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Engineering LibreTexts

12.9: Further Considerations

  • Page ID
    36303
  • Other methods for constructing phase diagrams

    Although the easiest way to investigate phase transformations is by the use of time temperature cooling curves, there are many ways to investigate these changes. This is most helpful when observing solid to solid phase changes, as these take longer to reach equilibrium, and so the cooling of a material must take place at a slower rate in order to get accurate results. Unfortunately, as the rate of cooling decreases, it gets harder to detect the latent heat being released, and so other methods must be sought.

    The thinking behind most of the methods is the same; as the material changes phase, there will be changes in its physical properties. As such, the phase change can be detected by observing a property as the temperature is reduced. Examples of this are density, electrical or thermal resistance, optical properties, Young's modulus and damping efficiency. Another frequently used measure is that of interplanar spacings in the crystalline phases, which can be measured using X-ray diffractometry.

    These techniques have been used to map out phase diagrams, but they are always time consuming, especially during the solid transformations.

    Dangers in interpretation of phase diagrams

    An essential point to remember in phase diagrams is that during normal or fast cooling, results may not be as expected in the diagram. Both the theory and the experiments to construct phase diagrams rely on the assumption that the system is in equilibrium, which is rarely the case, as this only occurs properly when the system is cooled very slowly. In order to reach full equilibrium, the solute in the solid phases must stay completely uniform throughout the cooling. However, in most systems, if the system is not cooled quickly, the phase diagram will give fairly accurate results. In addition, near the eutectic, the results become even closer to the phase diagram, as the liquid solidifies at nearly the same time.

    The non equilibrium conditions can sometimes be of benefit however, as microstructures at higher temperatures in a phase diagrams may sometimes be preserved to lower temperatures by fast cooling, i.e. quenching, or unstable microstructures may occur during fast cooling which can be useful when hardening an alloy.

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