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Solidification structure and primary Al-Fe-Si particles in direct-chilled-cast aluminium alloys
Ultramicroscopy North-Holland,
22 (1987) Amsterdam
239
239-250
SOLIDIFICATION STRUCTURE AND PRIMARY Al-Fe-Si IN DIRECT-CHILLED-CAST ALUMINIUM ALLOYS
PARTICLES
P. SKJERPE and J. GJONNES Department
of Physics, University of Oslo, Blindern, Oslo 3, Norway
and Y. LANGSRUD Center for Industrial Research, P.O. Box 350, Blindern, Oslo 3, Norway Received 26 September 1986; presented at Conference
April 1986
The primary Al-Fe-Si particles formed during solidification of commercial Al alloys may have considerable influence on fabrication properties. A variety of stable and metastable phases occur, depending upon alloy composition and solidification rate. Crystal structures are known only for some of the phases. The particles found after DC-casting of a commercially pure Al-0.25%Fe-O.l3%Si alloy have been examined. TEM specimens were prepared both by foil-thinning and by a novel extraction technique where the matrix is dissolved in butanol. In samples taken from selected positions in the ingot corresponding to different solidification rates, the most common phases were identified by selected-area diffraction as bee cu-Al-Fe-Si, Al,Fe, bet Al,Fe and the complicated “Al,Fe” particles. Other phases were found in minor quantities. Composition of the particles was determined by energy-dispersive X-ray analysis. High-resolution electron microscopy, selected-area and convergent-beam diffraction were used for confirmation and determination of Bravais lattices, space groups and several fault structures. The space group 14/mmm is proposed for Al,Fe.
1. Introduction
Primary particles with aluminum, iron and silicon as the main elements are present in virtually all commercial aluminum alloys. They are formed during solidification as a eutectic outlining the primary grain or dendritic structure. With iron and silicon concentrations of some tenths of a percent the particles may constitute as much as one per cent of the volume - and affect alloy properties during fabrication and service in many ways. The equilibrium phase diagram of the ternary system Al-Fe-Si provides only limited guidance as to what phases are formed during casting: in addition to the three equilibrium intermetallic phases appearing in the diagram, fig. 1, particles of more than a dozen metastable phases may be 0304-3991/87/$03.50
(North-Holland
found in cast alloys, depending upon solidification rate and alloy composition. Electron microscopy is the main tool for characterization of such particles, and average Bravais lattices and composition have been reported for many particle types, table 1. Crystal structures have been determined only for a few, mainly equilibrium phases. The single-crystal X-ray studies may include uncertainties associated with defects, and with the small difference in scattering power between aluminum and silicon. There are thus many details still missing in the picture of this technologically important alloy system: crystal structure of most of the particle types and the nature and role of internal defects and of the transformations which occur in the particles. We .can attack this deficiency along different avenues: One is to prepare ternary alloys of pre-
0 Elsevier Science Publishers B.V. Physics Publishing Division)
240 Table 1 Crystallographic Name
P. Skjerpe et ul. / Solidification
data for Al-Fe
and Al-Fe-Si
phases
structure and primary AI - Fe - Si particles in DC-cast AI
occurring
in aluminum
alloys
Bravais lattice, space group
Lattice parameters
(A)
Al,Fe
Monoclinic,
a b c p
A1,Fe
bet
a = 8.84 c= 21.6
[I81
Al,Fe,
Monoclinic
a = 8.90 b = 6.35 c= 6.32 B = 93.4
[61
A1,Fe
Orthorhombic
(I = b= c=
if91
Al,Fe
Unknown
a-Al-Fe-Si
bee Im3
a =
12.56
PI
ol-Al-Fe-Si
Cubic Pm3
a = 12.42
[91
a’-Al-Fe-%
Hexagonal
a = 12.30 c = 26.20
1101
q,-Al-Fe-Si
Orthorhombic, c-centered
a = 12.70 b = 36.20 c = 12.70
[20,11
u,-Al-Fe-Si
Monoclinic c-centered
a = b= c= =
[31
q,-Al-Fe-Si
Monoclinic
a = 12.50 b = 12.30 c= 19.70 p=111
PO1
/3-Al-Fe-Si
Monoclinic
(I = 6.12 b = 6.12 c= 41.50 /3 = 91.0
DLI
fi*-Al-Fe-Si
Monoclinic
LI= 8.90 b = 4.90 c= 41.60 fl= 92.0
WI
Al,FeSi
Monoclinic c-centered
(I = 17.80 b = 10.25 c= 8.90 fl=132
1211
Al 4 FeSi 2
Tetragonal
a = c=
[Ill
Refs.
