Thermal Equilibrium Diagrams

The subject of phase equilibrium was briefly discussed in Chapter 7 to introduce the reader to this very important field of study in as painless a manner as possible. The reader will have acquired a general impression of the significance of equilibrium diagrams, since no useful examination of the microstructures of alloys is possible without reference to the appropriate diagram, which, for this reason, becomes one of the metallurgists most useful tools . With its aid he can predict with precision the structure of a given alloy at any given temperature provided that the alloy s thermal-treatment history is known. In the thermodynamical sense, equilibrium refers to the state of balance which exists, or which tends to be attained, between the phases in the structure of an alloy after a chemical or physical change has taken place. In some cases equilibrium may not be reached for long periods after the change has begun. In fact, rapid cooling (or quenching ) to room temperature may suppress the chemical or physical change involved to such an extent that equilibrium will never be reached so long as the alloy remains at room temperature. Reheating (or tempering ) followed by slow cooling to room temperature will allow the physical or chemical change to proceed in some degree towards completion; the extent to which equilibrium is thus attained being dependent upon the temperature reached, the time the alloy remains at that temperature and the rate of cooling to room temperature or at whatever other temperature equilibrium is being studied.