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Unformatted text preview: Lab 04 Binary Alloy Phase Diagrams Objectives 1. To understand how phase diagrams are constructed from cooling curves 2. To compare the as-solidified microstructures of different alloys in a binary alloy system Overview This lab is designed to illustrate the experimental determination of binary alloy phase diagrams. Students generate cooling curves from a series of lead-antimony alloys to establish phase field boundary temperatures and compare this data to as-solidified microstructures to establish compositions. Equipment Thermal Processing 1. Pyrex glass tube, 15 125 mm, containing Pb-Sb alloy, labeled, with "lava plug" 2. Pyrex glass tubing, 4 mm diameter, 5 long, sealed on one end, to serve as thermocouple sleeve, with brass stop collar 3. Chromel+Alumel thermocouple 4. Coil furnace with power-control rheostat 5. Aluminum mold, glass slide in mold, with individual mold heater assembly 6. Safety gloves, goggles, tongs 7. Computer data acquisition equipment Metallography 8. Optical microscope with 100 and 500 objectives; Stereo microscope with 7 and 30 objectives 9. Microscope slides, plasticene mounting aid 10. Etching reagent: 16 parts glycerol; 1 part acetic acid; 1 part nitric acid E 45 Laboratory Manual Copyright 2010, Berkeley Professor Ronald Gronsky Background Phase Diagrams Phase diagrams are used to depict the equilibrium state of a multi-component system, normally as a function of tem- perature and composition. For example, solid water (ice) and liquid water are two different phases that have the same composition, but differ in atomic arrangements at their respective equilibrium temperatures. Tin has two distinct solid state phases with different atomic structures. Brine (a liquid solution of water and salt) has a different composition and structure than a mixture of solid salt and ice. The set of curves in a phase diagram delineate the temperature and composition boundaries within which specific phases will exist. For points on the curves, the two (or at the eutectic temperature, three) phases that are separated by the curve coexist in "equilibrium. Cooling Curves Pure elements have well-defined single melting temperatures, whereas alloys (materials containing more than one element) do not. If heat is applied to a pure metal, its temperature will rise as heat is absorbed until the melting point is reached. Then the heat will be absorbed at a constant temperature until melting is complete, after which the tem- perature will again begin to rise. The heat absorbed during the melting process is called the heat of fusion. Cooling the metal to the melting point will result in the release of the heat of fusion, which will halt the cooling until solidifi- cation is complete. This is apparent in the cooling curve for pure Pb shown in Fig. 1....
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- Fall '08