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HRTEM and ADF-STEM of precipitates at peak-ageing in cast A356 aluminium alloy
Journal of Alloys and Compounds 496 (2010) 478–487
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HRTEM and ADF-STEM of precipitates at peak-ageing in cast A356 aluminium alloy N. Chomsaeng a,∗ , M. Haruta b , T. Chairuangsri c,∗ , H. Kurata b , S. Isoda b,∗∗ , M. Shiojiri d a
Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand d Professor Emeritus of Kyoto Institute of Technology, 1-297 Wakiyama, Ohyamazaki, Kyoto 618-0091, Japan b c
a r t i c l e
i n f o
Article history: Received 4 September 2009 Received in revised form 8 February 2010 Accepted 11 February 2010 Available online 18 February 2010
a b s t r a c t Precipitates at peak-ageing in an A356 Al–Mg–Si alloy cast by a semi-solid process have been studied by high-resolution transmission electron microscopy (HRTEM) and annular dark-field scanning transmission electron microscopy (ADF-STEM). The major precipitate (ppt) at peak-ageing is the monoclinic  coincide
or pre- . Its orientation relationship with the fcc-Al matrix is [0 0 1]Al //[0 1 0]ppt , (0 2 0)Al // (6 0 1)ppt coincide
Keywords: Cast A356 aluminium alloy High-resolution transmission electron microscopy Annular dark-field scanning transmission electron microscopy Precipitate Semi-solid process
and (2 0 0)Al // (4¯ 0 3)ppt , equivalent to [0 0 1]Al //[0 1 0]ppt , (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt . The habit direction of this precipitate is [0 0 1]Al //[0 1 0]ppt forming facet planes on (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , in which very good atomic matching was found. The contrast of the precipitate in ADF-STEM is reversed to the typical atomic number contrast. To understand such contrast, a dynamical simulation based on the multi-slice method was therefore performed using different atomic stacking models and successfully explain the reverse contrast of the experimental images of the precipitate. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Al–Mg–Si alloys are an important group of aluminium alloys. Wrought Al–Mg–Si alloys of 6xxx series are alternatives to steels for automotive body applications since they show good combination of formability, corrosion resistance and weldability [1]. Cast A356 (Al–0.3 wt%Mg–7 wt%Si) and A357 (Al–0.5 wt%Mg–7 wt%Si) alloys have widespread applications for structural components in the automotive, aerospace and general engineering industries because of their excellent castability, corrosion resistance and particularly high strength-to-weight ratio in the heat-treated condition [2,3]. Cast alloys of 3xx series have similar primary alloying elements to wrought Al–Mg–Si alloys of 6xxx series. They are age-hardenable and the strengthening is known to be based on a precipitation process. Their important and significant age-hardening response has stimulated numerous studies of precipitation in these alloys. Due to the minute scale of ageing precipitates in these Al–Mg–Si aluminium alloys, electron microscopy has been an indispensable method in ageing precipitation studies [4–15], of which
includes conventional transmission electron microscopy (CTEM), high-resolution transmission electron microscopy (HRTEM) and annular dark field in scanning transmission electron microscopy (ADF-STEM). Most studies have been focused on wrought Al–Mg–Si alloys. For the wrought alloys without excess Si or the so-called balanced alloys, it has been reported that the precipitation sequence is independent of the composition [16,17]. Despite remaining controversy, a precipitation sequence that is generally accepted is (e.g. [16,18]):
SSS ␣ → GP zones (spheres or needles) →  (needles)
→  (rods) → (plates, Mg2 Si or non-stoichiometric Mgx Siy ) SSS denotes “supersaturated solid solution” and GP denotes “Guinier–Preston” [19]. A precipitation sequence for wrought Al–Mg–Si alloys containing excess Si was proposed [20] as: SSS ␣ → (Mg + Si) co-clusters or (plate-like/spherical)GP-I
→  or GP-II(needles) →  (rods) + Si + others → (Mg2 Si)plates + Si
∗ Corresponding author. Tel.: +66 53 943401; fax: +66 53 892262. ∗∗ Corresponding author. Tel.: +81 774 38 3051; fax: +81 774 38 3055. E-mail addresses: [email protected] (N. Chomsaeng), [email protected] (T. Chairuangsri), [email protected] (S. Isoda). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.084
Additional quarternary (Q) phases can form as a result of Cu addition. A complex decomposition sequence for wrought
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Al–Mg–Si–Cu alloys with a balanced Mg to Si ratio has been proposed [21] as:
SSS ␣ → clusters or co-clusters of Mg + Si → GP zones → 
→  + Q → (Mg2 Si) + Q(Al4 Cu2 Mg8 Si7 )
Here, Q denotes a quaternary phase and Q is its precursor. Other precursors of the quarternary phase were also reported, denoted as QP and QC [22]. Relatively few studies have been attempted for studying cast Al–Mg–Si alloys by electron microscopy. To the best of our recognition on the literature, none has been reported on using HRTEM and ADF-STEM to study precipitation in cast A356 aluminium alloys. The purpose of this work is therefore to utilize HRTEM and ADF-STEM to study the precipitate at peak ageing in an experimental alloy equivalent to cast A356 Al–Mg–Si alloy. Discussion on contrast in images from these techniques is also given for reasonable interpretation of structural images, which will be beneficial to extend our understanding on contrast mechanism, especially in ADF-STEM. The contrast in ADF-STEM, particularly the contrast in HAADF-STEM (high-angle ADF-TEM) is related directly to atomic number of constituent atom (Z-contrast) [23]. However, there are many factors for the contrast to deviate from the simple Z-contrast. ADF-STEM contrast depending on the Debye–Waller factor has been reported for GaAs [24], and a related change in contrast has been reported at the interface of a-Si/c-Si due to the strain field [25]. Moreover, effects of electron channeling on ADF-STEM intensity have been reported in La2 CuSnO6 [26]. Accordingly, dynamical image simulations such as multi-slice simulation are essentially important for precise interpretation on structures in ADF-STEM images by including at least the effect of the Debye–Waller factor as well as the effect of channeling process of electrons.
2. Materials and methods 2.1. Materials A cast A356 aluminium alloy with the composition of Al–7.0 wt%Si– 0.34 wt%Mg–0.12 wt%Ti–0.11 wt%Fe–0.01 wt%Cu was prepared. The alloy was cast by the sloped cooling plate method, which is a semi-solid casting process, and details have been reported elsewhere [27]. To study ageing precipitation, solution treatment was performed for 4 h at 540 ◦ C in an air furnace followed by quenching in hot water at 80 ◦ C and artificial ageing was performed at 160 ◦ C in an oil bath for 12–48 h followed by cold water quenching. Hardness test was carried out on aged samples and the peak-ageing time giving the maximum hardness was 18–24 h. The samples aged at 160 ◦ C for 18–24 h were therefore used to prepare specimens for studying precipitates at peak-ageing by HRTEM and ADF-STEM.
