Transactions of the Indian Institute of Metals

, Volume 65, Issue 6, pp 777–782

Microstructures of Directionally Solidified Al–Ag–Cu Ternary Eutectics

Authors

    • Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Materialphysik im Weltraum
  • Lorenz Ratke
    • Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Materialphysik im Weltraum
Technical Paper

DOI: 10.1007/s12666-012-0172-3

Cite this article as:
Dennstedt, A. & Ratke, L. Trans Indian Inst Met (2012) 65: 777. doi:10.1007/s12666-012-0172-3

Abstract

Different three-phase microstructures are observed in directionally solidified Al–Ag–Cu ternary eutectics composed of an Ag2Al, an Al2Cu and a solid solution α-Al phase. A survey from regular to irregular structures is given and some special, new structural defects are observed and discussed. An approach for the quantitative description of the rich variety of complex three-phase microstructures is suggested including the defects. Shape factor and specific surface area of the phase particles are determined and compared for different microstructures.

Keywords

Al–Ag–CuTernary eutecticsDirectional solidificationMicrostructures

1 Introduction

Ternary eutectic alloys exhibit a much richer variety of microstructures compared to binary ones. Whereas in regular binary eutectics simply fibrous and lamellar arrangements appear, ternary eutectics can show three phases being arranged in fibers, two fibers embedded in a lamella, two lamellae embedded in a matrix, ladder structures of two phases in a matrix, two phases parallel and one phase orthogonal to it etc. A short summary can be found in [1]. In reality the variety is much greater due to growth defects, growth instabilities, transient and kinetic effects. Whereas for binary eutectics the seminal paper of Jackson and Hunt [2] still is fundamental for understanding of eutectic growth and the recently discovered variants of the basic microstructures are described in many papers of the group of Faivre [35], there is only one attempt made by Himemiya and Umeda [6] to describe ternary eutectics. They extended the Jackson and Hunt model for simple geometrical arrangements of the three phases and concluded that also in ternary eutectics the Jackson–Hunt relationships for the spacing as a function of solidification velocity holds. Real ternary eutectics are much more complex as mentioned and their structure generally fails to be describable by the Himemiya approach. This paper is therefore devoted to describe the microstructures quantitatively and tries to define parameters, which could be helpful to discriminate between different structures.

2 Materials and Methods

For this study we used alloys from the AlAgCu system, whose phase diagram was assessed recently by Witusiewicz et al. [7, 8]. The AlAgCu system shows a eutectic at 774 K having three phases: an Al-rich phase, a Ag2Al rich phase and the Al2Cu phase. All three phases form in this system a nf–nf eutectic. The phase volume fractions in equilibrium are 55 % α-Al, 16 % Ag2Al and 29 % Al2Cu [9]. As shown by Genau and Ratke [9], solid-state diffusion of silver behind the eutectic front change the phase fraction to almost equal amounts of the three phases.

2.1 Solidification

The eutectic Al–Ag–Cu alloy was prepared by gradually melting and mixing of the appropriate amounts of the components Al (4N9), Ag and Cu in pure form (3N9) and casting them into a stainless steel mold with a cylindrical cavity of 12 mm diameter and 125 mm length. The chemical composition of each cast sample was determined at both the upper and the lower end using AAS. The results in Table 1 show that there is a segregation of Ag to the bottom and Al to the top. The rods used for directional solidification in the ARTEMIS facility [10] had a length of 95 mm and a diameter of 8 mm. The ARTEMIS facility consists of an upper and a lower heating element with an aerogel crucible in between. Thermal connection between the two heating elements is provided by the sample, which is first fully molten and then directionally solidified. Due to the transparency of the silica aerogel the progress of the solidification front can be observed optically. Two different solidification velocities with the same gradient were aimed for in one sample. Evaluation of the thermal radiation from the sample revealed steady velocity-gradient-pairs of 0.2 μm/s with 2.2 K/mm for the bottom part of the sample and 0.6 μm/s with 3 K/mm for the upper part.
Table 1

Chemical compositions of eutectic and of the three samples estimated by AAS

 

Ag (wt%)

Cu (wt%)

Al (wt%)

Eutectic

42.1

17.6

40.3

Sample, upper part

40.42 ± 0.14

16.05 ± 0.08

43.53

Sample, lower part

43.06 ± 0.11

17.14 ± 0.05

39.8

Sample, used for v = 0.34 μm/s

37.00 ± 0.36

14.87 ± 0.15

48.13

For each velocity one cross section was prepared by cutting, grinding and polishing. These cross sections were investigated using scanning electron microscopy (SEM) images. Image analysis was performed with the analySIS pro software [11]. In addition, in this paper two images are shown from a cross section of a sample solidified with a velocity of 0.35 μm/s and a gradient of 3.5 K/mm. Chemical composition of that sample was slightly off-eutectic and is also presented in Table 1.

