Environmental Engineering Reference
In-Depth Information
lium alloys in liquid mercury environments exhibit delayed failure. Delayed fail-
ure has also been reported for AISI 4130 steel in molten lithium. Investigations
have shown that a significant period of incubation or inactivity exists during
which the exposed specimens suffer no permanent change of mechanical proper-
ties, which indicates that the role of diffusion is insignificant in this type of em-
brittlement. Age-hardable alloys exhibit the lowest time of fracture in the maxi-
mum hardened state. The susceptibility increases with prior strain or cold work.
It is generally believed that grain boundary penetration of the solid metal by
the liquid metal occurs in the presence of applied stress and some critical depth
of penetration gives rise to brittle fracture propagation. However, the mechanism
for this has not been established.
7.2.4 Mechanisms of LME
There are three models proposed for the mechanism of liquid metals embrittle-
ment. These are:
1.
Reduction in surface energy model
2.
Stress-assisted dissolution model
3.
Adsorption-induced reduction in tensile cohesion/shear strength model
Reduction in Surface Energy Model
According to this model, the liquid metal embrittlement is associated with a re-
duction in the surface energy of the solid metal as a result of the adsorption of
liquid metal species [1,6]. The interfacial energy plays a big role in the generation
of fracture and a truly brittle fracture should be associated with energies of the
order of 10
3
ergs/cm
2
or less. Values of the similar order have been calculated
in several embrittling systems, e.g., Cu-liquid Bi, brass-liquid Bi, Fe-liquid Li,
Cu-liquid Li, and brass-liquid Hg.
The model envisages that the fracture propagates simply by the progressive
rupture of atomic bonds and the creation of new surface. The opponents of this
model argue that the energy involved in crack propagation are greater than the
surface energy of the solid by several orders of magnitude and the proposed
model does not suggest how this energy involved in plastic deformation is low-
ered so that a brittle crack propagation can be accomplished.
Stress-Assisted Dissolution Model
In this model, embrittlement has been envisaged to be an outcome of a very rapid
localized dissolution process occurring at the crack tip under the influence of an
applied stress. Volume diffusion of the dissolved solute through the liquid con-
trols the propagation or the crack. Crack velocities of the order of tens of centime-
ters per second have been claimed to be achieved through the increased solubility
of the solid at the crack tip [7].
