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2014 ANNUAL REPORT 103 while the radii of the clusters in both steels are similar 0.97 m 0.23 m and 0.9 m 0.16 m for UFG and CG steels respectively the density of clusters in the UFG steels 1.2 1024 m-3 is about twice that of CG steel 6.71023 m-3 Figure 3. Researchers believe this is due to the shorter distance defects have to diffuse in UFG before they reach the grain boundary and the resulting lower probability of defect recombination in the matrix. No clusters were observed before irradiation indicating that their formation was radiation-induced. Vickers micro hardness tensile test and dislocation density results are shown inTable 1. The average micro hardness values for CG steel increased by 62 after irradiation compared to only 8.6 for the UFG steel Figure 4.Tensile test results revealed that CG yield strength increased by 132 after irradiation and its ductility decreased by 82 while the yield strength of UFG steel increased by 30 and the ductility reduced by 56 Figure 5.Although irradiation hardening was minute in the UFG steel compared to its CG counterpart the irradiation-induced embrittlement is clearly apparent in the UFG steel after irradiation albeit the percentage decrease in the ductility of UFG steel is quite less than that of its CG counterparts. According to Odette and Lucas 6 the primary mechanism for embrittle- ment in ferritic steels is the hardening produced by nanometer-sized features that develop as a consequence of radiation exposure. Since our results showed that there is no significant change in dislocation density in UFG steel after irradiation the observed hardening in UFG steel is likely a product of the high density of irradiation-induced Mn-Si-enriched clusters found in UFG steel Table 2. To gain a better understanding of the effect of irradiation on microstructural changes in the steels mechanical properties refer to Alsabbagh et. als 2014 journal article 7 for a discus- sion of how the increase in yield stress Dosedpa m-2 Yield Strength MPa Tensile Strength MPa Vickers Hardness MPa Ductility CG 0 1.06 0.131014 29617 39018 128413 633 1.37 4.26 0.561014 68720 75420 208151 111 UFG 0 9.50 1.241014 7759 98011 308854 182 1.37 8.98 1.391014 100949 106043 335368 81 Table 1. Mechanical properties for both CG and UFG steels before and after irradiation. Figure 4. Micro hardness before and after irradiation for both UFG and CG low carbon steel Oro-Ash MPa Dis MPa Calculated MPa Measured Mpa CG Steel 137 44 245 35 382 57 391 27 UFG Steel 230 90 -20 73 210 115 234 50 Table 2. Estimated strength increments for different strengthening mechanisms for both CG and UFG steels.