Liquid metal embrittlement (also known as LME and liquid metal induced embrittlement) is a phenomenon of practical importance, where certain
ductile
Ductility refers to the ability of a material to sustain significant plastic deformation before fracture. Plastic deformation is the permanent distortion of a material under applied stress, as opposed to elastic deformation, which is reversi ...
metal
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
s experience drastic loss in
tensile ductility or undergo
brittle fracture when exposed to specific liquid metals. Generally,
tensile stress
In continuum mechanics, stress is a physical quantity that describes forces present during deformation. For example, an object being pulled apart, such as a stretched elastic band, is subject to ''tensile'' stress and may undergo elongati ...
, 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 (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
in the presence of liquid
gallium
Gallium is a chemical element; it has Chemical symbol, symbol Ga and atomic number 31. Discovered by the French chemist Paul-Émile Lecoq de Boisbaudran in 1875,
elemental gallium is a soft, silvery metal at standard temperature and pressure. ...
.
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
steel
Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
s experience ductility losses and cracking during
hot-dip galvanizing
Hot-dip galvanization is a form of galvanization. It is the process of coating iron and steel with zinc, which alloys with the surface of the base metal when immersing the metal in a bath of molten zinc at a temperature of around . When expos ...
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
Cadmium is a chemical element; it has chemical symbol, symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12 element, group 12, zinc and mercury (element), mercury. Like z ...
-coated parts operating at high temperature. This phenomenon is known as solid metal embrittlement.
[Kolman, D.G. (2003), pp. 393-397.]
Characteristics
Mechanical behavior
Liquid metal embrittlement 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 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 the final fracture.
]
Metal chemistry
It is believed that there is specificity in the solid-liquid metal 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. The 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. The addition of third elements to the liquid metal may increase or decrease the embrittlement and alter 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
Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...
and steel
Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...
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 boundaries
In materials science, a grain boundary is the interface between two grains, or crystallites, in a polycrystalline material. Grain boundaries are two-dimensional crystallographic defect, defects in the crystal structure, and tend to decrease the ...
of the solid metal and alteration of the grain boundary properties. Accordingly, maximum LME is seen in cases where alloy addition elements have saturated the grain boundaries of the solid metal.[ The hardness and deformation behaviour of the solid metal affects 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 kinetics 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 Glikman says that absorption of the liquid metal on the solid metal induces dissolution and inward diffusion. Under stress, these processes lead to crack nucleation
In thermodynamics, nucleation is the first step in the formation of either a new Phase (matter), thermodynamic phase or Crystal structure, structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically def ...
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 the 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, with the exception of Robertson,[ utilize the concept of an adsorption-induced surface energy lowering of the solid metal as the central cause of LME. They have succeeded in predicting many of the phenomenological observations. However, quantitative prediction of LME is still elusive.
]
Mercury embrittlement
The most common liquid metal to cause embrittlement is mercury, as it is a common contaminant in the processing of hydrocarbons
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic; their odor is usually faint, and may b ...
in petroleum reservoir
A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the prese ...
s. The embrittling effects of mercury were first recognized by Pliny the Elder
Gaius Plinius Secundus (AD 23/24 79), known in English as Pliny the Elder ( ), was a Roman Empire, Roman author, Natural history, naturalist, and naval and army commander of the early Roman Empire, and a friend of the Roman emperor, emperor Vesp ...
circa 78 AD. Mercury spills present an 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. Spilled elemental mercury can be immobilized and made relatively harmless by silver nitrate
Silver nitrate is an inorganic compound with chemical formula . It 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. It was once called ''lunar causti ...
.
On 1 January 2004, the Moomba, South Australia
Moomba is a restricted access company town located in the Australian state of South Australia within the gazetted locality of Gidgealpa about north of the state capital of Adelaide. It is operated by Santos Limited for the purpose of explor ...
, natural gas processing
Nature is an inherent character or constitution, particularly of the ecosphere or the universe as a whole. In this general sense nature refers to the laws, elements and phenomena of the physical world, including life. Although humans are part ...
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.[ Alt URL: ]
Popular culture
Liquid metal embrittlement plays a central role in the novel '' Killer Instinct'' by Joseph Finder
Joseph Finder (born October 6, 1958) is an American thriller writer. His books include '' Paranoia'', '' Company Man'', '' The Fixer'', '' Killer Instinct'', ''Power Play'', and the Nick Heller series of thrillers. His novel ''High Crimes'' was ...
.
In the film '' Big Hero 6'', Honey Lemon, voiced by Genesis Rodriguez
Génesis Rodríguez Pérez (born July 29, 1987) is an American actress. She began her career playing leading roles in the Telemundo telenovelas '' Prisionera'' (2004), '' Dame Chocolate'' (2007) and '' Doña Bárbara'' (2008–2009).Aquino, Tar ...
, uses liquid metal embrittlement in her lab.
See also
* Embrittlement
*Hydrogen embrittlement
Hydrogen embrittlement (HE), also known as hydrogen-assisted cracking or hydrogen-induced cracking (HIC), is a reduction in the ductility of a metal due to absorbed hydrogen. Hydrogen atoms are small and can Permeation, permeate solid metals. O ...
References
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Building defects
Materials degradation
Fracture mechanics