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