phase diagrams lab - EMA 3013C Lab 2: Phase Diagrams...

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EMA 3013C Lab 2: Phase Diagrams Introduction A system is at equilibrium when it reaches a condition such that the state of the system no longer changes unless acted upon by some external phenomenon. This equilibrium condition is seldom reached. What then is the point in studying equilibrium phase diagrams? Equilibrium phase diagrams give insight to the tendency of the system. Information for systems that approach equilibrium can be approximated from the phase diagrams. For systems removed from equilibrium conditions, phase diagrams show where the system ‘wants’ to be, and how far from that condition it is. Read sections 9.1 – 9.17, and 12.7 from your text book (Callister, 7 th ed.). This should be review to you, but if it is not, read it carefully and work through the examples. The information and exercises will help you in many of your classes. At first glance, phase diagrams may appear to be theoretical derivations. You have seen how free energy curves may be used to determine phase diagrams, however this is rarely, if ever, done in practice. Nearly all phase diagrams are developed empirically, and those that are not, have to be verified through experimentation. Several techniques exist to determine phase diagrams. Thermal analysis is one of the simplest and most widely used techniques, and it is the focus of this lab, so this technique will be described last. Other techniques include direct observation, dip sampling, fracture at liquation, dilatometry, metallographic examination, x-ray diffraction, and electrical resistivity. The direct observation technique involves heating or cooling a sample, and watching for melting or solidification. This technique is suitable for classroom demonstrations, but is plagued with experimental error. The instant or occurrence of a transformation is difficult to pinpoint accurately with optical observations, which in turn will result in erred transformation temperatures. Dip sampling is one of the oldest techniques. Liquidus and solidus points are determined by holding a sample at a fixed temperature in the liquid plus solid range until equilibrium is established. Then the phases are separated by canting and/or filtering, and compositions are analyzed. This technique is difficult in many systems due to elevated temperatures and dendritic formation. Fracture at liquation is useful in determining solidus curves and eutectic points.
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This note was uploaded on 11/16/2011 for the course EMA 3013C taught by Professor Bourne during the Spring '10 term at University of Florida.

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phase diagrams lab - EMA 3013C Lab 2: Phase Diagrams...

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