fig. 3 (b) and (c). Although the number of GPI[l] zones, the structure of which have been identied without ambiguity, is also limited (approx. ten) in the
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000~00 I0241 Fig. 4. Top view of a (024) plane. Solid circles are Cu atoms. Hatch& circles indicate a hypothetica second Cu hyer.
present study, we would like to conclude that the majority of the GP[I] zones in Al-Cu alloys consists of single layered Cu atoms. This conclusion agrees with that reached previously by observation of individually resolved spot images of GP[l] zones on (204) during successive field evaporation [IQ 3.2. GP[2] On the tip surface, the prominent planes in regions where the Al matrix is imaged arc (220). Even on these relatively large surface planes, the probability of linding GP@] zones near the center of top (2203 planes is small hecause of the subs@nti~ thickness of GP[2] zones compared with the diameter of the top (220) plane, which is normally 5nm or l&s. However, by repeated trials we were able to observe two GP[2] zones interesting the (022) plane, Fig+ 5. Since such line contrasts as in Fig. 5 are never observed in FIM images of pure Al, these lines are apparently from GP[2) zones. The zones are parallel to the (200) plane and perpcndicul8r to the (022) surface. The two par&e1 l&ht lines in Fig. 5 must be due to protruding Cu atoms for the reason given previously. The separation of the two lines is now estimated from the local tip radius which can be obtained by counting the number of step rings between two poles making a known angle. The local tip radius along the [IOO] ZoQe line between the (022) and (024) poles in Fig_ 5 {a) is c&&ted to be about 34 nm and the
Fig. 5. (200) GP[2] zones obscrvexlon the (022) surface. (a) Two bright lines due to Cu atoms observed at 3.8 kV. (b) Imaged at 4.8 kV. The zone ends near the center of the (022) surface. See text for an estimate of the separation of the tines.
WADA er uf.: FIM OBSERVATION OF GP ZONES
distance between the poles is about I1 nm. Assuming that the magnification along the [loo] zone line is the same as that along a direction perpendicular to the zone line, the separation of the bright lines on the (022) plane in Fig. 5 (a) is estimated to be about I. I nm. Similarly, the local radius in Fig. 5 (b) is about 31 nm and the separation of the lines is about 0.9 nm. An alternative way to obtain the magnification is to measure the separation of atoms on the (135) or (T35) planes. On the (135) plane, for example, the atomic rows along the [ 1211 and [21T] directions are recognized and the separation of the atoms along these directions should be (&/2)-a, (a: lattice constant) which is 0.5 nm. When the magnification thus obtained on the (135) plane is used, the separation of the lines is approximately 0.9 nm for both Fig. 5 (a) and (b). Since the two parallel Cu layers protrude from the surface, it is likely that the local magnification along the direction perpendicular to the bright lines is higher than that determined by the above two methods. That is, the true spacing could be smaller than these estimated values. However, this local magnification should not be higher by a significant factor than these estimates. Thus, the separations of the Cu layers in Fig. 5 (a) and (b) are probably slightly less than 0.9 nm. It should be noted that the bright lines in Fig. 5 (a) are not continuous and the spacings between the aligned bright spots are apparently larger than a/,/% the atomic spacing along [OlT]. One may argue that the Cu layers are not pure Cu layers but contain some Al atoms, and these Al atoms have field-evaporated selectively. However, since we have often found that a GP[2] image is unstable under an applied field, it is also possible that a fraction of protruding Cu atoms has simply evaporated. In fact, the same GP[2] zone at a different depth (revealed by successive field evaporation) exhibited two continuous lines. For this reason, we hesitate at this moment to discuss the composition of a GP[2] zone which, however, apparently consists of two mainly Cu (200) planes. Now, we will discuss the structure of the GP[2] zones in Fig. 5. When two Cu (200) planes are separated by some matrix layers, two types of images
Fig. 6. Two Cu layers on (200) intersecting the (022). The plane of the paper is (022 and atoms marked by a horizontal bar are located at ( j 2/4)n above and below the paper plane. (a) Three (200) Al layers are between the Cu layers. (b) Four Al layers are between two Cu layers and only the left-hand Cu layer protrudes from the (022) surface. (c) Same as (b) after the top Al (022) layer and left-hand protruding Cu atoms have been removed.
