Environmental Engineering Reference
In-Depth Information
favorably oriented slip plane S-P near the tip. F represents the increasing applied
force. Chemisorption may occur as such or only after the A-A 0 bonds have been
strained to some critical value. The bonds get weaker because of the electronic
rearrangement and the crack propagates if the applied stress exceeds the reduced
breaking strength. The liquid metal then gets stably chemisorbed on the fracture
surface and surface diffusion of the liquid metal atoms over the chemisorbed
layer, which is a low-activation energy process, feeds the crack tip with the neces-
sary embrittling species. The crack propagates through repetition of the process.
The shear strength of the atomic bonds at the crack tip may also get reduced
because of chemisorption. The reduced shear strength facilitates nucleation of
dislocations or slip at low stresses at or near the crack tip. The increased plasticity
produces a large localized plastic strain ahead of the crack tip that is effective
in nucleating voids at precipitations, inclusions, or at subboundaries in single
crystals. The coalescence of the voids produces a ductile fracture. The localized
increased plasticity results in an overall reduction in the strain at failure. The
process has been schematically represented in Fig. 7.12.
7.2.5 Preventive Measures
Prevention or reduction in the occurrence of liquid metal embrittlement can be
achieved by the following means:
1.
Introduction of impurity atoms in the solid having more affinity for sharing
electrons with the liquid metal atoms than for the solid metal atoms has
been successful in reducing embrittlement in some cases. Examples are the
addition of phosphorus to Monel to reduce embrittlement in liquid mercury
or the addition of lanthanides to leaded steels.
The addition of gold to silver inhibits its embrittlement by gallium, pre-
sumably because of the formation of strong Au-Ga or Ag-Au bonds at the
surface which counters the weakening influence of gallium on Ag-Ag bonds.
2.
As discussed in the section, ''Effect of Solute Additions to the Liquid
Metal,'' the addition of a second metal to the embrittling liquid decreases
the embrittlement in some cases.
3.
Providing an effective barrier in between the solid and liquid metals is an
effective measure. The barrier may be a metallic one that is not embrittled
by the liquid metal concerned, or a ceramic or a covalent material coating.
4.
Cladding with a soft, high-purity metal is at times employed. An example
would be zircaloy clad with pure zirconium to resist embrittlement in liquid
cadmium.
5.
Elimination of the embrittling liquid metal, particularly where a chance con-
tamination produces LME, is advised (see example given in Section 7.2.6).
6.
Reduction in the level of applied or residual stress below the static endurance
limit, where possible, is an obvious measure to reduce embrittlement.
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