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Unformatted text preview: 1 Chapter P. Phase diagrams. After studying this chapter, you will be able to P1. Sketch the phase diagram of a solid solution and analyze this diagram: determine the composition and amount of each phase for a given overall composition and temperature. P2. Sketch the phase diagram of a simple eutectic alloy and define the eutectic point, liquidus, solidus and solvus lines. Analyze this diagram in terms of number, composition and amount of the phases. P3. Sketch a simple phase diagram with an intermediate compound and analyze the diagram in terms of number, composition and amount of phases. P4. Analyze the phase diagram of the iron-carbon system that underlies steel. In Chapter 5, we have seen that metals are strengthened by introducing foreign atoms, either randomly dispersed in the host metal, or in the form of precipitates. In Chapter 4, we have found that a high electric resistance is obtained by the introduction of impurities. The deliberate introduction of foreign atoms is called alloying. In order to process the alloys or utilize alloys effectively, it is important to know what one obtains when one melts two metals together and lets them solidify to form an alloy. This is the topic of this chapter. For simplicity, we will restrict ourselves to binary alloys, which are composed of two metals. When two metals are molten together and solidified, the alloy can take three principal forms: a solid solution, a eutectic or a compound. This is illustrated in Figure P.1. In a solid solution , the atoms of both elements A and B occupy random positions in the crystal. The classic example is the copper nickel alloy. The solid solution is obtained because the strength of the Cu Ni bond is intermediate between those of the Cu Cu and the Ni Ni bonds. A eutectic is formed when the A B bond is weaker than the A A and the B B bond and each component has a low solubility in the other. (It is the solid equivalent of a water-oil mixture). In this case, the liquid solution solidifies by separating into two distinct phases, one rich in element A and one rich in element B. This will be illustrated by the lead tin or the bismuth tin alloys, which are the materials used as solder. Materials that form eutectics are commonly used for precipitation strengthening. A compound is obtained when the A B bond is stronger than the A - A and the B B bonds. This will be illustrated by the semiconductor Gallium Arsenide. 2 Figure P.1. Schematic illustration of differing configurations of A and B atoms in binary alloys. (C ). Solid solution, (D) Segregation in a eutectic. (E) Compound formation. The behavior of such alloys is codified in the phase diagrams. A phase diagram is a drawing that shows what phases an alloy will form at all temperatures and compositions in thermodynamic equilibrium, which is the state that nature aspires to and is obtained if sufficient time is allowed. When the alloy is cooled rapidly, the equilibrium may not be obtained and another form of alloy will be formed. equilibrium may not be obtained and another form of alloy will be formed....
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This note was uploaded on 10/03/2009 for the course E e 344l taught by Professor Libera during the Spring '09 term at Stevens.
- Spring '09