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are expected, depending on the number of the matrix layers, between the Cu. layers. This is illustrated in Fig. 6. When an odd number of (200) atomic layers are present between the Cu layers, Fig. 6 (a) is one of the stable configurations of the top (022) plane at&r field evaporation. The two bright lines observed in Fig. 5 must be the result of this configuration. Since the spacing between the bright lines is estimated to be slightly less than about 0.9nm, it is very likely that three matrix layers are present between the Cu layers, although the true spacing should be approx. 0.8 nm. If these layers are Al layers, the structure corresponds to the classical model of GP[2] by Gerald [3]. However, this cannot be absolutely confirmed by the images in Fig. 5, because we expect, in principle, the same image even if the sandwiched three (200) layers are Cu-rich layers. Nevertheless, considering the many observations of GP[2] zones using an electron microscope [8, I I], we believe that the GP[2] zones in Fig. 5 are those described by Gerald’s model. Further, that the sandwiched layers are Al atoms can be definitely shown in the following case. When an even number of (200) matrix layers exist between two Cu layers of a GP[2] zone, the manner in which Cu atoms protrude from a (022) plane becomes different from Fig. 6 (a). in this case one can discuss whether the (200) layers between Cu layers are Al layers or not. Phillips [5J has shown the presence of GP[2] zones which contain four {200) layers between Cu-rich layers, in addition to those with three (200) layers between Cu-rich layers. For example, if the four (200) layers between two Cu layers are Al layers, Fig. 6 (b) or (c) is the stable configuration after field evaporation; only one Cu layer protrudes from the (022) surface. We will not observe two parallel bright images. When the top (022) Al layer and the protruded Cu atoms in Fig. 6 (b) or (c) are evaporated together, as in the case of the evaporation of a GP[I] zone in Fig. 2, the other Cu layer appears as shown in Fig. 6 (c) or (h) and gives a single bright line image at a slightly shifttd position. Therefore, in order to discern such a GP[2] zone, one must continuously follow the changes in the image as the surface atoms are removed by field evaporation. Figure 7 (a) is a video-recorded image of a GP[2] zone observed near the (022) plane. This image is a single frame from a continuous video-recording taken during field evaporation at a rate of a few (022) layers per second. As the (022) planes field-evaporate, the zigzag pattern due to the zone moves toward the (022) pole. The bright spot disappears when it reaches the end of the zone near the pole and a new spot appears at the other end, while keeping the same zigzag pattern which is moving toward the (022) pole. Although the ring patterns due to edge atoms of atomic steps on the (022) planes are not clear on the recorded image, the possible positions of the steps are given in Fig. 7 (b), where protruding Cu atoms are also indicated. The above observation can be
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WADA et ul.:
FIM OBSERVATION
OF GP ZONES 4. SUMMARY
GP[I] and GP[2] zones in an aged AUwt%Cu alloy have been examined by FIM using [OOl] oriented single crystal specimens. It has been shown that Cu atoms forming a zone are imaged by FIM when they protrude from the surface. Clear FIM images of GP zones can be produced when the zones intersect the centers of large surface planes; when the spacing of Cu atoms in a zone at a surface is large, individual Cu atoms are resolved. When it is short, a bright line appears. All the GP[l] zones, clearly seen and analyzed, have been identified to consist of single (200) Cu planes. Two GP[2] zones formed on the (200) planes have been observed as two bright lines on the (022) surface. The separation of the lines indicates that the zones consist of two Cu layers with three (200) Al layers between them. A zigzag pattern image of a GP[2] zone parallel to (200) has also been observed on the (022) surface and it has been shown that this’ image can be explained by two Cu layers separated by four (200) Al layers.
Fig. 7. (a) A video-recorded image of a GP[Z] zone. parallel to (200), observed near the (022) plane at an applied voltage of 15 kV. Note the zigzag pattern due to the presence of two Cu layers, indicating the presence of an even number of (200) Al layers. For details see text.
explained as follows. On the plane “a” in Fig. 7 (b), one of the Cu layers protrudes as in Fig. 6 (b) and on the upper and lower planes “b”, the other Cu layer protrudes. As the edge atoms of the (022) planes field-evaporate aud atomic steps move toward the (022) pole, the zigzag image of the Cu layers also move toward the pole. This observation of the zigzag image clearly indicates that the zone consists of two Cu layers separated by an even number of Al layers. We will roughly estimate the spacing of the two Cu layers of the GP[2] zone in Fig. 7. For the applied voltage of 15kV, at which Fig. 7 (a) was recorded, the local tip radius around the (022) region of this ahoy is normally in the range from 80 to 90 nm according to more clearly resolved images of other specimens. The separation of the Cu layers on the FIM Screen is about 0.7 mm. The magnification is given by q = R/(/?r), where R is the distance between the tip and the screen, 8cm in the present study, r the tip radius and /I a geometrical factor which is usually between 1.5 and I .8 1161. Then q = 5.0-6.7 x le.
