Environmental Engineering Reference
In-Depth Information
ment can also occur by a ductile dimpled rupture mode in certain steels, copper
alloys, and aluminum alloys.
The failure arising from LME may be instantaneous or may take place after
a certain lapse of time after the exposure of the stressed metal in the liquid metal
environment. The former is treated as the ''classical'' LME and the latter is often
referred to as
delayed failure
or
static fatigue
. In either case, the presence of
stress is a requirement, which may be tensile, shear, or tortional in nature, but
not compressive. In this respect, LME is analogous to stress corrosion cracking
(SCC), but the propagation of fracture is much faster in LME than in SCC. Inter-
granular penetration of liquid metal may render a solid metal brittle even in the
absence of stress if sufficient time is allowed, e.g., aluminum by liquid gallium,
but this is not considered as LME. On the other hand, stressed aluminum in
contact with liquid gallium breaks immediately and provides an example for
LME.
7.2.1 Characteristics of LME
Effect on Stress-Strain Curve
The elongation and reduction in area of the metal or alloy are lowered as a result
of LME. The fracture stress is also reduced and in cases of severe embrittlement
may be less than the yield stress of the material. However, there is no change
in the yield strength and strain-hardening behavior of the solid metal. The stress-
strain curve remains unaltered up to the point of failure. The liquid metal acts
only to limit the total ductility before fracture or the stress at fracture if the failure
occurs below the normal yield point. This is illustrated in Fig. 7.1 for the embrit-
tlement of copper by various molten Bi-Pb alloys and in Fig. 7.2 for the embrittle-
ment of various Fe-Al alloys in liquid mercury. The failure of mild steel occurs
at only 2-3% elongation in molten lithium, but the lower yield point, upper yield
point, and yield point elongation remain unaffected.
Fracture Morphology
The fracture mode changes from ductile to brittle as a result of LME. In polycrys-
talline materials, the brittle intergranular mode is the most common, e.g.,
-brass
in mercury or aluminum in liquid gallium. However, mixed inter- and transgranu-
lar cracking has been encountered in some systems, e.g., aluminum alloy 2024-
T4 in Hg-Zn amalgam [1]. Single crystals essentially fail by cleavage fracture
in liquid metal environments, e.g., Cd in liquid indium. As mentioned earlier,
ductile intergranular dimpled fracture has been encountered in some steels, cop-
per alloys, and aluminum alloys. The fracture surfaces show complete coverage
by liquid metal, which is sometimes difficult to detect as well as to remove for
metallographic examination. It may be pointed out here that apart from the con-
tamination of the fracture surface, there is little or no intergranular penetration
of the liquid metal into the solid metal.
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