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A TEM study of a copper rich phase grown on an Al3Zr dispersoid in a spray formed AlSiC composite
Acra mater. Vol. 44, No. 6, pp. 24X-2455, 1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in Great Britain. All rights reserved
Pergamon 0956-7151(95)00339-8
1359-6454/96 $15.00 + 0.00
A TEM STUDY OF A COPPER RICH PHASE GROWN ON AN Al,Zr DISPERSOID IN A SPRAY FORMED Al/Sic COMPOSITE P. VERMAUT LERMAT,
URA CNRS
No 1317, ISMRa,
and P. RUTERANA
6 Boulevard
du Mar&ha1
Juin, 14050 CAEN
Cedex, France
(Received 2 February 1995; in revised form 2 August 1995)
Abstract-A
copper rich aluminium zirconium phase encountered in a spray formed Al/Sic composite has been investigated. It was found to grow epitaxially on the (001) faces of the A1,Zr dispersoids. The measured concentrations by EDS, down to an accuracy of 1% did not correspond to a known compound, the new phase has a composition of Al,Cu,Zr,. By careful diffraction and image simulation, and comparison with experimental results, we obtain a spatial group of P4/mmm and two very closely related atomic distributions inside the unit cell. The structure is close to that of Al,Zr with the same a parameter, however the c parameter is 1.157 nm. R&um&Une phase Aluminium Zirconium riche en Cuivre, observte dans un composite Al/Sic obtenu par mtthode Osprey, a et& Ctudike par microscopic tlectronique en transmission. Cette phase a crO en epitaxie sur les faces (001) d’un precipite Al,Zr. Sa composition: Al,Cu,Zr,, obtenue par analyse EDS avec une prkcision de l’ordre du pourcent, ne correspond a aucun compos& connu. La comparaison des images haute r&solution et des clichts de diffraction expkrimentaux avec leurs simulations a conduit au groupe spatial P4/mmm et a deux, trts proches, distributions atomiques dans la maille. La structure obtenue est proche de celle de Al,Zr avec un paramktre a identique et un paramCtre c valant 1.157nm.
Zusammenfassung-Eine Kupfer-hochhaltige
Aluminium Zirconium Phase, durch das Osprey-Verfahren wurde in einem Al/Sic Verbundwerkstoff, mittels der Durchstrahlungs electronenmikroscopie und untersucht. Diese Phase erschien durch epitaxiales Wachstum auf der (OOl)-Ebene einer A1,Zr Auscheidung. Seine Zuzammensetzung, Al,Cu,Zr,, die durch EDS mit einer prozent-Genauigkeit, ermittelt werden konnte, entspricht keinem bekannten Komponent. Der Vergleich van Hochaufliisungsund-Streuungsbildern mit Simulation fiihrt zu der Raumgruppe P4/mmm sowie zu zwei, sehr benahrten atomaren Verteilungen im Gitter. Die endeckte Struktur ist der von Al,Zr, mit demselben a-Parameter, und einen 1.157 nm wertigen c-Parameter, sehr nahe. hergestellt, beobachtet
1. INTRODUCTION
A composite material, made of 13 vol % SIC particles in an aluminium alloy with 6 wt% Cu, 2 wt% Mn, 0.35 wt% Mg and 1 wt% (Zr, V, Ti, Cr, Ag) matrix, is in practical use. In the reinforcement of the matrix, it is known that Cu, Mg and Ag contribute to the hardening by precipitation, while the role of the other elements is to make dispersoids. These later elements can act as heterogeneous nucleation sites for intermediate phases in alloys such as AlLCu-Zr [l]. However, large A1,Zr dispersoids are also known to form at grain boundaries as do Mn-rich ones. In the studied samples, they were found to be aligned along the extrusion direction and a Scanning Electron Microscopy (SEM) investigation showed their size to range from 0.5 Transmission Electron to 1 jrn [2]. In this Microscopy (TEM) work, they were always found to be rather large, and to have a complex mor-
phology, including possible cracks which have been filled with pure Al [3]. The formation of dispersoids itself can be interesting to understand. In our material, it seems that they form during the cooling step. In later treatments, they may continue to grow or even serve as sites for precipitation. This was found to be particularly true for the A1,Zr dispersoids. Due to the extrusion. some of them have been found to be preferentially oriented with the c axis perpendicular to the extrusion direction. In some cases, on top of these dispersoids a different phase has grown which contains copper.. An Energy Dispersion Spectroscopy (EDS) analysis showed us quickly that it was a new phase. It is clear that due to the small size of the crystallites (a few tens of nanometres), one can hardly use other techniques for the structure and chemical analysis. Therefore, diffraction, High Resolution Electron Microscopy (HREM) and image simulation were used in order to determine the structure and atomic
2445
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VERMAU’J
and RI;JERANA:
HREM
composition and distribution for this phase. In the following. we discuss the work that was carried out and present the models of structure and composition used. 2. EXPERIMENTAL
The studied samples were taken from a commercial Al/Sic composite. The alloy was made by Alusuisse for use in composite f’abrication by Osprey preform [4]. The composition was Al 6-6.3% Cu. l&2.1% Mn, 0.35% Ag, 0.3% Mg, 0.8-l% (Cr, Ti, V, Zr). The Osprey preform consisted of mixing the atomized molten alloy and SIC particles, and quenching onto a mandrel. The fabrication details can be found in Ref. [5]. The composite was next extruded and then aged at 190X (T6) after solution treatment at 520°C for 20 min (T4) followed by water quenching. We examined the samples after extrusion, T4 and T6 Tempers. The TEM samples were made in the usual way by mechanically polishing 3 mm discs down to 100 pm, and then dimpling to lo-20 pm followed by ion milling at LN, temperature. Observations were carried out on an ABT 002 Electron Microscope equipped with a thin window EDS facility. The quantitative analysis was made using the commercial programs and we only worked on the K-lines, for which the theoretical models are well known. The microscope was operated at 200 kV, the smallest spot size for EDS was 2 nm and the point-to-point resol-
Fig. 1. BF image of a Zr based dispersoid
SI’l.I)I
utlon
01
lYi:\>~
was O.iS nm. High resolutton as ;I focat series using a focu> step of IO nm starting from the minimum contrast image. Optical diffraction wa\ carried out on the negatives for focus determination. Selected Area Ditfraction Patterns (SADP) and HREM image simulations were performed using EMS software written by P. Stadetmann [6]. images
for
4 f 11 KiCH
HREM
WCI‘C recorded
3. RESULTS
The precipitate studied was a Zr-based dispersoid with a complex morphology, as shown in the BF image in Fig. I. It was split into parallel plates separated by matrix channels. At1 plates have the same orientation as if they originally had formed one crystallite. The presence of low intensity (002) and (006) spots in SADP of these plates [Fig. 2(a)] shows that they have the high temperature stable DO,, structure [7]. The matrix channel thickness varies from 5 to 20 nm. and the thickness of the At,Zr plates from 30 to 120 nm. At the external interfaces, the dispersoid is coated with a different phase which is epitaxial on the (001) A1,Zr phase. EDS analysis showed that it contains more Cu than Zr. The thickness of this layer is about 170 nm. The whole dispersoid does not present a simple orientation relationship with the matrix whereas the interface is still planar. This can be due to a recrystallisation of the aluminium matrix after the extrusion process. In
showing
a morphology
of parallel
plates.
