Forms of Metallic Corrosion
71
few monolayers of liquid atoms are all that is necessary for LME. Even a few
micrograms of liquid can cause LME.
3.11.1.2 Factors Influencing LME
There are several factors that influence liquid metal embrittlement (LME),
including:
Grain size.
•
The yield stress and fracture stress of a metallic material
normally bear a linear relationship with the inverse square root of
grain diameter. The same relationship holds true for LME. A linear
decrease in fracture strength as a function of the square root of the
average grain diameter has been observed for copper and iron in
molten lithium, 70–30 brass in mercury, and zinc in mercury, indicating that coarse-grained materials are more susceptible to LME.
The grain size dependence of LME is indicative of a reduction in
cohesive strength of the material rather than an effect of the penetration or dissolution of liquid into the grain boundary.
Temperature.
•
Except for a few cases of embrittlement caused by the
vapor phase, LME takes place at temperatures above the melting
point of the liquid metal. In the vicinity of the melting point of the
liquid metal, LME is relatively temperature insensitive. At high temperatures, brittle-to-ductile transition occurs in many systems over
a temperature range, and the ductility is restored. The brittle-toductile transition temperature depends on the presence of a notch,
grain size, and strain rate. The transition temperature increases in
the presence of notches. An increase in strain rate and a decrease in
grain size increase the transition temperature.
Strain rate.
•
In addition to its effect on the brittle-to-ductile transition temperature, the strain rate may be an important factor for the
occurrence of LME. The effect of strain rate appears to be related to
the increase in yield strength, and this corresponds to an increase in
LME susceptibility.
Alloying.
•
Some metals are embrittled in their pure state (such as zinc
by mercury, and aluminum by liquid gallium). On the other hand,
pure iron is not embrittled by mercury, and pure copper is relatively
immune in liquid mercury (coarse-grained copper is embrittled).
However, iron becomes susceptible to embrittlement in mercury
when alloyed with more than 2% silicon, 4% aluminum, or 8% nickel.
When copper is alloyed with zinc, aluminum, silicon, or gallium, its
susceptibility to LME significantly increases. The same occurs when
zinc is alloyed with a small amount of copper or gold in mercury.
The increase in yield strength of the metal on alloying is considered
responsible for the increased susceptibility. The high-strength alloys
71
few monolayers of liquid atoms are all that is necessary for LME. Even a few
micrograms of liquid can cause LME.
3.11.1.2 Factors Influencing LME
There are several factors that influence liquid metal embrittlement (LME),
including:
Grain size.
•
The yield stress and fracture stress of a metallic material
normally bear a linear relationship with the inverse square root of
grain diameter. The same relationship holds true for LME. A linear
decrease in fracture strength as a function of the square root of the
average grain diameter has been observed for copper and iron in
molten lithium, 70–30 brass in mercury, and zinc in mercury, indicating that coarse-grained materials are more susceptible to LME.
The grain size dependence of LME is indicative of a reduction in
cohesive strength of the material rather than an effect of the penetration or dissolution of liquid into the grain boundary.
Temperature.
•
Except for a few cases of embrittlement caused by the
vapor phase, LME takes place at temperatures above the melting
point of the liquid metal. In the vicinity of the melting point of the
liquid metal, LME is relatively temperature insensitive. At high temperatures, brittle-to-ductile transition occurs in many systems over
a temperature range, and the ductility is restored. The brittle-toductile transition temperature depends on the presence of a notch,
grain size, and strain rate. The transition temperature increases in
the presence of notches. An increase in strain rate and a decrease in
grain size increase the transition temperature.
Strain rate.
•
In addition to its effect on the brittle-to-ductile transition temperature, the strain rate may be an important factor for the
occurrence of LME. The effect of strain rate appears to be related to
the increase in yield strength, and this corresponds to an increase in
LME susceptibility.
Alloying.
•
Some metals are embrittled in their pure state (such as zinc
by mercury, and aluminum by liquid gallium). On the other hand,
pure iron is not embrittled by mercury, and pure copper is relatively
immune in liquid mercury (coarse-grained copper is embrittled).
However, iron becomes susceptible to embrittlement in mercury
when alloyed with more than 2% silicon, 4% aluminum, or 8% nickel.
When copper is alloyed with zinc, aluminum, silicon, or gallium, its
susceptibility to LME significantly increases. The same occurs when
zinc is alloyed with a small amount of copper or gold in mercury.
The increase in yield strength of the metal on alloying is considered
responsible for the increased susceptibility. The high-strength alloys