2.2 Microstructural Characterization

We calculated several parameters to describe the structures. Since the three phases in AlAgCu alloy exhibit three distinct grey shades in the SEM images, one can use thresholding to determine in micrographs the area and thus the volume fraction of each phase. The shape factor F of a phase can be calculated from the perimeter U of each phase particle and the enclosed area A:
$$ F = \frac{{U^{ 2} }}{{ 4 {{\uppi}}A}} $$
The shape factor of the phases is one possible parameter, the specific interface perimeter another. The perimeter U is simply dived by the enclosed area A yielding Sv = U/A.

3 Results and Discussion

Figures 1, 2, 3, 4, 5, 6, and 7 show some microstructures observed in the directionally solidified ternary Al–Ag–Cu alloy. The dark grey areas reflect the Al phase, the grey areas are the Al2Cu phase and the bright areas are the Ag2Al phase. In some images areas of the Al phase show some mottle contrast, which reflects plate- or needle-like Ag2Al precipitations [9].
https://static-content.springer.com/image/art%3A10.1007%2Fs12666-012-0172-3/MediaObjects/12666_2012_172_Fig1_HTML.jpg
Fig. 1

Brick-like structure observed in the sample solidified with 0.2 μm/s (2.2 K/mm)

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Fig. 2

Half-lamellar structure observed in the sample solidified with 0.6 μm/s (3 K/mm)

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Fig. 3

H-structure observed in the sample solidified with 0.34 μm/s (3.5 K/mm)

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Fig. 4

Irregular structure observed in the sample solidified with 0.6 μm/s (3 K/mm)

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Fig. 5

Changed phase fractions in H-structure observed in the sample solidified with 0.34 μm/s (3.5 K/mm)

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Fig. 6

Brick-like structure with highlighted features, observed in the sample solidified with 0.2 μm/s (2.2 K/mm)

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Fig. 7

Microstructure details: a ring-like structure observed in the sample solidified with 0.2 μm/s (2.2 K/mm), b Y-junction observed in the sample solidified with 0.2 μm/s (2.2 K/mm), c development of a new lamella between two existing ones, observed in the sample solidified with 0.2 μm/s (2.2 K/mm), and d dead end of a lamella observed in the sample solidified with 0.2 μm/s (2.2 K/mm)

Figure 1 shows an aligned structure that is called ladder or brick-like structure and which is expected for slowly solidified ternary Al–Ag–Cu alloy [9, 12, 13]. The two intermetallic phases Ag2Al and Al2Cu alternate in a row and thus one could imagine them as one lamella consisting of two sub-lamellae. In between the Al phase separates the intermetallic super-lamella. Figure 2 shows a structure with a degree of misalignment compared to the brick-like structure in Fig. 1. The two-phase lamellae with the intermetallic phases are internally unaligned whereas the Al phase still develops long phase particles which are arranged in parallel and can be identified as lamellae. The microstructure in Fig. 3 shows an alignment of one of the intermetallic phase. Thus Al2Cu can be thought as threaded phase particles (the direction of such a thread is drawn in Fig. 3). The Ag2Al phase is arranged in between the Al2Cu particles in such a way that the Al phase still is in lamellae and thus Ag2Al developed different shapes sometimes mimic an H or a doubled H. Figure 4 shows in contrast to Fig. 2 all three phases in an unaligned and irregular way. The intermetallic phases Ag2Al and Al2Cu are irregularly connected. Thus a higher number of intermetallic neighbors is possible and the particles of the Al phase are smaller and curved in comparison to the more ordered structures.

The area fractions were calculated in several images of all three solidification velocities presented in this paper and are shown in Table 2 and compared to the theoretical values from the phase diagram. These values agree with the values obtained by Cooksey and Hellawell [14] as well as by Genau and Ratke [9].
Table 2

Area fractions estimated in images of the different cross sections as well as the phase volume fractions in equilibrium

 

Ag2Al (vol%)

Al2Cu (vol%)

Al (vol%)

v = 0.2 μm/s

30.8 ± 0.4

37.6 ± 1.4

32.2 ± 0.9

v = 0.34 μm/s

28.7 ± 4.8

41.2 ± 9.1

30.1 ± 4.7

v = 0.6 μm/s

27.5 ± 0.8

35.8 ± 0.7

38.3 ± 1.9

Equilibrium

16

29

55

The microstructure in Fig. 5 shows again threaded Al2Cu particles and lamellae of the Al phase. Again Ag2Al particles with an H or double-H shape are visible. However, the phase fractions in this image differ strongly from that in the other images. In the image of Fig. 5 the fractions of the intermetallic phases increased (37.2 vol% for Ag2Al and 52.9 vol% for Al2Cu) in accordance with a decreasing fraction of the Al phase (9.9 vol%).