Fig. 2. Bright-field TEM image shows precipitates in the sample after ageing for 18 h at 160 ◦ C. The white arrowheads indicate the needles oriented along three orthogonal directions.
2.2. Sample preparation Cast samples were cut with a cutting machine, Labotom 3, Struers. Standard grinding and polishing methods were used to get mirror surface and a solution of 10 ml hydrofluoric acid and 90 ml distilled water was used as an etchant to reveal microstructures in light microscopy (LM), which was studied using an Olympus BX60M light microscope. TEM thin foils were prepared by twin-jet polishing, Tenupol-5, Struers, using 5 vol% perchloric acid in absolute ethanol solution at −15 to −5 ◦ C, applied at 37 V and approximately 16 mA. A JEM-2010FS and a JEM-9980TKP1 operated at 200 kV were utilized for HRTEM and ADF-STEM investigations. The spherical aberration Cs of the probe-forming lens was 1 mm. The inner and outer angles of the low-angle annular dark field (LAADF) detector are 20 and 60 mrad, whereas those of the high-angle annular dark field (HAADF) detector are 64 and 170 mrad, respectively. The multi-slice calculations of ADF images and electron propagation in crystalline materials were carried out using the Win HREM v. 3.0 software package [28].
3. Results and discussion 3.1. General observation of microstructure The microstructure from LM of the as-cast and peak-aged samples was shown in Fig. 1. Microstructural constituents are mainly the primary austenite and the eutectic structure of Si and austenite.
Fig. 1. Light micrographs show the microstructure of (a) as-cast sample and (b) aged sample at 160 ◦ C for 18 h.
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A spheroidal structure was formed in the as-cast sample through the semi-solid casting. After ageing, precipitation occurred substantially within the primary austenite, but it is not revealed by LM. 3.2. HRTEM and ADF-STEM Fig. 2 is a bright-field TEM micrograph of the microstructure within the primary austenite in the as-cast and peak-aged samples in which the precipitation with a weak contrast can be clearly observed. Shape of the typical precipitate is apparently needle-like with a length of about 10 nm, lying in the Al matrix along three orthogonal variants of 0 0 1Al as shown with the white arrowheads.
HRTEM images revealed different orientations of precipitates in association with peak-ageing. However, the type that was most clearly found is that given in Fig. 3(a) with its needle axis parallel to the electron beam direction. The image was taken at the Scherzer focus of 59 nm. Dark spots arrayed regularly in the matrix correspond to the lattice of Al matrix atoms. The inset is a fast Fourier transform (FFT) image from the precipitate together with that from the Al matrix, from the marked area by the white boarder in the figure. The experimental nano-diffraction pattern from the same precipitate is given in Fig. 3(b), in agreement to the FFT inset in Fig. 3(a). The diffraction was taken from an area of 3–4 nm in size. Indexing of the nano-diffraction pattern suggests that these precipitates at the peak-ageing can be regarded as pre- [12,13] or
Fig. 3. HREM image (a) shows precipitates in the sample aged at 160 ◦ C for 18 h and the Fourier transform (inset) of the area marked by the white boarder. The image was taken at the Scherzer focus of 59 nm and dark contrast corresponds to positions of atoms. Experimental nano-diffraction pattern in reversed contrast (b) is from the precipitate within the area marked by the white boarder. Schematic of the diffraction (c) shows the monoclinic pattern superimposed on the face-centered cubic pattern along the [0 0 1]Al //[0 1 0]ppt zone axes. Coincidence between (0 2 0)Al //(6 0 1)ppt and (2 0 0)Al //(4¯ 0 3)ppt is evident. Corresponding stereographic projection of the pattern is given in (d) along the [0 0 1]Al //[0 1 0]ppt zone axes. The orientation relationship of [0 0 1]Al //[0 1 0]ppt , (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt are satisfied as reported by Wang et al. [32]. This is also equivalent to [0 0 1]Al //[0 1 0]ppt , [3 1 0]Al //[0 0 1]ppt and [2¯ 3 0]Al //[1 0 0]ppt reported by Matsuda et al. [33] and Andersen et al. [34], as shown in the equivalent stereogram in (e).
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Fig. 3. (Continued ).
 [11,18]. The lattice parameters obtained in this study are closer to the  with the empirical formula Mg5 Si6 reported by Zandbergen et al. [11]. From such a precipitate, Al, Mg and Si signals were detected by energy dispersive spectroscopy (EDS), though their accurate compositional ratio could not be determined. These confirm that the precipitate at peak-ageing giving the maximum hardness is the “ precipitate being consistent with that reported in wrought alloys [20,29,30], not the  precipitate as previously reported in other cast alloys [31]. Details of the lattice parameters of this precipitate are shown in Table 1, in comparison to those reported for the pre- [12,13] and the  [11]. The lattice parameters of precipitate were measured by a calibration using the fcc-Al lattice on [0 0 1]Al as a = 0.405 nm on the same HRTEM image. When considering the similarity in the a-axis dimension, the present precipitate might be the  rather the pre- . Schematic illustration of the nano-diffraction patterns is given in Fig. 3(c). The orientation relationship (OR) between this precipitate (ppt) and the Al matrix
that this OR satisfies [0 0 1]Al //[0 1 0]ppt , (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , which has been reported previously [32], and also that reported by Matsuda et al. [33] and Andersen et al. [34], i.e. [0 0 1]Al //[0 1 0]ppt , [3 1 0]Al //[0 0 1]ppt and [2¯ 3 0]Al //[1 0 0]ppt . Note that the directions [3 1 0]Al //[0 0 1]ppt and [2¯ 3 0]Al //[1 0 0]ppt are perpendicular to the [0 0 1]Al //[0 1 0]ppt and are lying on the (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , respectively, as shown in Fig. 3(e). An enlarged HRTEM image of the precipitate is given in Fig. 4(a). Facet planes parallel to the needle axe are (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt . A plane view of atomic positions along the [0 1 0]ppt of the crystal structure of the  (Mg5 Si6 ) precipitate was created based on atomic positions of Mg and Si atoms suggested by Zandbergen et al. [11] in Fig. 4(b) and given in Table 2. The model in Fig. 4(b) was inversed as shown in Fig. 4(c) and consequently superimposed on the enlarged image. Fair agreement can be seen in the inserted image of Fig. 4(a), where the half of the unit cell is illustrated. However, some difference is obviously at the four Si atoms, at y = 0 and y = 0.5, in the middle of the half unit cell, which is still under examination but presumably owing to an overlapping of Al matrix layer above or beneath the precipitate. Moreover, a parallelism of (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt is of inter-
coincide
can be proposed as [0 0 1]Al //[0 1 0]ppt , (0 2 0)Al
// (6 0 1)ppt
coincide
and (2 0 0)Al // (4¯ 0 3)ppt . The habit direction (or the direction parallel to the needle axe) is [0 0 1]Al //[0 1 0]ppt . Stereographic projection representing this OR is shown in Fig. 3(d). It can be seen Table 1 Crystal structure and lattice parameter of  precipitate and its precursors. Precipitate
Pre-  Pre- or 
Crystal structure
Monoclinic (C2/m) Monoclinic (C2/m) Monoclinic (C2/m)
Lattice parameter
Ref.
