PSI - Issue 5

Kulbir Singh et al. / Procedia Structural Integrity 5 (2017) 294–301 Kulbir Singh/ StructuralIntegrity Procedia 00 (2017) 000 – 000

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due to higher dislocation density and reduction in effective shear stress). Up to 250 K in non irradiated case and 200 K in irradiated case the τ c is governed by τ self and hence remains almost constant. Once the τ LT starts contributing the value of τ c starts increasing and finally leads to significant increase in τ RSS or τ app value. In irradiated case τ LT contribution starts from lower temperature to internal stress, this is mainly due to presence of more obstacles for dislocation movement (forest dislocations and irradiation defects). The higher value of τ self , significant contribution of τ LT from lower temperature range and its higher magnitude leads to significant increase in the τ RSS or τ app value in irradiated case as compared to non-irradiation case. It is evident from this figure that the numerical model is predicting well the effect of irradiation hardening i.e. the increase in yield point as compared to non-irradiation case.

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Fig. 4. (a) Variation of average length of screw dislocation on primary slip system for non-irradiated and irradiated case; (b) Temperature dependence of equivalent stress corresponding to 2 % strain.

The average length of screw dislocation ( l s ) for different temperatures as a function of strain on the primary slip system is shown in Fig. 4a, for non-irradiated and irradiated case. In absence of irradiation, its value decreases with increasing temperature and strain level, due to dislocations multiplication. At higher temperature ranges the value of l s is governed by l min and it remains almost constant (for a given temperature) as it is function of τ eff only. In irradiated case, presence of large number of irradiation defects (interstitial loops) hinders the motion of dislocations hence the average length of screw dislocation is smaller as compared to non-irradiated case. It is interesting to observe that value of l s increases with straining in case of irradiation, whereas it is decreasing in non-irradiated case. This is mainly due to presence of obstacles. The l s value decreases with straining in non-irradiated case due to increase in forest dislocation density. In irradiated case annihilation of irradiation defects to reduce their number with straining dominates on the increase in forest dislocation which leads to net effect of increase in l s value.

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Fig. 5. (a) Strain in secondary slip system vs. strain in primary slip system; (b) Temperature dependence of secondary slip participation in terms of ratio of secondary slip system to primary slip system for non-irradiated and irradiated case.

The numerically predicted equivalent stress corresponding to 2 % plastic strain for non-irradiated and irradiated case is shown in 4b. The hardening effect is more pronounced above 150 K and increases with the straining temperature. The current model is also able to predict the strain localization effect due to irradiation (see Fig. 5a). The ratio ɛ sss / ɛ pss as a function of temperature is shown in Fig. 5b. In non-irradiated case the participation of secondary slip systems increases from straining temperature of 100 K ( ɛ sss / ɛ pss = 0.38) to 250 K ( ɛ sss / ɛ pss = 0.67). Above this temperature the contribution of secondary slip systems starts decreasing. In the presence of defect dispersions however