1. NEEP 541 – Irradiation Creep
Fall 2002
Jake Blanchard

2. Outline
Creep

3. Effect of Irradiation on Thermal Creep
Solid lines=irradiated
Dashed lines=unirradiated

4. Irradiation Creep
Irradiation-enhanced creep=augmentation of thermal creep
Irradiation-induced creep=development of creep under conditions under which thermal creep is absent

5. Two Phenomena
Dislocation climb=dislocations absorb or emit point defects
Dislocation glide=dislocation motion by pure slip
glide
climb

6. 4 Mechanisms SIPN=stress-induced preferential nucleation
SIPA=stress-induced preferential absorption
PAG=preferential absorption glide
Cascade-induced creep

7. 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

8. 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

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

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

11. PAG Dislocations climb preferentially due to defect bias
Net climb velocity

12. Correlations
Typically:

13. Dependence on Swelling Rate

14. Interstitials vs. Vacancies
Vacancies (Em=1.63 eV)
Interstitials Em=0.09 eV

15. 316 SS
Accelerated thermal creep

16. Irradiation Creep at Low T
20% CW 316 Stainless

17. “Disappearing Creep”

18. 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)

19. Effect of Irradiation on Rupture Life – austenitic steel
Larson-Miller
T[C+log(tR)]=constant