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NEEP 541 – Irradiation Creep
Fall 2002Jake Blanchard
Outline Creep
Effect of Irradiation on Thermal Creep
Solid lines=irradiated
Dashed lines=unirradiated
Irradiation Creep Irradiation-enhanced
creep=augmentation of thermal creep
Irradiation-induced creep=development of creep under conditions under which thermal creep is absent
Two Phenomena Dislocation climb=dislocations
absorb or emit point defects Dislocation glide=dislocation
motion by pure slip
climb
glide
4 Mechanisms SIPN=stress-induced preferential
nucleation SIPA=stress-induced preferential
absorption PAG=preferential absorption glide Cascade-induced creep
Notes Creep is anisotropic and volume
conservative Therefore, irradiation creep
requires preferential dislocation motion
Either loops are nucleated in preferential directions or they grow in preferential directions
SIPN Assumes loops are preferentially nucleated on
planes perpendicular to the stress Loops grow by defect absorption Growth is independent of stress, so strain
should continue if stress is removed Hence model is applicable only to transients
S
kT
nb
dt
d
d
dc
3
9
2
SIPN
S
kT
nb
dt
d
d
dc
3
9
2
# of interstitials in critical loop
Loop density
Swelling rate
Total dislocation
density
SIPA Defects absorbed preferentially
by dislocations of particular orientations
Mechanism relies on interaction between defects and elastic stress fields around dislocations
Gdt
d c
PAG Dislocations climb preferentially due
to defect bias
ELf
CDzCDzb
v
vf
vvviiDi
Net
climb velocit
y
Correlations Typically:
64
n
A
SDBnc
th
cirr
Dependence on Swelling Rate
Interstitials vs. Vacancies
Vacancies (Em=1.63
eV)Interstitials
Em=0.09 eV
316 SS
Accelerated thermal creep
Irradiation Creep at Low T
20% CW 316 Stainless
“Disappearing Creep”
Why does creep disapear? Without swelling, dislocation and loop
microstructures become progressively more anisotropic (important to SIPA)
When voids begin to form, interstitials feed this microstructure faster than if voids weren’t present
Hence, irradiation creep accelerates as swelling begins
As swelling increases, microstructure becomes more isotropic (so SIPA stops)
Effect of Irradiation on Rupture Life – austenitic steel
Larson-MillerT[C+log(tR)]=constan
t
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