a
b
c
ˇ
1.478 or 1.460 nm 1.516 nm 1.568 nm
0.405 nm 0.405 nm 0.405 nm
0.640 or 0.674 nm 0.674 nm 0.675 nm
105.3◦ or 106.8◦ 105.3◦ 105.3◦
[13] [11] This study
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Table 2 Parameters for simulation. Parameters
Description
Atomic position parameter [11]
Size of super-cells Accelerate voltage Aperture radius Cs Defocus Dark-field detector
Atom
x
y
z
B (Å2 )
Mg(1) Mg(2) Mg(3) Si(1) Si(2) Si(3)
0 0.3459 0.4299 0.0565 0.1885 0.2172
0 0 0 0 0 0
0 0.089 0.652 0.649 0.224 0.617
0.5 1.0 0.8 1.1 0.5 2.5
Width 3.645 nm × length 3.645 nm × thickness 80.595 nm 200 kV 13.2 mrad 1 mm 59 nm under 20–60 mrad (LAADF) and 64–170 mrad (HAADF)
est. The atomic arrangement in these precipitate and Al matrix planes were consequently compared in Fig. 4(d) and (e), in which the atoms with no shading and stripped by “1” are those on the interested planes. Very good matching was found with the mis-
fit of 5.0% along [3 1 0]Al //[0 0 1]ppt direction on (1¯ 3 0)Al //(1 0 0)ppt and 6.9% along [2¯ 3 0]Al //[100]ppt direction on (3 2 0)Al //(0 0 1)ppt . Lower interfacial energy of lattice–lattice interaction can therefore be expected by the parallelism of these planes, so that the interface can be stabilized. Since ADF-STEM is a developing analytical tool on structures at atomic scale, low-angle annular dark field (LAADF) and high-angle annular dark field (HAADF) in STEM were adopted in the present sample as given in Fig. 5(a) and (b), respectively. HAADF-STEM is applied more frequently for analyzing structures due to its simple Z-contrast formation, even though not always so simple. In comparison to the HAADF-STEM, the LAADF-STEM image gives different but useful information about structures depending on the detector angles. As shown in the present LAADF-STEM image, total feature of image is similar to that of HRTEM, indicating that the LAADF-STEM provides structural information basically on the  phase in the present case. Even though no more details on atomic positions can be extracted from these STEM images, the reason for the darker contrast of the precipitate as compared to that of the Al matrix in HAADF-STEM, and vice versa in LAADF-STEM, is of interest in analysis of materials structures as well as from the microscopic
Fig. 4. Noise-filtered image of the precipitate marked by the white boarder in Fig. 3 is given in (a). Faceted faces parallel to the needle axis are (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt . The atomic model of the  (Mg5 Si6 ) by Zandbergen et al. [11] (b) was inversed as (c) and superimposed on the noise-filtered image where lattice parameters were slightly adjusted as a = 1.568 nm, b = 0.405 nm, c = 0.675 nm and ˇ = 105.3◦ to fit as inset in (a). Good agreement on the positions of Mg atoms is obtained, but the positions of Si atoms of the precipitates can be slightly different from those in the Mg5 Si6 . The atomic arrangement in the faceted planes parallel to the needle axis, (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , is compared in (d) and (e), respectively. The needle axis is [0 0 1]Al //[0 1 0]ppt where perfect matching of atomic spacing (0.203 nm) is obvious without misfit. Very good fit of Mg and Al atoms on those planes is achieved with 5.0% misfit along [3 1 0]Al for (1¯ 3 0)Al //(1 0 0)ppt and 6.9% misfit along [2¯ 3 0]Al for (3 2 0)Al //(0 0 1)ppt .
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Fig. 4. (Continued ).
viewpoint. In general, contrast of ADF image, especially HAADF, an atom with a higher atomic number should show a brighter contrast comparing to one with a lower atomic number (Z-contrast [35]), when both have the same Debye–Waller factor. For example, the Si(2) atom in Table 2 should be brighter in the HAADF image than the Mg(1) atom. On the other hand, HAADF-STEM simulations often show that atoms with a smaller Debye–Waller factor might have a tendency to exhibit a brighter contrast comparing to those of the same element with a larger Debye–Waller factor, as in the case between the Si(2) and Si(3) in Table 2. However, the atomic numbers of Mg, Al and Si are very close, hence it should be difficult to distinguish simply in the Z-contrast HAADF-STEM image. Moreover, the bulk density of the  (Mg5 Si6 ) precipitate is slightly lower than that of the Al matrix. Strain field contrast of ADF depends on sample thickness and collection angle [25,36]. However, the bright contrast of precipitate in LAADF-STEM of Fig. 5(b) cannot be understood straightforward. The multi-slice simulation is indispensable
in understanding of the experimental contrasts. For the big change in ADF-STEM contrasts in HAADF and LAADF, two possibilities can be suggested. Firstly, the growth of the precipitate causes some distortion or strain field in the Al matrix as well as the precipitate itself [29,37]. This is difficult to prove especially for LAADF-STEM image in which many phenomena are involved. The image in Fig. 5(a) as well as its Fourier-filtered image in Fig. 5(c) show very similar contrast with HRTEM image in Fig. 4(a), indicating that the main contrast might be originated not from a strain in the precipitate. Secondly, there is a dynamical effect in combination with a channeling effect, in which an overlapping between the precipitate and the Al matrix along the incident beam direction should be another important factor. This relates to a positional image effect of precipitate in matrix. Simulation was therefore performed to explain the peculiar contrast in Fig. 5 by using the Win HREM software (HREM Research Inc.) based on the multi-slice method and the Materials Studio®
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Fig. 5. LAADF (a) and HAADF (b) images of the same precipitate in Fig. 4. Atomic resolution is not achieved, but the periodicity showing the monoclinic lattice type can be seen in LAADF. A reverse contrast to that typically expected from the Z-contrast mechanism in ADF image is obtained. (c) The noise-filtered LAADF image on the monoclinic lattice.
