Environmental Engineering Reference
In-Depth Information
Requirements for Embrittlement
The general requirements for LME to occur in a ductile metal are as follows:
1. There must be intimate contact or wetting of the solid metal by the liquid
metal.
2. The solid metal must be sufficiently stressed to produce plastic deformation.
3. There should be an adequate supply of liquid metal.
An intimate contact between the solid metal and the liquid metal is the most
critical and mandatory condition for LME. This is required to initiate embrittle-
ment as well as to ensure the presence of the liquid metal at the tip of the propagat-
ing crack to cause brittle failure. A thin oxide film on the surface of the solid
metal may intervene in the process of embrittlement. Local breakdown of the
film by the acting stress or plastic deformation would expose freshly created
surface and establish a true interface between the solid and the liquid metals. A
few metals wet oxides, e.g., molten gallium will wet aluminum oxide and molten
lithium will wet iron oxides; the specificity of the environment may be attributed
to this factor.
The surface wetting by a liquid metal can lead to intergranular penetration
originating at the interface. In general, this can only happen when the grain
boundary dihedral angle contained by two liquid-solid surface tension vectors
approaches zero. On the other hand, wetting over large areas can be achieved as
long as the dihedral angle is less than 90
. Grain boundary penetration is therefore
not a natural consequence of wetting and is also not relevant to the occurrence
of LME, as has been pointed out earlier. The wettability can be enhanced by
alloying additions to the liquid metal. For example, it is difficult to make pure
mercury embrittle aluminum alloys because of the large contact angle between
mercury and aluminum oxide. Additions of a few percent of zinc or gallium
reduces the contact angle and renders the wetting action more effective.
Some amount of plastic deformation is required for the cracking process (dis-
cussed later in Section 7.2.4). A general yielding of the solid metal is not envis-
aged. The yielding or deformation may be localized in a few grains. For this
purpose some stable obstacle to slip serving as stress concentrator should be
present. The grain boundaries,a twin or a kink band produced during deformation,
second phase particles, and notches can provide such a situation.
An adequate supply of liquid metal is necessary to adsorb at the obstacle and
subsequently at the propagating crack tip. The gross amount of liquid need not
be large; a few monolayers of liquid metal atoms are necessary for LME and it
has been reported that even micrograms of liquid lead can cause LME in 75-
mm-thick tubes [3]. If the solid metal is notch-brittle, liquid metal is not required
to be present at the propagating crack tip because the crack, once initiated, will
propagate in a brittle manner.
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