Liquid metal embrittlement (LME) is a condition where certain metals lose tensile ductility or seem to undergo brittle fractures when tested in the presence of specific liquid metals. In order to induce embrittlement, a tensile stress is needed. This most often occurs either during fabrication or during hot-dip galvanizing.
The effects of liquid metal embrittlement can be felt even in solid state. This can be evident when a metal, such as a cadmium-coated part that operates at high temperatures is brought close to its melting point. This state can be called solid metal embrittlement.
Liquid metal embrittlement is characterized by the decrease in true fracture stress, threshold stress intensity or strain to fracture when it’s tested in liquid metals. Reduction in fracture strain is temperature dependent, thus the test temperature is decreased.
There are limited mutual solubilities for the metals to cause liquid metal embrittlement. Excess solubilities result in sharp crack propagation difficulties, although wetting of the solid surfaces is not prevented. The oxide layer on the metal surface prevents good contact between the two metals, thus stopping liquid metal embrittlement. Note that metal combinations that form inter-metallic compounds don’t cause liquid metal embrittlement.
There are many theories that try to explain liquid metal embrittlement, with the major ones being:
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Dissolution-diffusion model: This model states that the absorption of the liquid metal induces an inward diffusion and dissolution. When subjected to stress, the process can lead to nucleation of the crack and propagation.
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Brittle fracture theory: This theory proposes that the liquid metal atom absorption at the crack tip tends to propagate the crack as well as weakening the inter-atomic bond.
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Gordon model: This model was postulated based on the diffusion-penetration of the liquid metal atoms. This can cause failure of the material when cracks are nucleated and grow under stress.
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Ductile failure model: This model predicted that the absorption of liquid metal could lead to weak atomic bonds. Nucleation of dislocations moves under stress, piling-up and work hardens the solid. The dissolution also helps nucleate voids that grow under stress, causing ductile failure.