Environmental Engineering Reference
In-Depth Information
Figure 7.4
Variation of ductility of polycrystalline cadmium as a function of indium
content of mercury-indium surface coatings at 25°C [8].
Embrittlement has been observed even below the melting point of the embrit-
tling metal, i.e., even when it is in the solid state and is in intimate contact with
the ''solid'' metal of the couple. Embrittlement by solid cadmium of titanium
and steel [2] has been reported. Presumably, the embrittlement is caused by the
vapor phase of the solid metal with the lower melting point. Although this type
of embrittlement is called
solid metal-induced embrittlement
(SMIE), this appar-
ently follows the same mechanism of LME.
Severity of Embrittlement
The severity of embrittlement is related to the chemical nature of the embrittling
species and depends on such factors as strength, alloying elements, and grain
size, which determine the properties of the solid metal. It does not depend on
the time of exposure to the liquid metal before testing or whether the liquid is
presaturated with the solid. However, the severity as well as the occurrence or
nonoccurrence of LME depend on the presence or absence of stress concentration
such as preexisting cracks or flaws in the solid metal. High-stress concentrations
lead to a greater severity of embrittlement.
With regard of the dependence of severity on the chemical nature of the em-
brittling species, there appears to be a correlation with the electronegativities of
the metals involved. Zinc (electronegativity 1.6) is more severely embrittled by
liquid gallium (1.6) than by mercury (1.9). Cadmium (1.7) is most severely em-
brittled by liquid indium (1.7), less by liquid gallium (1.6), and not at all by
thallium (1.8) or mercury (1.9). The embrittlement of pure aluminum by various
