Liquid metal embrittlement

Liquid Metal 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.^[1] 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.^[2] 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.^[3]

Liquid metal embrittlement effects can be observed even in solid state, when one of the metals is brought close to its melting point; eg. cadmium-coated parts operating at high temperature.

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Characteristics

Mechanical Behaviour

Liquid metal embrittlement or LME is characterized by the reduction in the true fracture stress and/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.^[2] 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.^[4] 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.

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 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.^[2] 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 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.

Mechanisms

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

• The dissolution-diffusion model of Robertson ^[5] and Glickman ^[6] says that adsorption 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,^[7] Westwood and Kamdar ^[8] proposed that the adsorption of the liquid metal atoms at the crack tip weakens inter-atomic bonds and propagates the crack.
• Gordon ^[9] 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 ^[10] and Popovich ^[11] 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 mercury. 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. [1]

Popular Culture

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

Conclusion

Many instances of severe embrittlement of normally ductile materials in the presence of specific liquid metals have been noticed. Numerous investigations on this phenomenon were carried out during the last 100 years resulting in a clear understanding of the phenomenology of the process. A broad understanding of the atomic mechanism of LME also exists. However, a universally applicable theory of the process is yet to evolve.

References

1. ^ J. Huntington, Inst. Metals, 11 (1914), 108
2. ^ ^{a b c} B. Joseph, M. Picat, and F. Barbier, Eur. Phys. J. AP 5 (1999), 19
3. ^ M. H. Kamdar, Treatise on Material Science and Technology, Academic Press, Vol. 25 (1983), 361
4. ^ Liquid metal assisted cracking of galvanized steel work, Topic Paper, SC / T / 04 / 02, Standing Committee on Structural Safety, London, U. K. June 2004, website: www.scoss.org.uk
5. ^ W. M. Robertson, Trans. Met. Soc. AIME, 236 (1966), 1478
6. ^ E. E. Glickman and Y. V. Goryunov, Sov. Mater. Sci., (1978), 355
7. ^ N. S. Stoloff and T. L. Johnston, Acta Met., 11 (1963), 251
8. ^ A. R. C. Westwood and M. H. Kamdar, Phil. Mag. 8 (1963), 787
9. ^ P. Gordon and H. H. Ann, Met. Trans., A 13 (1982), 457
10. ^ S. P. Lynch, Acta Met., 36 (1988), 2639
11. ^ V. V. Popovich, and I. G. Dmukhovskaya, Sov. Mater. Sci., (1987), 535

See also

• Embrittlement