Pure metals are very soft. They can only be strengthened by grain refinement and work hardening, which provide a limited hardening effect. This is due to the intrinsic properties of Gold (for metallurgists, it has a high stacking fault energy). For this reason, alloys have been used to increase strength and hardness.
In Asia, 22 and 24-karat gold alloys dominate the market. These account for 40% of total gold jewelry fabrication, and their softness is a major weakness. Microalloying helps to solve this issue.
What is microalloying?
Micro-alloying is the process of alloying with tiny amounts of additions to improve properties dramatically. There are a number of 24K gold and platinum alloys on the market that show high hardness and strength, despite minute additions.
Alloy Description | Fineness | Alloying Additions | Hardness / HV |
High-strength Pure Gold (Mitsubishi) | 999 | Ca, Gd | 55 (annealed), 123 (cold-worked) |
Hard 24K (Mintek) | 995 | Co, Sb | 32 (annealed), 100 (cold-worked), 130 (aged) |
Pure Gold | 997/998.5 | Gd, Ca | 63 (annealed), 106 (cold-worked), 145 (aged) |
Pure Silver | 998.6 | Gd, Ca | 156 (cold-worked) |
Pure Platinum | 997 | Gd, Ca | 242 (as-cast) |
Fine Gold | 999 | – | 20–30 (annealed), 50 (cold-worked) |
Fine Silver | 999 | – | 22 (annealed) |
Fine Platinum | 999 | – | 30 (annealed) |
Fine Palladium | 999 | – | 40 (annealed) |
We typically micro-alloy gold with light metals such as calcium or rare-earth metals at levels below 0.5 wt.%. Micro-alloys in silver and palladium are also possible.
How does micro-alloying work?
The alloying elements are only slightly soluble in the main metal. This means a very small fraction of alloying addition introduces a large fraction of the second phase. The large fraction of the second phase can be achieved in one of two ways:
Using light metals
Light metals such as lithium, potassium, calcium, and magnesium are very light (small atoms) and so have a low density.
A low-weight fraction of these elements still introduces a large number of atoms. For example, there is seven times the number of lithium atoms to gold atoms for the same mass.
Some of these elements have minimal solubility in gold and easily form intermetallic phases.
Hence, a large number of precipitates can be added with a small amount of alloying.
Gold-Calcium and possibly Gold-Potassium are the most viable, as their intermetallics have a high gold weight fraction.
We need this to have a high volume fraction of precipitates but remain in the "micro" section of alloying.
Using rare-earth metals
Rare-earth metals have limited solubility in gold. They form eutectics and intermetallic compounds. Examples include cerium, lanthanum, and dysprosium.
These 'heavy' rare-earth elements have limited solubility that falls with temperature.
At the eutectic temperature, the solubility is around 0.5%
At room temperature, there is minimal solid solubility.
Such alloys could be age-hardenable by precipitating a fine dispersion of the inter-metallic.
There is little effect on the color of these alloys, as the alloying additions are so small.
Uses of Micro-alloyed Gold
Micro-alloyed golds can be cast, but hardness is optimized when worked (wrought condition) and age-hardened, where appropriate. The high strength makes them useful for lobster claws, difficult chain designs, etc., which would be difficult to perform with conventional 24K gold. Because of its hardness, 24K gold is easier to polish.
These alloys do also have some limitations. To prevent microalloying elements' oxidation, melting must be done in an inert atmosphere. These materials cannot also be simply remelted and recycled without losing strength. The precipitates lose their strengthening effect due to oxidation.
24K gold cannot be easily soldered as solders themselves are not 24K Gold. Some 22K gold solders exist, and in some countries, can be used and remain within hallmarking rules.
Microalloying of Silver and Platinum
Micro-alloyed Silver
Most of the same principles of micro-alloying Gold also apply to Silver. Many contain Gadolinium and are heat treatable.
Micro-alloyed Platinum
Platinum-based micro alloys also exist. In addition to Gadolinium and Calcium, very small additions of cerium or samarium help improve hardness, luster, and tarnish resistance. The platinum content is a minimum of 99% since the Pt-Ce phase diagram shows the same limited solubility as the gold alloys.
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