VERMAUT and RUTERANA:
HREM STUDY OF A Cu-RICH PHASE
2441
3.1.1. The composition. A quantitative EDS analysis of this phase gives the 60.5% Al, 25.5% Cu, 14% Zr atomic composition or Al,Cu,Zr,. Using the same conditions (detection configuration, spot size, specimen thickness), in the matrix area close to the precipitate, one obtains a Cu concentration lower than 2%. So, the Cu signal from the specimen holder, which would be identical in both cases, is small. To our knowledge no phase with such a composition was reported in the literature. The only ternary known which has the cubic Ll, compound is Al,CuZr,, a = 0.404 nm. It structure with a lattice parameter has been shown by Schubert et al. [8] that the addition of Cu in A1,Zr gives the Al,CuZr, phase and leads to a stabilization of the low temperature Ll, structure [7]. 3.1.2. The dzfiaction. The diffraction patterns along the [ 1 lo],,,,, direction of the two phases [Figs 2(a) and (b)] are very similar. Both present a 2 mm point group symmetry. The copper-rich phase grows on the (001) plane of the A1,Zr precipitate which has a tetragonal structure. HREM images of the interface (Fig. 3) shows it to be planar and without any misfit dislocations, or other defects over more than 200 nm, meaning that the two lattices fit perfectly on the basal planes. These two facts lead us to assume, for the Cu-rich phase, a tetragonal structure similar to the A1,Zr one. Therefore, the c-parameter which is 1.732 nm for Al,Zr, is 1.157 nm for the new phase. Moreover, the spots intensity sequencies along the [OOl] direction are different for
Fig. 2. Diffraction pattern along the [l lo] axis (a) of Al,Zr, showing visible (002) and (006) spots; (b) of the &-rich phase.
the following, phase.
we will identify
the structure
of this
3.1. IdentiJication of the &-rich phase In order to identify this phase, we simultaneously carried out EDS, diffraction and HREM work. Unfortunately, it was not possible to carry out a convergent beam analysis due to the large lattice parameter of the compound. Moreover, the only available zone axis was the [l lo]. So, after determining the composition of this phase, a trial and error search of the most suitable atomic position in the unit cell was carried out. We tried to match the diffraction as well as the high resolution experimental images.
Fig. 3. HREM image of the Al,Zr/Cu-rich phase interface. The defocus is -49 nm.
the two phases. For Al&. only the even spots are visible. The (002) and (006) spots present a low intensity but are not extinguished meaning that the dispersoid has the stable DO?, tetragonal structure and not the cubic LIZ one. The (004) and the (008) ones have a higher intensity with a maximum for (008). For the Cu-rich phase, no extinctions are observed [Fig. 2(b)]. Only the (004) spot presents a low intensity. The strongest spots are the (73) ones. and (003) and (TO) have comparable mtensities.
i.2.
The HREM
study of’the Cu-rich phase
3.2.1. The experimental shows two HREM images
parameters. Figure 4 and the corresponding
opttcal diffractograms. The tirst [Fig. 4(a)] shows. intensity up tc\ m its optical diffractogram, 3 nm -‘. which corresponds to a - I6 nm defocus. The second [Fig. 4(b)] presents a minimum close to 3.5 nm ’and a broad high intensity up to 4.5 nm ’ For a weak phase object. it corresponds to the Coherent Transfer Function of our microscope (C’s = 0.4 mm, s( = 0.8 mrad) for a -49 nm defocus. The first defocus corresponds, for a weak phase object. to the minimum of contrast, and the second is just between the Scherzer focus and the first contrast inversion. To estimate the thickness of the studied area, we can see on the BF image (Fig. 1) that the examined area is located before the first minimum
Fig. 4(a) See caption oppositr.
VERMAUT and RUTERANA:
Fig. 4. IIREM images of the Q-rich
HREM STUDY OF A Cu-RICH PHASE
2449
phase with their optical diffraction: (a) at - 16 nm defocus; (b) at - 49 nm defocus.
in the image. This later point has been estimated by drawing the transmitted beam intensity versus specimen thickness. The minima (Fig. 5) do not go to zero, but the intensity is small from 10 to 20 nm thickness and beyond 40nm. So, we can suppose that the specimen thickness of the studied area is lower than 9 nm. To verify these parameters, HREM image simulations of an Al,Zr structure have been carried out at - 16 and -49 nm defocus, for thicknesses from 1 to 27 nm, and compared to the experimental images. The two experimental images, at the two focus values, have a similar contrast. In the two cases, we can observe an alternate contrast of low and high intensity of the basal planes and an increase of these variations with the
specimen thickness. Simulated HREM images with -49 nm defocus, shown in Fig. 6, are in good agreement with the experimental image (Fig. 3) for specimen thickness ranging from 2.3 to 13.6nm. Beyond 10 nm, one can observe a shift in the high intensity fringes which is not observed on experimental images in the studied area. For larger thicknesses (20-30 nm), one lattice plane in four shows higher intensity, which is not observed. Therefore, we are not at the second maxima of the transmitted beam intensity (Fig. 5). Comparing with the unit cell, one can say that for - 16 nm defocus the highest intensity basal lattice fringes are Al containing atomic planes, whereas at -49 nm defocus the highest intensity fringes are
2450
VERMAUT and RUTERANA:
HREM STUDY
OF A (‘u-RICH
PHASE
AI3Zr phase Transmitted Ream Intensity
0.
J
0.
$1
.2
.3
.4
.5
.6
Thickness [nm] *
.7
.n
.v
1.
100
Fig. 5. Transmitted beam intensity versus specimen thickness.
Al$Cr Af=-49 nm
Fig. 6. A1,Zr HREM image simulations for -49 nm defocus.
Specimen
thickness ranges from 13.6 nm (image t).
