New Metallographic Method for Estimation of Ordering and Lattice Parameter in Ternary Eutectic Systems
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Ternary eutectics develop a rich variety of microstructures depending on the solidification conditions. This article describes a new procedure to analyze the three-phase arrangement in metallographic sections following solidification; this method provides a clear definition of both the interphase spacing and the degree of ordering. Distance and angle between the phase areas are calculated after determination of the centers of mass for each phase area. A polar plot of these data allows an easy determination of the spacing and the order. The new method is discussed with both experimental as well as simulated microstructure images. It is first tested on artificial phase arrangements, which are fully ordered, semi-ordered, or random.
KeywordsStructural characterization Alloys Microstructure Microscopy Al–Ag–Cu
In three component systems with a ternary eutectic point, three phases can solidify simultaneously from the melt in a eutectic reaction. In simple binary eutectic systems only two morphologies are possible, either lamellae or fibrous. However, ternary eutectic alloys exhibit a much richer variety of possible morphologies. For instance, the three phases can be arranged in fibers, two fibers embedded in a lamella, two lamellae embedded in a matrix, a ladder structure of two phases in a matrix, two phases parallel and one phase orthogonal to it and many others. The phases can be arranged almost regularly until completely disordered and these might have simple surface geometries like circular, ellipsoid, rectangular in a cross section, or very complex folded ones [1, 2, 3]. Thus, these systems are ideally suited for studying the fundamental science of a highly complex, multiphase solidification process.
Al–Ag–Cu is often used as model system for these kinds of studies. The phase diagram has recently been validated very carefully  and the three phases solidifying, namely the Ag2Al-, the Al2Cu θ-phase, and the α-aluminum solid solution phase, grow in a non-facetted manner (in contrast to, for instance, the eutectic in the AlSiCu system). As the thermodynamics in this system are also well known, 3D phase-field modeling can be utilized to probe stability limits and complex pattern formation, and to compare the simulated microstructures with real microstructures.
The microstructures observed in AlAgCu eutectic alloys depend on solidification velocity and composition, as to be expected from the theory of eutectics [3, 5, 6]. McCartney et al.  identified a “semi-regular brick-type structure” for the Al–Ag–Cu system. In contrast to this, several different structures, even in one cross section, have been identified by the authors in previous studies on this alloy system [3, 8]. The structures were classified as ordered and irregular, but also spirals, cobble pavement structures, and others were found.
One essential issue in ternary eutectics is how to unambiguously describe the structure and transition between the observed varieties, such that one can try to understand the physical mechanisms behind them utilizing microstructure modeling. Appropriate descriptors of microstructures include the distances between the phases, distance between like and unlike phases, the shape factor of the phase in a cross section, the orderly arrangement of phases, area fraction, grain orientation, density of triple points, and angle distribution at triple points. In attempting to describe the ternary microstructure, a description of the separation and distances between two areas of the same phase in simulated as well as in real cross sections is developed. In collaboration with this description, an analysis of the orderly arrangement is possible. The measure should include distances in different directions, largely in horizontal and in vertical direction. In this article, a new method will be presented which allows for a rapid determination of an average distance in different directions between (nearly) regular ordered particles.
Results and Discussion
Images of real ternary eutectics, as well as simulated ones, were used for evaluation of the new method shown in this article. The SEM images in Fig. 1(a–c) show structures obtained via directional solidification of Al–Ag–Cu melt of near-eutectic composition. In Fig. 1(a), the phase particles are regularly arranged and such a pattern can be called a ladder structure [3, 6, 7]. The structure consists of two lamellas. One lamella is made of Al, the other one is composed of the two intermetallic phases in a lamellar arrangement. Such an arrangement was discussed by Himemiya and Umeda . They defined a parameter ε, which is the ratio of λ2/λ1. Here, λ1 is the distance between the lamellas of the α-Al solid solution and λ2 the distance between Ag2Al and Al2Cu in the intermetallic lamellae. To simplify the discussion here, in this article, the distance between the intermetallic lamellas was used as λ1. Thus, it is possible to use one of the intermetallic phases for the estimation of both distances λ1 and λ2 (compare arrows in Fig. 2b). Himemiya and Umeda  calculated a theoretical value of 0.45. However, with images of McCarntey they obtained a value of 0.75. Genau and Ratke  found experimentally a value of 0.62. Figure 2(a–c) shows simulated structures which again include three different phases represented as white, gray, and black areas. Pattern of the phase particles were generated. These images, in addition to the images of the real microstructures, are used to test the new method presented in this study.
In the second case (Fig. 5b) of the semi-regular lattice, the distance can only be read in one direction. The third case showed a completely irregular arrangement of points. The pattern observed in this polar coordinate system has the appearance of a cloud and an exclusive distance cannot be detected. In this respect, the polar plots of distances and angles clearly showed the transition from a regular, ordered structure to an irregular unordered one.
In addition to the distance between the particles, plots in Fig. 5(d, f) show the direction or orientation of the lamellas. The direction is reflected by the dominant linear accumulation of points. Thus, an orientation of about 60° is observed in the polar plot in Fig. 5(d) generated by the image in Fig. 1(a). In contrast, for Fig. 1(c), an orientation of 90° is indicated in Fig. 5(f). The curvature of the less dominant linear accumulation of points in Fig. 5(f) is caused by the mixed misalignment between the Ag2Al particles in the different intermetallic lamellas. If there were only one type of misalignment between the lamellas, then the centers of mass would form a lattice of points comparable to that one shown in Fig. 3, but with an angle γ differing from 90°. In that more simple case, the less dominant linear accumulation would not be curved and the angle γ could be read from the difference between the angles of both accumulation lines.
In general, the development of patterns in the polar coordinate system can be explained by the positioning of every single point to the pole of the system and then plotting all points being neighbors to this point. If the surroundings of the points are equal, a smooth pattern arises. The more different the surroundings are, the more chaotic the resulting pattern will be. Thus, this evaluation method provides a quick method to distinguish between ordered and disordered structures. The determination of average distances, directions as well as angles between directions is possible by evaluation of patterns of ordered structures.
In this article, a method is shown to estimate distances in a more or less ordered arrangement of areas in a 2D image. The surroundings of every phase particle are plotted around the pole of a polar coordinate system. Due to the overlap of these points, an accumulation of lines appears in the polar plots. These lines reflect both distances between the particles as well as the orientation of the rows of the particles. Decreasing the order in the microstructure image from regular, to semi-regular, to irregular arrangements, results in a concomitant transition in the polar plot from a regular pattern to a disordered cloud of points.
This procedure allows for a simple and direct evaluation of both the average distances in a microstructure and the order in the arrangements of phase areas. It can be used for a variety of microstructures where any type of phase particles or areas are arranged, such as a dendritic structure in 2D cross sections to reveal differences between ordered and disordered arrangements and to easily estimate the primary stem separation .
The authors gratefully acknowledge the financial support by the Deutsche Forschungsgemeinschaft under Contract Numbers RA537/14-1 and NE822/14-1.
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