Liquid metal embrittlement#Mercury embrittlement

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 {{nobr|air{{hsp}}/{{hsp}}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 the final fracture.

It is believed that there is specificity in the solid-liquid metal combinations experiencing LME.{{cite report |title=Topic Paper SC/T/04/02: Liquid metal assisted cracking of galvanized steel work |publisher=Standing Committee on Structural Safety |date=June 2004}} 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 and steel alloys.

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 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.

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.

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.{{cite journal |last=Kamdar |first=M. H. |title=Embrittlement by Liquid and Solid Metals |journal=Proceedings of the Symposium |editor-first=M. H. |editor-last=Kamdar |publisher=Metallurgical Society of AIME |location=Warrendale, PA |date=1984 |page=149}}{{cite journal |title=Crack growth behavior of a high strength aluminum alloy during LME by gallium |last1=Benson |first1=B. A. |last2=Hoagland |first2=R. G. |name-list-style=amp |date=1989 |journal=Scripta Metallurgica |volume=23 |issue=11 |page=1943 |doi=10.1016/0036-9748(89)90487-0}}{{cite journal |title=Fracture mechanics method for determining the crack propagation resistance of embrittled aluminum bicrystals |last1=Kargol |first1=J. A. |last2=Albright |first2=D. L. |name-list-style=amp |date=May 1975 |journal=Journal of Testing and Evaluation |volume=3 |number=3 |page=173|doi=10.1520/JTE10649J }}{{cite journal |title=Liquid-Metal Embrittlement of Type 316L Stainless Steel by Gallium as Measured by Elastic-Plastic Fracture Mechanics |last1=Kolman |first1=D.G. |last2=Chavarria |first2=R. |name-list-style=amp |date=March 2004 |journal=Corrosion |volume=60 |issue=3 |pages=254–261 |doi=10.5006/1.3287729}}

Many theories have been proposed for LME. The major ones are listed below;

• The dissolution-diffusion model of Robertson{{cite journal |title=Propagation of a Crack Filled with Liquid Metal |last=Robertson |first=W. M. |date=November 1966 |journal=Transactions of the Metallurgical Society of AIME |volume=236 |number=11 |page=1478}} and Glikman{{cite journal |title=Mechanism of embrittlement by liquid metals and other manifestations of the Rebinder effect in metal systems |last1=Glikman |first1=E.É. |last2=Goryunov |first2=Yu.V. |name-list-style=amp |date=July 1978 |journal=Soviet Materials Science |volume=14 |issue=4 |pages=355–364 |doi=10.1007/BF01154710}} says that absorption of the liquid metal on the solid metal induces dissolution and inward diffusion. Under stress, these processes lead to crack nucleation and propagation.
• The brittle fracture theory of Stoloff and Johnson,{{cite journal |title=Crack propagation in a liquid metal environment |last1=Stoloff |first1=N. S. |last2=Johnston |first2=T. L. |name-list-style=amp |date=1963 |journal=Acta Metallurgica |volume=11 |issue=4 |pages=251–256 |doi=10.1016/0001-6160(63)90180-9}} Westwood and Kamdar{{cite journal |title=Concerning liquid metal embrittlement, particularly of zinc monocrystals by mercury |last1=Westwood |first1=A. R. C. |last2=Kamdar |first2=M. H. |name-list-style=amp |date=1963 |journal=Philosophical Magazine |volume=8 |number=89 |pages=787–804 |doi=10.1080/14786436308213836|bibcode=1963PMag....8..787W }} proposed that the adsorption of the liquid metal atoms at the crack tip weakens inter-atomic bonds and propagates the crack.
• Gordon{{cite journal |title=The mechanisms of crack initiation and crack propagation in metal-induced embrittlement of metals |last1=Gordon |first1=Paul |last2=An |first2=Henry H. |name-list-style=amp |date=March 1982 |journal=Metall Mater Trans A |volume=13 |issue=3 |pages=457–472 |doi=10.1007/BF02643354|bibcode=1982MTA....13..457G }} 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{{cite journal |title=Environmentally assisted cracking: Overview of evidence for an adsorption-induced localised-slip process |last=Lynch |first=S. P. |date=1988 |journal=Acta Metallurgica |volume=36 |issue=10 |pages=2639–2661 |doi=10.1016/0001-6160(88)90113-7}} and Popovich{{cite journal |title=The embrittlement of metals and alloys being deformed in contact with low-melting alloys (A review of foreign literature) |last1=Popovich |first1=V. V. |last2=Dmukhovskaya |first2=I. G. |name-list-style=amp |date=1987 |journal=Soviet Materials Science |volume=23 |issue=6 |pages=535–544 |doi=10.1007/BF01151882}} 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.

The most common liquid metal to cause embrittlement is mercury, as it is a common contaminant in the processing of hydrocarbons in petroleum reservoirs.{{cite conference|title=Mercury Liquid Metal Embrittlement Of Alloys For Oil And Gas Production And Processing|url=https://onepetro.org/NACECORR/proceedings-abstract/CORR10/All-CORR10/NACE-10294/126967|first1=Raymundo|last1=Case|first2=Dale R.|last2=McIntyre|date=14 March 2010}} The embrittling effects of mercury were first recognized by Pliny the Elder circa 78 AD.{{cite book |author=C. Plinius Secundus |others=Translated by Philemon Holland |date=1964 |orig-year=78 AD |title=Naturalis Historia |trans-title=The History of the World, or The Natural History |language=Latin |publisher=McGraw Hill}} 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.{{cite report |url=http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA043160 |title=A Chemical Treatment for Mercury Accidentally Spilled in Aircraft |last=Allsopp |first=H. J. |date=31 January 1977 |publisher=Royal Aircraft Establishment |via=DTIC |url-status=dead |archive-url=https://web.archive.org/web/20070927191708/http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA043160 |archive-date=2007-09-27}}

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.{{cite press release |author= |title=Moomba Plant Update |date=2004-03-05 |website=Santos |location=Adelaide, South Australia |url=http://www.santos.com/Archive/NewsDetail.aspx?p=121&id=412 |access-date=2013-01-18 |archive-url=https://archive.today/20130216182806/http://www.santos.com/Archive/NewsDetail.aspx?p=121&id=412 |archive-date=2013-02-16 |url-status=dead}} Alt URL: {{cite press release |url=https://www.sec.gov/Archives/edgar/vprr/04/9999999997-04-008108 |title=Moomba Plant Update |date=2004-03-05 |author=Santos |via=SEC |access-date=2013-01-18 |archive-url=https://web.archive.org/web/20160424015909/http://www.sec.gov/Archives/edgar/vprr/04/9999999997-04-008108 |archive-date=2016-04-24 |url-status=dead}}

Liquid metal embrittlement plays a central role in the novel Killer Instinct by Joseph Finder.

In the film Big Hero 6, Honey Lemon, voiced by Genesis Rodriguez, uses liquid metal embrittlement in her lab.

• Embrittlement
• Hydrogen embrittlement

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Category:Fracture mechanics