2.3 (image
a) to
Zr containing atomic planes. In the following, simulations will be carried out at - 16 and -49 nm for a specimen thickness ranging from 2.3 to 13.6 nm. 3.2.2. The Cu-rich phase images. The HREM images of the Cu-rich phase present a contrast revealing a superstructure. At the minimum of contrast [Fig. 4(a)], one can observe an alternated set of basal lattice planes with one in three having higher intensity. On each side of the higher intensity fringes, large dark fringes are present, in which low intensity fringes are visible at low thicknesses. At -49 nm defocus [Fig. 4(b)], the pattern is identical to that of an Al,Zr HREM image. Only the lattice plane intensity changes. The superstructure presents a 6 basal lattice plane periodicity. The first half has a contrast similar to that of Al,Zr, in the intensity distribution: two high intensity lattice planes, as in Al,Zr, may contain Zr, and a lower intensity lattice plane with Al atoms in the centre. This intensity difference increases with the specimen thickness. The second half shows a similar structure with lower intensity. In this part, the three lattice fringes seem to have the same intensity and are closer than in the first part. When we superimpose the Al,Zr HREM image pattern on the superstructure image and a good fit can be observed for the first half (high intensity) but not for the second half. So, the basal lattice plane distance of the first half is equal to the Al,Zr one, say 0.2165 nm, but it is smaller for the second half. This observation is confirmed by the cina ratio which is 0.960 with n = 3 for the Cu-rich phase, and 1.079 with n = 4 for the DO,, Al,Zr structure. As it is quite difficult to measure the distances between lattice fringes
VERMAUT
HREM
and RUTERANA:
STUDY
OF A &-RICH
PHASE
C
_‘.‘. ::~:~:~:~Zr atom 0 ._‘_‘_. AI atom
f---
A-type
plane
-
R-type
plane
b
a
a Fig. 7. (a) Crystal
b
structure
model of Al,Zr. (b) Al,Zr (110) plane showing
‘b
a*
b
a
the Zr atoms in the (110) plane.
Model A
Model B
0
Cu atom
@
Atomic site randomly occupied by Al or Cu atoms Zr atom Al atom
a
b Model C Fig. 8. Atomic
distribution
models
of the Cu-rich
phase.
2451
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VERMAUT
and RUTERANA:
HREM STUDY OF A <-u-RIC‘H PHASE
with sufictent accuracy. several hypotheses have been taken into account, based on these measurements, in order to generate model unit cells for the structure. 3.3. The Cu-rich phase unit cell The A1,Zr unit cell is an addition of four facecentred cubes of Cu,Au type, along the c-direction with one in two shifted by l/2 [I lo] [Fig. 7(a)]. This shift gives a triangle and rectangle arrangement of the Zr atoms in the (110) planes, as shown [Fig. 7(b)] [9]. Its spatial group is 14/mmm and it contains 16 atoms (12 Al, 4 Zr). The resulting structure is a stacking of basal lattice planes alternating one kind of atom (Al) plane (type A), and two kinds of atom (Al. Zr) planes (type B) [Fig. 7(a)]. Moreover, particularly for the DO,, structure, the (Al, Zr) basal lattice planes are corrugated in two planes: one Al plane and one Zr plane, shifted by 0.024nm in the c-direction. Simulations of diffraction pattern and HREM images of A1,Zr in our experimental conditions are not sensitive. Therefore, in the following, this shift will be neglected for the Cu-rich phase unit cell construction. As the HREM images and the diffraction patterns of A1,Zr and Cu-rich phase are similar, we suppose that the unit cells have the same structure, say the addition of face-centred cubes of Cu,Au type in the c-direction. The Cu-rich phase contains only three cubes as it has a 6 basal lattice plane periodicity, and 12 atoms per unit cell, deduced from EDS analysis which gives the Al,Cu,Zr, composition. The smaller number of basal lattice planes in the unit ceil reduces the centred Bravais mode to primitive. As shown in HREM images, the first half of the unit cell is identical to the Al,Zr, meaning it is made up of two Zr containing B-type planes (2 x 1 Al, 2 x 1 Zr) and between them, an A-type plane (1 x 2 Al) [Fig. 7(b)]. Then, there are three Al atoms and three Cu atoms left in order to fill the second part of the unit cell. The introduction of Cu atoms in the unit cell decreases the symmetry of the AI,Zr structure. To take into account the four-fold axis symmetry of the system, there are only two different possibilities for positioning the atoms (Models A and B in Fig. 8) on the remaining three planes: (1) three planes (2 A and 1 B-type) with 0.5 Al and 0.5 Cu occupancy level of the atomic sites (Model A, Fig. 8); (2) one A-type plane of Cu atoms; one B-type plane of Al and Cu atoms; one A-type plane of Al atoms (Model B, Fig. 8). A third possibility the four-fold symmetry
of atomic position and sets Cu atoms
keeps at the
centre of the squares of the B-type planes, however the first half of the unit cell which was similar to the AI,Zr structure. is modified (Model C. Fig. 8). It gives two R-type planes with Cu and Zr atoms, and one with Al and Cu atoms. All other solutions will break the four-fold symmetry axis. Due to the rectangular and triangular arrangement of the Zr atoms in the (110) planes. the second and third solutions give several possibilities, breaking or not breaking the mirror parallel to the c-direction. Then, the spatial groups are P4/mmm or P4mm. For the first solution. the non-symmetric distribution of the CLI atoms always breaks this mirror and the spatial group is then P4mm. Measurement of the distances between lattice fringes on HREM images indicates that the second
c
a 0
Atomic site
4 dl 4 d2 dz 6
a
b Fig. 9. The two hypotheses a and b, made on the interplanar distances in the Cu-rich phase.
VERMAUT
and RUTERANA:
HREM
half of the Cu-rich phase unit cell is smaller than the first. The smaller size of the Cu atoms may explain this variation in the interplanar distances. Considering that the chemical nature of the planes is an important factor in their distance, two hypotheses can be made: unit cell has three different interplanar distances dl, d2 and d3, where dl + d2 + d3 = c/2 [Fig. 9(a)]. ??the unit cell has only two different interplanar distances dl and d2, where dl fd2 = c/3 [Fig. 9(b)]. The spatial group is then reduced to P4mm.
STUDY
OF A Cu-RICH
PHASE
2453
Table I. Sets of possible interplanar distances for which the calculated structure factors match the experiment. These results obtained from different curves are similar for the three models A, B and C, of the distribution of the chemical species dl +d2+d3=c/2
Model
dlfd2=c/3
(om)
A, B and C
dl = 0.17 d3 = 0.208-0.216
(nm) dl = 0.19 d3 = 0.2
dl ~0.17
??the
A simple evaluation of the structure factor, for (OO/) reflections, of the different atomic distribution models combined with the two hypotheses on the interplanar distances allows us to determine the value of the distances d 1, 62 and d3, for which SADP simulations match with experiments. The calculations were carried out for an interplanar distance in the first half of the unit cell around 0.2165 nm, which is that of Al,Zr, and the other distances ranging from 0.1 to 0.22 nm. The usual expression was used for the calculation of the structure factors: FkrG= h,f(O) exp[ -Zrci(Zu,
The following criteria used to select good models are deduced from the SAD pattern of the Cu-rich phase in Fig. 2(b). ??an
increase
of the spots intensity
from (001) to
(003); ??a lower intensity for (004) than for (001); ??an equal intensity between (003) and (110). The results of the calculations can be represented in a curve for each atomic distribution combined with
Model B
+ Rv, + /w,)].