software (Accelrys Software Inc.). Parameters including atomic positions and the Debye–Waller factor (B) were taken from Zandbergen [11] as given Table 2. Other parameters related to the utilized microscope were also given in the table. So as to consider the position effect on image contrasts, three different models (I, II and III) were assumed as illustrated in Fig. 6(a); the precipitate layer is located at the top, bottom or middle of the sample. The socalled “super-cells” for the simulation were consequently created as shown in Fig. 6(b). The thickness of the TEM sample in this study is about 80 nm as determined from a low loss electron energy-loss spectrum. The depth of the precipitate in the needle axis direction was given as 10.125 nm according to its longitudinal length measured from the experimental HRTEM images, which is equivalent to 25 unit cells in the [0 1 0]ppt direction. The thickness of Al overlapping with the precipitate was then assumed to be up to 70.470 nm, which is equivalent to 174 unit cells of the fcc-Al. The total thickness
of the super-cells is therefore 80.595 nm, which should be enough to see the intensity variation effect in ADF image simulation as compared to the thickness of the TEM sample. To execute the calculation in a reasonable time for the large super-cells, calculation of the contrast were performed to compare only among two different areas in the super-cells; (i) Al atom near Si(1) position in the precipitate and (ii) Al atoms far outside the precipitate, as denoted in the atomic plan view in Figs. 4(b) and 6(b). The Al atom near Si(1) is a representative position on precipitate being expected brightest in contrast through many trial simulations on various beam positions and the Al column in the matrix is as a reference. The results from the simulation are summarized in Fig. 7, in which the horizontal and vertical axes represent depth in the super-cells and intensity, respectively. As shown in the figure, the intensity change is somewhat entangled with increasing the thickness. It can be seen that, as the thickness increased, the precipitate can possess the fairly lower
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Fig. 6. Three different super-cell models for simulation of intensity in ADF images are illustrated in (a), using the Materials Studio® software. The position of the precipitate within the Al matrix is set at the top (I), the bottom (II) and the middle (III) of the sample, respectively. The depth of 10.125 nm (25 unit cells in the [0 1 0]ppt ) is given, according to the length of the precipitate observed experimentally in HRTEM. The thickness of the Al matrix overlapping the precipitate is assumed as 70.470 nm (174 unit cells), giving the total thickness of the sample of 80.595 nm. Plan view of the model is shown in (b). Simulation for comparing channeling contrast was performed on two different positions, i.e. (i) Al atom near Si(1) in the precipitate and (ii) Al atom far outside the precipitate.
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Fig. 7. (a–c) HAADF and (d–f) LAADF intensity variation versus depth in the multi-slice method is simulated based on the three super-cell models in Fig. 6. As the thickness increased, the precipitate can possess the fairly lower intensity in HAADF for the models I and III, whereas it can possess the higher intensity in LAADF for the models II and III. Therefore, the peculiar contrast of the precipitate in the Al matrix observed experimentally can be explained as the model III situation in Fig. 6.
intensity in HAADF for the models I and III, whereas it can possess the higher intensity in LAADF for the models II and III. Therefore, the only possibility for the peculiar contrast of the precipitate in the Al matrix observed experimentally in Fig. 5 is the model III, where the precipitate situates inside the Al matrix. Accordingly, the contrast of HAADF-STEM image (Fig. 7(c)) of the precipitate exhibits lower than that of the Al matrix at the thickness above 35 nm, but higher in LAADF-STEM (Fig. 7(f)) as shown in the present experiment. The origin of such a strange contrast variation is come from the dynamical and the channeling effects in the crystalline precipitate situated in a crystalline matrix, of which the dynamical simulation is indispensable in structure analysis. By assuming
the  with the formula Mg5 Si6 , the experimental images of the precipitate can be explained consistently in the present case. The LAADF-STEM is a potential tool in atomic structure analysis of precipitates in combination with HAADF-STEM, though there remain practical problem to be solved in detailed simulation of big unit cells. 4. Conclusions The precipitate most frequently found at peak-ageing of the cast A356 aluminium alloy is the  precipitate or its precursors, not the  as previously reported.
