Also known as liquid metal induced embrittlement, LME occurs when normally ductile metals experience a drastic increase in brittleness after exposure to certain liquid metals.
The embrittlement is generally induced after experiencing tensile stress. However, there are some exceptions. For example, aluminum when exposed to liquid gallium does not experience embrittlement. Even when in the solid state embrittlement effects can be observed if one of the metals is brought close to their melting point, known as solid metal embrittlement.
Effects of LME
LME induces many negative mechanical effects when tested in the presence of liquid metals, such as reducing the true fracture stress or strain to fracture when compared to air or vacuum tests. Because the reduction in mechanical properties is generally temperature dependent, a “ductility trough” when the temperature is decreased.
What Causes LME?
In order to cause embrittlement, the mutual solubilities for the two metals in question should be limited because excess solubility makes sharp crack propagation difficult, but no solubility at all prevents the wetting of the solid surfaces by the liquid metal and therefore prevents LME entirely. Additionally, the presence of an oxide layer on the solid metal surface (as is the case in pure aluminum and stainless steel) prevents contact between the two metals and stops LME as well.
Alloying a pure solid metal alters the LME that it experiences. Some elements will increase the effect while others may mitigate it. The alloying element is generally segregated to the grain boundaries of the solid boundaries where it alters the properties of the grain boundaries. Therefore, the greatest LME effects are generally seen when alloying elements saturate the grain boundaries of the solid metal.
Examples of LME
An important and very common example of LME occurs in mercury and was first recorded by Pliny the Elder around 78 CE. Mercury spills are particularly dangerous in the aerospace industry, because the commonly used aluminum-zinc-magnesium-copper alloy DTD 5050B is particularly susceptible to LME by liquid mercury. However, spilled mercury can be neutralized and rendered mostly harmless by silver nitrate. More recently, a natural gas processing plant in South Australia suffered a fire in 2004 caused by liquid metal embrittlement of a heat exchanger inlet nozzle by elemental mercury which ultimately led to failure of the heat exchanger and release of flammable gas.
Understanding the potential effects of a failure due to liquid metal embrittlement is very important when engineering a process. Therefore, all possible means of reducing the chances of such a failure should be evaluated for both efficiency and cost. The cheapest and most effective solution should be chosen based on available risk analyses.
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