The atomic scattering factors f(0) for each type of atom were deduced from structure factors, calculated using EMS, for unit cells containing only one type of atom, and having the following interplanar distances: d3 = 0.2165 nm and dl = d2 = 0.181 nm [Fig. 9(a)]. The influence of dl was found to be less than 5% for the extreme cases of dl = 0.1 or 0.262 nm.
3.4nm
2.2 nm Af-16 d3=2.16 A
??FOOl F002
45
0
40 35 b ‘, 30 5
25
g
20
z*
15
nm
F003 F004
?? FllO
10 5 0
3.4 Nn
2.2 Nn he-49
Fig. 10. Typical curve showing variations of structure factor of particular reflections versus the d 1 interplanar distance. A good match can be observed with the experimental sequence
for d 1 = 0.17 nm.
nm
Fig. 11. HREM image simulation of the model B with dl =0.17 and d3 =0.216nm for a -49nm defocus. The specimen thicknesses are 2.2 and 3.4 nm. One can observe the non-symmetric fringe intensity distribution in the second part of the unit cell.
2454
VERMAUT
and RUTERANA:
HREM STIJDY 01, .A (‘u-RICH PHASE:
the two hypotheses on the interplanar distances. A typical curve obtained for a unit cell with A-type atomic distribution and three different interplanar distances is shown in Fig. 10. From this curve one can obtain the value of the interplanar distances for which the calculated structure factors match with the precedingly cited criteria. Three sets of interplanar distances have been selected and are presented in Table 1.
4.DISCUSSION AND CONCLUSIONS SAD pattern simulations have been carried out for the various models. A good match is obtained for most of the models. Only unit cells with dl = 0.19 and d3 = 0.2 nm for which the high intensity of the (OO/) spots with G > 4 do not match with experiment can be easily rejected. HREM image simulations were carried out for each model and for the two defocus values with specimen thickness ranging from 2.3 to 13.6 nm. In the case of B atomic distribution and unit cells with only two different interplanar distances, where the mirror in the c-direction is broken, image simulations do not match. As an example, Fig. 11 shows HREM image simulations made with a model B unit cell at -49 nm defocus, the lattice fringes of the second part of the cell do not present the same intensity, and this discrepancy increases with specimen thickness. The same observation can be made on simulations at the minimum of contrast. Finally, only two models present a good match with experimental SAD patterns (Fig. 12) and HREM image simulations (Fig. 13). They have the same spatial group P4/mmm. One presents three planes randomly occupied by Al or Cu atoms and the other one presents two planes containing Cu and Zr atoms and one plane containing Cu and Al atoms separated by Al planes. In these two cases, the 6 basal lattice planes of the unit cells are not equidistant, probably because of their different chemical nature. The phase observed, coating an Al,Zr dispersoid, in a T6 tempered spray formed Al/Sic composite is identified. Its composition is found to be Al,Cu3Zr, with a tetragonal structure. The spatial group is P4/mmm and the parameters are a = aA,+ and c = 1.157 nm. Interplanar distances of several high intensity reflections are given in Table 2. A large number of unit cells have been investigated to determine atomic positions, using HREM image and SAD pattern simulations. Two solutions came out with two different atomic distributions. HREM image and SADP simulations did not allow one to decide between these two solutions. The atomic positions are given in Table 3 for both models. The formation of such a phase with Al substituted by Cu in model A can be compared with that of
Model A with dl=O.l7and
.
0
0..
.
.a..
.w.
d3zO.216 nm
.
??
.e..
.*
82 T
..m....i;...@#@.* * *.m .e.e...*
??
??
??
. . .@. . .
??
gl : ( 0, 0, I)! g2 : ( 1, -1, 0) Zone axis : [ 1, 1, O]
a
Model C with dl=0.17
and d3z0.216 nm
?? ????? ?? ? ?? ? ???? ? ? ?
??*o*
??*o-o
gl : ( 0, 0, 1) /g2 : ( 1, -1, 0)
[ 1, 1, 01
Zone axis:
b Fig. 12. SAD pattern simulations (a) of the model A, (b) of the model C, matching with experiments.
the Al,CuZr, compound, but with an excess of Cu and Al in comparison to Zr concentration. This excess may explain the tetragonal structure of this phase instead of the cubic structure of the Al,CuZr, compound.
VERMAUT
and RUTERANA:
HREM
STUDY
OF A Cu-RICH
Model C
Model A
A+-16 nm
AC-16 nm
2.2 Ml
3.4 nm
2.2 Nn
3.4 nm A+49 nm
de-49 nm Fig. 13. HREM
image simulations
Table 2. Crystallographic hkl
d (nm)
001 100 003 110 112 113 123 133
1.157 0.4014 0.3857 0.2838 0.2548 0.2286 0.1627 0.1206
3.4 nm
2.2 run
3.4 nm
2.2 nm
2455
PHASE
of the models A and C matching thicknesses are 2.2 and 3.4 nm.
data of the &-rich
Table 3. Atomic
phase
AI,Cu,Z, Tetragonal P4/mmm (123) a =0.4014nm c = 1.157nm c/a = 2.88 12 atoms per unit cell
Acknowledgements-The authors acknowledge P. Jensen and M. Roulin of Alusuisse Lonza Research and Development Centre of Neuchausen Switzerland for having provided the samples and for many encouraging discussions, G. Nouet for fruitful discussions and encouragement, and F. Osterstock for the German version of the abstract. REFERENCES 1. M. Kanno and B. Ou, Mater. Trans., JIM 32, 445 (1991). 2. P. Vermaut and P. Ruterana, J. Microscopy 177, 387 (1994). 3. P. Vermaut and P. Ruterana, Proc. XIII Inr. Congress on Electron Microscopy in Paris (edited by B. Jouffrey and C. Collier) Paris 1997: les editions de Physique. Vol. 2A, pp. 687-688 (1994).
Model C
with experiments.
positions
The specimen
of the two selected models A and C
Atom
X
Y
Z
Al CU Al CU Zr Al 0.5 Ala.5 Cu 0.5 Ala.5 Cu 0.5 AlLO. CU Al Zr
0 112 l/2 t/2 0 l/2 0 112 l/2 l/2 0
0 l/2 0 112 0 0 0 t/2 0 112 0
0 0 0.146 0.313 0.313 t/2 0 0 0.146 0.313 0.313
Al
l/2
0
-
112
4. R. W. Ewans, A. G. Leatham and R. G. Brooks, Powder metall. 28, 13 (1985). 5. P. S. Jensen and W. Khal, Proc. XII Int. Rise Symp. on Material Science Metal Matrix Composites-Process ing, Microstructure and Properties (edited by D. Jull Jensen et al.), p. 182. Rise National Laboratory, Roskilde, Denmark (1991). 6. P. A. Stadelmann, Ultramicroscopy 21, 131 (1987). 7. S. Srinivasan, P. B. Desch and B. Schwarz, Scripm metall. 25, 2513 (1991). 8. K. Schubert A. Raman and W. Rossteutscher, Naturwiss. 51, SO6 (1964). 9. W. B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, p. 322. Wiley-Interscience, New York (1972).
