BINARY PHASE DIAGRAMSRelation withFree Energy-Composition curves

A SIMPLE PHASE DIAGRAMA and B are completely miscible in both liquid and solid states,Both are ideal solutions

SYSTEMS WITH A MISCIBILITY GAPFree energy curve for a system in which: The liquid phase is ideal But for solid phaseHmix > 0(i.e. A and B atoms dislike each other)Therefore, at low temperatures (T3), the free energy curve for the solid assumes a negative curvature in the middle.The solid solution is thus most stable as a mixture of two phases and with compositions e and f.

SYSTEMS WITH A MISCIBILITY GAPAt higher temperatures, when TSmix becomes larger: e and f will approach each other and eventually disappear (Fig 1.30d).The ( + ) region is known as MISCIBILITY GAP.

SYSTEMS WITH A MISCIBILITY GAPThe effect of +ve Hmix in the solid is also apparent at higher temperatures where; It gives rise to a minimum melting point mixture(simply because the A and B atoms dislike each other and, therefore, its easy to break them apart when they are in mixture form )

Simple Eutectic SystemIf the Hsmix is much larger than zero, the miscibility gap can extend into the liquid phase.Simple eutectic phase diagram results.

Simple Eutectic SystemA similar phase diagram can result when A & B have different crystal structures

Ordered AlloysThis effect (ordering) arises when, Hmix < 0 In these systems melting will be more difficult in the alloys and a maximum melting point mixture may appear.If the attraction between unlike atoms is very strong, the ordered phase may extend as far as the liquid. This type of alloy has a tendency to order at low temperatures.

PHASE DIAGRAMS WITH INTERMEDIATE PHASESWhen stable intermediate phases can form,Extra free energy curves appear in the phase diagram

PHASE DIAGRAMS WITH INTERMEDIATE PHASESWhen stable intermediate phases can form,Extra free energy curves appear in the phase diagramPeritectic transformation

PHASE DIAGRAMS WITH INTERMEDIATE PHASESAn interesting result of the common tangent construction is that:The stable composition range of the phase in the phase diagram need not include the composition with the minimum free energy, but is determined by the relative free energies of the adjacent phases, Figure 1.35.e.g. Phase in the Cu-Al system is usually denoted as CuAl2 although the composition XCu = 1/3, XAl = 2/3 is not covered by the field on the phase diagram.

PHASE DIAGRAMS WITH INTERMEDIATE PHASESFormation of stable intermediate compounds will restrict primary solid solubility.

The likelihood of the formation of such compounds in an alloy system is related to the chemical affinity of the participating elements and will be increased if one element is more electronegative and the other more electropositive.The width of the shaded area represent the extent of primary solid solubility

it becomes more restricted the greater the stability of the intermediate phase.

SPINODAL DECOMPOSITIONPhase separation with no barrier to nucleation

For compositions at Xo below a critical temperature, the alloy will be unstable because small fluctuations in composition that produce A-rich and B-rich regions will cause the total free energy to decrease.

Therefore, uphill diffusion takes place.