Thermal Analysis to determine a phase diagram (click to enlarge)
Condensed Binary Systems and Thermal Analysis are described in sections 6.4 and 6.5 of the Physical Chemistry textbook.
While most of us may not realise it, phase and phase transformations are ubiquitous in everyday life. The most simplistic illustration is a saucepan filled with boiling water – here the bubbles of steam formed by the water changes from liquid to vapour phase.
During snow and icy conditions, salt is spread on the roads to lower the melting point of the water by changing its composition and causing a phase change. Bubbles rising in a glass of beer signify a change in phase, i.e. gases dissolve in the beer forming a separate phase.
Phase diagrams provide valuable information about melting, casting, crystallization, and other phenomenon. Phase equilibrium diagrams are plots of the relationship between temperature, pressure and composition.
Phase diagrams and phase transformation are used in the understanding of how microstructure evolves and their properties in relation to manufacturing and engineering processes. The details of the thermal history controls the way phase transformation takes place. The processing of most materials involves a thermal history such as the thermal history of solidification which is cooling from a high temperature process.
Using Cooling Curves to Construct Phase Diagrams
The cooling curve method is one of the oldest and simplest methods to determine phase diagrams and phase transition temperatures. This is achieved by recording temperature (T) of a material versus time as it cools from its molten state through solidification (at constant pressure).
Whenever a phase change takes place in a metal or alloy, the total energy content changes because cooling or heating is the process of evolution or absorption of heat.
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Supposed you allow a pure metal to cool down until it has all solidified (i.e. cooled under near equilibrium conditions from the liquid state), plotting its temperature as a function of time, the resulting cooling curve will show a plateau (B-C); this is also known as thermal arrest. The plateau corresponds to the beginning (at point B) and end of solidification (at point C).
Sometimes the liquid may cool to a temperature below its freezing point before crystallization occurs and this is called supercooling (this is explained in the Physical Chemistry book on page 6-12).
While the process of solidification begins, the temperature drops and remains there until solidification is complete (C to D).
Most alloys will solidify from the molten state over a range of temperatures. The cooling curve will thus have liquid-solid transition points at two different temperatures representing the beginning and end of solidification.
Pb-Sn alloy is a good example of a eutectic alloy system. Eutectic is a ‘term’ used to describe two components which are completely soluble in each other in the liquid state, but only partially soluble in the solid state.
In a eutectic system, the “eutectic alloy” composition has the lowest melting point in the system. It is lower than the melting points of either of the pure components. Figure 6.16 (below) in the Physical Chemistry book illustrates the gold-silicon system which is another example of a eutectic system.
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Many eutectic alloy systems have been found to be useful as solders. A typical old-fashioned solder is Pb-Sn comprising 40% lead and 60% tin. This ‘combination’ is close to the idealized eutectic composition having a low melting point (comprising 38% lead and 62% tin). Because this alloy system melts and freezes cleanly over a very limited temperature range, they have been found to be useful for electrical work.
In determining the phase diagrams for alloys (in which solid-to-solid transformations take place) other methods are used instead of the cooling curve method of thermal analysis. That is because solid state transformations are often sluggish and the heat change is too small to be readily detected by cooling curves.