and P (deg)
C2/m
Ccmm
= 15.49 = 8.08 = 12.48 = 107.75
6.49 7.44 8.79
[41
WI
P63/mmc
27.95 30.62 20.73 97.74
6.14 9.48
P. Skjerpe et al. / Solidification
structure andprimaly
Al-
Fe- Siparticles
in DC-cast Al
241
I-
>/
I-
/\ 1 1
1 2
I 3
4
I 5
I 6
I 7 SILICON
Fig. 1. Al-rich comer of Al-Fe-Si
I 8
9
10
Si
I
I
11
12
13
14
WT %
phase diagram, liquidus surface after Rivlin and Raynor [12]. Arrows indicate solidification paths.
determined compositions under controlled solidification conditions. Another is to study the structures appearing in industrial material and relate the observations of structural features to process parameters. We have followed the latter course, aiming at a quantitative and detailed description of microstructures in industrial material as well as of the internal structure of particles. Our investigation therefore includes examination on several scales, ranging from optical metallography to high-resolution electron microscopy and convergent-beam electron diffraction. An extraction technique based on complete dissolution of the aluminum matrix was used for preparation of electron microscope specimens. The combination of different techniques is seen as essential both for a complete description of the microstructures and for relating the crystal and defect structure to the local conditions leading to their formation in the alloy. In the present paper we emphasize this connection between various techniques for characterization of structure at different levels as a starting point for detailed structure studies of particles, phases and defects. Electron microscopy results, using HREM and CBED, are presented from particles with Al and Fe as main constituents.
2. Materials and methods Samples from DC (“direct-chilled”)-cast aluminum of several alloys with Fe content 0.25 wt% and different amounts of Si were supplied by ASV, Sunndalsora, Norway. The samples were available from different positions through the 1050 x 600 mm ingot, viz. 10, 25, 50 and 100 mm from the ingot surface. The present results were obtained mainly from the 0.25wtS Fe-O.l3wt% Si alloy. Optical characterization of the microstructures was carried out both on anodized specimens and by etching, so as to reveal both grain and dendritic structure and particle distribution. From the optical micrographs, the scale of these structures could be determined with an IBAS image analyzer, which also was used for determination of overall particle concentrations. Computer calculations of local solidification times across the ingot were available and could be compared with cooling rates, C, estimated from the measured dendritic spacing, D, using the standard relation DC“ = K. Specimens for electron microscopy were obtained both as thin foils by a jet-polishing technique in an A7 electrolyte (70% methanol, 20% glycerol, 10% perchloric acid) and by a special extraction technique recently developed by Simen-
P. Skjerpe et al. / Solidification
242
structure undprima~
Al- Fe- Si purticles in DC-cast AI
sen et al. [14]. In the latter method the aluminum matrix is dissolved in a continuous flow of isobutanol, which is redistilled in a glass apparatus. The particles are collected on a filter; via redispersion in butanol they may be collected on suitable support for electron microscopy. The method has several advantages: Larger particles are not lost, as may be the case in voil preparation. There is no interference from aluminum matrix in EDS analysis, even for quite small particles. Shapes are often more easily seen than in a foil where particles may have been cut during preparation. An optimum density of particles is readily obtained, with many more particles than can be found in a foil. The filters also provide excellent specimens for other techniques, e.g. XRMA and other spectroscopic techniques, X-ray powder diffraction, Coulter counters, etc. Matrix concentrations can be determined by analysis of the solvent. Electron microscopy was carried out in a JEM 2OOOFX operated at 200 kV and equipped with a TN 2000 EDS unit. Conversion of X-ray intensity ratios into concentrations was performed using the Cliff-Lorimer equation c*/c*
= k,, (IA/IB >.