Pergamon 0956-7151(95)00339-8
1359-6454/96 $15.00 + 0.00
A TEM STUDY OF A COPPER RICH PHASE GROWN ON AN Al,Zr DISPERSOID IN A SPRAY FORMED Al/Sic COMPOSITE P. VERMAUT LERMAT,
URA CNRS
No 1317, ISMRa,
and P. RUTERANA
6 Boulevard
du Mar&ha1
Juin, 14050 CAEN
Cedex, France
(Received 2 February 1995; in revised form 2 August 1995)
Abstract-A
copper rich aluminium zirconium phase encountered in a spray formed Al/Sic composite has been investigated. It was found to grow epitaxially on the (001) faces of the A1,Zr dispersoids. The measured concentrations by EDS, down to an accuracy of 1% did not correspond to a known compound, the new phase has a composition of Al,Cu,Zr,. By careful diffraction and image simulation, and comparison with experimental results, we obtain a spatial group of P4/mmm and two very closely related atomic distributions inside the unit cell. The structure is close to that of Al,Zr with the same a parameter, however the c parameter is 1.157 nm. R&um&Une phase Aluminium Zirconium riche en Cuivre, observte dans un composite Al/Sic obtenu par mtthode Osprey, a et& Ctudike par microscopic tlectronique en transmission. Cette phase a crO en epitaxie sur les faces (001) d’un precipite Al,Zr. Sa composition: Al,Cu,Zr,, obtenue par analyse EDS avec une prkcision de l’ordre du pourcent, ne correspond a aucun compos& connu. La comparaison des images haute r&solution et des clichts de diffraction expkrimentaux avec leurs simulations a conduit au groupe spatial P4/mmm et a deux, trts proches, distributions atomiques dans la maille. La structure obtenue est proche de celle de Al,Zr avec un paramktre a identique et un paramCtre c valant 1.157nm.
Zusammenfassung-Eine Kupfer-hochhaltige
Aluminium Zirconium Phase, durch das Osprey-Verfahren wurde in einem Al/Sic Verbundwerkstoff, mittels der Durchstrahlungs electronenmikroscopie und untersucht. Diese Phase erschien durch epitaxiales Wachstum auf der (OOl)-Ebene einer A1,Zr Auscheidung. Seine Zuzammensetzung, Al,Cu,Zr,, die durch EDS mit einer prozent-Genauigkeit, ermittelt werden konnte, entspricht keinem bekannten Komponent. Der Vergleich van Hochaufliisungsund-Streuungsbildern mit Simulation fiihrt zu der Raumgruppe P4/mmm sowie zu zwei, sehr benahrten atomaren Verteilungen im Gitter. Die endeckte Struktur ist der von Al,Zr, mit demselben a-Parameter, und einen 1.157 nm wertigen c-Parameter, sehr nahe. hergestellt, beobachtet
1. INTRODUCTION
A composite material, made of 13 vol % SIC particles in an aluminium alloy with 6 wt% Cu, 2 wt% Mn, 0.35 wt% Mg and 1 wt% (Zr, V, Ti, Cr, Ag) matrix, is in practical use. In the reinforcement of the matrix, it is known that Cu, Mg and Ag contribute to the hardening by precipitation, while the role of the other elements is to make dispersoids. These later elements can act as heterogeneous nucleation sites for intermediate phases in alloys such as AlLCu-Zr [l]. However, large A1,Zr dispersoids are also known to form at grain boundaries as do Mn-rich ones. In the studied samples, they were found to be aligned along the extrusion direction and a Scanning Electron Microscopy (SEM) investigation showed their size to range from 0.5 Transmission Electron to 1 jrn [2]. In this Microscopy (TEM) work, they were always found to be rather large, and to have a complex mor-
phology, including possible cracks which have been filled with pure Al [3]. The formation of dispersoids itself can be interesting to understand. In our material, it seems that they form during the cooling step. In later treatments, they may continue to grow or even serve as sites for precipitation. This was found to be particularly true for the A1,Zr dispersoids. Due to the extrusion. some of them have been found to be preferentially oriented with the c axis perpendicular to the extrusion direction. In some cases, on top of these dispersoids a different phase has grown which contains copper.. An Energy Dispersion Spectroscopy (EDS) analysis showed us quickly that it was a new phase. It is clear that due to the small size of the crystallites (a few tens of nanometres), one can hardly use other techniques for the structure and chemical analysis. Therefore, diffraction, High Resolution Electron Microscopy (HREM) and image simulation were used in order to determine the structure and atomic
2445
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VERMAU’J
and RI;JERANA:
HREM
composition and distribution for this phase. In the following. we discuss the work that was carried out and present the models of structure and composition used. 2. EXPERIMENTAL
The studied samples were taken from a commercial Al/Sic composite. The alloy was made by Alusuisse for use in composite f’abrication by Osprey preform [4]. The composition was Al 6-6.3% Cu. l&2.1% Mn, 0.35% Ag, 0.3% Mg, 0.8-l% (Cr, Ti, V, Zr). The Osprey preform consisted of mixing the atomized molten alloy and SIC particles, and quenching onto a mandrel. The fabrication details can be found in Ref. [5]. The composite was next extruded and then aged at 190X (T6) after solution treatment at 520°C for 20 min (T4) followed by water quenching. We examined the samples after extrusion, T4 and T6 Tempers. The TEM samples were made in the usual way by mechanically polishing 3 mm discs down to 100 pm, and then dimpling to lo-20 pm followed by ion milling at LN, temperature. Observations were carried out on an ABT 002 Electron Microscope equipped with a thin window EDS facility. The quantitative analysis was made using the commercial programs and we only worked on the K-lines, for which the theoretical models are well known. The microscope was operated at 200 kV, the smallest spot size for EDS was 2 nm and the point-to-point resol-
Fig. 1. BF image of a Zr based dispersoid
SI’l.I)I
utlon
01
lYi:\>~
was O.iS nm. High resolutton as ;I focat series using a focu> step of IO nm starting from the minimum contrast image. Optical diffraction wa\ carried out on the negatives for focus determination. Selected Area Ditfraction Patterns (SADP) and HREM image simulations were performed using EMS software written by P. Stadetmann [6]. images
for
4 f 11 KiCH
HREM
WCI‘C recorded
3. RESULTS
The precipitate studied was a Zr-based dispersoid with a complex morphology, as shown in the BF image in Fig. I. It was split into parallel plates separated by matrix channels. At1 plates have the same orientation as if they originally had formed one crystallite. The presence of low intensity (002) and (006) spots in SADP of these plates [Fig. 2(a)] shows that they have the high temperature stable DO,, structure [7]. The matrix channel thickness varies from 5 to 20 nm. and the thickness of the At,Zr plates from 30 to 120 nm. At the external interfaces, the dispersoid is coated with a different phase which is epitaxial on the (001) A1,Zr phase. EDS analysis showed that it contains more Cu than Zr. The thickness of this layer is about 170 nm. The whole dispersoid does not present a simple orientation relationship with the matrix whereas the interface is still planar. This can be due to a recrystallisation of the aluminium matrix after the extrusion process. In
showing
a morphology
of parallel
plates.