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The orientation relationship (OR) between the precipitate and the Al matrix is proposed as [0 0 1]Al //[0 1 0]ppt , coincide
coincide
(0 2 0)Al // (6 0 1)ppt and (2 0 0)Al // (4¯ 0 3)ppt , equivalent to [0 0 1]Al //[0 1 0]ppt , (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt . The habit direction parallel to the needle axe is [0 0 1]Al //[0 1 0]ppt . Facet planes parallel to the needle axe are (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , in which very good atomic matching was found and should therefore lead to low energy interfaces. Different atomic stacking in the direction parallel to the electron beam for atoms with different Z and Debye–Waller factor can lead to a reverse contrast to the typical Z-contrast expected in ADF image of the  precipitate embedded in the Al matrix. Acknowledgements This work was partly supported by the Commission on Higher Education, Thailand, and also by Grants-in-Aid for Scientific Research Grants Nos. 19310071 and 20550188, and for JSPS Fellowship No. 20-145 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] G.B. Burger, A.K. Gupta, P.W. Jeffrey, D.J. Lloyd, Mater. Charact. 35 (1995) 23. [2] W.S. Miller, L. Zhuang, J. Bottema, A.J. Wittebrood, P. De Smet, A. Haszler, A. Vieregge, Mater. Sci. Eng. A280 (2000) 37. [3] Q.G. Wang, Metall. Mater. Trans. 34A (2003) 2887. [4] G. Thomas, J. Inst. Met. 90 (1961) 57. [5] J.P. Lynch, L.M. Brown, M.H. Jacobs, Acta Metall. 30 (1982) 1389. [6] S.D. Dumolt, D.E. Laughlin, J.C. Williams, Scripta Mater. 18 (1984) 1347. [7] I. Dutta, S.M. Allen, J. Mater. Sci. Lett. 10 (1991) 323. [8] K. Matsuda, S. Tada, S. Ikeno, T. Sato, A. Kamio, Scripta Metall. Mater. 32 (8) (1995) 1175.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
HRTEM and ADF-STEM of precipitates at peak-ageing in cast A356 aluminium alloy N. Chomsaeng a,∗ , M. Haruta b , T. Chairuangsri c,∗ , H. Kurata b , S. Isoda b,∗∗ , M. Shiojiri d a
Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand d Professor Emeritus of Kyoto Institute of Technology, 1-297 Wakiyama, Ohyamazaki, Kyoto 618-0091, Japan b c
a r t i c l e
i n f o
Article history: Received 4 September 2009 Received in revised form 8 February 2010 Accepted 11 February 2010 Available online 18 February 2010
a b s t r a c t Precipitates at peak-ageing in an A356 Al–Mg–Si alloy cast by a semi-solid process have been studied by high-resolution transmission electron microscopy (HRTEM) and annular dark-field scanning transmission electron microscopy (ADF-STEM). The major precipitate (ppt) at peak-ageing is the monoclinic  coincide
or pre- . Its orientation relationship with the fcc-Al matrix is [0 0 1]Al //[0 1 0]ppt , (0 2 0)Al // (6 0 1)ppt coincide
Keywords: Cast A356 aluminium alloy High-resolution transmission electron microscopy Annular dark-field scanning transmission electron microscopy Precipitate Semi-solid process
and (2 0 0)Al // (4¯ 0 3)ppt , equivalent to [0 0 1]Al //[0 1 0]ppt , (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt . The habit direction of this precipitate is [0 0 1]Al //[0 1 0]ppt forming facet planes on (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , in which very good atomic matching was found. The contrast of the precipitate in ADF-STEM is reversed to the typical atomic number contrast. To understand such contrast, a dynamical simulation based on the multi-slice method was therefore performed using different atomic stacking models and successfully explain the reverse contrast of the experimental images of the precipitate. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Al–Mg–Si alloys are an important group of aluminium alloys. Wrought Al–Mg–Si alloys of 6xxx series are alternatives to steels for automotive body applications since they show good combination of formability, corrosion resistance and weldability [1]. Cast A356 (Al–0.3 wt%Mg–7 wt%Si) and A357 (Al–0.5 wt%Mg–7 wt%Si) alloys have widespread applications for structural components in the automotive, aerospace and general engineering industries because of their excellent castability, corrosion resistance and particularly high strength-to-weight ratio in the heat-treated condition [2,3]. Cast alloys of 3xx series have similar primary alloying elements to wrought Al–Mg–Si alloys of 6xxx series. They are age-hardenable and the strengthening is known to be based on a precipitation process. Their important and significant age-hardening response has stimulated numerous studies of precipitation in these alloys. Due to the minute scale of ageing precipitates in these Al–Mg–Si aluminium alloys, electron microscopy has been an indispensable method in ageing precipitation studies [4–15], of which
includes conventional transmission electron microscopy (CTEM), high-resolution transmission electron microscopy (HRTEM) and annular dark field in scanning transmission electron microscopy (ADF-STEM). Most studies have been focused on wrought Al–Mg–Si alloys. For the wrought alloys without excess Si or the so-called balanced alloys, it has been reported that the precipitation sequence is independent of the composition [16,17]. Despite remaining controversy, a precipitation sequence that is generally accepted is (e.g. [16,18]):
SSS ␣ → GP zones (spheres or needles) →  (needles)
→  (rods) → (plates, Mg2 Si or non-stoichiometric Mgx Siy ) SSS denotes “supersaturated solid solution” and GP denotes “Guinier–Preston” [19]. A precipitation sequence for wrought Al–Mg–Si alloys containing excess Si was proposed [20] as: SSS ␣ → (Mg + Si) co-clusters or (plate-like/spherical)GP-I
→  or GP-II(needles) →  (rods) + Si + others → (Mg2 Si)plates + Si
∗ Corresponding author. Tel.: +66 53 943401; fax: +66 53 892262. ∗∗ Corresponding author. Tel.: +81 774 38 3051; fax: +81 774 38 3055. E-mail addresses: [email protected] (N. Chomsaeng), [email protected] (T. Chairuangsri), [email protected] (S. Isoda). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.02.084
Additional quarternary (Q) phases can form as a result of Cu addition. A complex decomposition sequence for wrought
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Al–Mg–Si–Cu alloys with a balanced Mg to Si ratio has been proposed [21] as:
SSS ␣ → clusters or co-clusters of Mg + Si → GP zones → 
→  + Q → (Mg2 Si) + Q(Al4 Cu2 Mg8 Si7 )
Here, Q denotes a quaternary phase and Q is its precursor. Other precursors of the quarternary phase were also reported, denoted as QP and QC [22]. Relatively few studies have been attempted for studying cast Al–Mg–Si alloys by electron microscopy. To the best of our recognition on the literature, none has been reported on using HRTEM and ADF-STEM to study precipitation in cast A356 aluminium alloys. The purpose of this work is therefore to utilize HRTEM and ADF-STEM to study the precipitate at peak ageing in an experimental alloy equivalent to cast A356 Al–Mg–Si alloy. Discussion on contrast in images from these techniques is also given for reasonable interpretation of structural images, which will be beneficial to extend our understanding on contrast mechanism, especially in ADF-STEM. The contrast in ADF-STEM, particularly the contrast in HAADF-STEM (high-angle ADF-TEM) is related directly to atomic number of constituent atom (Z-contrast) [23]. However, there are many factors for the contrast to deviate from the simple Z-contrast. ADF-STEM contrast depending on the Debye–Waller factor has been reported for GaAs [24], and a related change in contrast has been reported at the interface of a-Si/c-Si due to the strain field [25]. Moreover, effects of electron channeling on ADF-STEM intensity have been reported in La2 CuSnO6 [26]. Accordingly, dynamical image simulations such as multi-slice simulation are essentially important for precise interpretation on structures in ADF-STEM images by including at least the effect of the Debye–Waller factor as well as the effect of channeling process of electrons.
2. Materials and methods 2.1. Materials A cast A356 aluminium alloy with the composition of Al–7.0 wt%Si– 0.34 wt%Mg–0.12 wt%Ti–0.11 wt%Fe–0.01 wt%Cu was prepared. The alloy was cast by the sloped cooling plate method, which is a semi-solid casting process, and details have been reported elsewhere [27]. To study ageing precipitation, solution treatment was performed for 4 h at 540 ◦ C in an air furnace followed by quenching in hot water at 80 ◦ C and artificial ageing was performed at 160 ◦ C in an oil bath for 12–48 h followed by cold water quenching. Hardness test was carried out on aged samples and the peak-ageing time giving the maximum hardness was 18–24 h. The samples aged at 160 ◦ C for 18–24 h were therefore used to prepare specimens for studying precipitates at peak-ageing by HRTEM and ADF-STEM.
