Environmental Engineering Reference
In-Depth Information
first-order size effect, i.e., the self-interstitials atoms have a larger relaxation
strain and hence a stronger size effect interaction with the edge dislocations, and
(2) the Eshelby elastic strain effect, i.e., in the presence of an internal stress
interstitials have a larger inhomogeneity interaction with those edge dislocations
whose Burger vectors are aligned in the direction of the stress axis. Such interac-
tion causes more interstitials to be attracted to, and annihilated at, dislocations
aligned with their Burger vectors in the direction of the applied tensile stress
resulting in their climb. For vacancies, the bias is just the opposite; therefore, the
corresponding excess vacancies are annihilated at dislocations with their Burger
vectors perpendicular to the applied tensile stress leading to their climb as well.
Climb-Induced Glide Mechanisms
In a crystallographically isotropic material with isotropic dislocation distribution,
the climb of dislocations by itself causes no creep strain, but this enables disloca-
tions to break loose of obstacles. Stress then causes slip of the glissile disloca-
tions, resulting in creep strain. Under such circumstances, the creep rate is con-
trolled by climb but the main source of the strain is the dislocation glide. A
number of models based on the concept of climb-enabled glide have been pro-
posed, i.e., I-creep model [19], PAG model [20], climb-controlled glide (CCG)
model [21], and climb-induced yield (CIY) model [22].
Stress-Induced Gas-Driven Mechanism
Coupled with the growth of gas bubbles on the grain boundaries orthogonal to
the stress axis, extension of grains may result from suitably oriented dislocation
loops. This has been termed a ''jacking mechanism'' [23], and is important when-
ever conditions suitable for gas bubble growth prevail.
9.6 IRRADIATION STRENGTHENING AND EMBRITTLEMENT
Particle irradiation brings about large changes in some of the mechanical proper-
ties of metals. The yield stress increases, leading to the phenomenon of irradiation
strengthening or hardening. The flow stress also shows an increase at least in the
initial stages of plastic deformation. The fracture behavior changes, exhibiting
a transition from ductile to brittle features, and the ductile-to-brittle transition
temperature shows an increase on irradiation.
The stress-strain curves of irradiated and unirradiated single crystals of copper
are shown in Fig. 9.7. The increases in yield stress and flow stress at a lower
deformation level are clearly visible, although after appreciable plastic deforma-
tion the difference in the curves essentially disappears. However, in some metals
the whole stress-strain curve is appreciably altered by irradiation, as is shown
for nickel in Fig. 9.8. In copper, iron, and zinc single crystals the critical shear
stress has been observed to increase progressively with exposure to irradiation.
