It is well known that certain metals are susceptible to a phenomenon known as liquid-metal embrittlement (LME), a.k.a. liquid-metal-induced embrittlement or liquid-metal cracking (LMC), when exposed to other metals in the liquid (or solid) state. The embrittlement of aluminum when it comes in contact with liquid mercury is a classic example, which is why any mercury-filled device is prohibited on an aircraft (due to fears over loss of structural integrity of the aircraft in flight).

 

Other examples include:

  • Carbon steels and stainless steels, which are susceptible to LME by zinc and lithium
  • Copper and copper alloys, which are susceptible to liquid-metal cracking by mercury and lithium
  • Aluminum and aluminum alloys, which are susceptible to LME from mercury and zinc

 

While the exact mechanisms of embrittlement are complicated (addressed in part 2), the penetration by the embrittling agent is normally intergranular and the requirements for embrittlement tend to vary depending on the materials involved.

 

LME effects can be observed even in the solid state when one of the metals is brought close to its melting point. This type of phenomenon is also known as solid-metal-induced embrittlement.

 

The minimal conditions required for LME of steels are:

 

  1. The alloy must be in a state of tension (applied or internal).
  2. The surface must be clean and free of oxides (which act as a barrier).
  3. The embrittling species (liquid) must intimately wet the metal surface.

 

The embrittling effect is most severe when high hardness exists (above 40 HRC). How fast a material will fail due to LME depends on many factors. Under certain conditions, fracture can take place in seconds (Fig. 2). Crack growth and propagation rates in the range of 0.82-3.3 feet/second (0.25-1.0 meter/second) have been measured. An incubation period and a slow pre-critical crack-propagation stage generally precede final fracture.

 

It is not uncommon for certain steels to experience ductility losses and cracking during manufacturing processes such as hot-dip galvanizing or during subsequent fabrication.

 

 

References

 

  1. de Rosset, William S., “Use of Liquid Metal Embrittlement (LME) for Controlled Fracture,” Army Research Laboratory, ARL-TR-4976, September 2009.
  2. C. J. McMahon, Jr., “Brittle Fracture of Grain Boundaries,” Interface Science 12, 141-146, 2004.
  3. Bogner, B., G. Rorvik, and L. Marken, “Bolt Failures – Case Histories from the Norwegian Petroleum Industry,” Microscopy and Microanalysis, Volume 11 Supplement S02, August 2005.
  4. Wikipedia (www.wikipedia.com)
  5. Kolman, D. G., “Environmentally Induced Cracking,” Liquid-Metal Embrittlement,” ASM Handbook, Volume 13A, Corrosion: Fundamentals, Testing and Protection, ASM International, 2003, pp. 381-392.