Liquid Metal Embrittlement (LME)

For LME to occur, several conditions must be met: the presence of a tensile stress, intimate contact between the solid and liquid metals, and a specific metallurgical pairing. A classic and dramatic example is the embrittlement of aluminum by liquid gallium. A small drop of gallium on an aluminum part under stress can cause it to fracture almost instantly. The technical novelty of understanding LME was the recognition that fracture could be driven by a surface energy phenomenon rather than bulk material properties alone. The liquid metal atoms effectively ‘unzip’ the atomic bonds at the crack tip by lowering the energy required for crack propagation.

The severity of LME depends on factors like temperature, stress level, and the specific solid-liquid metal couple. The solubility between the two metals is a key factor; systems with low mutual solubility are often highly susceptible. Historically, unexpected failures in structures like galvanized steel frameworks or in components where mercury thermometers broke were later attributed to LME. This knowledge became foundational for materials selection and compatibility assessment in engineering design, especially in aerospace and nuclear fields where dissimilar metals are often in close proximity.