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NEEP 541 – Irradiation Creep

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

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NEEP 541 – Irradiation Creep

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  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

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