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Arab Journal of Nuclear Science and Applications, 46(2), (107-114) 2013 The rate of change of χp with irradiation dose (δ χp / δ D) is calculated from the slopes of fig.6 and its dependence on the deformation temperature Tw is shown in fig 7. From this figure it is clear that (δ χp / δ D) decreases with increasing the deformation temperature. Work-Hardening Coefficient p (MPa) 2 x 10 1.8 5 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 S.R = 1.5x10 -3 S -1 303K 373K 398K 423K 448K 473K 523K 0 500 1000 1500 2000 2500 Dose D (kGy) ( p / D ) (MPa 2 /kGy) 771 40 38 36 34 32 30 28 26 24 S.R = 1.5x10 -3 S -1 22 350 400 450 500 550 Deformation Temperature T (K) w Fig. (6): Irradiation dose dependence of Fig. (7): Dependence of the rate of change the work-hardening coefficient χp of χp with irradiation dose on the for Al-5356 alloy specimens tested deformation temperature Tw. at different deformation temperatures. (b) The Activation Energy (Q) The energy activating the deformation process may be evaluated from the variation of the yield stress σy as related to the deformation temperature Tw through the kinetic rate equation (19) : σy = k [ε˙ exp (Q/RT)] m ( 1 ) where k is a material constant, ε˙ is the strain rate, R is the universal gas constant and m is the strain rate sensitivity index. To estimate the value of the activation energy Q for the deformation process, the standard Arrhenious plot of ln σy versus 1000/T is to be constructed as shown in fig. 8. From the slopes of the obtained straight lines, the mean value of the activation energy Q was found 80 kJ/mole independent on the irradiation dose which is in accordance with that obtained in similar work by Peng Yong-yi (20) . ln y 4.5 4.4 4.3 4.2 4.1 4.0 3.9 S.R = 1.5x10 -3 S -1 2000 kGy 1000 kGy 500 kGy unirradiated 3.8 1.8 2.0 2.2 2.4 2.6 2.8 3.0 1000/T K -1 Fig. (8): Arrhenius plot of lnσy vs. 1000/T for Al-5356 alloy.
Arab Journal of Nuclear Science and Applications, 46(2), (107-114) 2013 (1) Effect of γ-Irradiation DISCUSSION As has been mentioned before, the primary cause of radiation damage is the displacement of atoms and electrons due to the creation of the induced defects (13) . Defects produced during irradiation have an influence upon the structural order- disorder and other properties (21) . In ordered lattice, migration of vacancies is difficult, so impurity- vacancy recombination process during γ-irradiation more likely to take place. Hence, these defects (vacancy centers) would redistribute the fine structure of β-phase configuration and the dislocations become more able to multiply (15) . This may lead to more strengthening of the irradiated samples. Furthermore, the increase of the irradiation dose leads to an increase in the creation of the induced defects. Thus the increase in the work hardening parameters σy, σf and χp consequently the decrease in the total strain εT can be explained. On the other hand, although, the presence of some retained Mg, Cu, Si atoms in the solid solution treated samples besides β (Al3Mg2) phase as has been detected from X-ray diffraction analysis which cause an anchoring of the dislocations created during the plastic deformation, the creation of the induced defects (vacancies) as a result of γ-irradiation makes it easier for the dislocations to interact with these vacancies rather than anchored by the other entities. So, the disappearance of the serration behavior by irradiation may be accounted for. (2) Effect of Deformation Temperature It is well known that increasing deformation temperature of strained material results in: (i) An increase of the thermal agitation for the dislocation glide motion and easier overcoming the obstacles leading to the increase of dislocation slip distance (22) . (ii) The annihilation of the present dislocations as well as dislocation induced by plastic strain once they are formed (23) . (iii) Increasing the rate of reorientation of the remaining dislocations in a more relaxed configuration aligned in the direction of the applied stress (24) . From these viewpoints, the decrease in the work hardening parameters σy, σf, χp and increase in εT with increasing the deformation temperature Tw (Figs.3, 4 and 6) can be explained. Also, the disappearance of the PLC phenomenon at higher deformation temperatures may be retained due to the thermal agitation which assists the dislocations to breakaway from the binning centers. (3) The Activation Energy (Q) The value of the energy activating the deformation process Q calculated from fig.6 was found to be ~ 80 kJ/mole which is in accordance with that needed for the activation energy of grain boundary diffusion in aluminum alloy and this in agreement with the results obtained in other works (20, 25- 27) . CONCLUSIONS The main conclusions to be drawn from this study may be summarized as follows: (i) The WHP σy, σf, χp and εT were found to be affected by γ-irradiation. Where σy, σf and χp increased with increasing of the irradiation dose D, while εT decreased. This is due to creation of the induced defects leading to redistribute the fine structure of β-phase configuration and the dislocations become more able to multiply. 771
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Page 5: Yield Stress y (MPa) 85 80 75 70 6

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