In order to separate the partly overlapping Al and Si peaks, a least-squares fit program supplied by Tracer Northern was used. The program calculates the best combination of reference peaks that fits an observed spectrum. The correction factors were calculated from standard expressions, as given in ref. [15]. For determination of Bravais lattices from selected-area patterns, the crystal was tilted around two independent axes in reciprocal space. The best geometrical lattice was determined by a leastsquares procedure. In this way the uncertainties associated with reading goniometer angles were eliminated. Convergent-beam diffraction patterns were taken in order to derive symmetry information. 3. Results 3.1. Metallography,
particle distribution,
analysis
In optical micrographs of cast material (fig. 2), the particles outline a dendritic structure. From
Fig. 2. Optical
micrograph of cast structure, ingot surface. 65 x
5-10
mm from
the observed interdendritic spacings (fig. 3) local cooling rates can be estimated. At higher optical magnification and in low-magnification TEM, the particles are seen as plates, needles or with other, more complicated, characteristic shapes. Relative fractions of the commonly occurring types at different positions in the ingot were determined both in thin foils, using SAD, and from extracted specimens, using EDS with SAD as a check. Results for the 0.13% Si alloy are presented in table 3. The average compositions of the different types were obtained by quantitative evaluation of the EDS spectra. In addition to AI, Fe and Si, 0.1-l!% Ni was found in all particles. Unfortunately copper could not be determined, due to a background signal from the instrument. The compositions were in reasonable agreement with previously reported values, e.g.: (Al, Si),.,(Fe, Ni), (Al, Si),,(Fe, Ni) and (Al, Si),.,(Fe, Ni) for the three phases usually
P. Skjerpe et al. / Solidification DAS,
siructure and primaty
243
Al - Fe - Si particles in DC-cast Al
3.2. Electron microscopy and diffraction of particles
pm
t
Low-magnification micrographs of extracted particles reveal typical morphologies. Fig. 4a of an Al,Fe particle shows how the characteristic dendrite-like shape is much more readily seen than in the foil specimen (fig. 4b). The aim of structure characterization with electron microscopy was to confirm the equilibrium crystal structures assumed for particles classified as Al,Fe, a-Al-Fe-Si, etc., to seek structure models for the metastable intermetallic phases as well as describe the internal defect structure of the particles. To this end high resolution microscopy, spot patteins and CBED were used. Particles of the equilibrium phase Al,Fe appear as (100) plates in the extracted specimens. Spot pattern intensities were found to agree qualitatively with kinematical calculations based on the monoclinic structure which Black [4] had determined with X-ray investigation. Diffraction patterns indicate twins and faults to be frequent on both (001) and (100) planes. Evidence for the latter was not readily obtained from the (100) plates, but in thinned foils the [OlO] orientation was sometimes found. The diffraction pattern, fig. 5a, taken along the monoclinic axis shows twinning on (100) as well as streaks along c* and c+. The micrograph, fig. 5b, shows a Tl:O) twin and several faults on (100). Some main contrast features in the micrographs may be interpreted with reference to the X-ray structure, but extensive calculations will be needed for detailed comparison with the observed images. There are no distinct “block” or similar features which can be discerned in these large and complicated unit cells, where repetition distance in the beam direction typically is lo-15 A or more.
0
50
311, ., .5p.
,
,
,
Distance
Fig.
3. Secondary
‘p?
from
, ,
.,p,
surface,
.
, ,2p.
)
mm
dendrite spacing in ,um as function distance from ingot surface.
of
called Al,Fe, Al,Fe and Al,Fe respectively. Absorption corrections were not applied; at thickness 1000 A or less the correction in iron content will be less than 1 wt%.
Table 2 Chemical composition of material spectrography; all concentrations wt%, others in ppm(wt)
Ni 43 Cu 23 B 21 Cr 18 Zr 16 Co 16 Snll
Fe 0.25 Si 0.13 Zn 185 Ti 174 Ga 125 V 62 Mn 47 Pb 44
Table 3 Relative amounts
10 mm (foil) 25 mm (extracted) 50 mm (foil) 100 mm (extracted) 100 mm (foil)
as determined by emission over 10 ppm Fe and Si in
of phases
at different
position
Al,Fe
AI,Fe
55 48
33 37 20 28 17
in the ingot;
SAD measurements
in foil specimens;
Al,Fe
a-Al-Fe-Si
14 22
67 63 80 3 8
EDS in extracted
samples
a”-Al-Fe-Si
N
4
18 42 15 52 23
244
Fig. 4. (a) Al,Fe
P. Skjerpe et al. / Solidification
structure andprimary
Al- Fe- Si particles in DC-crrrt Al
particle (B) with characteristic dendrite shape together with an (Y,-Al-Fe-S Al,Fe particle in foil specimen.
High-resolution images of an unknown structure, e.g. fig. 6, taken along the (111) body diagonal in the bet A1,Fe may then yield little direct indication about the atomic arrangement in the unit cell. Diffraction information is therefore essential. SAD patterns from different crystals may vary considerably, as seen from figs. 7a and 7b, also along (111) direction: fig. 7a includes mainly the funda-
particle (A) in extraction specimen. (b)
mental spots from the reported bet unit cell; in fig. 7b there are many extra spots which cannot be indexed readily as multiples of the basic unit cell, and thus indicate incommensurate modulations of the structure in the (110) directions - see also the micrograph, fig. 6. CBED patterns were then taken in order to determine possible space groups. Fig. 8, taken
245
P. Skjerpe et al. / Solidification structure and primary Al- Fe - Si particles in DC-cast Al
Fig. 5. Diffraction
pattern
(a) and micrograph
(b) of a faulted AI,Fe
along the c-axis, shows two sets of mirror planes. Since no systematic absences appear, apart from those resulting from the body-centered Bravais lattice, only two space groups appear possible: 14/mmm and I4mm. No direct evidence for a third mirror, normal to c, was obtained. Therefore tests for center of symmetry were carried out, as demonstrated by fig. 9. From the combination of the reciprocity relation and the inversion oper-
crystal,
showing
many faults and a twinned
region, T.
ation (see, e.g., Steeds [16]) it follows that the intensity distributions within the g and g disks should be similar if there is a center of symmetry: 1,(/r + g) = I,(k). From the similarity of the g and g disks in fig. 9 it is concluded that the basic structure of Al,Fe has a center of symmetry and hence that the space group is 14/mmm. Attempts to accommodate the nearly 120 atoms (tentatively 22 Fe + 96 Al) in the corresponding unit cell
246
P. Skjerpe et al. /Solidification
Fig. 6. Electron
micrograph
structure andprimq
of Al,Fe
crystal,
indicate the structure to be quite different from either the Al,Fe or the Al&o, structure which was proposed for particles with similar composition by Simensen and Vellasamy [6].
