NEEP 541 � Irradiation Creep by C0vcd6US

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									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

              d c
                   2  b3n   d 
                               
                   
                   9 kT   S
                           
               dt           d
   SIPN
# of interstitials in            Loop density
   critical loop


          d  c
               2b n  3   

                 kT   S
                          d
           dt  9     d

     Total dislocation             Swelling
         density                     rate
SIPA
   Defects absorbed preferentially by
    dislocations of particular orientations
   Mechanism relies on interaction
    between defects and elastic stress
    fields around dislocations

              d  c
                       
                   
               dt      G
       PAG
          Dislocations climb preferentially due to
           defect bias

              vf
            
                D
            v  zi Di Ci  zv Dv Cv 
  Net
 climb
velocity       b
                        
             f  L
                         E
Correlations
   Typically:

                    
             B  DS
             c
             irr

             A
             c
             th
                     n


           n  46
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

								
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