Phase diagrams are graphical representations of what phases are present in an alloy (or ceramic) system at various temperatures, pressures, and compositions. Phase diagrams are constructed using the information gathered from cooling curves. Cooling curves are time
temperature plots generated for various alloy compositions and provide information about
phase transition temperatures. In this chapter the emphasis has been placed on temperature-composition binary equilibrium phase diagrams. These diagrams tell us which phases are
present at different compositions and temperatures for slow cooling or heating conditions
that approach equilibrium. In two-phase regions of these diagrams the chemical compositions
of each of the two phases is indicated by the intersection of the isotherm with the phase boundaries. The weight fraction of each phase in a two-phase region can be determined
by using the lever rule along an isotherm (tie line at a particular temperature).
In binary equilibrium isomorphous phase diagrams the two components are completely soluble in each other in the solid state, and so there is only one solid phase. In binary equilibrium alloy (ceramic) phase diagrams, invariant reactions involving three phases in equilibrium often occur. The most common of these reactions are:
Eutectic reaction: L→ α + β
Eutectoid reaction: α→ β + γ
Peritectic reaction: α + L→ β
Peritectoid reaction: α + β→ γ
Monotectic reaction: L1→ α + L2
In many binary equilibrium phase diagrams, intermediate phase(s) and/or compounds are
present. The intermediate phases have a range of compositions, whereas the intermediate
compounds have only one composition. If the components are both metal, the intermediate
compound is called an intermetallic.
During the rapid solidification of many alloys, compositional gradients are created and cored structures are produced. A cored structure can be eliminated by homogenizing the cast alloy for long times at high temperatures just below the melting temperature of the
lowest melting phase in the alloy. If the cast alloy is overheated slightly so that melting
occurs at the grain boundaries, a liquated structure is produced. This type of structure is undesirable since the alloy loses strength and may break up during subsequent working.