Thus, the spacing between the two Cu planes is between 1.O and 1.4 nm. Therefore, this GP[2] zone should contain either four or six Al layers between the Cu planes. Since the local magnification ratio at a zone is higher than the average, as mentioned before, it is likely that four Al {200} planes exist between the Cu planes.
,4cknow/e&emenrs-Ryuji Uemori collaborated with us in the preliminary stage of the present work. This study was supported by a Grant-in-Aid for Sclentitic Research from the Ministry of Education and Culture (No. 5846209).
REFERKNCES 1. G. D. Preston, Proc. R. Sot. Lend. A 167,526 (1938). 2. A. &tinier, Ann. Phys. 12, 161 (1939). J. de Phys. Radium 8, 122 (1942). 3. V. Gerold, 2. Metallk. 45, 593, 599 (1954). Aclo crwtullogr. 11, 230 (1958). 4. R. B. Nicholson and J. Nutting, Phil. Msg. 3, 531 (1958). 5. V. A. Phillips, Acfu metall. 23, 751 (1975). 6. H. Yoshida, D. J. H. Coekayne and M. J. Whelan, Phil. ‘J-
Mug. 34, 89 (1976).
7. X. Auvray, P. Georgopoulos and J. B. Cohen, Acta metoll. 29, 1061 (1981). 8. T. Sate, Y. Kojima and T. Takahashi, J. Japan Inst. Metals 45, 803 (1981). 9. K. Osamura. Y. Murakama. T. Sate. T. Tabahashi, T. Abe and K. Hirano, Actu metoll. 31, 1669 (1983). 10. T. Abe, K. Miyaxaki and K-I. Hirano, Acta metall. 30, 357 (1982). II. T. Sato and T. Takahashi, Trans. Japan Inst. Metals 24, 386 (1983).
12. E. Matsubara and J. B. Cohen, Acta metoll. 13. E. D. Boyes, A. R. Waugh, P. J. Turner. P. F. Mills and M. J. Southon, 24h Int. Field Emksion Symp., p. 26, Oxford University (1977). 14. B. Ralph, S. A. Hill, M. J. Southon, M. P. Thomas and A. R. Waugh. Ultramicroscopy 8, 36 (1982). IS. T. Mori, M. Wada, H. Kita, R. Uemori, S. Horie, A. Sato and 0. Nishikawa, Jup. J. Appt. Phys. 22, L203 (1983).
16. E. W. Miiller and T. T. Tsong. Field Ion Microscopy, Principles and Applicutions. Elsevier. New York (1969). 17. R. Wagner, Field-Ion Microscopy, p. 21. Springer, Berlin (1982). 18. M. Wada, H. Kita, T. Mori and 0. Nishikawa, 31sr Int. Field Emission Symp.. Paris, France. Univ. Paris (I 984).
1985
CopyrightQ
wo1-6160/85 $3.00 + 0.00 1985 Pcrgamon Press Ltd
FIM OBSERVATION OF GP ZONES IN AN Al--4%Cu ALLOY M. WADA, I-L KITA and T. MORI Department of Materials sdence and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan (Eeceived 23 February 1985)
Abstrset-GP
zones in an aged AH%Cu alloy have been examined by a field ion microscope. When the spacing of Cu atoms is large on the observing plane, as, for example, on {024}, { 113) or { 135) planes, which a GP[l] ZOI#intersects, the images of individual Cu atoms constituting the zone are clearly resolved. A change in image contrast upon mive field evaporation has shown that Cu atoms forming a GP[I] zone are brightly hnaged by prafarentially ionizing image gas atoms when those Cu atoms protrude from an otherwise smooth plane. When they do not protrude, a dark contrast appears because of the larger elazronegativity of Cu. All the Gfll] zones, observed and analyzed with respect to their structures, are identified to consist of a sh~gle Cu {2OOjplane. GP[2] zones with two Cu planes separated by three or four (200) matrix planes are observed. R&am&-Nous avons observb par microscopic d &mission de champ les zones GP dans un a&age Al+$u vieilli. L.orsque l’espacement des atomes de cuivre est grand dans les plans d’observation qui coupent tme zone GP [I& comme c’est le cas par exempk des ptans @24}, (113) ou {ISS), on peut
clairement r&sot&e Its images des atomes de cuim individuek qui constituent la zone. Un ebangement du contra& de l’image au tours d%vaporations sucussives a montrC gut les atomcs de euivre formant une zone GP [I] sent &g&s en clsir par ionisation pr&rentielle d’atomes de gaz, lorsque as atomes de cuivre d&assent d’une surf&e qui au&mat serait plane. Lorsqu’ils ne dipassent pas, il apparait un contraste sombre B cause de la plus grande &ctronCgativiti du Cu. Toutes Ies zones GP [I] dent nous avons observ& et anal* la structure coasistaient en un seul plan d’atomea de Cu (200). Nous avons &galementobservk des zones GP [2], awx deux plans de Cu &arts par trois ou quatre plans (200) de
mat&e. aufder b&&a&&~ Ebene (z.B. auf Feldionenm&ros&oP untemucbt.Wenn fy Abstaad der &-&me (0241, (113) oder f135)), wekha die GmmeMn~ton-Zonen, bier mit GP [IJ be&&et, &n&let, dann ‘~ilichdie~A~~a~Eine~imBildlcootrastnrchvenchisdmen f~~~ P~~f~~ weist darauf bin, da83die Cu-Atome dna GP [lE&te 4 ehler bevO~gten IonWlmg dea Bildgases bier hell ahgebikiet we&n, da die Cu-Atome aus da amst &men Ebenc herausngen. Ragan sie nicht heraus, dann entsteht wegen der hi%eren Elektronegativitiit des Cu ein dunkkr Kontrast. SImtliehe beobaehtcten GP [l&&men, die auf ihre Struktur hia aaalysieti
