Liquid metal embrittlement
Encyclopedia
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 tested in the presence of 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
. People have studied this phenomenon from the beginning of the 20th century. Many of its phenomenological characteristics are known and several mechanisms were proposed to explain it. The practical significance of liquid metal embrittlement is revealed by the observation that several steels experienced 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.
Liquid metal embrittlement effects can be observed even in 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.
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.
of the solid metal and altering the grain boundary properties. Accordingly, maximum LME is seen in cases where alloy additions elements have saturated the grain boundaries of the solid metal. The hardness and deformation behaviour of the solid metal affect its susceptibility to LME. Generally harder metals are more severely embrittled. Grain size greatly influences LME. Solids with larger grains are more severely embrittled and the fracture stress varies inversely with the square root of grain diameter. Also the brittle to ductile transition temperature is increased by increasing grain size.
of LME.
An increase in strain rate increases the upper limit temperature as well as the crack propagation rate. In most metal couples LME does not occur below a threshold stress level.
Testing typically involves tensile specimens but more sophisticated testing using fracture mechanics specimens is also performed .
All of these models utilize the concept of an adsorption-induced surface energy lowering of the solid metal as the central cause of LME. They succeeded in predicting many of the phenomenological observations. However, a quantitative prediction of LME is still elusive.
. Its spills present especially significant danger for airplanes. The aluminium-zinc-magnesium-copper alloy DTD 5050B is especially susceptible. The Al-Cu alloy DTD 5020A is less susceptible. Elemental mercury spilled can be immobilized and made relatively harmless by silver nitrate
. http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA043160
On 1 January 2004, the Moomba, South Australia
, natural gas processing plant operated by Santos
suffered a major fire. The gas release that led to the fire was caused by the failure of a heat exchanger (cold box) inlet nozzle in the liquids recovery plant. The failure of the inlet nozzle was due to liquid metal embrittlement of the train B aluminium cold box by elemental mercury.
" by Joseph Finder
.
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
when tested in the presence of 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
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
in the presence of liquid gallium
Gallium
Gallium is a chemical element that has the symbol Ga and atomic number 31. Elemental gallium does not occur in nature, but as the gallium salt in trace amounts in bauxite and zinc ores. A soft silvery metallic poor metal, elemental gallium is a brittle solid at low temperatures. As it liquefies...
. People have studied this phenomenon from the beginning of the 20th century. Many of its phenomenological characteristics are known and several mechanisms were proposed to explain it. The practical significance of liquid metal embrittlement is revealed by the observation that several steels experienced ductility losses and cracking during hot dip galvanizing
Hot-dip galvanizing
Hot-dip galvanizing is a form of galvanization. It is the process of coating iron, steel, or aluminum with a thin zinc layer, by passing the metal through a molten bath of zinc at a temperature of around 860 °F...
or during subsequent fabrication. Cracking can occur catastrophically and very high crack growth rates have been measured.
Liquid metal embrittlement effects can be observed even in solid state, when one of the metals is brought close to its melting point; e.g. cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
-coated parts operating at high temperature. This phenomenon is known as solid metal embrittlement.
Mechanical behaviour
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 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.Metal chemistry
It is believed that there is specificity in the solid-liquid metals combinations experiencing LME. There should be limited mutual solubilitiesSolubility
Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a solid, liquid, or gaseous solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on...
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
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
alloys.
Metallurgy
Alloying of the solid metal alters its LME. Some alloying elements may increase the severity while others may prevent LME. The action of the alloying element is known to be segregation to grain boundariesGrain boundary
A grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are defects in the crystal structure, and tend to decrease the electrical and thermal conductivity of the material...
of the solid metal and altering the grain boundary properties. Accordingly, maximum LME is seen in cases where alloy additions elements have saturated the grain boundaries of the solid metal. The hardness and deformation behaviour of the solid metal affect its susceptibility to LME. Generally harder metals are more severely embrittled. Grain size greatly influences LME. Solids with larger grains are more severely embrittled and the fracture stress varies inversely with the square root of grain diameter. Also the brittle to ductile transition temperature is increased by increasing grain size.