VERMAUT and RUTERANA:
HREM STUDY OF A Cu-RICH PHASE
2441
3.1.1. The composition. A quantitative EDS analysis of this phase gives the 60.5% Al, 25.5% Cu, 14% Zr atomic composition or Al,Cu,Zr,. Using the same conditions (detection configuration, spot size, specimen thickness), in the matrix area close to the precipitate, one obtains a Cu concentration lower than 2%. So, the Cu signal from the specimen holder, which would be identical in both cases, is small. To our knowledge no phase with such a composition was reported in the literature. The only ternary known which has the cubic Ll, compound is Al,CuZr,, a = 0.404 nm. It structure with a lattice parameter has been shown by Schubert et al. [8] that the addition of Cu in A1,Zr gives the Al,CuZr, phase and leads to a stabilization of the low temperature Ll, structure [7]. 3.1.2. The dzfiaction. The diffraction patterns along the [ 1 lo],,,,, direction of the two phases [Figs 2(a) and (b)] are very similar. Both present a 2 mm point group symmetry. The copper-rich phase grows on the (001) plane of the A1,Zr precipitate which has a tetragonal structure. HREM images of the interface (Fig. 3) shows it to be planar and without any misfit dislocations, or other defects over more than 200 nm, meaning that the two lattices fit perfectly on the basal planes. These two facts lead us to assume, for the Cu-rich phase, a tetragonal structure similar to the A1,Zr one. Therefore, the c-parameter which is 1.732 nm for Al,Zr, is 1.157 nm for the new phase. Moreover, the spots intensity sequencies along the [OOl] direction are different for
Fig. 2. Diffraction pattern along the [l lo] axis (a) of Al,Zr, showing visible (002) and (006) spots; (b) of the &-rich phase.
the following, phase.
we will identify
the structure
of this
3.1. IdentiJication of the &-rich phase In order to identify this phase, we simultaneously carried out EDS, diffraction and HREM work. Unfortunately, it was not possible to carry out a convergent beam analysis due to the large lattice parameter of the compound. Moreover, the only available zone axis was the [l lo]. So, after determining the composition of this phase, a trial and error search of the most suitable atomic position in the unit cell was carried out. We tried to match the diffraction as well as the high resolution experimental images.
Fig. 3. HREM image of the Al,Zr/Cu-rich phase interface. The defocus is -49 nm.
the two phases. For Al&. only the even spots are visible. The (002) and (006) spots present a low intensity but are not extinguished meaning that the dispersoid has the stable DO?, tetragonal structure and not the cubic LIZ one. The (004) and the (008) ones have a higher intensity with a maximum for (008). For the Cu-rich phase, no extinctions are observed [Fig. 2(b)]. Only the (004) spot presents a low intensity. The strongest spots are the (73) ones. and (003) and (TO) have comparable mtensities.
i.2.
The HREM
study of’the Cu-rich phase
3.2.1. The experimental shows two HREM images
parameters. Figure 4 and the corresponding
opttcal diffractograms. The tirst [Fig. 4(a)] shows. intensity up tc\ m its optical diffractogram, 3 nm -‘. which corresponds to a - I6 nm defocus. The second [Fig. 4(b)] presents a minimum close to 3.5 nm ’and a broad high intensity up to 4.5 nm ’ For a weak phase object. it corresponds to the Coherent Transfer Function of our microscope (C’s = 0.4 mm, s( = 0.8 mrad) for a -49 nm defocus. The first defocus corresponds, for a weak phase object. to the minimum of contrast, and the second is just between the Scherzer focus and the first contrast inversion. To estimate the thickness of the studied area, we can see on the BF image (Fig. 1) that the examined area is located before the first minimum
Fig. 4(a) See caption oppositr.
VERMAUT and RUTERANA:
Fig. 4. IIREM images of the Q-rich
HREM STUDY OF A Cu-RICH PHASE
2449
phase with their optical diffraction: (a) at - 16 nm defocus; (b) at - 49 nm defocus.
in the image. This later point has been estimated by drawing the transmitted beam intensity versus specimen thickness. The minima (Fig. 5) do not go to zero, but the intensity is small from 10 to 20 nm thickness and beyond 40nm. So, we can suppose that the specimen thickness of the studied area is lower than 9 nm. To verify these parameters, HREM image simulations of an Al,Zr structure have been carried out at - 16 and -49 nm defocus, for thicknesses from 1 to 27 nm, and compared to the experimental images. The two experimental images, at the two focus values, have a similar contrast. In the two cases, we can observe an alternate contrast of low and high intensity of the basal planes and an increase of these variations with the
specimen thickness. Simulated HREM images with -49 nm defocus, shown in Fig. 6, are in good agreement with the experimental image (Fig. 3) for specimen thickness ranging from 2.3 to 13.6nm. Beyond 10 nm, one can observe a shift in the high intensity fringes which is not observed on experimental images in the studied area. For larger thicknesses (20-30 nm), one lattice plane in four shows higher intensity, which is not observed. Therefore, we are not at the second maxima of the transmitted beam intensity (Fig. 5). Comparing with the unit cell, one can say that for - 16 nm defocus the highest intensity basal lattice fringes are Al containing atomic planes, whereas at -49 nm defocus the highest intensity fringes are
2450
VERMAUT and RUTERANA:
HREM STUDY
OF A (‘u-RICH
PHASE
AI3Zr phase Transmitted Ream Intensity
0.
J
0.
$1
.2
.3
.4
.5
.6
Thickness [nm] *
.7
.n
.v
1.
100
Fig. 5. Transmitted beam intensity versus specimen thickness.
Al$Cr Af=-49 nm
Fig. 6. A1,Zr HREM image simulations for -49 nm defocus.
Specimen
thickness ranges from 13.6 nm (image t).