worden sind, butanden aus elner &zigen Cu+OO~-Ebene. GP I2l_zonen mit zwei Cu-Ebeaea, die durch drei oder vkr (200~Matdxebenen getmnnt waren, wurden au& beobachtct. 1. INTRODUCIlON GP zones in an aged Al-Cu alloy are Cu-rich layers
formed on (2OOf planes fl, 21. The zone structures have been extensively studied by X-ray diRraction and transmission electron microscopy [l-9]. It ap pears to have been genually accepted that a GP[l] zone is a plate-like single layer of Cu atoms and that a GPt2J zone consists of two layers of Cu atoms, separated by three Al layers (Gerald’s model). However, several groups have recently reexamined GP zone structures in Al-Cu alloys and reported the existence of GP zones having structuras different from the above mentioned simple ones. For example, from measurement of diffuse X-ray scattering, together with simulation of atom arrangements, Auvray, Georgopoulos and Cohen have concluded that some 20-“/,of GP[I] zones have multi-layered Cu atoms [?J. GPflf zones with multi-layered Cu atoms have also been observed on a high resolution electron
microscope by Sato, Kojima and Takahashi and Sato er al. [g. 91. In addition, using field ion microscopy (FIM), Abe er al. have constructed a structure of a GP[l] zone which is described by multi-layered Cu atoms [lo], more complicated than those reported by Auvray ef at. and Sato et a!. [7-91. Silty, GP[2] zone structures, different from the Gerold model, have been observed by transmission electron microscopy [S, 1I ] and constructed to account for observed X-ray diffuse scattering by Matsubara and Cohen [12]. In short, the structure of GP zones in AI-Q alloys has not been resolved yet. More observations are needed to achieve a better understanding of this subject. In thii paper, we will report FIM observations of GP[I] and GP[2] zones in an aged AI-4”/,Cu alloy. Although some FIM studies have been made on GP zones in Al-01 alloys [IO, 13,141, additional observations complement the previous ones, because the number of 20nes observed is naturatly limited in FIM
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studies. Our observations will also fill the vacuum that has existed in the above mentioned subject between different studies. For example, by X-ray and transmission electron microscopy, most of the GP[l] zones have been identified to be single layered (200) Cu planes [7,8], while the structural analysis by Abe ef al. with FIM was limited to a multi-layered zone, Moreover, GP[I] zone images reported in past FIM studies were not clearly resolved. However, as shown previously [ 151,under a certain circumstance, individual Cu atoms can be resolved on an FIM image. Together with additional observations, we will show the existence of single layered GP[l] zones. We will also report some observed GP[2] zone structures. An analysis of the structure of a GP zone with FIM involves discussion of the image formation. The principle of FIM image formation is that imaginggas atoms, usually He or Ne, are preferentially field-ionized on protruding surface atoms above which the local electric field is high, and the ionized gas atoms are accelerated toward a screen and form an image [16]. In an alloy, there is an additional problem of selective field ionization of the imaging gas on surface atoms of particular species of the alloy [ 17. Therefore, when one studies GP zones structures by FIM, one has to discuss at the same time why Cu atoms are discernible in the Al matrix. We will also conduct a brief discussion of this subject, when we analyze the structure of GP zones. 2. EXPERIMENTAL An Al-&tQZu single crystal was grown by the Bridgman method and homogenized at 793 K for cut 1.7 x 106s. The was ilIt9 crystal 10 x 0.6 x 0.6 mm square rod samples with the longer axis parallel to [OOl]. After solution treatment at 793 K for 7.2 x I@ s and subsequent quenching into ice water. the samples were aged at 403 K for 6 x 104s to produce mainly GP[l] zones. Some samples were further aged at 443 K for 1.8 x 10 s. With this second aging most GP zones should have been converted to GP[2] zones. FIM specimens were prepared by elcctropolishing in a solution of HCl (1), HN03 (1) and HZ0 (2) (N 5 V d.c.) or HNO, (1) and HZ0 (1) (N 5 V a.c.). The imaging gas was 10e3 Pa of Ne and the background pressure was about lo-’ Pa. The specimen temperature during FIM observation was 20-30 K. 3. RESULTS AND DlSCUSSION 3.1. GP[I] On FIM images of [OOI] oriented specimens, regions where the Al matrix is clearly imaged are restricted approximately to triangular areas cornered by, for example, {022), (I 13) and (T13) poles. There are four such regions around the (002) pole. On other areas, the matrix is quite dark and a FlM image can not be easily analyzed in detail.
