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Binary phase diagrams "map out" the relationship between composition and phase occurrence as a function of temperature under equilibrium conditions that involve very slow heating and cooling rates. While metals production involves faster heating and cooling rates, non-equilibrium conditions, the phase diagram remains the starting point for our understanding of alloy systems as it teaches us about melting and solidification temperatures, the solidification sequence, equilibrium phases that can form, solubility limits for alloy or impurity additions and dissolution temperatures for second phases.The Al-Si phase diagram, a diagram of considerable commercial significance, is the starting point for developing more complex alloys and for understanding their behavior. Si is a common impurity in aluminum alloys, but it is also a deliberate alloying addition in both cast and wrought Al alloys. Cast Al alloys can contain Si in amounts from about 5 to 22 weight percent. At this level, Si improves the fluidity and castability of aluminum. The 3xx.x and the 4xx.x cast alloys (US designations of the Aluminum Association) are very popular commercially. Hypereutectic (alloys with greater than 12.6% Si, the eutectic composition) contain primary Si particles that provide improved wear resistance. Near-eutectic Al-Si alloys are popular casting alloys and can be cast using a variety of techniques: sand casting, die casting and permanent mold casting are most common.The binary Al-Si phase diagram was first studied by Fraenkel of Germany in 1908. It is a relatively simple binary diagram where there is very little solubility at room temperature for Si in Al and for Al in Si. Thus, the terminal solid solutions are nearly pure Al and Si under equilibrium conditions. The currently accepted diagram, Figure 1, is based on the study by Murray and McAlister [1] in 1984. The melting points of Al and Si are, respectively, 660.45 and 1414 C, while the eutectic reaction occurs at 12.6 wt. % Si and 577 1 C. The maximum solubility of Si in Al occurs at the eutectic temperature and is 1.65 wt. %. At least up to the late 1950's, the eutectic was believed to be at 11.6 wt. % Si. Modification of the eutectic by very small Na additions has been known since the 1920s. Modifiers like Na shift the eutectic to higher silicon contents, around 14 wt. %, preventing precipitation of hypereutectic Si while refining the structure of the eutectic. This increases both strength and ductility substantially.The talk will present the microstructure of Al-Si alloys up to 50% Si, as well as illustrate the effect of various minor additions that modify the eutectic. Examples of the microstructure of some of these alloys are given in Figures 1 to 6.

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Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

The use of color in metallography has a long history with color micrographs published over the past eighty-some years. Examples of natural color in metals are rare. Gold and copper exhibit yellow color under bright field illumination. Color can be produced using optical methods, as in dark field illumination, polarized light and differential interference contrast illumination. The microstructure of metals with non-cubic crystal structures can be examined without etching using polarized light but color is not always observed. The specimen must be prepared completely free of residual damage for color to be observed, and even then, some non-cubic metals still exhibit little color. However, many metals and alloys can be etched with reagents that deposit an interference film on the surface that creates color in bright field illumination. If it is difficult to grow such a film to the point where the color response is excellent, the color can be enhanced by examination with polarized light, perhaps aided with a sensitive tint filter (also called a lambda plate or first-order red filter). There are a number of electrolytic etching reagents that can be used to produce color. Second-phase constituents can be colored and viewed with bright field. Anodizing aluminum specimens with Barker's reagent, or similar solutions, does not produce an interference film, as color is not observed in bright field. This procedure produces fine etch pitting on the surface. The grain structure can be seen in black and white in polarized light, and in color if a sensitive tint plate is added. Heat tinting is a more universal procedure for growing an interference film on the surface of a metallic specimen. Also, the Pepperhoff interference film method, where a dielectric film with a high refractive index is produced by vapor deposition onto the specimen surface to a thickness that produces interference effects, is universally applicable. This technique makes features with little differences in reflectivity, or very low reflectivity, visible in color.There are some standard chemical etchants that produce gray-scale images in bright field, but when viewed with polarized light and sensitive tint, produce color images. There are two main examples of this behavior. First, if an etchant produces grain contrast effects, rather than a "flat etch" (only phase boundaries, grain boundaries and/or twin boundaries are visible), a color image will be observed when polarized light and sensitive tint is used. Second, any fine lamellar structure, such as exhibited by all eutectoid transformation products, will exhibit color when viewed with polarized light and sensitive tint. But, the main way to yield color micrographs is the use of "tint" etchants, also called "stain" etchants.Color is more useful than black and white because the human eye is sensitive to only about 40 shades of gray from white to black, but a myriad of colors with variations in hue, saturation and intensity. Metals with non-cubic crystal structures are often difficult to etch and obtain a co...