Issue 50

K. Singh et alii, Frattura ed Integrità Strutturale, 50 (2019) 319-330; DOI: 10.3221/IGF-ESIS.50.27 323 correction factor l f is introduced [9]. This factor scales with average spacing due to the presence of irradiation defects and is multiplied with total strain rate value given in Eqn. 3 (accounting an effect on velocity). ,1 2 s m l min irr irr l f min l N d      =             (11) Dislocations and dislocation-loops density evolution Strain dependence is used to define the dislocation density evolution based on storage (leads to an increase in flow stress) and annihilation (leads to dynamic recovery) phenomenon of dislocation as [11, 18]: 1 1 Λ d y d b      = −     (12) Λ is dislocation mean free path defined as 0 1 1 Λ A A A A self eff obs self forest K K          = − +     and y is temperature dependent material parameter accounting for dynamic recovery expressed as 2 1 1 eff prop y y b   = + . Temperature dependence of y is accounted by using a harmonic average of critical distance in thermal ( 2 eff b   ) and athermal regime ( prop y ). For irradiated materials, the current investigation focuses on dislocation loops (sessile), which are primary irradiation- induced defects. It is assumed that the material has pre-known dislocation loops density ( irr irr irr N d  = ) due to radiation exposure. Further, these defects will interact with the dissociations to induce strain hardening. A gradual reduction in the irradiation defect density on the active slip plane due to their interaction with mobile dislocations is accounted. Rate of change of irr  [8] is defined as: irr irr    = − (13) with  as a loop annihilation parameter. Increase in the value of tot  (accounted in Eqn. 12) and m  is generally observed due to the presence of irradiation defects in the material [12, 19]. The increase in m  may be attributed to the interaction of irradiation defects with screw dislocation lines. The rate of increase of mobile dislocations due to interaction with irradiation defects is expressed as: ( ) 0 m irrr t r     = (14) where  is the number of screw-loop interaction which produces dislocation source before irradiation loop’s removal from lath and 0 r is an initial ratio of irr  over m  . The resistance against the dislocation motion is dependent upon the size (increases for higher defect size) of the defects (loops) along with their number density. Size effect of irradiation defects is incorporated by introducing size factor size f [9] defined as: , 1 irr size r r d f max s d   =      with ( ) , 1 irr irr r irr irr lim N d s min N d   =       (15)