4. Discussion The extraction technique is seen to be very useful for preparation of electron microscope
Al- Fe-Siparticles
seen along (ill),
showing
in DC-cast Al
complicated
disorder
specimens of intermetallic phases in aluminum alloys, improving the statistics of analysis as well as providing stable specimens for high-resolution and analytical microscopy. Although no extensive analysis of statistics was carried out, the comparison between results from foils and extraction specimens seems to support the claim by Simensen et al. [14] that the extraction technique is quantitative and that no appreciable loss of material from the particles occurs.
P. Skjerpe et al. / Solidification
Fig. 7. Diffraction patterns from Al,Fe,
structure and primary Al- Fe-Si
particles in DC-cast Al
(111) projections, showing many superstructure reflections in (b).
Fig. 8. CBED pattern of AI,Fe
along 001, showing mirror planes
241
248
P. Skjerpe et al. / Solidification
structure and primary Al- Fe- Si particles m DC-rust Al
Fig. 9. CBED pattern
of AI,Fe,
Particle types and compositions could be readily determined as function of distance from the ingot surface and compared with the estimated cooling rates. At the high solidification rates in the region between 10 and 50 mm, A1,Fe and (YAl-Fe-Si were most frequent, with no A1,Fe occurring. As the cooling rate decreases towards the center of the ingot, the equilibrium phase Al,Fe
showing
center of symmetry.
appears and, in addition, the highly disordered phase A1,Fe - which may have some resemblance to Al,Fe - a nearly stable structure in the system. The formation of metastable and silicon-rich phases as the cooling rate increases is known from other investigations [1,2]. The micrographs and diffraction patterns of A1,Fe agree with the published structure. Faults
P. Skjerpe et al. / Solidification structure andprimaty
and twins on the (001) planes are observed in HREM images. For the phase Al,Fe, the value m = 4.2 is found; the space group 14/mmm is deduced from CBED patterns. Further work is needed in order to substantiate structure models based upon combined diffraction and microscope evidence. An account of the structure and microstructure observations of the particles related to the solidification process will be published elsewhere [ 171.
[7] [8] [9] [lo] [ll] [12] [13] [14] [15]
[16]
References [l] H. Westengen, Z. Metal& 73 (1982) 360. [2] R.M.K. Young and T.W. Clyne, Scripta Met. 15 (1981) 1211. [3] A.L. Dons, Z. Metal& 76 (1985) 151. [4] P.J. Black, Acta Cryst. 8 (1955) 43, 175. 15) I. M&i, H. Kosuge and K. Nagahama, J. Japan. Inst. L. Metals 25 (1975) 1. [6] C.J. Simensen and R. Vellasamy, Z. Metallk. 68 (1977) 428.
[17] [18] [19] [20] [21]
Al- Fe- Siparticles
in DC-cast Al
249
L.K. Walford, Acta Cryst. 18 (1965) 287. M. Cooper, Acta Cryst. 23 (1967) 1106. M. Cooper and K. Robinson, Acta Cryst. 20 (1966) 614. K. Robinson and P.J. Black, Phil. Mag. 44 (1953) 1392. G. Phragmen, J. Inst. Metals 77 (1950) 498. V.G. Rivlin and G.V. Raynor, Intern. Met. Rev. 3 (1981) 133. C.Y. Sun and L.F. Modolfo, J. Inst. Metals 95 (1967) 384. C.J. Simensen, P. Fartum and A. Andersen, Fresenius Z. Anal. Chem. 319 (1984) 286. J.I. Goldstein, in: Introduction to Analytical Electron Microscopy, Eds. J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum, New York, 1979). J.W. Steeds, in: Quantitative Electron Microscopy, Eds. J.N. Chapman and A.J. Craven (Scottish Universities Summer School in Physics, 1984). P. Skjerpe, Met. Trans., in press. H. Kosuge and I. Mizukami, J. Japan. Inst. L. Metals 22 (1972) 437. E.H. HoIlingsworth, G.R. Frank, Jr. and R.E. WiIlett, Trans. Met. Sot. AIME 224 (1962) 188. P. Liu, T. Thorvaldsen and G.L. Dunlop, in: Proc. SCANDEM 85, p. 35. D. Munson, J. Inst. Metals 95 (1967) 217.