Physico-chemical properties
The interfacial energy between the solid and liquid metals and the grain boundary energy of the solid metal greatly influence LME. These energies depend upon the chemical compositions of the metal couple.Test parameters
External parameters like temperature, strain rate, stress and time of exposure to the liquid metal prior to testing affect LME. Temperature produces a ductility trough and a ductile to brittle transition behaviour in the solid metal. The temperature range of the trough as well as the transition temperature are altered by the composition of the liquid and solid metals, the structure of the solid metal and other experimental parameters. The lower limit of the ductility trough generally coincides with the melting point of the liquid metal. The upper limit is strain rate sensitive. Temperature also affects the kineticsChemical kinetics
Chemical kinetics, also known as reaction kinetics, is the study of rates of chemical processes. Chemical kinetics includes investigations of how different experimental conditions can influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition...
of LME.
An increase in strain rate increases the upper limit temperature as well as the crack propagation rate. In most metal couples LME does not occur below a threshold stress level.
Testing typically involves tensile specimens but more sophisticated testing using fracture mechanics specimens is also performed .
Mechanisms
Many theories have been proposed for LME. The major ones are listed below;- The dissolution-diffusion model of Robertson and Glickman says that adsorption of the liquid metal on the solid metal induces dissolution and inward diffusion. Under stress these processes lead to crack nucleationNucleationNucleation is the extremely localized budding of a distinct thermodynamic phase. Some examples of phases that may form by way of nucleation in liquids are gaseous bubbles, crystals or glassy regions. Creation of liquid droplets in saturated vapor is also characterized by nucleation...
and propagation. - The brittle fracture theory of Stoloff and Johnson, Westwood and Kamdar proposed that the adsorption of the liquid metal atoms at the crack tip weakens inter-atomic bonds and propagates the crack.
- Gordon postulated a model based on diffusion-penetration of liquid metal atoms to nucleate cracks which under stress grow to cause failure.
- The ductile failure model of Lynch and Popovich predicted that adsorption of the liquid metal leads to weakening of atomic bonds and nucleation of dislocations which move under stress, pile-up and work harden the solid. Also dissolution helps in the nucleation of voids which grow under stress and cause ductile failure.
All of these models utilize the concept of an adsorption-induced surface energy lowering of the solid metal as the central cause of LME. They succeeded in predicting many of the phenomenological observations. However, a quantitative prediction of LME is still elusive.
Mercury embrittlement
The most common liquid metal causing embrittlement is mercuryMercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
. Its spills present especially significant danger for airplanes. The aluminium-zinc-magnesium-copper alloy DTD 5050B is especially susceptible. The Al-Cu alloy DTD 5020A is less susceptible. Elemental mercury spilled can be immobilized and made relatively harmless by silver nitrate
Silver nitrate
Silver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...
. http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA043160
On 1 January 2004, the Moomba, South Australia
Moomba, South Australia
Moomba is a Santos-owned gas exploration and processing town located in the Cooper and Eromanga Basins, in central Australia, approximately 770 kilometres north of Adelaide....
, natural gas processing plant operated by Santos
Santos Ltd.
Santos Ltd. is a large Australian oil and gas exploration company. Its name is an acronym for South Australia Northern Territory Oil Search.-Operations:...
suffered a major fire. The gas release that led to the fire was caused by the failure of a heat exchanger (cold box) inlet nozzle in the liquids recovery plant. The failure of the inlet nozzle was due to liquid metal embrittlement of the train B aluminium cold box by elemental mercury.
Popular culture
Liquid metal embrittlement plays a central role in the novel "Killer InstinctKiller Instinct (novel)
Killer Instinct is a thriller novel written by Joseph Finder published in May 2006 by Orion Publishing Group. It debuted at #13 on the New York Times bestseller list with competition from The DaVinci Code and won the International Thriller Writers Award for best novel in 2007....
" by Joseph Finder
Joseph Finder
Joseph Finder is an American writer of several thrillers set in a business environment. His books include Paranoia, Company Man, Killer Instinct and Power Play...
.