Fig. 2. Bright-field TEM image shows precipitates in the sample after ageing for 18 h at 160 ◦ C. The white arrowheads indicate the needles oriented along three orthogonal directions.
2.2. Sample preparation Cast samples were cut with a cutting machine, Labotom 3, Struers. Standard grinding and polishing methods were used to get mirror surface and a solution of 10 ml hydrofluoric acid and 90 ml distilled water was used as an etchant to reveal microstructures in light microscopy (LM), which was studied using an Olympus BX60M light microscope. TEM thin foils were prepared by twin-jet polishing, Tenupol-5, Struers, using 5 vol% perchloric acid in absolute ethanol solution at −15 to −5 ◦ C, applied at 37 V and approximately 16 mA. A JEM-2010FS and a JEM-9980TKP1 operated at 200 kV were utilized for HRTEM and ADF-STEM investigations. The spherical aberration Cs of the probe-forming lens was 1 mm. The inner and outer angles of the low-angle annular dark field (LAADF) detector are 20 and 60 mrad, whereas those of the high-angle annular dark field (HAADF) detector are 64 and 170 mrad, respectively. The multi-slice calculations of ADF images and electron propagation in crystalline materials were carried out using the Win HREM v. 3.0 software package [28].
3. Results and discussion 3.1. General observation of microstructure The microstructure from LM of the as-cast and peak-aged samples was shown in Fig. 1. Microstructural constituents are mainly the primary austenite and the eutectic structure of Si and austenite.
Fig. 1. Light micrographs show the microstructure of (a) as-cast sample and (b) aged sample at 160 ◦ C for 18 h.
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A spheroidal structure was formed in the as-cast sample through the semi-solid casting. After ageing, precipitation occurred substantially within the primary austenite, but it is not revealed by LM. 3.2. HRTEM and ADF-STEM Fig. 2 is a bright-field TEM micrograph of the microstructure within the primary austenite in the as-cast and peak-aged samples in which the precipitation with a weak contrast can be clearly observed. Shape of the typical precipitate is apparently needle-like with a length of about 10 nm, lying in the Al matrix along three orthogonal variants of 0 0 1Al as shown with the white arrowheads.
HRTEM images revealed different orientations of precipitates in association with peak-ageing. However, the type that was most clearly found is that given in Fig. 3(a) with its needle axis parallel to the electron beam direction. The image was taken at the Scherzer focus of 59 nm. Dark spots arrayed regularly in the matrix correspond to the lattice of Al matrix atoms. The inset is a fast Fourier transform (FFT) image from the precipitate together with that from the Al matrix, from the marked area by the white boarder in the figure. The experimental nano-diffraction pattern from the same precipitate is given in Fig. 3(b), in agreement to the FFT inset in Fig. 3(a). The diffraction was taken from an area of 3–4 nm in size. Indexing of the nano-diffraction pattern suggests that these precipitates at the peak-ageing can be regarded as pre- [12,13] or
Fig. 3. HREM image (a) shows precipitates in the sample aged at 160 ◦ C for 18 h and the Fourier transform (inset) of the area marked by the white boarder. The image was taken at the Scherzer focus of 59 nm and dark contrast corresponds to positions of atoms. Experimental nano-diffraction pattern in reversed contrast (b) is from the precipitate within the area marked by the white boarder. Schematic of the diffraction (c) shows the monoclinic pattern superimposed on the face-centered cubic pattern along the [0 0 1]Al //[0 1 0]ppt zone axes. Coincidence between (0 2 0)Al //(6 0 1)ppt and (2 0 0)Al //(4¯ 0 3)ppt is evident. Corresponding stereographic projection of the pattern is given in (d) along the [0 0 1]Al //[0 1 0]ppt zone axes. The orientation relationship of [0 0 1]Al //[0 1 0]ppt , (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt are satisfied as reported by Wang et al. [32]. This is also equivalent to [0 0 1]Al //[0 1 0]ppt , [3 1 0]Al //[0 0 1]ppt and [2¯ 3 0]Al //[1 0 0]ppt reported by Matsuda et al. [33] and Andersen et al. [34], as shown in the equivalent stereogram in (e).
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Fig. 3. (Continued ).
 [11,18]. The lattice parameters obtained in this study are closer to the  with the empirical formula Mg5 Si6 reported by Zandbergen et al. [11]. From such a precipitate, Al, Mg and Si signals were detected by energy dispersive spectroscopy (EDS), though their accurate compositional ratio could not be determined. These confirm that the precipitate at peak-ageing giving the maximum hardness is the “ precipitate being consistent with that reported in wrought alloys [20,29,30], not the  precipitate as previously reported in other cast alloys [31]. Details of the lattice parameters of this precipitate are shown in Table 1, in comparison to those reported for the pre- [12,13] and the  [11]. The lattice parameters of precipitate were measured by a calibration using the fcc-Al lattice on [0 0 1]Al as a = 0.405 nm on the same HRTEM image. When considering the similarity in the a-axis dimension, the present precipitate might be the  rather the pre- . Schematic illustration of the nano-diffraction patterns is given in Fig. 3(c). The orientation relationship (OR) between this precipitate (ppt) and the Al matrix
that this OR satisfies [0 0 1]Al //[0 1 0]ppt , (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , which has been reported previously [32], and also that reported by Matsuda et al. [33] and Andersen et al. [34], i.e. [0 0 1]Al //[0 1 0]ppt , [3 1 0]Al //[0 0 1]ppt and [2¯ 3 0]Al //[1 0 0]ppt . Note that the directions [3 1 0]Al //[0 0 1]ppt and [2¯ 3 0]Al //[1 0 0]ppt are perpendicular to the [0 0 1]Al //[0 1 0]ppt and are lying on the (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , respectively, as shown in Fig. 3(e). An enlarged HRTEM image of the precipitate is given in Fig. 4(a). Facet planes parallel to the needle axe are (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt . A plane view of atomic positions along the [0 1 0]ppt of the crystal structure of the  (Mg5 Si6 ) precipitate was created based on atomic positions of Mg and Si atoms suggested by Zandbergen et al. [11] in Fig. 4(b) and given in Table 2. The model in Fig. 4(b) was inversed as shown in Fig. 4(c) and consequently superimposed on the enlarged image. Fair agreement can be seen in the inserted image of Fig. 4(a), where the half of the unit cell is illustrated. However, some difference is obviously at the four Si atoms, at y = 0 and y = 0.5, in the middle of the half unit cell, which is still under examination but presumably owing to an overlapping of Al matrix layer above or beneath the precipitate. Moreover, a parallelism of (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt is of inter-
coincide
can be proposed as [0 0 1]Al //[0 1 0]ppt , (0 2 0)Al
// (6 0 1)ppt
coincide
and (2 0 0)Al // (4¯ 0 3)ppt . The habit direction (or the direction parallel to the needle axe) is [0 0 1]Al //[0 1 0]ppt . Stereographic projection representing this OR is shown in Fig. 3(d). It can be seen Table 1 Crystal structure and lattice parameter of  precipitate and its precursors. Precipitate
Pre-  Pre- or 
Crystal structure
Monoclinic (C2/m) Monoclinic (C2/m) Monoclinic (C2/m)
Lattice parameter
Ref.
