Binary Phase Diagrams
A binary phase diagram consists of two variables, temperature and percentage composition. Although, phased diagrams will vary according to different pressures, binary phase diagrams are normally viewed at constant 1 atm pressure. A copper-silver system means that the diagram has copper and silver as the two components on the x-axis. The temperature is found on the vertical while weight percent (wt %) of alloy forms the horizontal axis. There are ternary diagrams which have three components (three different metals) but these are a lot more complex.
As temperature and alloy composition change, different phases of the alloy might appear. This formation of phases depends upon free energy. This is a whole different topic. Suffice to say that phase which are stable at prevalent conditions, they will form. Take iron-carbon system; at 727 degrees Celsius and 0.78 wt% carbon in iron, austenite phase forms because this is stable under these conditions. But if we take a look at the alloy at temperatures less than 727 degrees Celsius, we find austenite phase transforms to pearlite as it is more stable at the prevailing temperature. To put it simply, the phase with lower Gibbs free energy is more stable and hence more likely to form. This term Gibbs free energy depends on Enthalpy and Entropy. Enthalpy is total internal energy of a system + the product of pressure and volume. Entropy defines the rate of disorder in a system. It depends on purity of the system and temperature. Temperature and composition govern the two ‘E’ terms and by extension control the Gibbs free energy. This means at temperatures less than 727 degrees Celsius and 0.78 wt% carbon in iron-carbide system, pearlite has lower Gibbs free energy than the Austenite phase which is why austenite to pearlite transformation occurs.
Now the binary phase diagrams are all depicted at an equilibrium state meaning that cooling rates during phase transformations are extremely low i.e. the decrease in temperatures which leads to the change in the phase is very slow. This almost never happens in real situations. In reality, the cooling rates which coincide with the change in phases are never at an equilibrium temperature meaning, the map or the data gathered from a phase diagram is not accurate. In practical situations cooling rates are usually too fast to be assumed as equilibrium. Take the iron carbide phase diagram, it shows that austenite phase of iron and carbon at a eutectoid composition (0.78% carbon) transforms to pearlite at 727 degrees Celsius, but under normal conditions with faster cooling rates this transformation occurs at lower temperature.
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