Fig. 1. (a) A GP(l] zone, parallel to (200), as observed on the (022) plane as a single bright line. Imaged at 8.8 kY. (b) At -a slightiy higher voltage, edge atoms of the (022) planes are removed and the size of the top (022) is reduced. (c) ARer the complete evaporation of this (022) plane, a dark line contrast appears. (d) After the top (022) plane in (c) is removed, the bright line again appears. Figure 1 shows a series of FIM images as surface atoms around a GP[l] zone are removed by field evaporation. The bright line in the center of the (022) pole, such as in Fig. 1 (a), (b) and (d), and the dotted images along this line at the concentric steps of the (022) planes undoubtediy originated from the edge of a GP[l] zone on the surface. For the reason given below, we think that this zone consists of a single layer of Cu atoms. The zone is parallel to the (200) plane. The spacing between the Cu atoms on the (022) plane is too small for the Cu atoms to be resolved into isoiated bright spots. As the applied field is increased, edge atoms of the (022) planes field-evaporate and the concentric rings due to the edge atoms move toward the (022) pole as shown in Fig. 1 (b). When the top Al plane is reduced to a smali critical size, both the top Al plane and the row of Cu atoms on the plane field-evaporate almost simultaneously, resulting in the image shown in Fig. 1 (cc).It is interesting to note that the removal of the protruding Cu atoms, which we will discuss later, is always accompanied by the removal of the top matrix plane on the (022) surface. In Fig. 1 (c) a dark line is observed at the position where a bright line was observed before the evaporation, Fig. 1 (a). As the top plane in Fig. 1 (c) is removed, a bright line appears again as shown in Fig. 1 (d). This evaporation sequence is schematically illustrated in Fig. 2 which shows a cross-section of the (022) plane. During a short time period between Fig. 2 (b) and Fig. 2 (c), the remaining top At plane is removed together with the row of Cu atom protruding from the plane, resulting in a flat (022) plane as shown in
WADA Ed al.:
FIM OBSERVATION OF GP ZONES
t0111
L,
Cl001
Fig. 2. Field evaporation sequence of (022) layers intersected by a (200) GP[ I] wne. The plane of the paper is (02) and atoms marked by a horizontal bar are located at (G/4)0 above and below the paper plane (a: lattice constant). Solid circles an Cu atoms. The most probable
sequence of the evaportion is (a)-*(b)+(c)+(d). The (022) layers are numbered from the top layer in (a).
Fig. 2 (c). Figure 1 (c) corresponds to Fig. 2 (c) and this indicates that the ionization probability of NC on a flat (022) plane must be lower for Cu atoms than for the Al matrix. The origin of the dark line contrast in Fig. 1 (c) is quite probably due to the higher electronegativity of Cu atoms. Electrons are redistributed to the Cu atoms of the GP zone and the ionization probability of Ne gas on the surrounding Al atoms is increased (selective field ionization). If a
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bright line image such as in Fig. 1 (a) were not produced by protrudin$,,Cu atoms but instead by those forming a smooth plane such as shown in Fig. 2 (c). then Fig. I (c) must have been the result of the atomic arrangement in Fig. 2 (a’). The top layer in Fig. 2 (a’) consists only of Al atoms. It is highly unlikely that this configuration results in the image seen in Fig. 1 (c). Thus, we conclude that Cu atoms of a GP zone are brightly imaged only when they protrude from the Al matrix. The protrusion is the result of a stronger binding between Cu atoms and preferential field evaporation of surrounding Al atoms. (This discussion is slightly different from our previous one [18], although the conclusion is essentially the same.) When the spacing of protruding Cu atoms of a GP[ l] zone is sufficiently large, each Cu atom can be resolved into an isolated spot as shown in Fig. 3. Here, GP[l] zones formed on the (200), (020) and (020) and observed on the (024), (113) and (TX), respectively, are shown. The spacings between Cu atoms are 0.91, 0.64 and 1.03 nm on the (024), (113) and (TX) planes, respectively. The spacings between the bright spots in Fig. 3 compare very favorably with these calculated atom spacings. (For magnification, refer to the discussion of GP[2] zones.) It must be emphasized here that clear GP[ I] images such as in Fig. 3 can be obtained only when zones
Fig. 3. GP[I] wnes obsarsd on &t&matplanes. (a) The zone is on the (200) plane and is observed on the (024) surf= at 10.4 kV. (b) The wne is on the (020) phme and is observed 011the (113) surface at 7.0 kV. (c) The zone is on the (020) plane and is observedon the 035) surfaceat 10.5kV. Cu atoms an resolved on each surface.
