Liquid metal embrittlement

Liquid metal embrittlement, also known as liquid metal induced embrittlement, is a phenomenon of practical importance, where certain ductile metals experience drastic loss in tensile ductility or undergo brittle fracture when exposed to specific liquid metals. Generally, a tensile stress, either externally applied or internally present, is needed to induce embrittlement. Exceptions to this rule have been observed, as in the case of aluminium in the presence of liquid gallium. This phenomenon has been studied since the beginning of the 20th century. Many of its phenomenological characteristics are known and several mechanisms have been proposed to explain it. The practical significance of liquid metal embrittlement is revealed by the observation that several steels experience ductility losses and cracking during hot-dip galvanizing or during subsequent fabrication. Cracking can occur catastrophically and very high crack growth rates have been measured.

Similar metal embrittlement effects can be observed even in the solid state, when one of the metals is brought close to its melting point; e.g. cadmium-coated parts operating at high temperature. This phenomenon is known as solid metal embrittlement.

Liquid metal embrittlement or LME is characterized by the reduction in the threshold stress intensity, true fracture stress or in the strain to fracture when tested in the presence of liquid metals as compared to that obtained in air / vacuum tests. The reduction in fracture strain is generally temperature dependent and a "ductility trough" is observed as the test temperature is decreased. A ductile-to-brittle transition behaviour is also exhibited by many metal couples. The shape of the elastic region of the stress-strain curve is not altered, but the plastic region may be changed during LME. Very high crack propagation rates, varying from a few centimeters per second to several meters per second are induced in solid metals by the embrittling liquid metals. An incubation period and a slow pre-critical crack propagation stage generally precede final fracture.

It is believed that there is specificity in the solid-liquid metals combinations experiencing LME. There should be limited mutual solubilities for the metal couple to cause embrittlement. Excess solubility makes sharp crack propagation difficult, but no solubility condition prevents wetting of the solid surfaces by liquid metal and prevents LME. Presence of an oxide layer on the solid metal surface also prevents good contact between the two metals and stops LME. The chemical compositions of the solid and liquid metals affect the severity of embrittlement. Addition of third elements to the liquid metal may increase or decrease the embrittlement and alters the temperature region over which embrittlement is seen. Metal combinations which form intermetallic compounds do not cause LME. There are a wide variety of LME couples. Most technologically important are the LME of aluminum and steel alloys.

...
Wikipedia