22 (1987) Amsterdam
239
239-250
SOLIDIFICATION STRUCTURE AND PRIMARY Al-Fe-Si IN DIRECT-CHILLED-CAST ALUMINIUM ALLOYS
PARTICLES
P. SKJERPE and J. GJONNES Department
of Physics, University of Oslo, Blindern, Oslo 3, Norway
and Y. LANGSRUD Center for Industrial Research, P.O. Box 350, Blindern, Oslo 3, Norway Received 26 September 1986; presented at Conference
April 1986
The primary Al-Fe-Si particles formed during solidification of commercial Al alloys may have considerable influence on fabrication properties. A variety of stable and metastable phases occur, depending upon alloy composition and solidification rate. Crystal structures are known only for some of the phases. The particles found after DC-casting of a commercially pure Al-0.25%Fe-O.l3%Si alloy have been examined. TEM specimens were prepared both by foil-thinning and by a novel extraction technique where the matrix is dissolved in butanol. In samples taken from selected positions in the ingot corresponding to different solidification rates, the most common phases were identified by selected-area diffraction as bee cu-Al-Fe-Si, Al,Fe, bet Al,Fe and the complicated “Al,Fe” particles. Other phases were found in minor quantities. Composition of the particles was determined by energy-dispersive X-ray analysis. High-resolution electron microscopy, selected-area and convergent-beam diffraction were used for confirmation and determination of Bravais lattices, space groups and several fault structures. The space group 14/mmm is proposed for Al,Fe.
1. Introduction
Primary particles with aluminum, iron and silicon as the main elements are present in virtually all commercial aluminum alloys. They are formed during solidification as a eutectic outlining the primary grain or dendritic structure. With iron and silicon concentrations of some tenths of a percent the particles may constitute as much as one per cent of the volume - and affect alloy properties during fabrication and service in many ways. The equilibrium phase diagram of the ternary system Al-Fe-Si provides only limited guidance as to what phases are formed during casting: in addition to the three equilibrium intermetallic phases appearing in the diagram, fig. 1, particles of more than a dozen metastable phases may be 0304-3991/87/$03.50
(North-Holland
found in cast alloys, depending upon solidification rate and alloy composition. Electron microscopy is the main tool for characterization of such particles, and average Bravais lattices and composition have been reported for many particle types, table 1. Crystal structures have been determined only for a few, mainly equilibrium phases. The single-crystal X-ray studies may include uncertainties associated with defects, and with the small difference in scattering power between aluminum and silicon. There are thus many details still missing in the picture of this technologically important alloy system: crystal structure of most of the particle types and the nature and role of internal defects and of the transformations which occur in the particles. We .can attack this deficiency along different avenues: One is to prepare ternary alloys of pre-
0 Elsevier Science Publishers B.V. Physics Publishing Division)
240 Table 1 Crystallographic Name
P. Skjerpe et ul. / Solidification
data for Al-Fe
and Al-Fe-Si
phases
structure and primary AI - Fe - Si particles in DC-cast AI
occurring
in aluminum
alloys
Bravais lattice, space group
Lattice parameters
(A)
Al,Fe
Monoclinic,
a b c p
A1,Fe
bet
a = 8.84 c= 21.6
[I81
Al,Fe,
Monoclinic
a = 8.90 b = 6.35 c= 6.32 B = 93.4
[61
A1,Fe
Orthorhombic
(I = b= c=
if91
Al,Fe
Unknown
a-Al-Fe-Si
bee Im3
a =
12.56
PI
ol-Al-Fe-Si
Cubic Pm3
a = 12.42
[91
a’-Al-Fe-%
Hexagonal
a = 12.30 c = 26.20
1101
q,-Al-Fe-Si
Orthorhombic, c-centered
a = 12.70 b = 36.20 c = 12.70
[20,11
u,-Al-Fe-Si
Monoclinic c-centered
a = b= c= =
[31
q,-Al-Fe-Si
Monoclinic
a = 12.50 b = 12.30 c= 19.70 p=111
PO1
/3-Al-Fe-Si
Monoclinic
(I = 6.12 b = 6.12 c= 41.50 /3 = 91.0
DLI
fi*-Al-Fe-Si
Monoclinic
LI= 8.90 b = 4.90 c= 41.60 fl= 92.0
WI
Al,FeSi
Monoclinic c-centered
(I = 17.80 b = 10.25 c= 8.90 fl=132
1211
Al 4 FeSi 2
Tetragonal
a = c=
[Ill
Refs.
