Once a gold alloy has been annealed to restore its' ductility and water quenched, it can often be hardened by annealing at a low temperature (say 400˚C). A similar effect can often be achieved in Sterling Silver.
This allows a highly workable "fully-soft condition" alloy, which, once semi-finished (shaped), can then be hardened to ensure excellent scratch resistance and durability. The material can then be polished and finished.
How do these alloys get their strength?
These alloys are "precipitate hardened". The presence of a second phase within the microstructure (notably at grain boundaries) has a significant strengthening effect by inhibiting deformation.
Plastic (irreversible) deformation occurs by layers of atoms slide over one another. This is known as "slip". Slip leads to a change in the shape and orientation of grains in the microstructure and, ultimately, a change in the shape of the material.
A type of crystal defect known as dislocations is necessary for slip to occur. These types of defects are specific to each crystal. Adding a second phase with a different crystal structure makes it difficult for these dislocations to move. As a result, the material is more resistant to deformation.
What happens during the processing?
When we anneal the alloy to restore ductility, the metal has a single-phase microstructure. Copper and Silver exist in the gold crystal lattice as substitutional solutes. This provides some strengthening effect.
The alloy is then quenched (cooled quickly in water), so the microstructure is "frozen-in". At high temperatures, the atoms have the energy to rearrange themselves. By cooling quickly, they don't have sufficient time to diffuse (move) around and form second phases.
Practice: Avoiding "over-aging"?
Achieving the optimum or "peak" hardening is a function of temperature and time. As we anneal for longer, the precipitates grow larger.
When the precipitates are small, they provide little resistance to deformation. The precipitates can be easily deformed or "cut". When the precipitates are too large (coarse). The precipitates do not interact strongly with dislocations, so the strengthening effect is very weak.
There is a fixed amount of solute that can form precipitates. As a result, as the precipitates get larger, there are fewer of them. The best distribution is achieved when there is a distribution of many small precipitates – a fine microstructure.
Particular alloys
Gold Alloys
18K and 14K Gold-copper alloys can be increased by age-hardening treatments. Gold and Copper can form an "ordered" intermetallic phase. The gold and copper occupy particular positions in the lattice; gold and copper sit in alternate layers.
By quenching the alloy after annealing, we prevent this phase from forming on slow cooling. We can then re-anneal (age) the alloy at a lower temperature to carefully control the precipitation and form a finely dispersed array of precipitates. Typically, we heat at about 280-300°C for 3-4 hours.
Composition / wt.% | Color | Annealed Hardness / HV | Aged Hardness / HV |
18K Gold (12.5% Ag, 12.5% Cu) | Yellow | 150 | 230 |
18K Gold (4.5% Ag, 20.5% Cu) | Red | 165 | 325 |
14K Gold (20.5% Ag, 21% Cu) | Yellow | 190 | 270 |
14K Gold (9% Ag, 32.5% Cu) | Red | 160 | 260 |
18K: Annealed at 550°C (1022°F), aged at 360°C (680°F) for 1 hour
14K: Annealed at 650°C (1202°F), aged at 260°C (500°F) for 1 hour
Higher fineness gold alloys have insufficient copper alloying to form these ordered precipitates.
Silver Alloys
Silver alloys can also be hardened, but soldering hinders the use of this strengthening technique.
Platinum Alloys
Most 950 platinum alloys are not amenable to age-hardening treatment, but some heat-treatable alloys have been developed. These alloys contain indium or gallium, which form small second-phase precipitates. Hardness of up to 360 HV can be achieved.
Summary
The key points are as follows:
Metals deform plastically by "slip". Layers of atoms can slide over one another caused by defects called dislocations.
Second-phase precipitates can hinder the passage of dislocations, so they have a strengthening effect.
The size and distribution of precipitates significantly affect their strengthening effect.
A fine well-dispersed array of precipitates has the best strengthening effect. This can be achieved by quenching an annealed alloy and aging at a high temperature within the two-phase field.
The time and temperature of aging are important to controlling the precipitate distribution.