It's cracked! How on earth did that happen?

Even in the best workshops, a piece can crack. While a defect or crack is obvious, the cause is often not. The cause of the crack could be due to the behavior of the specific alloy or the history of its processing. Many different phenomena can lead to similar-looking cracks.

Consequences of cracking

For metallurgists, a crack is a puzzle to solve, but for a jeweler, it has many undesirable consequences:

• Item is unusable – it must be scrapped

• Costly repair or replacement

• Loss of time and perhaps reputation

• The concern of repeated occurrence if left unsolved

The issue of defect and cracking is not unique to jewelry; it is a problem that pervades all metallurgical manufacturing, from submarines to medical implants.

Cracking in an 18K Gold Sample. Source: Santa Fe Symposium.

Types of cracks

There are two main types of cracking:

• Intergranular fracture

  • The cracks grow along grain boundaries, so the fracture surfaces are rough.

  • This is a ductile metal fracture, and the fracture surfaces are dull.

• Transgranular fracture

  • The cracks cut in straight lines across and through grains. The surfaces are quite smooth.

  • This is often associated with the fracture of very brittle metals, and the fracture surfaces are shiny.

Cracking of jewelry materials can occur during and after manufacture. The problem is that cracking in jewelry materials is usually intergranular in nature (cracks grow around the grains along the boundaries). Still, the cause can vary widely, making tracing the cause difficult.

Why do jewelry materials crack?

The answer is simply that the imposed stresses exceed the material's mechanical strength, or the material is strained so quickly that the material can't deform fast enough.

When a force is applied around a defect, the bonds adjoining the defect must accommodate the stress of the absent bonds and therefore experience a larger force.

Cracks often occur and begin at defects. These reduce the stress value at which failure occurs. A defect disrupts the crystal lattice structure, so there are fewer atomic bonds to bear the load at a given point in the lattice – the stress is concentrated. Cracking will occur when the stress concentration is high enough to break the bonds.

Inclusions at the surface can act as crack initiation sites. Source: Santa Fe Symposium.

Hard particles such as inclusions or second phases at the surface (Figure 4) or within the alloy can act as stress raisers, i.e., they amplify the local stresses in the adjacent metal, which means that cracks can initiate and grow more easily around them, even when the imposed stresses are lower than those normally leading to crack formation.

Causes of cracking

Cracks can be due to several causes, including:

1. Mechanical overworking

2. Embrittlement by impurities, including gases

3. Casting and working defects and inclusions

4. Stress corrosion cracking

5. Quench cracking in castings

6. Fire cracking

The various causes of defects, many of which manifest themselves as cracking during working and manufacturing operations, can be attributed to the following factors:

• Poor quality start materials

  • Recycled scrap often causes contamination and possible embrittlement.

• Poor melting practice

  • Casting defects such as pipes and/or gas porosity and blisters, incorporation of inclusions, excessive shrinkage porosity, and chemical segregation

• Poor ingot or material working practice

  • Incorrect working procedure for the characteristics of the specific alloy

  • It can also lead to surface defects, such as laps, that develop into cracks.

• Incorrect annealing practice

• Residual (internal) stress is possibly linked to a corrosive environment.

  • This can be generated mechanically or thermally and lead to phenomena such as stress corrosion cracking, quench cracking and fire cracking.