Liquid Metal Embrittlement
Liquid metal embrittlement is a generic term which refers to at least three distinctly different phenomena. The type of embrittlement which has received most attention in recent years appears to arise from a process in which atoms of the liquid adsorb preferentially at sites of high stress concentration, leading to a reduction in cohesive energy, and premature fracture. In this form, fracture is virtually instantaneous; the path can be either intergranular or transgranular, and embrittlement is increased by increasing the grain size or lowering the number of slip systems of the substrate. A ductile-to-brittle transition similar to that manifested by body-centered cubic alloys is noted with changes in test temperature. One may inhibit or enhance adsorption-type embrittlement by a given liquid metal by means of selected solute additions to the liquid.
Liquid metal embrittlement sometimes occurs by diffusion-controlled processes such as grain boundary penetration. This type of behavior, which always leads to intergranular failure, has been treated theoretically by grain boundary wetting concepts and by a stress-enhanced dissolution model. Diffusion along boundaries may also play a role in delayed failure (static fatigue) phenomena, as there is evidence of surface notching during exposure of susceptible solids to liquid metals.
Refractory metals are subject to a corrosion-type attack by boiling mercury at temperatures greater than 600°C; this represents the third major class of liquid metal embrittlement phenomena. The interrelationship among and the distinguishing features of the various forms of embrittlement will be discussed.
KeywordsLiquid Metal Solid Solution Alloy Tilt Boundary Cohesive Stress Liquid Gallium
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