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FIM OBSERVATION OF GP ZONES
appear near the center of atomically smooth and reasonably large low index top planes. In these planes, the extent of the protrusion of a GP zone is large enough to produce a discernible cuntrast, On the other hand, if a GP zone appears on a higher index plane or if a zone image extends over many atomic steps, the protrusion may not be large enough for the imaging of only Cu atoms and the evaporation at the zone may become more complicated because of the presence of atomic steps of the matrix. Therefore, the analysis of images on such regions is diffcult and may not be reliable even if attempted. Abe et of. have constructed a detailed structure of B GP[I] zone by a sequence of images obtained by controlled field evaporation [IO]. It appears that the image of the zone analyzed was, however, off the center of 8 flat top plane. For these reasons, we think that the GP[l] zone structure constructed by Abe ef al. requires ~nsider~tion before un~~rno~ acceptance. For similar reasons, we restricted ourselves to GP zones which show relatively clear images around the centers of smooth and large planes. These GP[l] zones can be identified to consist essentially of single layered (200) Cu atoms. If the GP[I] in Fig. 1 should contain two adjacent (200) Cu planes and were supposed tu be represented by the solid circled plane in Fig. 2 and either ad¢ open circled plane, an image such as Fig. 1 (c) would never have been observed. Also consider the GP[l] zones seen in Fig. 3 (a). If it were a two-layered zone, the protruding Cu atoms would be rep-ted by the two rows of solid and hatched circIes in Fig. 4. Then, there would he no reason for the bright spots being resolved with B spacing of N 0.91 nm, The spacing of the brig& mfi would become -WSntn, which seem9 Wow the resolution, or a continuous broad line contrast would be produced. That the GP[l] zone observed in Fig. 3 (a) consists of a single row of Cu atoms enables one to see the resolved spots of the individual Cu atoms. A similar discussion can be applied to the zones in Fig. 3 (b) and (c). Although the number of GPI[l] zones, the structure of which have been identied without ambiguity, is also limited (approx. ten) in the
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000000 0
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000~00 I0241 Fig. 4. Top view of a (024) plane. Solid circles are Cu atoms. Hatch& circles indicate a hypothetica second Cu hyer.
present study, we would like to conclude that the majority of the GP[I] zones in Al-Cu alloys consists of single layered Cu atoms. This conclusion agrees with that reached previously by observation of individually resolved spot images of GP[l] zones on (204) during successive field evaporation [IQ 3.2. GP[2] On the tip surface, the prominent planes in regions where the Al matrix is imaged arc (220). Even on these relatively large surface planes, the probability of linding GP@] zones near the center of top (2203 planes is small hecause of the subs@nti~ thickness of GP[2] zones compared with the diameter of the top (220) plane, which is normally 5nm or l&s. However, by repeated trials we were able to observe two GP[2] zones interesting the (022) plane, Fig+ 5. Since such line contrasts as in Fig. 5 are never observed in FIM images of pure Al, these lines are apparently from GP[2) zones. The zones are parallel to the (200) plane and perpcndicul8r to the (022) surface. The two par&e1 l&ht lines in Fig. 5 must be due to protruding Cu atoms for the reason given previously. The separation of the two lines is now estimated from the local tip radius which can be obtained by counting the number of step rings between two poles making a known angle. The local tip radius along the [IOO] ZoQe line between the (022) and (024) poles in Fig_ 5 {a) is c&&ted to be about 34 nm and the
Fig. 5. (200) GP[2] zones obscrvexlon the (022) surface. (a) Two bright lines due to Cu atoms observed at 3.8 kV. (b) Imaged at 4.8 kV. The zone ends near the center of the (022) surface. See text for an estimate of the separation of the tines.