and P (deg)
C2/m
Ccmm
= 15.49 = 8.08 = 12.48 = 107.75
6.49 7.44 8.79
[41
WI
P63/mmc
27.95 30.62 20.73 97.74
6.14 9.48
P. Skjerpe et al. / Solidification
structure andprimaly
Al-
Fe- Siparticles
in DC-cast Al
241
I-
>/
I-
/\ 1 1
1 2
I 3
4
I 5
I 6
I 7 SILICON
Fig. 1. Al-rich comer of Al-Fe-Si
I 8
9
10
Si
I
I
11
12
13
14
WT %
phase diagram, liquidus surface after Rivlin and Raynor [12]. Arrows indicate solidification paths.
determined compositions under controlled solidification conditions. Another is to study the structures appearing in industrial material and relate the observations of structural features to process parameters. We have followed the latter course, aiming at a quantitative and detailed description of microstructures in industrial material as well as of the internal structure of particles. Our investigation therefore includes examination on several scales, ranging from optical metallography to high-resolution electron microscopy and convergent-beam electron diffraction. An extraction technique based on complete dissolution of the aluminum matrix was used for preparation of electron microscope specimens. The combination of different techniques is seen as essential both for a complete description of the microstructures and for relating the crystal and defect structure to the local conditions leading to their formation in the alloy. In the present paper we emphasize this connection between various techniques for characterization of structure at different levels as a starting point for detailed structure studies of particles, phases and defects. Electron microscopy results, using HREM and CBED, are presented from particles with Al and Fe as main constituents.
2. Materials and methods Samples from DC (“direct-chilled”)-cast aluminum of several alloys with Fe content 0.25 wt% and different amounts of Si were supplied by ASV, Sunndalsora, Norway. The samples were available from different positions through the 1050 x 600 mm ingot, viz. 10, 25, 50 and 100 mm from the ingot surface. The present results were obtained mainly from the 0.25wtS Fe-O.l3wt% Si alloy. Optical characterization of the microstructures was carried out both on anodized specimens and by etching, so as to reveal both grain and dendritic structure and particle distribution. From the optical micrographs, the scale of these structures could be determined with an IBAS image analyzer, which also was used for determination of overall particle concentrations. Computer calculations of local solidification times across the ingot were available and could be compared with cooling rates, C, estimated from the measured dendritic spacing, D, using the standard relation DC“ = K. Specimens for electron microscopy were obtained both as thin foils by a jet-polishing technique in an A7 electrolyte (70% methanol, 20% glycerol, 10% perchloric acid) and by a special extraction technique recently developed by Simen-
P. Skjerpe et al. / Solidification
242
structure undprima~
Al- Fe- Si purticles in DC-cast AI
sen et al. [14]. In the latter method the aluminum matrix is dissolved in a continuous flow of isobutanol, which is redistilled in a glass apparatus. The particles are collected on a filter; via redispersion in butanol they may be collected on suitable support for electron microscopy. The method has several advantages: Larger particles are not lost, as may be the case in voil preparation. There is no interference from aluminum matrix in EDS analysis, even for quite small particles. Shapes are often more easily seen than in a foil where particles may have been cut during preparation. An optimum density of particles is readily obtained, with many more particles than can be found in a foil. The filters also provide excellent specimens for other techniques, e.g. XRMA and other spectroscopic techniques, X-ray powder diffraction, Coulter counters, etc. Matrix concentrations can be determined by analysis of the solvent. Electron microscopy was carried out in a JEM 2OOOFX operated at 200 kV and equipped with a TN 2000 EDS unit. Conversion of X-ray intensity ratios into concentrations was performed using the Cliff-Lorimer equation c*/c*
= k,, (IA/IB >.
In order to separate the partly overlapping Al and Si peaks, a least-squares fit program supplied by Tracer Northern was used. The program calculates the best combination of reference peaks that fits an observed spectrum. The correction factors were calculated from standard expressions, as given in ref. [15]. For determination of Bravais lattices from selected-area patterns, the crystal was tilted around two independent axes in reciprocal space. The best geometrical lattice was determined by a leastsquares procedure. In this way the uncertainties associated with reading goniometer angles were eliminated. Convergent-beam diffraction patterns were taken in order to derive symmetry information. 3. Results 3.1. Metallography,
particle distribution,
analysis
In optical micrographs of cast material (fig. 2), the particles outline a dendritic structure. From
Fig. 2. Optical
micrograph of cast structure, ingot surface. 65 x
5-10
mm from
the observed interdendritic spacings (fig. 3) local cooling rates can be estimated. At higher optical magnification and in low-magnification TEM, the particles are seen as plates, needles or with other, more complicated, characteristic shapes. Relative fractions of the commonly occurring types at different positions in the ingot were determined both in thin foils, using SAD, and from extracted specimens, using EDS with SAD as a check. Results for the 0.13% Si alloy are presented in table 3. The average compositions of the different types were obtained by quantitative evaluation of the EDS spectra. In addition to AI, Fe and Si, 0.1-l!% Ni was found in all particles. Unfortunately copper could not be determined, due to a background signal from the instrument. The compositions were in reasonable agreement with previously reported values, e.g.: (Al, Si),.,(Fe, Ni), (Al, Si),,(Fe, Ni) and (Al, Si),.,(Fe, Ni) for the three phases usually
