Radiation Hardening and Plastic Instability in Neutron Irradiated CuCrZr
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Radiation Hardening and Plastic Instability in Neutron Irradiated CuCrZr Dan. J. Edwards1 and Bachu. N. Singh2 1 Materials Development Group Pacific Northwest National Laboratory, MSIN P8-15 Richland, WA 99352, U.S.A. 2 Materials Research Department Risø National Laboratory Roskilde Denmark, DK-4000 ABSTRACT The effect of post-irradiation annealing at 300°C for 50 hours on the microstructure and mechanical properties of CuCrZr irradiated to 0.3 dpa at 100°C has been evaluated. The postirradiation annealing restores some ductility and work hardening to the material as well as lowers the yield strength, however it does not completely remove the effects of irradiation. A comparison of the microstructural features and mechanical properties in the as-irradiated condition and in the post-irradiation annealed case highlights the fact that the observed microstructure does not necessarily correlate with the changes in tensile behavior, most notably in the removal of the yield point and lowering of the yield stress after annealing. INTRODUCTION Radiation hardening in metals and alloys is an important phenomenon that has been studied for over 40 years [1-6], but only recently have attempts been made to apply our knowledge of cascade damage to understanding the effect of irradiation on microstructure and hardening. Singh, Foreman and Trinkhaus reviewed the subject of radiation hardening in metals and alloys [4]. The most commonly accepted approach used to describe radiation hardening is the Dispersed Barrier Hardening model (DBH model), which is derived from the early work of Seeger [1]. The DBH model relates the increase in yield strength to the stress needed to force dislocations to bow around the obstacles in order to bypass them. The validity of this model, however, was recently questioned [4] when applied to irradiated metals and alloys containing a high density of small, shearable defects such as very small dislocation loops and stacking fault tetrahedra. In particular, the formation of a yield point often occurs in irradiated metals and alloys, but this has never been effectively addressed by the DBH model. Singh, Trinkhaus and Foreman recently proposed a different model for explaining the increase in yield strength and the formation of an upper and lower yield point [4]. Their model, called Cascade-Induced Source Hardening (CISH), indicates that the increase in yield strength and the eventual formation of a yield drop in irradiated metals and alloys arises from pinning of the Frank-Read sources by small clusters of SIA and loops. In materials exhibiting a yield drop, the source hardening mechanism will control the yield behavior, not the increased resistance to dislocation glide due to an array of obstacles on the slip plane as assumed in the DBH model. Note that the decoration around the dislocations is a distinct but separate feature of the microstructure of irradiated materials, analogous to the Cottrell atmospheres that produce yield point behavior in low-carbon steels and pure iron [4]. BB4.2.1
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