Methods to Determine Phase Diagrams

Exercise 5C. #4        

There are two methods to determine phase diagrams. These are the static and dynamic method. Both methods explore, understand, and investigate systems. Static or quenching method is necessary for the investigation of systems of which phase transition is very slow thus needs long-term homogenization. On the other hand, the dynamic method is inappropriate for systems having slow phase transitions.

        Static method has various approach in exploring, understanding and investigating samples. These analysis are the following:

1.        Metallographic analysis is one of the keys to determine phase diagrams. Its microstructure examinations studies the number of phases and invariant reaction types. Examination of characteristics such as shape, composition, size, distribution, color, orientation and hardness is possible. The diagram below briefly explains how metallographic analysis is done.

[pic 1]

The surface of specimen is cleaned and polished. Small samples need matrix such as polymer resin and other mounting materials. This would then be polished with emery paste of Al2O3 , Cr2O3 , Fe2O3, MgO, or carborundum. Last is the microstructure etching to identify the different phases involved.

2.        Optical Microscopy is a very old method in determining phases. This method of analysis uses visible light and a system of lenses to magnify a very small sample. This sort of examination can reveal volume fraction, microstructure type, homogeneity and surface contamination. Limitation of this method are (1) the restricted magnification of only ≤2000X and (2) absence of crystal structure information of phases.

3.        Scanning electron microscopy is a method of producing images of the sample by scanning it with a beam of electrons. Preparing the sample’s size depends upon the size of the specimen chamber. It can be solid or bulk as long as it will fit the chamber.

4.        Energy dispersive X-ray analysis is used for elemental analysis and chemical characterization of the sample. The x-rays emitted after the excitation of the sample obtains a quantitative and qualitative information about the sample. This type of analysis provides information about composition of individual phases, distribution of alloying elements and micro-inhomogeneity.

        Dynamic method also has types of analysis, namely:

1.        High temperature metallurgy which studies physical and chemical behaviorial properties of metallic compounds, intermetallic compounds and mixtures.

2.        DTA involves enthalpy change (absorption and release of heat), thus, it is important to monitor thermal properties of sample being studied. A test and reference sample are heated and cooled under the same conditions.

3.        Dilatometry is useful for phase transition of alloys. This technique uses change in volume relative to the transition. It also measures the shrinkage and expansion of materials as it is heated or cooled at a fixed rate.

4.        Differential scanning calorimetry (DSC) is a technique for determining the energy absorbed and released by a sample as a function of time or temperature. DSC provides a calorimetric measurement of transition energy and temperature. DSC can determine liquidus line, solidus line, other phase transition points and also to measure thermodynamic parameters such as enthalpy, entropy and specific heat. One disadvantage of DSC is having a lower temperature range of 175-1100°C than that of DTA.

...