Before you read this, we recommend reading "What is a metal? The basic structure".
An alloy is a mixture of two or more pure metals. Alloying additions (deliberately added elements) can be chosen for many reasons. These include:
Hardening and strengthening: Alloying can dramatically improve strength in several ways. This is the primary reason for alloying in Jewelry.
Control physical properties: Control the melting temperature and temperature range of melting.
Reduce cost: Precious metals are expensive.
Color change: Most alloys have a silvery-white appearance, but gold alloys are a great example of how alloying can dramatically change color.
Deoxidizing agents: Help to prevent other elements from reacting with oxygen () and remove dissolved oxygen (which causes porosity during casting).
Improving fluidity: Alloying can help the liquid alloy flow better and therefore improve form filling (how easily the liquid fills the mold)
Grain Refinement: Reducing grain size can help improve strength and appearance.
Microstructure of Alloys
Pure metals form single-phase microstructures. An alloy can have a microstructure with one or more phases.
When an alloy exists as a single phase, it typically forms a "solid solution." The added element will occupy a random position in the crystal lattice. In most precious metal alloys, the added element will substitute for another atom in the crystal lattice. If the element differs in size, then the lattice will become strained.
If the element has a dissimilar size or very different chemistry or we add too much of it, then it may be energetically favorable to form a second phase. This phase may have either a different composition, crystal structure, or both.
A two-phase system can form via either:
Phase separation – where two phases form from one original phase (solid or liquid) normally during cooling.
Precipitation – Where a second phase (with a different composition) forms within another. In a solid, these precipitates forms at grain boundaries.
Precipitation of a second phase is the most common and is an extremely useful method of strengthening. Two-phase alloys are stronger but typically less ductile.
Strengthening effect of alloying
Crystalline structures deform plastically (irreversibly) by a process known as slip. Layers of atoms in the crystal can easily slide over one another to accommodate the shape change. As a result, grains re-orient and change shape, leading to an overall shape change of the material.
In single-phase alloys (where the alloying addition is a substitutional solute), the strain induced by the substitutional atom makes it more difficult for the layers of atoms to move. A larger size mismatch means a stronger strengthening effect. This is known as "solid-solution" strengthening.
In two-phase systems, the layers of atoms are different in the different phases, and so have different strengths and cannot "slide" in the same way. This is known as precipitate hardening; the "second phase" precipitates from the melt during cooling (or typically during annealing) and hardens the alloy.
Solid-solution strengthening is limited in its effect; we can only substitute so many atoms (there is also an upper limit on size mismatch) before the strain in the lattice is too great, and it is more energetically favorable for a second phase to form.
Precipitate hardening can have a much greater strengthening effect, but these alloys are far less ductile. The strengthening effect can be carefully optimized during working and annealing (processing).
Changing the color
Metals, except for a few, are typically a silvery-white color. However, the color of gold alloys is extremely sensitive to composition.
Adding Nickel or Palladium in substantial quantities turns the alloy white, adding copper gives it a reddish hue, and adding silver gives the alloy a yellowish/green hue.
For example, a wide range of colors can be changed by changing the proportions of gold, silver, and copper. Pure gold is at the top corner of the triangle, copper on the bottom right, and silver on the bottom left. Horizontal lines illustrate the gold content for 18, 14, and 9 karats on this triangle.
There are no such things as white gold mines! Notice that alloys with lower karatage have a wider range of possible colors.
Changing casting conditions
Alloying additions make the alloy less dense (most elements are lighter than gold) and can dramatically change its melting point. Most alloys melt or solidify over a temperature range, whereas pure metals melt at a single temperature.
In general, the melting (liquidus) temperature falls as the alloying additions increase. The temperature at which an alloy melts and the size of the temperature range over which the alloy melts affect the processing conditions, the microstructure of the alloy, and, therefore, its properties.
Impurities
Of course, not all elements present in these alloys are deliberately added. Many elements may be there as impurities. These tend to segregate and accumulate at grain boundaries. This results in embrittlement, as they hinder "slip" and plastic deformation; the material will just crack along the grain boundaries (inter-granular fracture).
Typical impurities include lead, silicon, sulfur, tin, selenium, and bismuth. Oxygen and hydrogen can often dissolve into the liquid alloy and then evaporate on cooling, which results in casting porosity.
Key Points
The key points are:
An alloy contains two or more elements.
An alloy can consist of one or more phases. These phases have either differing compositions, crystal structures, or both. They form part of the description of the microstructure alongside the grain structure.
Alloying helps to strengthen precious metals; pure metals are very soft and are not suitable for most jewelry.
Alloying can also be used to make casting easier, improve appearance, change color or reduce cost.