P. Skjerpe et al. / Solidification DAS,
siructure and primaty
243
Al - Fe - Si particles in DC-cast Al
3.2. Electron microscopy and diffraction of particles
pm
t
Low-magnification micrographs of extracted particles reveal typical morphologies. Fig. 4a of an Al,Fe particle shows how the characteristic dendrite-like shape is much more readily seen than in the foil specimen (fig. 4b). The aim of structure characterization with electron microscopy was to confirm the equilibrium crystal structures assumed for particles classified as Al,Fe, a-Al-Fe-Si, etc., to seek structure models for the metastable intermetallic phases as well as describe the internal defect structure of the particles. To this end high resolution microscopy, spot patteins and CBED were used. Particles of the equilibrium phase Al,Fe appear as (100) plates in the extracted specimens. Spot pattern intensities were found to agree qualitatively with kinematical calculations based on the monoclinic structure which Black [4] had determined with X-ray investigation. Diffraction patterns indicate twins and faults to be frequent on both (001) and (100) planes. Evidence for the latter was not readily obtained from the (100) plates, but in thinned foils the [OlO] orientation was sometimes found. The diffraction pattern, fig. 5a, taken along the monoclinic axis shows twinning on (100) as well as streaks along c* and c+. The micrograph, fig. 5b, shows a Tl:O) twin and several faults on (100). Some main contrast features in the micrographs may be interpreted with reference to the X-ray structure, but extensive calculations will be needed for detailed comparison with the observed images. There are no distinct “block” or similar features which can be discerned in these large and complicated unit cells, where repetition distance in the beam direction typically is lo-15 A or more.
0
50
311, ., .5p.
,
,
,
Distance
Fig.
3. Secondary
‘p?
from
, ,
.,p,
surface,
.
, ,2p.
)
mm
dendrite spacing in ,um as function distance from ingot surface.
of
called Al,Fe, Al,Fe and Al,Fe respectively. Absorption corrections were not applied; at thickness 1000 A or less the correction in iron content will be less than 1 wt%.
Table 2 Chemical composition of material spectrography; all concentrations wt%, others in ppm(wt)
Ni 43 Cu 23 B 21 Cr 18 Zr 16 Co 16 Snll
Fe 0.25 Si 0.13 Zn 185 Ti 174 Ga 125 V 62 Mn 47 Pb 44
Table 3 Relative amounts
10 mm (foil) 25 mm (extracted) 50 mm (foil) 100 mm (extracted) 100 mm (foil)
as determined by emission over 10 ppm Fe and Si in
of phases
at different
position
Al,Fe
AI,Fe
55 48
33 37 20 28 17
in the ingot;
SAD measurements
in foil specimens;
Al,Fe
a-Al-Fe-Si
14 22
67 63 80 3 8
EDS in extracted
samples
a”-Al-Fe-Si
N
4
18 42 15 52 23
244
Fig. 4. (a) Al,Fe
P. Skjerpe et al. / Solidification
structure andprimary
Al- Fe- Si particles in DC-crrrt Al
particle (B) with characteristic dendrite shape together with an (Y,-Al-Fe-S Al,Fe particle in foil specimen.
High-resolution images of an unknown structure, e.g. fig. 6, taken along the (111) body diagonal in the bet A1,Fe may then yield little direct indication about the atomic arrangement in the unit cell. Diffraction information is therefore essential. SAD patterns from different crystals may vary considerably, as seen from figs. 7a and 7b, also along (111) direction: fig. 7a includes mainly the funda-
particle (A) in extraction specimen. (b)
mental spots from the reported bet unit cell; in fig. 7b there are many extra spots which cannot be indexed readily as multiples of the basic unit cell, and thus indicate incommensurate modulations of the structure in the (110) directions - see also the micrograph, fig. 6. CBED patterns were then taken in order to determine possible space groups. Fig. 8, taken
245
P. Skjerpe et al. / Solidification structure and primary Al- Fe - Si particles in DC-cast Al
Fig. 5. Diffraction
pattern
(a) and micrograph
(b) of a faulted AI,Fe
along the c-axis, shows two sets of mirror planes. Since no systematic absences appear, apart from those resulting from the body-centered Bravais lattice, only two space groups appear possible: 14/mmm and I4mm. No direct evidence for a third mirror, normal to c, was obtained. Therefore tests for center of symmetry were carried out, as demonstrated by fig. 9. From the combination of the reciprocity relation and the inversion oper-
crystal,
showing
many faults and a twinned
region, T.
ation (see, e.g., Steeds [16]) it follows that the intensity distributions within the g and g disks should be similar if there is a center of symmetry: 1,(/r + g) = I,(k). From the similarity of the g and g disks in fig. 9 it is concluded that the basic structure of Al,Fe has a center of symmetry and hence that the space group is 14/mmm. Attempts to accommodate the nearly 120 atoms (tentatively 22 Fe + 96 Al) in the corresponding unit cell
246
P. Skjerpe et al. /Solidification
Fig. 6. Electron
micrograph
structure andprimq
of Al,Fe
crystal,
indicate the structure to be quite different from either the Al,Fe or the Al&o, structure which was proposed for particles with similar composition by Simensen and Vellasamy [6].