In Fig. 6 another image of a brick-like structure is shown with several defects, which are highlighted by circles. These defects are shown in detail in the Fig. 7a–d. First, in Fig. 7a, a ring-like structure is observed where the two-phase lamellae of the intermetallic phases are arranged to a circle in 2D or a tube and a spiral in 3D. Tilt structures in binary eutectics (2D) were first observed by Kassner [15] after a sudden velocity change. Simple tilting of all three phases in ternary eutectics might be impossible, but a possible analogue to tilting could be the formation of spirals as conjectured by Choudhury [16]. Figure 7b shows the development of several two-phase lamellae out of one. There are some Ag2Al particles which have three instead of two neighboring Al2Cu particles. Every neighboring Al2Cu particle is a starting point for a separate two-phase lamella. In Fig. 7c two Al2Cu particles with three neighboring Ag2Al particles are present. The additional Ag2Al particle between the Al2Cu particles is the starting point for a further two-phase lamella. Contrary to these well connected lamellae also dead ends are often observed. One example of such a dead end is shown in detail in Fig. 7d. Additionally, in this image a further type of connection between two lamellae is visible. Two different particles of the neighboring two-phase lamellae are broadened and touch each other. The shape of such a connection could be compared to two T-s and may be called “double-T junction” in contrast to the connections in Fig. 7b, c which may be called “Y-junctions”. However, a clear distinction between such connections (or junctions) is not easily possible. Therefore an approach is suggested to consider every intermetallic particle and count the neighboring intermetallic particles. Every particle with more than two neighbors does not fit to the brick-like structure and is counted. A comparison between the two intermetallic phases was executed. In the case of the higher solidification velocity 70 % of these irregular particles were found in the Ag2Al phase whereas with the lower solidification velocity the fractions of these particles were nearly equal in both phases. For a more detailed differentiation between the connections it may be wise to consider also the number of neighbors for the neighboring particles. However, this will be a task for further investigations.

Shape factors and specific surface areas were calculated for every complete particle in three different images of both solidification velocities. Figures 8 and 9 show the probabilities obtained for both parameters of the two intermetallic phases. In all cases the probability curve is S-shaped. The shape factors of Ag2Al particles are in the case of the lower solidification velocity smaller than that at higher solidification velocity. That means in the first case 90 % of the particles possesses a shape factor between 1.4 and 3.1, whereas in the second case 90 % are between 1.3 and 5.0. In the case of the Al2Cu particles the opposite behavior was observed: 90 % are between 1.4 and 4.6 for the lower velocity, whereas the higher velocity reveals 90 % of the shape factors between 1.2 and 3.8. Shape factors determined in different images of one cross section show the same behavior concerning the probability. The factor F equals to 1 in the case of a circle and to 1.27 in the case of a square and is influenced by both length-to-width ratio and roundness of the particles. Shape factors of Ag2Al particles are in the case of the lower solidification velocity smaller than at higher solidification velocity. In the case of the Al2Cu particles the opposite behavior is observed. Low F values reflect more rounded particles as shown by many Al2Cu particles in the case of the higher solidification velocity. This can also be seen comparing SEM images shown in Figs. 1 and 2 where the Al2Cu particles in the ladder structure are more rectangular. The detected probabilities of shape factor raise the question if there is a solidification velocity where the probability of both shape factors becomes equal and how such a structure would look like.
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Fig. 8

Shape factors determined for particles of Ag2Al phase and Al2Cu phase solidified with two different velocities

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Fig. 9

Specific surface areas of particles of Ag2Al phase and Al2Cu phase solidified with two different velocities

Specific surface area is for both intermetallic phases lower in the case of the lower solidification velocity (in the mean 0.40 compared to 0.72). This is a well-known effect of the solidification velocity [6]. The specific surface area is inversely proportional to a characteristic length scale (here spacing) and thus should decrease with the square root of the solidification velocity. There is, however, also an effect by the structure developed. In every image a lower specific surface area is detected for Ag2Al which should be caused by the stronger curvature of these particles. This curvature is developed by addition of Ag2Al precipitations from the Al phase directly onto the existing Ag2Al particles as it was described by Genau and Ratke [9].

Both types of probability curves shown in this paper show differences between the different structures observed in samples solidified with different velocities. These differences get more pronounced in the case of the shape factor. The shape factor F on the specific surface area Sv are related via \( F = \frac{1}{{ 4 {{\uppi}}}} \times S_{\text{v}} \times U \). Thus one can say that the multiplication with the perimeter leads to a better discrimination between the different structure types.

4 Conclusions

Microstructures in directionally solidified Al–Ag–Cu ternary eutectics were shown. On one hand the decreasing ordering from brick-like to irregular structure and on the other hand some defects in the ordered brick-like structure were presented. For every particle in the images the shape factor as well as the specific surface area were calculated and plotted to a probability curve. Discrimination between different structure types was possible using these factors.

Acknowledgments

We wish to thank Natalie Jacobs for section preparations and measurements. The support by “Deutsche Forschungsgemeinschaft” (DFG; project No. RA 537/14-1) is gratefully acknowledged.

Copyright information

© Indian Institute of Metals 2012