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Unformatted text preview: Chapter 10: Phase Diagrams Show figures 10-1 and 10-2 and discuss the difference between a component and a phase. A component is a distinct chemical entity, such as Cu, Ni, NiO or MgO. A phase is a chemically and structurally homogeneous portion of the microstructure. A phase often contains more than one component. Figure 10-1 illustrates a two-component (sugar, water), two-phase (sugar solution, solid sugar) system. Figures 10-2 illustrates a system with two-components (Cu, Ni) and two phases (liquid, solid solution). In some cases, the two different phases could have the same crystal structure, but the two materials are immiscible because of differing atomic radii, electronegativity, valence, etc. Clearly, solid, liquid and gas also represent different phases. Phase diagrams: Phase diagrams map the number and types of phases of phases that are present, the composition of each phase, and the microstructures that exist. For practical reasons, this course will only cover binary (two-component) phase diagrams, although of course many materials of interest contain more than two components. Type 316 stainless steel has the following composition (weight %): C (0.10), Mo (2.5), Ni (11), Cr (18), Fe (68.4). The Cu-Ni and binary phase diagram (Figure 10.2) is the simplest type of binary phase diagrams for two metals. Figure 10.2 shows that Cu and Ni are mutually soluble at room temperature throughout the entire range of compositions. Remember from Chapter 5 that we discussed the Hume-Rothery rules for deciding when two metals will be soluble, and we concluded that Cu and Ni form a perfect solid solution. The detailed comparison was: R Cu = 0.128 nm, R Ni = 0.125 nm, 2.4% difference. Electronegativities of Cu and Ni are 1.9 and 1.8, respectively. Both have a FCC crystal structure. Valences are +1, +2. Our conclusion (Chapter 5) that Cu and Ni form a complete solid solution is represented graphically by Figure 10.2. When two metals do not form a complete solid solution, the phase diagram will become more complex. The Al 2 O 3- Cr 2 O 3 phase diagram (Figure 10.21) is the simplest type of binary phase diagram for two ceramic materials. Figure 10.21 is exactly analogous to Figure 10.2. While we did not explicitly discuss Hume-Rothery-type rules for predicting ceramic solubility, similar guidelines do exist. We expect a new criterion, the same stoichiometric ratio of anions to cations, to replace the criterion of having similar valence. From the inside cover of the text, the ionic radii of Al and Cr 0.053 and 0.063 nm. The borderline between the liquid phase and two-phase (liquid + solid) region is termed the liquidus line, while the borderline between the solid phase and two-phase (liquid + solid) region is termed the solidus line. In the two-phase region, given the temperature, we can determine the composition of each of the phases by drawing a tie line, as illustrated in figure 10.2b. Example Problem: At point B (35 wt% Ni, 1250ºC) on figure 10.2b, what are the mass fraction liquid and the mass At point B (35 wt% Ni, 1250ºC) on figure 10....
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- Spring '08