a
b
c
ˇ
1.478 or 1.460 nm 1.516 nm 1.568 nm
0.405 nm 0.405 nm 0.405 nm
0.640 or 0.674 nm 0.674 nm 0.675 nm
105.3◦ or 106.8◦ 105.3◦ 105.3◦
[13] [11] This study
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Table 2 Parameters for simulation. Parameters
Description
Atomic position parameter [11]
Size of super-cells Accelerate voltage Aperture radius Cs Defocus Dark-field detector
Atom
x
y
z
B (Å2 )
Mg(1) Mg(2) Mg(3) Si(1) Si(2) Si(3)
0 0.3459 0.4299 0.0565 0.1885 0.2172
0 0 0 0 0 0
0 0.089 0.652 0.649 0.224 0.617
0.5 1.0 0.8 1.1 0.5 2.5
Width 3.645 nm × length 3.645 nm × thickness 80.595 nm 200 kV 13.2 mrad 1 mm 59 nm under 20–60 mrad (LAADF) and 64–170 mrad (HAADF)
est. The atomic arrangement in these precipitate and Al matrix planes were consequently compared in Fig. 4(d) and (e), in which the atoms with no shading and stripped by “1” are those on the interested planes. Very good matching was found with the mis-
fit of 5.0% along [3 1 0]Al //[0 0 1]ppt direction on (1¯ 3 0)Al //(1 0 0)ppt and 6.9% along [2¯ 3 0]Al //[100]ppt direction on (3 2 0)Al //(0 0 1)ppt . Lower interfacial energy of lattice–lattice interaction can therefore be expected by the parallelism of these planes, so that the interface can be stabilized. Since ADF-STEM is a developing analytical tool on structures at atomic scale, low-angle annular dark field (LAADF) and high-angle annular dark field (HAADF) in STEM were adopted in the present sample as given in Fig. 5(a) and (b), respectively. HAADF-STEM is applied more frequently for analyzing structures due to its simple Z-contrast formation, even though not always so simple. In comparison to the HAADF-STEM, the LAADF-STEM image gives different but useful information about structures depending on the detector angles. As shown in the present LAADF-STEM image, total feature of image is similar to that of HRTEM, indicating that the LAADF-STEM provides structural information basically on the  phase in the present case. Even though no more details on atomic positions can be extracted from these STEM images, the reason for the darker contrast of the precipitate as compared to that of the Al matrix in HAADF-STEM, and vice versa in LAADF-STEM, is of interest in analysis of materials structures as well as from the microscopic
Fig. 4. Noise-filtered image of the precipitate marked by the white boarder in Fig. 3 is given in (a). Faceted faces parallel to the needle axis are (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt . The atomic model of the  (Mg5 Si6 ) by Zandbergen et al. [11] (b) was inversed as (c) and superimposed on the noise-filtered image where lattice parameters were slightly adjusted as a = 1.568 nm, b = 0.405 nm, c = 0.675 nm and ˇ = 105.3◦ to fit as inset in (a). Good agreement on the positions of Mg atoms is obtained, but the positions of Si atoms of the precipitates can be slightly different from those in the Mg5 Si6 . The atomic arrangement in the faceted planes parallel to the needle axis, (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , is compared in (d) and (e), respectively. The needle axis is [0 0 1]Al //[0 1 0]ppt where perfect matching of atomic spacing (0.203 nm) is obvious without misfit. Very good fit of Mg and Al atoms on those planes is achieved with 5.0% misfit along [3 1 0]Al for (1¯ 3 0)Al //(1 0 0)ppt and 6.9% misfit along [2¯ 3 0]Al for (3 2 0)Al //(0 0 1)ppt .
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Fig. 4. (Continued ).
viewpoint. In general, contrast of ADF image, especially HAADF, an atom with a higher atomic number should show a brighter contrast comparing to one with a lower atomic number (Z-contrast [35]), when both have the same Debye–Waller factor. For example, the Si(2) atom in Table 2 should be brighter in the HAADF image than the Mg(1) atom. On the other hand, HAADF-STEM simulations often show that atoms with a smaller Debye–Waller factor might have a tendency to exhibit a brighter contrast comparing to those of the same element with a larger Debye–Waller factor, as in the case between the Si(2) and Si(3) in Table 2. However, the atomic numbers of Mg, Al and Si are very close, hence it should be difficult to distinguish simply in the Z-contrast HAADF-STEM image. Moreover, the bulk density of the  (Mg5 Si6 ) precipitate is slightly lower than that of the Al matrix. Strain field contrast of ADF depends on sample thickness and collection angle [25,36]. However, the bright contrast of precipitate in LAADF-STEM of Fig. 5(b) cannot be understood straightforward. The multi-slice simulation is indispensable
in understanding of the experimental contrasts. For the big change in ADF-STEM contrasts in HAADF and LAADF, two possibilities can be suggested. Firstly, the growth of the precipitate causes some distortion or strain field in the Al matrix as well as the precipitate itself [29,37]. This is difficult to prove especially for LAADF-STEM image in which many phenomena are involved. The image in Fig. 5(a) as well as its Fourier-filtered image in Fig. 5(c) show very similar contrast with HRTEM image in Fig. 4(a), indicating that the main contrast might be originated not from a strain in the precipitate. Secondly, there is a dynamical effect in combination with a channeling effect, in which an overlapping between the precipitate and the Al matrix along the incident beam direction should be another important factor. This relates to a positional image effect of precipitate in matrix. Simulation was therefore performed to explain the peculiar contrast in Fig. 5 by using the Win HREM software (HREM Research Inc.) based on the multi-slice method and the Materials Studio®
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Fig. 5. LAADF (a) and HAADF (b) images of the same precipitate in Fig. 4. Atomic resolution is not achieved, but the periodicity showing the monoclinic lattice type can be seen in LAADF. A reverse contrast to that typically expected from the Z-contrast mechanism in ADF image is obtained. (c) The noise-filtered LAADF image on the monoclinic lattice.
