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Unformatted text preview: B D C I H G E F 6.3 Eutectic Phase Diagrams - M icrostructure Objectives • Draw the microstructures developed on cooling for a eutectic phase diagram. The simple phase diagram shown in Figure 6.2.1 is actually pretty rare. Most phase diagrams are more complicated than this, with limited solubility of the two components in each other. Figure 6.3.1 shows the Pb-Sn phase diagram. This type of phase diagram is called a eutectic phase diagram. There are a few special things to notice about this phase diagram. First, at low temperatures you will see that the two components have limited solubility in each other. For example, at 100° C the solubility limit of tin dissolved in lead is only about 5 wt%; if any more tin is added two phases will form. This is just like our original example of sugar and water, except that instead of a liquid phase and a solid phase, for lead and tin there are two solid phases. As a shorthand, for metallic phase diagrams we name the different solid phases from left to right across the phase diagram with Greek letters. There is no special meaning to the α phase on the Pb-Sn phase diagram. It’s just easier to say “ α solid” than “the solid that has a composition ranging from pure lead to 18.3 wt% lead and 81.7 wt% tin”. Figure 6.3.1: The lead-tin phase diagram. Also notice what happens as you cool down from high temperatures at 30 wt% Sn. At this composition you start in the liquid phase, go to a two phase region of solid plus liquid, then...
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This note was uploaded on 03/30/2010 for the course EMA 3010 taught by Professor Unknown during the Spring '08 term at University of Florida.
- Spring '08