WADA er uf.: FIM OBSERVATION OF GP ZONES
distance between the poles is about I1 nm. Assuming that the magnification along the [loo] zone line is the same as that along a direction perpendicular to the zone line, the separation of the bright lines on the (022) plane in Fig. 5 (a) is estimated to be about I. I nm. Similarly, the local radius in Fig. 5 (b) is about 31 nm and the separation of the lines is about 0.9 nm. An alternative way to obtain the magnification is to measure the separation of atoms on the (135) or (T35) planes. On the (135) plane, for example, the atomic rows along the [ 1211 and [21T] directions are recognized and the separation of the atoms along these directions should be (&/2)-a, (a: lattice constant) which is 0.5 nm. When the magnification thus obtained on the (135) plane is used, the separation of the lines is approximately 0.9 nm for both Fig. 5 (a) and (b). Since the two parallel Cu layers protrude from the surface, it is likely that the local magnification along the direction perpendicular to the bright lines is higher than that determined by the above two methods. That is, the true spacing could be smaller than these estimated values. However, this local magnification should not be higher by a significant factor than these estimates. Thus, the separations of the Cu layers in Fig. 5 (a) and (b) are probably slightly less than 0.9 nm. It should be noted that the bright lines in Fig. 5 (a) are not continuous and the spacings between the aligned bright spots are apparently larger than a/,/% the atomic spacing along [OlT]. One may argue that the Cu layers are not pure Cu layers but contain some Al atoms, and these Al atoms have field-evaporated selectively. However, since we have often found that a GP[2] image is unstable under an applied field, it is also possible that a fraction of protruding Cu atoms has simply evaporated. In fact, the same GP[2] zone at a different depth (revealed by successive field evaporation) exhibited two continuous lines. For this reason, we hesitate at this moment to discuss the composition of a GP[2] zone which, however, apparently consists of two mainly Cu (200) planes. Now, we will discuss the structure of the GP[2] zones in Fig. 5. When two Cu (200) planes are separated by some matrix layers, two types of images
Fig. 6. Two Cu layers on (200) intersecting the (022). The plane of the paper is (022 and atoms marked by a horizontal bar are located at ( j 2/4)n above and below the paper plane. (a) Three (200) Al layers are between the Cu layers. (b) Four Al layers are between two Cu layers and only the left-hand Cu layer protrudes from the (022) surface. (c) Same as (b) after the top Al (022) layer and left-hand protruding Cu atoms have been removed.
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are expected, depending on the number of the matrix layers, between the Cu. layers. This is illustrated in Fig. 6. When an odd number of (200) atomic layers are present between the Cu layers, Fig. 6 (a) is one of the stable configurations of the top (022) plane at&r field evaporation. The two bright lines observed in Fig. 5 must be the result of this configuration. Since the spacing between the bright lines is estimated to be slightly less than about 0.9nm, it is very likely that three matrix layers are present between the Cu layers, although the true spacing should be approx. 0.8 nm. If these layers are Al layers, the structure corresponds to the classical model of GP[2] by Gerald [3]. However, this cannot be absolutely confirmed by the images in Fig. 5, because we expect, in principle, the same image even if the sandwiched three (200) layers are Cu-rich layers. Nevertheless, considering the many observations of GP[2] zones using an electron microscope [8, I I], we believe that the GP[2] zones in Fig. 5 are those described by Gerald’s model. Further, that the sandwiched layers are Al atoms can be definitely shown in the following case. When an even number of (200) matrix layers exist between two Cu layers of a GP[2] zone, the manner in which Cu atoms protrude from a (022) plane becomes different from Fig. 6 (a). in this case one can discuss whether the (200) layers between Cu layers are Al layers or not. Phillips [5J has shown the presence of GP[2] zones which contain four {200) layers between Cu-rich layers, in addition to those with three (200) layers between Cu-rich layers. For example, if the four (200) layers between two Cu layers are Al layers, Fig. 6 (b) or (c) is the stable configuration after field evaporation; only one Cu layer protrudes from the (022) surface. We will not observe two parallel bright images. When the top (022) Al layer and the protruded Cu atoms in Fig. 6 (b) or (c) are evaporated together, as in the case of the evaporation of a GP[I] zone in Fig. 2, the other Cu layer appears as shown in Fig. 6 (c) or (h) and gives a single bright line image at a slightly shifttd position. Therefore, in order to discern such a GP[2] zone, one must continuously follow the changes in the image as the surface atoms are removed by field evaporation. Figure 7 (a) is a video-recorded image of a GP[2] zone observed near the (022) plane. This image is a single frame from a continuous video-recording taken during field evaporation at a rate of a few (022) layers per second. As the (022) planes field-evaporate, the zigzag pattern due to the zone moves toward the (022) pole. The bright spot disappears when it reaches the end of the zone near the pole and a new spot appears at the other end, while keeping the same zigzag pattern which is moving toward the (022) pole. Although the ring patterns due to edge atoms of atomic steps on the (022) planes are not clear on the recorded image, the possible positions of the steps are given in Fig. 7 (b), where protruding Cu atoms are also indicated. The above observation can be
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FIM OBSERVATION
OF GP ZONES 4. SUMMARY
GP[I] and GP[2] zones in an aged AUwt%Cu alloy have been examined by FIM using [OOl] oriented single crystal specimens. It has been shown that Cu atoms forming a zone are imaged by FIM when they protrude from the surface. Clear FIM images of GP zones can be produced when the zones intersect the centers of large surface planes; when the spacing of Cu atoms in a zone at a surface is large, individual Cu atoms are resolved. When it is short, a bright line appears. All the GP[l] zones, clearly seen and analyzed, have been identified to consist of single (200) Cu planes. Two GP[2] zones formed on the (200) planes have been observed as two bright lines on the (022) surface. The separation of the lines indicates that the zones consist of two Cu layers with three (200) Al layers between them. A zigzag pattern image of a GP[2] zone parallel to (200) has also been observed on the (022) surface and it has been shown that this’ image can be explained by two Cu layers separated by four (200) Al layers.
