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An introduction to Phase Diagrams

Updated: Oct 21, 2022

A phase diagram is essential to a metallurgist's "toolkit'. They tell us in what state and where we will find each element within a material when in "equilibrium" (i.e. it's lowest energy and most stable state) for a given composition, temperature (and pressure). This can be reached by heating at a given temperature or by cooling very slowly.

A phase is defined as a portion of a system with the same structure, properties and composition, and that is different to the rest of a system.

Why are they relevant to a jeweler? A phase diagram is important because it helps to tells us:

  • Will the alloy be single phase or two phase?

  • Can the alloy be age hardened?

  • At what temperature should we anneal our metal to restore ductility?

  • Will adding more alloying additions introduce a deleterious phase?

Explaining Phase Diagrams

Single Phase Solid Solution: Gold–Silver Phase Diagram

For some compositions, the phase diagram is quite simple – the two elements form a complete single phase solid solution across all possible compositions. Gold & Silver form such a solid solution, where the silver sits as a substitutional soluite on the gold lattice or vice versa.

The gold-silver phase diagram shows complete solid solubility.

The lines on the diagram illustrate the boundary between two regions, and the regions between the lines are for a particular stable phase or combination of phases:

  • Liquidus – The temperature at which the liquid begins to solidify on cooling or when the material is entirely liquid on heating.

  • Solidus – The temperature at which the solid begins to melt on heating or when the material is entirely solid on heating.

The region between the solidus and liquids is a two-phase region where the liquid and solid co-exist. This is the melting range. Normally, a narrow melting range is desirable.

We can determine the proportion and composition of solid and liquid at a particular temperature using the phase diagram. It is not necessary to know, but you can read more here.

Two-phase solid solution: Gold–Copper Phase Diagram

Gold and copper form a series of "ordered" intermetallic phases, and the liquids and solidus have a complicated shape (a pronounced dip in the middle). Compared to Gold-Silver, the melting range is narrow.

The gold-copper phase diagram shows a solid solution but ordered precipitates form on cooling. Data below 200˚C incomplete.

On cooling, gold and copper initially form a single-phase alloy at all compositions. But at lower temperatures, around 400˚C, ordered intermetallic phases are formed. These low-temperature phases are stronger, harder, and less ductile. They are relevant to why we quench after annealing.

A binary eutectic system: Ag-Cu phase diagram

Compared to gold and silver, silver and copper have limited solubility. As we've seen elsewhere, silver atoms are very different in size from copper atoms.

The silver-copper phase diagram is an excellent example of a eutectic system

In the central region of composition "space," there is a two-phase microstructure. A silver-rich phase and a copper-rich phase will form, and the proportion of them depends on the overall composition. Both phases contain both copper and silver. These two-phase microstructures are often stronger and more difficult to work with.

The key feature of the silver-copper system is that it contains a eutectic point. At this point, the solidus and liquidus meet. Two solid phases form from a single liquid at a single temperature. Normally, only pure metals melt at a single temperature; melting generally occurs over a range in alloys,

Eutectic compositions are typically used as solders because these alloys have a far lower melting point than the two or more pure metals from which they are made.

Three-element alloys – A ternary phase diagram

When there are three elements, we must plot a ternary diagram. In this case, the phase diagram forms a 3D graph. The triangular cross-section of the prism gives us the composition at a particular temperature. These isothermal triangular sections can be plotted at every temperature and vertically stacked to form a 3D graph in the shape of a prism.

We can use Gold-Copper-Silver as an example. This alloy system forms the basis for most karat gold alloys. The ternary diagram shows the stable phases at all compositions for a particular temperature (an isothermal section).

Gold-silver-copper ternary phase diagram. Source: Santa Fe Symposium

We can re-cut the prism horizontally to find the possible microstructures at a particular temperature for all compositions, or we can cut vertically to find the possible microstructures at a particular range of compositions for all temperatures.

Intrusion of the two-phase region in the gold-silver-copper system as a function of temperature (overlays of horizontal sections of the ternary phase diagram). Source: Santa Fe Symposium

Horizontal sections show that depending on composition, 18, 14, 10, and 9K gold-silver-copper alloys can be either single-phase or two-phase.

Three possible sections would be for 10K (37.5 wt.% Au), 14K (58.5 wt.% Au), and 18K (75 wt.% Au). Again, we can see the significant region of two-phase structures at these three karats.

Source: Santa Fe Symposium


  • Phase diagrams plot the expected phases at a given temperature and composition in the "equilibrium" case.

  • Phase diagrams can be used to help design alloy compositions and processing treatments to optimize the properties of jewelry alloys.


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