NEEP 541 – Irradiation Creep Fall 2002 Jake 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 glide climb

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

SIPN # of interstitials in critical loop Loop density Total dislocation density Swelling rate

SIPA Defects absorbed preferentially by dislocations of particular orientations Mechanism relies on interaction between defects and elastic stress fields around dislocations

PAG Dislocations climb preferentially due to defect bias Net climb velocity

Correlations Typically:

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-Miller T[C+log(tR)]=constant