4. Discussion The extraction technique is seen to be very useful for preparation of electron microscope
Al- Fe-Siparticles
seen along (ill),
showing
in DC-cast Al
complicated
disorder
specimens of intermetallic phases in aluminum alloys, improving the statistics of analysis as well as providing stable specimens for high-resolution and analytical microscopy. Although no extensive analysis of statistics was carried out, the comparison between results from foils and extraction specimens seems to support the claim by Simensen et al. [14] that the extraction technique is quantitative and that no appreciable loss of material from the particles occurs.
P. Skjerpe et al. / Solidification
Fig. 7. Diffraction patterns from Al,Fe,
structure and primary Al- Fe-Si
particles in DC-cast Al
(111) projections, showing many superstructure reflections in (b).
Fig. 8. CBED pattern of AI,Fe
along 001, showing mirror planes
241
248
P. Skjerpe et al. / Solidification
structure and primary Al- Fe- Si particles m DC-rust Al
Fig. 9. CBED pattern
of AI,Fe,
Particle types and compositions could be readily determined as function of distance from the ingot surface and compared with the estimated cooling rates. At the high solidification rates in the region between 10 and 50 mm, A1,Fe and (YAl-Fe-Si were most frequent, with no A1,Fe occurring. As the cooling rate decreases towards the center of the ingot, the equilibrium phase Al,Fe
showing
center of symmetry.
appears and, in addition, the highly disordered phase A1,Fe - which may have some resemblance to Al,Fe - a nearly stable structure in the system. The formation of metastable and silicon-rich phases as the cooling rate increases is known from other investigations [1,2]. The micrographs and diffraction patterns of A1,Fe agree with the published structure. Faults
P. Skjerpe et al. / Solidification structure andprimaty
and twins on the (001) planes are observed in HREM images. For the phase Al,Fe, the value m = 4.2 is found; the space group 14/mmm is deduced from CBED patterns. Further work is needed in order to substantiate structure models based upon combined diffraction and microscope evidence. An account of the structure and microstructure observations of the particles related to the solidification process will be published elsewhere [ 171.
[7] [8] [9] [lo] [ll] [12] [13] [14] [15]
[16]
References [l] H. Westengen, Z. Metal& 73 (1982) 360. [2] R.M.K. Young and T.W. Clyne, Scripta Met. 15 (1981) 1211. [3] A.L. Dons, Z. Metal& 76 (1985) 151. [4] P.J. Black, Acta Cryst. 8 (1955) 43, 175. 15) I. M&i, H. Kosuge and K. Nagahama, J. Japan. Inst. L. Metals 25 (1975) 1. [6] C.J. Simensen and R. Vellasamy, Z. Metallk. 68 (1977) 428.
[17] [18] [19] [20] [21]
Al- Fe- Siparticles
in DC-cast Al
249
L.K. Walford, Acta Cryst. 18 (1965) 287. M. Cooper, Acta Cryst. 23 (1967) 1106. M. Cooper and K. Robinson, Acta Cryst. 20 (1966) 614. K. Robinson and P.J. Black, Phil. Mag. 44 (1953) 1392. G. Phragmen, J. Inst. Metals 77 (1950) 498. V.G. Rivlin and G.V. Raynor, Intern. Met. Rev. 3 (1981) 133. C.Y. Sun and L.F. Modolfo, J. Inst. Metals 95 (1967) 384. C.J. Simensen, P. Fartum and A. Andersen, Fresenius Z. Anal. Chem. 319 (1984) 286. J.I. Goldstein, in: Introduction to Analytical Electron Microscopy, Eds. J.J. Hren, J.I. Goldstein and D.C. Joy (Plenum, New York, 1979). J.W. Steeds, in: Quantitative Electron Microscopy, Eds. J.N. Chapman and A.J. Craven (Scottish Universities Summer School in Physics, 1984). P. Skjerpe, Met. Trans., in press. H. Kosuge and I. Mizukami, J. Japan. Inst. L. Metals 22 (1972) 437. E.H. HoIlingsworth, G.R. Frank, Jr. and R.E. WiIlett, Trans. Met. Sot. AIME 224 (1962) 188. P. Liu, T. Thorvaldsen and G.L. Dunlop, in: Proc. SCANDEM 85, p. 35. D. Munson, J. Inst. Metals 95 (1967) 217.