2.3 (image
a) to
Zr containing atomic planes. In the following, simulations will be carried out at - 16 and -49 nm for a specimen thickness ranging from 2.3 to 13.6 nm. 3.2.2. The Cu-rich phase images. The HREM images of the Cu-rich phase present a contrast revealing a superstructure. At the minimum of contrast [Fig. 4(a)], one can observe an alternated set of basal lattice planes with one in three having higher intensity. On each side of the higher intensity fringes, large dark fringes are present, in which low intensity fringes are visible at low thicknesses. At -49 nm defocus [Fig. 4(b)], the pattern is identical to that of an Al,Zr HREM image. Only the lattice plane intensity changes. The superstructure presents a 6 basal lattice plane periodicity. The first half has a contrast similar to that of Al,Zr, in the intensity distribution: two high intensity lattice planes, as in Al,Zr, may contain Zr, and a lower intensity lattice plane with Al atoms in the centre. This intensity difference increases with the specimen thickness. The second half shows a similar structure with lower intensity. In this part, the three lattice fringes seem to have the same intensity and are closer than in the first part. When we superimpose the Al,Zr HREM image pattern on the superstructure image and a good fit can be observed for the first half (high intensity) but not for the second half. So, the basal lattice plane distance of the first half is equal to the Al,Zr one, say 0.2165 nm, but it is smaller for the second half. This observation is confirmed by the cina ratio which is 0.960 with n = 3 for the Cu-rich phase, and 1.079 with n = 4 for the DO,, Al,Zr structure. As it is quite difficult to measure the distances between lattice fringes
VERMAUT
HREM
and RUTERANA:
STUDY
OF A &-RICH
PHASE
C
_‘.‘. ::~:~:~:~Zr atom 0 ._‘_‘_. AI atom
f---
A-type
plane
-
R-type
plane
b
a
a Fig. 7. (a) Crystal
b
structure
model of Al,Zr. (b) Al,Zr (110) plane showing
‘b
a*
b
a
the Zr atoms in the (110) plane.
Model A
Model B
0
Cu atom
@
Atomic site randomly occupied by Al or Cu atoms Zr atom Al atom
a
b Model C Fig. 8. Atomic
distribution
models
of the Cu-rich
phase.
2451
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and RUTERANA:
HREM STUDY OF A <-u-RIC‘H PHASE
with sufictent accuracy. several hypotheses have been taken into account, based on these measurements, in order to generate model unit cells for the structure. 3.3. The Cu-rich phase unit cell The A1,Zr unit cell is an addition of four facecentred cubes of Cu,Au type, along the c-direction with one in two shifted by l/2 [I lo] [Fig. 7(a)]. This shift gives a triangle and rectangle arrangement of the Zr atoms in the (110) planes, as shown [Fig. 7(b)] [9]. Its spatial group is 14/mmm and it contains 16 atoms (12 Al, 4 Zr). The resulting structure is a stacking of basal lattice planes alternating one kind of atom (Al) plane (type A), and two kinds of atom (Al. Zr) planes (type B) [Fig. 7(a)]. Moreover, particularly for the DO,, structure, the (Al, Zr) basal lattice planes are corrugated in two planes: one Al plane and one Zr plane, shifted by 0.024nm in the c-direction. Simulations of diffraction pattern and HREM images of A1,Zr in our experimental conditions are not sensitive. Therefore, in the following, this shift will be neglected for the Cu-rich phase unit cell construction. As the HREM images and the diffraction patterns of A1,Zr and Cu-rich phase are similar, we suppose that the unit cells have the same structure, say the addition of face-centred cubes of Cu,Au type in the c-direction. The Cu-rich phase contains only three cubes as it has a 6 basal lattice plane periodicity, and 12 atoms per unit cell, deduced from EDS analysis which gives the Al,Cu,Zr, composition. The smaller number of basal lattice planes in the unit ceil reduces the centred Bravais mode to primitive. As shown in HREM images, the first half of the unit cell is identical to the Al,Zr, meaning it is made up of two Zr containing B-type planes (2 x 1 Al, 2 x 1 Zr) and between them, an A-type plane (1 x 2 Al) [Fig. 7(b)]. Then, there are three Al atoms and three Cu atoms left in order to fill the second part of the unit cell. The introduction of Cu atoms in the unit cell decreases the symmetry of the AI,Zr structure. To take into account the four-fold axis symmetry of the system, there are only two different possibilities for positioning the atoms (Models A and B in Fig. 8) on the remaining three planes: (1) three planes (2 A and 1 B-type) with 0.5 Al and 0.5 Cu occupancy level of the atomic sites (Model A, Fig. 8); (2) one A-type plane of Cu atoms; one B-type plane of Al and Cu atoms; one A-type plane of Al atoms (Model B, Fig. 8). A third possibility the four-fold symmetry
of atomic position and sets Cu atoms
keeps at the
centre of the squares of the B-type planes, however the first half of the unit cell which was similar to the AI,Zr structure. is modified (Model C. Fig. 8). It gives two R-type planes with Cu and Zr atoms, and one with Al and Cu atoms. All other solutions will break the four-fold symmetry axis. Due to the rectangular and triangular arrangement of the Zr atoms in the (110) planes. the second and third solutions give several possibilities, breaking or not breaking the mirror parallel to the c-direction. Then, the spatial groups are P4/mmm or P4mm. For the first solution. the non-symmetric distribution of the CLI atoms always breaks this mirror and the spatial group is then P4mm. Measurement of the distances between lattice fringes on HREM images indicates that the second
c
a 0
Atomic site
4 dl 4 d2 dz 6
a
b Fig. 9. The two hypotheses a and b, made on the interplanar distances in the Cu-rich phase.
VERMAUT
and RUTERANA:
HREM
half of the Cu-rich phase unit cell is smaller than the first. The smaller size of the Cu atoms may explain this variation in the interplanar distances. Considering that the chemical nature of the planes is an important factor in their distance, two hypotheses can be made: unit cell has three different interplanar distances dl, d2 and d3, where dl + d2 + d3 = c/2 [Fig. 9(a)]. ??the unit cell has only two different interplanar distances dl and d2, where dl fd2 = c/3 [Fig. 9(b)]. The spatial group is then reduced to P4mm.
STUDY
OF A Cu-RICH
PHASE
2453
Table I. Sets of possible interplanar distances for which the calculated structure factors match the experiment. These results obtained from different curves are similar for the three models A, B and C, of the distribution of the chemical species dl +d2+d3=c/2
Model
dlfd2=c/3
(om)
A, B and C
dl = 0.17 d3 = 0.208-0.216
(nm) dl = 0.19 d3 = 0.2
dl ~0.17
??the
A simple evaluation of the structure factor, for (OO/) reflections, of the different atomic distribution models combined with the two hypotheses on the interplanar distances allows us to determine the value of the distances d 1, 62 and d3, for which SADP simulations match with experiments. The calculations were carried out for an interplanar distance in the first half of the unit cell around 0.2165 nm, which is that of Al,Zr, and the other distances ranging from 0.1 to 0.22 nm. The usual expression was used for the calculation of the structure factors: FkrG= h,f(O) exp[ -Zrci(Zu,
The following criteria used to select good models are deduced from the SAD pattern of the Cu-rich phase in Fig. 2(b). ??an
increase
of the spots intensity
from (001) to
(003); ??a lower intensity for (004) than for (001); ??an equal intensity between (003) and (110). The results of the calculations can be represented in a curve for each atomic distribution combined with
Model B
+ Rv, + /w,)].
The atomic scattering factors f(0) for each type of atom were deduced from structure factors, calculated using EMS, for unit cells containing only one type of atom, and having the following interplanar distances: d3 = 0.2165 nm and dl = d2 = 0.181 nm [Fig. 9(a)]. The influence of dl was found to be less than 5% for the extreme cases of dl = 0.1 or 0.262 nm.
