Platinum alloys

Dr. Dmitri Kopeliovich

• Platinum
• Platinum-Iridium alloys
• Platinum-Cobalt alloys
• Platinum-Palladium alloys
• Platinum-Ruthenium alloys
• Platinum-Gold alloys
• Platinum-Tungsten alloys
• Platinum-Copper alloys
• Chemical compositions of some platinum alloys

Platinum (Pt) is a transition metal with an atomic number 78.
The atomic mass of platinum is 194.9647911(9) (the most abundant and stable ¹⁹⁵Pt isotope).
A nucleus of ¹⁹⁵Pt is composed of 78 protons and 117 neutrons.

Platinum is found in alluvial or secondary deposits mostly in chemically uncombined state (as a natural pure metal or platinum-iridium alloy).
Platinum occurs also in a combined form as PtS (platinum sulfide), PtAs₂ (platinum arsenide), PtTe₂ (platinum telluride), PtSb (platinum antimonide).
The most prominent properties of platinum:

• Chemical inertness. In contrast to most of metals platinum does not oxidize (tarnish, rust) in the atmosphere even at elevated temperatures. It does not react with most of acids and bases. Platinum may slowly react with cyanide solutions, strong alkalies, sulfur and some halogens. Platinum can also be dissolved in Aqua regia (a mixture of nitric and hydrochloric acid in the proportion 1:3). Due to its exceptional chemical stability platinum is referred to noble metals together with ruthenium, rhodium, palladium, silver, osmium, iridium and gold.

• Malleability and ductility. Platinum has an extraordinary ability to be compressed (malleable) or stretched (ductile). It is the most ductile metal.

to top

Pure platinum is rarely used in jewelry because of its low hardness (about 40 Vickers in annealed state).
Alloying platinum with iridium (Ir) has a strong hardening effect even at relatively low iridium concentrations.

• Pt950/Ir is the alloy containing 5%wt. of iridium (950 parts of platinum and 50 parts of iridium). The hardness of the alloy is 80 HV in annealed state and may reach 160 HV in cold worked state. On the other hand the alloy is very malleable and ductile what makes it easy to fabricate. Pt950/Ir alloy is used for fabrication handmade pieces and parts resistant to mechanical impacts. Because of its softness the alloy has poorer resistance to scratches.
• Pt900/Ir contains 10% of iridium. The alloy combine good hardness (110 HV in annealed state) and sufficient ductility. It is suitable for both handmade and cast pieces. Pt900/Ir alloy is the most popular due to the optimal combination of the mechanical properties and its aesthetic bright white color. Th alloy is wear and oxidation resistant.
• Pt850/Ir is even harder (160 HV) than Pt900/Ir. It is widely used in Japan for manufacturing jewelry findings.
• Pt800/Ir is the hardest Pt-Ir alloy (200 HV) used for fabrication fine meshes and chains.

to top

Alloying platinum with cobalt (Co) results in significant hardness increase due to the grain refining effect of cobalt.
Pt950/Co is the most popular Pt-Co alloy. It is the best platinum alloy for fabricating cast pieces due to its excellent Casting properties - fine and detailed products may be fabricated from the alloy. Pt950/Co has a high hardness (135 HV as annealed and up to 185 HV in cold worked state) and good scratch resistance. Unlike other platinum alloys Pt950/Co tarnishes when heated.

to top

Platinum-palladium alloys are traditionally popular in Japan and Hong Kong.
The Pt-Pd alloys are relatively soft therefore the pieces made of them are cold worked (burnished) in order to increase the surface hardness by cold.
The alloys have grey color. Rhodium Electroplating is used to improve their appearance.

• Pt950/Pd contains 5% of palladium (Pd). It is the softest Pt-Pd alloy with hardness 60 HV. It is used for fabricating fine and detailed castings.
• Pt900/Pd is harder than Pt950/Pd (80 HV). The mechanical properties and applications of the alloy are similar to those of Pt950/Ir.
• Pt850/Pd is used for fabrication of chains. Hardness of the alloy in annealed state is 90 HV.
• Pt900/Pd50/Cu50 is harder (110 HV) than the binary Pt-Pd alloys.
• Pt900/Pd70/Co30 has an increased hardness 125 HV.
• Pt850/Pd100/Co50 is the hardest Pt-Pd alloy with hardness 150 HV.

to top

Pt950/Ru is very hard alloy with the hardness 130 HV in annealed state and up to 210 HV in cold worked state. The Casting properties of the alloy are moderate therefore fine pieces can not be cast. The alloy has good wear and scratch resistances.

to top

Platinum-Gold alloys are heat treatable which means that they may be strengthened by precipitation hardening method: solution treatment at a temperature 1100-1800ºF (599-982ºC) followed by quenching and aging.
In hardened state Pt-Au alloys are brittle and not suitable for Forging.

• Pt950/Au is a general purpose alloy with the hardness 92 HB which may be increased to 143 HB by heat treatment.
• Pt900/Au contains 10% of Au. It’s hardness in anneled state is 143 HB. Precipitation hardening allows to increase the hardness to 222 HB.
• Pt900/Pd50/Au50 is more ductile than the alloy Pt900/Au. It may be easily forged.

to top

From all alloying elements tungsten (W) exerts the greatest effect on the platinum alloy hardness.
Platinum-tungsten alloys are used for fabricating springs (e.g. a tongue of a bracelet lock).
The alloys are heat treatable and may be strengthed by precipitation hardening (solution treatment-quenching-aging).
Hardness 300 HV may be achieved by the heat treatment.

The alloys available on the market:

• Pt950/W has a hardness 135 HV in annealed state.
• Pt900/W
• Pt850W
• Pt900/Pd50/W50 is a general purpose alloy having hardness 150 HV in the annealed state.

to top

Pt960/Cu alloy is a popular general purpose alloy in Germany an some other European countries.
The alloy is rarely used for casting since it has moderate Casting properties and also oxidizes in molten state.
Pt960/Cu alloy has good ductility and malleability. It’s hardness is 120 HV in the annealed state.

to top

to top

• Steels and cast irons
• Aluminum alloys
• Copper alloys
• Magnesium alloys
• Nickel alloys
• Titanium alloys
• Silver alloys
• Gold alloys

to Metals