Fig. 7. (a) A video-recorded image of a GP[Z] zone. parallel to (200), observed near the (022) plane at an applied voltage of 15 kV. Note the zigzag pattern due to the presence of two Cu layers, indicating the presence of an even number of (200) Al layers. For details see text.
explained as follows. On the plane “a” in Fig. 7 (b), one of the Cu layers protrudes as in Fig. 6 (b) and on the upper and lower planes “b”, the other Cu layer protrudes. As the edge atoms of the (022) planes field-evaporate aud atomic steps move toward the (022) pole, the zigzag image of the Cu layers also move toward the pole. This observation of the zigzag image clearly indicates that the zone consists of two Cu layers separated by an even number of Al layers. We will roughly estimate the spacing of the two Cu layers of the GP[2] zone in Fig. 7. For the applied voltage of 15kV, at which Fig. 7 (a) was recorded, the local tip radius around the (022) region of this ahoy is normally in the range from 80 to 90 nm according to more clearly resolved images of other specimens. The separation of the Cu layers on the FIM Screen is about 0.7 mm. The magnification is given by q = R/(/?r), where R is the distance between the tip and the screen, 8cm in the present study, r the tip radius and /I a geometrical factor which is usually between 1.5 and I .8 1161. Then q = 5.0-6.7 x le.
Thus, the spacing between the two Cu planes is between 1.O and 1.4 nm. Therefore, this GP[2] zone should contain either four or six Al layers between the Cu planes. Since the local magnification ratio at a zone is higher than the average, as mentioned before, it is likely that four Al {200} planes exist between the Cu planes.
,4cknow/e&emenrs-Ryuji Uemori collaborated with us in the preliminary stage of the present work. This study was supported by a Grant-in-Aid for Sclentitic Research from the Ministry of Education and Culture (No. 5846209).
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Mug. 34, 89 (1976).
7. X. Auvray, P. Georgopoulos and J. B. Cohen, Acta metoll. 29, 1061 (1981). 8. T. Sate, Y. Kojima and T. Takahashi, J. Japan Inst. Metals 45, 803 (1981). 9. K. Osamura. Y. Murakama. T. Sate. T. Tabahashi, T. Abe and K. Hirano, Actu metoll. 31, 1669 (1983). 10. T. Abe, K. Miyaxaki and K-I. Hirano, Acta metall. 30, 357 (1982). II. T. Sato and T. Takahashi, Trans. Japan Inst. Metals 24, 386 (1983).
12. E. Matsubara and J. B. Cohen, Acta metoll. 13. E. D. Boyes, A. R. Waugh, P. J. Turner. P. F. Mills and M. J. Southon, 24h Int. Field Emksion Symp., p. 26, Oxford University (1977). 14. B. Ralph, S. A. Hill, M. J. Southon, M. P. Thomas and A. R. Waugh. Ultramicroscopy 8, 36 (1982). IS. T. Mori, M. Wada, H. Kita, R. Uemori, S. Horie, A. Sato and 0. Nishikawa, Jup. J. Appt. Phys. 22, L203 (1983).
16. E. W. Miiller and T. T. Tsong. Field Ion Microscopy, Principles and Applicutions. Elsevier. New York (1969). 17. R. Wagner, Field-Ion Microscopy, p. 21. Springer, Berlin (1982). 18. M. Wada, H. Kita, T. Mori and 0. Nishikawa, 31sr Int. Field Emission Symp.. Paris, France. Univ. Paris (I 984).