Electron Irradiation-Induced Mechanical Property Changes in Reactor Pressure Vessel Alloys
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177 Mat. Res. Soc. Symp. Proc. Vol. 398 ©1996 Materials Research Society
occurring in energetic neutron-induced cascades. Comparison of embrittlement resulting from electron irradiation, in which CC production is negligible, versus neutron irradiation can thus provide some insight into the role that the CC have on CRP formation kinetics and embrittlement. Electron irradiations have been previously used to study microstructural and property changes (e.g. surface hardness) in various model PV alloys [4-l1]. In the present study, tensile property changes in model PV alloys induced by 10-MeV electron irradiation at a nominal temperature of 288°C are examined and compared with neutron irradiation data from the same materials. Unlike previous work, the use of high-energy electrons in this study is advantageous because a greater volume of material may be irradiated, allowing the study of bulk mechanical property effects such as yield stress. EXPERIMENTAL Two model PV alloys were used: material VH was a binary Fe-0.9 wt. % Cu alloy and material VD was a ternary Fe-0.9 wt. % Cu- 1.0 wt. % Mn alloy. Sheet mini-tensile specimens with a rectangular gage section 9 mm x 2 mm were precision die punched for use in the tensile tests. Electron irradiations were performed with 10-MeV electrons from a pulsed linac operating at 15 Hz. The beam was incident on samples in a vacuum chamber after passing through a thin aluminum foil window and a 12 mm diameter collimator. The sample chamber was backfilled with He gas held at a slight vacuum between -130 mm Hg and -510 mm Hg relative to atmospheric pressure. Sample temperature was controlled by varying the electron beam current incident on the samples, which varied between 14 g.A and 23 4iA. Assuming a damage cross section of 86 barns for 10-MeV electrons in a medium Z material [12], these beam currents corresponded to respective damage rates of 6.6x10 9 dpa s' and l.1xl0- dpa s1 (dpa=displacements per atom). Temperature was monitored using a thermocouple spot-welded to the middle of the gage section of a sample located near the center of the irradiated area defined by the collimator. In addition, thermocouples were used to monitor the temperature on the same sample's shoulders away from the gage section, which lay outside of the electron beam. Heat sinking at the clamped sample end resulted in a slightly asymmetric temperature distribution along the gage section. With the middle of the gage section held at 288°C, it was estimated, using the shoulder temperature measurements, that the temperatures at the extremities of the 9.0 mm long gage section were 281VC and 250'C, with the lower temperature at the clamped end of the sample. Neutron irradiation data on the VH and VD alloys for a single fluence were obtained from experiments performed in the University of Michigan Ford Nuclear Reactor (UM FNR) [13]. Samples were sealed in He-backfilled aluminum capsules and irradiated at 288°C. The neutron flux (E>1 MeV) was 7x10 1 cm.2 s-1 and the neutron fluence achieved was IxI0 19 cm"2. Assuming a di
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