software (Accelrys Software Inc.). Parameters including atomic positions and the Debye–Waller factor (B) were taken from Zandbergen [11] as given Table 2. Other parameters related to the utilized microscope were also given in the table. So as to consider the position effect on image contrasts, three different models (I, II and III) were assumed as illustrated in Fig. 6(a); the precipitate layer is located at the top, bottom or middle of the sample. The socalled “super-cells” for the simulation were consequently created as shown in Fig. 6(b). The thickness of the TEM sample in this study is about 80 nm as determined from a low loss electron energy-loss spectrum. The depth of the precipitate in the needle axis direction was given as 10.125 nm according to its longitudinal length measured from the experimental HRTEM images, which is equivalent to 25 unit cells in the [0 1 0]ppt direction. The thickness of Al overlapping with the precipitate was then assumed to be up to 70.470 nm, which is equivalent to 174 unit cells of the fcc-Al. The total thickness
of the super-cells is therefore 80.595 nm, which should be enough to see the intensity variation effect in ADF image simulation as compared to the thickness of the TEM sample. To execute the calculation in a reasonable time for the large super-cells, calculation of the contrast were performed to compare only among two different areas in the super-cells; (i) Al atom near Si(1) position in the precipitate and (ii) Al atoms far outside the precipitate, as denoted in the atomic plan view in Figs. 4(b) and 6(b). The Al atom near Si(1) is a representative position on precipitate being expected brightest in contrast through many trial simulations on various beam positions and the Al column in the matrix is as a reference. The results from the simulation are summarized in Fig. 7, in which the horizontal and vertical axes represent depth in the super-cells and intensity, respectively. As shown in the figure, the intensity change is somewhat entangled with increasing the thickness. It can be seen that, as the thickness increased, the precipitate can possess the fairly lower
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Fig. 6. Three different super-cell models for simulation of intensity in ADF images are illustrated in (a), using the Materials Studio® software. The position of the precipitate within the Al matrix is set at the top (I), the bottom (II) and the middle (III) of the sample, respectively. The depth of 10.125 nm (25 unit cells in the [0 1 0]ppt ) is given, according to the length of the precipitate observed experimentally in HRTEM. The thickness of the Al matrix overlapping the precipitate is assumed as 70.470 nm (174 unit cells), giving the total thickness of the sample of 80.595 nm. Plan view of the model is shown in (b). Simulation for comparing channeling contrast was performed on two different positions, i.e. (i) Al atom near Si(1) in the precipitate and (ii) Al atom far outside the precipitate.
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Fig. 7. (a–c) HAADF and (d–f) LAADF intensity variation versus depth in the multi-slice method is simulated based on the three super-cell models in Fig. 6. As the thickness increased, the precipitate can possess the fairly lower intensity in HAADF for the models I and III, whereas it can possess the higher intensity in LAADF for the models II and III. Therefore, the peculiar contrast of the precipitate in the Al matrix observed experimentally can be explained as the model III situation in Fig. 6.
intensity in HAADF for the models I and III, whereas it can possess the higher intensity in LAADF for the models II and III. Therefore, the only possibility for the peculiar contrast of the precipitate in the Al matrix observed experimentally in Fig. 5 is the model III, where the precipitate situates inside the Al matrix. Accordingly, the contrast of HAADF-STEM image (Fig. 7(c)) of the precipitate exhibits lower than that of the Al matrix at the thickness above 35 nm, but higher in LAADF-STEM (Fig. 7(f)) as shown in the present experiment. The origin of such a strange contrast variation is come from the dynamical and the channeling effects in the crystalline precipitate situated in a crystalline matrix, of which the dynamical simulation is indispensable in structure analysis. By assuming
the  with the formula Mg5 Si6 , the experimental images of the precipitate can be explained consistently in the present case. The LAADF-STEM is a potential tool in atomic structure analysis of precipitates in combination with HAADF-STEM, though there remain practical problem to be solved in detailed simulation of big unit cells. 4. Conclusions The precipitate most frequently found at peak-ageing of the cast A356 aluminium alloy is the  precipitate or its precursors, not the  as previously reported.
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The orientation relationship (OR) between the precipitate and the Al matrix is proposed as [0 0 1]Al //[0 1 0]ppt , coincide
coincide
(0 2 0)Al // (6 0 1)ppt and (2 0 0)Al // (4¯ 0 3)ppt , equivalent to [0 0 1]Al //[0 1 0]ppt , (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt . The habit direction parallel to the needle axe is [0 0 1]Al //[0 1 0]ppt . Facet planes parallel to the needle axe are (1¯ 3 0)Al //(1 0 0)ppt and (3 2 0)Al //(0 0 1)ppt , in which very good atomic matching was found and should therefore lead to low energy interfaces. Different atomic stacking in the direction parallel to the electron beam for atoms with different Z and Debye–Waller factor can lead to a reverse contrast to the typical Z-contrast expected in ADF image of the  precipitate embedded in the Al matrix. Acknowledgements This work was partly supported by the Commission on Higher Education, Thailand, and also by Grants-in-Aid for Scientific Research Grants Nos. 19310071 and 20550188, and for JSPS Fellowship No. 20-145 from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] G.B. Burger, A.K. Gupta, P.W. Jeffrey, D.J. Lloyd, Mater. Charact. 35 (1995) 23. [2] W.S. Miller, L. Zhuang, J. Bottema, A.J. Wittebrood, P. De Smet, A. Haszler, A. Vieregge, Mater. Sci. Eng. A280 (2000) 37. [3] Q.G. Wang, Metall. Mater. Trans. 34A (2003) 2887. [4] G. Thomas, J. Inst. Met. 90 (1961) 57. [5] J.P. Lynch, L.M. Brown, M.H. Jacobs, Acta Metall. 30 (1982) 1389. [6] S.D. Dumolt, D.E. Laughlin, J.C. Williams, Scripta Mater. 18 (1984) 1347. [7] I. Dutta, S.M. Allen, J. Mater. Sci. Lett. 10 (1991) 323. [8] K. Matsuda, S. Tada, S. Ikeno, T. Sato, A. Kamio, Scripta Metall. Mater. 32 (8) (1995) 1175.
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