05 Phase Diagrams and Phase Transformations

Reading binary phase diagrams

A phase is a physically and chemically distinct region of material, such as a liquid, a solid solution, or a compound. At fixed pressure, a binary phase diagram plots temperature against composition. Each region indicates either one stable phase or a combination of phases in equilibrium.

The boundaries distinguish important conditions:

• Liquidus: Above this line, the alloy is entirely liquid. On cooling through it, solidification begins.

• Solidus: Below this line, the alloy is entirely solid. Between the solidus and liquidus, liquid and solid coexist.

• Solvus: This boundary separates a single solid solution from a region in which two solid phases coexist.

A phase diagram describes equilibrium: it indicates stable phases and their compositions, but not how quickly a transformation occurs or what microstructure processing will produce.

Tie lines and phase fractions

In a two-phase region, a horizontal tie line at the temperature of interest gives the compositions of the two coexisting phases at its endpoints. The alloy’s overall composition generally differs from either phase composition. Their relative amounts can be calculated with the lever rule.

For a two-phase mixture with overall composition $C_0$, and phase compositions $C_\alpha$ and $C_\beta$, where $C_\alpha < C_0 < C_\beta$:

For example, if the tie-line endpoints are $20$ and $80$ wt% B and the alloy is $40$ wt% B, then $f_\alpha=\frac{80-40}{80-20}=\frac{2}{3}$ and $f_\beta=\frac{1}{3}$. These are phase fractions. The compositions of the phases come from the tie-line endpoints, not from the lever-rule calculation.

Solidification and invariant reactions

As a melt cools below its liquidus, solid nuclei form and grow. In an alloy, the first solid may have a different composition from the liquid. Continued growth and diffusion redistribute the components. If cooling is slow enough for diffusion to keep pace, the material approaches equilibrium. Faster cooling can leave composition gradients, called segregation or coring, and can produce finer structures.

A common binary invariant reaction is the eutectic reaction, in which one liquid transforms into two solid phases simultaneously:

An alloy at the eutectic composition solidifies at a single temperature into a fine mixture of the two solids. An off-eutectic alloy first forms a primary solid phase; the remaining liquid then transforms into the eutectic mixture. Other invariant reactions include the peritectic, in which liquid plus solid forms a different solid, and the eutectoid, in which one solid transforms into two solids in a solid-state system.

Heat treatment and steel microstructures

Heat treatment uses controlled heating, holding, and cooling to guide phase changes. The final structure depends on the thermodynamic driving force and kinetics, especially diffusion, nucleation, and growth. Time–temperature–transformation (TTT) and continuous-cooling-transformation (CCT) diagrams help predict transformations that an equilibrium phase diagram alone cannot specify.

Steel illustrates how the thermal path affects the products. Heating steel into the austenite phase field creates austenite. On slow cooling, austenite decomposes into ferrite and cementite. Near the eutectoid composition, these phases commonly form a layered microstructure called pearlite.

Faster cooling can suppress these diffusion-controlled products. A sufficiently rapid quench transforms austenite into martensite, a hard, nonequilibrium structure formed largely without long-range diffusion. Subsequent tempering—reheating martensitic steel below the austenite range—allows limited diffusion and reduces brittleness while retaining useful strength.

Other heat treatments and processing choices

Other heat treatments also alter microstructure. Annealing generally promotes recovery and recrystallization, reducing the effects of prior deformation; holding at high temperature can also grow grains.

Solution treatment and aging dissolves alloying elements at elevated temperature, then forms fine precipitates during a later hold, often increasing strength. The best schedule depends on alloy composition, section size, and the desired balance of properties.

Together, equilibrium diagrams and transformation knowledge explain different aspects of processing: diagrams identify equilibrium phases and their compositions, while temperature, time, and cooling rate influence how far transformations proceed and whether equilibrium products or metastable phases such as martensite form.