Embrittlement by Impurities
Certain impurities and alloying metals can cause embrittlement in all precious metals at low concentrations. This means the material has less inherent ductility and will fail before expected under moderate stress at room or hot working temperatures.
Precious Metal | Impurity Elements |
Silver (Ag) | Phosphorus, Lead, Sulfur, Selenium, Silicon, Tellurium |
Gold (Au) | Phosphorus, Lead, Sulfur, Silicon, Bismuth |
Platinum (Pt) | Carbon, Phosphorus, Lead, Sulfur, Silicon |
Palladium (Pd) | Carbon, Hydrogen, Lead, Sulfur, Silicon |
Many of these impurities are elements with low melting points. If present in the metal, they tend to lie preferentially on the grain boundaries. At grain boundaries, they form second phases or eutectics with low melting points. They often have extremely low solubility in the alloy itself.
Mechanism of cracking due to embrittlement
Most low-melting-point contaminants, such as silicon and lead, can cause embrittlement of gold, silver, platinum, and palladium alloys at very low concentrations. The presence of these particles impedes deformation and act as pinning sites.
The effect is magnified if the grain size of the alloy is large; there are many particles dispersed in very thin films around the grain boundaries. The effect is less pronounced in fine-grained alloys; they have the same number of particles but a much larger grain boundary area.
Often, these contaminants manifest themselves as cracking during metal-working operations.
Causes of impurities
Embrittling elements can occur in alloys from a number of sources:
Impure starting materials
Starting materials should be pure metals or pre-alloys (master alloys)
Purity should be a minimum of 99.9% to avoid embrittlement by certain rare-earth elements.
Gas content (especially oxygen for silver and hydrogen for palladium) should be low.
Check specific alloys: E.g., Tarnish-resistant Bright Silver should have selenium and tellurium levels below two ppm to avoid embrittlement.
Impure scrap
The most common issue – it is a continual source of contamination.
Common causes: refractory metals such as investment casting particles, oxides from dirty surfaces, lead-tin solder (and other solders) from repaired jewelry, sulfur contamination, and gas porosity.
The use of scrap to make new products should be strictly controlled and, preferably, should be subject to melting and analysis before use in making up new alloy ingots or recycled in investment casting.
Pick-up during processing from the atmosphere or contact with other surfaces
Carbon can be picked up during melting in graphite crucibles, which is a problem for palladium and platinum alloys and palladium-containing white golds in particular.
Silicon crucibles can also contaminate platinum and palladium melts if melted under reducing or neutral conditions.
Overalloying
Too frequently, melters tend to add “a bit extra” when alloying.
Very minor additions to alloys, such as grain refiners and fluidity promoters (e.g., silicon), can be deleterious if too much is added.
Customer use
Contact with liquid mercury leads to the whitening and embrittlement of karat golds and other precious metal alloys. Mercury forms amalgams with precious metals very quickly.
Understanding "Hot shortness"
Most of these elements that cause impurity form intermetallics and second phases, which often have low melting points because they form deep eutectics.
When these second phases have a far lower melting point than the bulk alloy, it means they melt first. The melting of grain boundaries before the rest of the material is known as incipient melting.
Hot shortness is where incipient melting occurs during hot working due to impurities. It is irreversible, resulting in material scrapping. Phosphorous is often a cause of incipient melting in sterling silver, and sulfur often does the same in platinum.
Porosity in Casting
Shrinkage Porosity
Ingot Casting
When a cast ingot solidifies, crystallization of the liquid results in shrinkage since atoms in most solids pack more densely than their respective liquid (ice being a notable exception).
In casting an ingot, the result of shrinkage is the formation of a central pipe at the top of the ingot. It is necessary to cut off this pipe before working the ingot. Otherwise, a central defect will be introduced, which will elongate on working and is likely to result in subsequent longitudinal cracking.
Pipe shrinkage can be minimized by not excessively overheating the alloy above the liquidus temperature. The temperature should not exceed 95˚C so that the shrinkage due to the sample cooling isn't too great. The shrinkage due to crystallization is fundamental and cannot be changed for a specific alloy.
High casting should generally be avoided since they encourage large grain sizes. This decreases the ductility of the alloy and, at the same time, it magnifies the effect of any low-melting-point impurities that might be present.
Investment Casting
In investment casting, the volume of the mold is filled, and so shrinkage often manifests itself as shrinkage porosity. This can be avoided by ensuring the feed of liquid metal to the casting is prevented by premature solidification in thinner sections or in the feed sprue.
The positioning and size of the sprues (into which the liquid is poured) are important to minimize their occurrence. Generally, sprues should be placed on thicker sections (whose centers are last to solidify and need the most material), and multi-feed sprues may be needed for complex-shaped castings.
Gas Porosity
Gases such as oxygen and hydrogen can readily dissolve in molten alloys. Upon cooling, the solubility of the gas decreases, and so it is ejected as gas bubbles. This can lead to gas porosity in the casting.
Gas porosity is often due to sulfur dioxide, oxygen, or other gases. Compared to shrinkage porosity, the pores are usually fine and round, whereas shrinkage pores resemble a dendritic shape.
The gas itself may arise from:
Dissolved gas in the start material or moisture
Gas dissolved during melting – Too high a temperature, non-protective atmospheres, a flux, or gas-based melting.
Sulfur dioxide usually originates from the chemical breakdown of gypsum-based material used to make the investment mold.
If the gas is present in the molten alloy before casting, such gas porosity will manifest itself throughout the casting, whereas if it occurs as a result of a chemical reaction with the investment during casting, it will manifest itself as a layer of porosity at the surface.
To minimize porosity, it is best practice to ensure clean burnout of the mold, use cleaned scrap in the melt charge, and minimize the casting and/or flask temperatures.
Gas porosity affects the tensile strength and ductility of castings. Porosity can also appear later in fabrication operations as surface blisters, defects, cracks, or internal porosity.
Porosity and blisters
Silver is especially prone to surface defects arising from porosity due to poor de-oxidation. Initial working may flatten the pores and cause small laminations and cracks on the surface. It can also close the porosity only for annealing operations to allow the gas to expand and reappear as pores and blisters on the surface.
Annealing silver containing dissolved oxygen or copper oxide inclusions in hydrogen-containing atmospheres can result in hydrogen absorption and the formation of pores due to the “steam” reaction between hydrogen and oxygen. It can also occur in karat gold:
$\rm 2H_2+O_2\to {H_2O}_{[gas]}$
$\rm H_2+Cu_2O\to2Cu+ {H_2O}_{[gas]}$
Inclusions and other Defects
Inclusions
Inclusions are undesirable foreign particles that act as stress raisers and give rise to cracks or failure during subsequent working.
They can be incorporated into the melt as oxides, refractory or even metallic particles from the casting equipment, by erosion of the crucible, furnace lining, stirring rods, or a reaction between a non-inert atmosphere and base metal alloying additions or grainers such as copper or zinc, or indium respectively.
Surface defects
Surface defects can be the starting point of cracks. They often arise due to poor melting and casting. A splash during casting that solidifies and sticks to the mold wall, surface inclusion, surface oxidation, or mechanical damage can all be the cause.
To avoid this, all ingot surfaces should be inspected and defects removed and cleaned away before working. If necessary, the surface might have to be milled to ensure it is clean and flat.