3.4nm
2.2 nm Af-16 d3=2.16 A
??FOOl F002
45
0
40 35 b ‘, 30 5
25
g
20
z*
15
nm
F003 F004
?? FllO
10 5 0
3.4 Nn
2.2 Nn he-49
Fig. 10. Typical curve showing variations of structure factor of particular reflections versus the d 1 interplanar distance. A good match can be observed with the experimental sequence
for d 1 = 0.17 nm.
nm
Fig. 11. HREM image simulation of the model B with dl =0.17 and d3 =0.216nm for a -49nm defocus. The specimen thicknesses are 2.2 and 3.4 nm. One can observe the non-symmetric fringe intensity distribution in the second part of the unit cell.
2454
VERMAUT
and RUTERANA:
HREM STIJDY 01, .A (‘u-RICH PHASE:
the two hypotheses on the interplanar distances. A typical curve obtained for a unit cell with A-type atomic distribution and three different interplanar distances is shown in Fig. 10. From this curve one can obtain the value of the interplanar distances for which the calculated structure factors match with the precedingly cited criteria. Three sets of interplanar distances have been selected and are presented in Table 1.
4.DISCUSSION AND CONCLUSIONS SAD pattern simulations have been carried out for the various models. A good match is obtained for most of the models. Only unit cells with dl = 0.19 and d3 = 0.2 nm for which the high intensity of the (OO/) spots with G > 4 do not match with experiment can be easily rejected. HREM image simulations were carried out for each model and for the two defocus values with specimen thickness ranging from 2.3 to 13.6 nm. In the case of B atomic distribution and unit cells with only two different interplanar distances, where the mirror in the c-direction is broken, image simulations do not match. As an example, Fig. 11 shows HREM image simulations made with a model B unit cell at -49 nm defocus, the lattice fringes of the second part of the cell do not present the same intensity, and this discrepancy increases with specimen thickness. The same observation can be made on simulations at the minimum of contrast. Finally, only two models present a good match with experimental SAD patterns (Fig. 12) and HREM image simulations (Fig. 13). They have the same spatial group P4/mmm. One presents three planes randomly occupied by Al or Cu atoms and the other one presents two planes containing Cu and Zr atoms and one plane containing Cu and Al atoms separated by Al planes. In these two cases, the 6 basal lattice planes of the unit cells are not equidistant, probably because of their different chemical nature. The phase observed, coating an Al,Zr dispersoid, in a T6 tempered spray formed Al/Sic composite is identified. Its composition is found to be Al,Cu3Zr, with a tetragonal structure. The spatial group is P4/mmm and the parameters are a = aA,+ and c = 1.157 nm. Interplanar distances of several high intensity reflections are given in Table 2. A large number of unit cells have been investigated to determine atomic positions, using HREM image and SAD pattern simulations. Two solutions came out with two different atomic distributions. HREM image and SADP simulations did not allow one to decide between these two solutions. The atomic positions are given in Table 3 for both models. The formation of such a phase with Al substituted by Cu in model A can be compared with that of
Model A with dl=O.l7and
.
0
0..
.
.a..
.w.
d3zO.216 nm
.
??
.e..
.*
82 T
..m....i;...@#@.* * *.m .e.e...*
??
??
??
. . .@. . .
??
gl : ( 0, 0, I)! g2 : ( 1, -1, 0) Zone axis : [ 1, 1, O]
a
Model C with dl=0.17
and d3z0.216 nm
?? ????? ?? ? ?? ? ???? ? ? ?
??*o*
??*o-o
gl : ( 0, 0, 1) /g2 : ( 1, -1, 0)
[ 1, 1, 01
Zone axis:
b Fig. 12. SAD pattern simulations (a) of the model A, (b) of the model C, matching with experiments.
the Al,CuZr, compound, but with an excess of Cu and Al in comparison to Zr concentration. This excess may explain the tetragonal structure of this phase instead of the cubic structure of the Al,CuZr, compound.
VERMAUT
and RUTERANA:
HREM
STUDY
OF A Cu-RICH
Model C
Model A
A+-16 nm
AC-16 nm
2.2 Ml
3.4 nm
2.2 Nn
3.4 nm A+49 nm
de-49 nm Fig. 13. HREM
image simulations
Table 2. Crystallographic hkl
d (nm)
001 100 003 110 112 113 123 133
1.157 0.4014 0.3857 0.2838 0.2548 0.2286 0.1627 0.1206
3.4 nm
2.2 run
3.4 nm
2.2 nm
2455
PHASE
of the models A and C matching thicknesses are 2.2 and 3.4 nm.
data of the &-rich
Table 3. Atomic
phase
AI,Cu,Z, Tetragonal P4/mmm (123) a =0.4014nm c = 1.157nm c/a = 2.88 12 atoms per unit cell
Acknowledgements-The authors acknowledge P. Jensen and M. Roulin of Alusuisse Lonza Research and Development Centre of Neuchausen Switzerland for having provided the samples and for many encouraging discussions, G. Nouet for fruitful discussions and encouragement, and F. Osterstock for the German version of the abstract. REFERENCES 1. M. Kanno and B. Ou, Mater. Trans., JIM 32, 445 (1991). 2. P. Vermaut and P. Ruterana, J. Microscopy 177, 387 (1994). 3. P. Vermaut and P. Ruterana, Proc. XIII Inr. Congress on Electron Microscopy in Paris (edited by B. Jouffrey and C. Collier) Paris 1997: les editions de Physique. Vol. 2A, pp. 687-688 (1994).
Model C
with experiments.
positions
The specimen
of the two selected models A and C
Atom
X
Y
Z
Al CU Al CU Zr Al 0.5 Ala.5 Cu 0.5 Ala.5 Cu 0.5 AlLO. CU Al Zr
0 112 l/2 t/2 0 l/2 0 112 l/2 l/2 0
0 l/2 0 112 0 0 0 t/2 0 112 0
0 0 0.146 0.313 0.313 t/2 0 0 0.146 0.313 0.313
Al
l/2
0
-
112
4. R. W. Ewans, A. G. Leatham and R. G. Brooks, Powder metall. 28, 13 (1985). 5. P. S. Jensen and W. Khal, Proc. XII Int. Rise Symp. on Material Science Metal Matrix Composites-Process ing, Microstructure and Properties (edited by D. Jull Jensen et al.), p. 182. Rise National Laboratory, Roskilde, Denmark (1991). 6. P. A. Stadelmann, Ultramicroscopy 21, 131 (1987). 7. S. Srinivasan, P. B. Desch and B. Schwarz, Scripm metall. 25, 2513 (1991). 8. K. Schubert A. Raman and W. Rossteutscher, Naturwiss. 51, SO6 (1964). 9. W. B. Pearson, The Crystal Chemistry and Physics of Metals and Alloys, p. 322. Wiley-Interscience, New York (1972).