On the Effect of Neutron Flux and Composition on Hardening of Reactor Pressure Vessel Steels and Model Alloys
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On the Effect of Neutron Flux and Composition on Hardening of Reactor Pressure Vessel Steels and Model Alloys G. R. Odette, G. E. Lucas, and D. Klingensmith Department of Mechanical and Environmental Engineering University of California, Santa Barbara Santa Barbara, CA 93106 ABSTRACT Single variable experiments on a set of reactor pressure vessel and model alloys with a range of controlled compositions show a systematic effect of neutron flux (φ) on hardening in the intermediate fluence (φt) regime. The data suggest that high φ and enhanced recombination by vacancy trapping by solutes delays the onset of radiation-enhanced precipitation hardening. INTRODUCTION Increases in transition temperature delineating the cleavage and ductile fracture regimes in nuclear reactor pressure vessels (RPV) as a consequence of exposure to high energy neutrons may be life limiting in some cases [1,2]. This transition temperature shift, or embrittlement, is generally proportional to the increase in the yield stress, which in turn is mediated by the evolution of fine-scale features which result from neutron radiation damage [3]. The evolution of the nanofeatures is controlled by both thermodynamic and kinetic factors. The kinetics intrinsically involve time, as well as irradiation temperature (T). Hence, the proper treatment of dose rate, or φ, effects is a key issue both for practical predictions of RPV embrittlement and for understanding the basic mechanisms underlying this phenomenon. There is substantial theoretical and experimental evidence for such dose rate effects [3-7]. However, several previous studies have suggested that the rate effects are most significant at high dose rates characteristic of accelerated test reactor irradiations. This φ-dependent regime varies with T, but is believed to lie above about 5x1015 n/m2-s at around 290°C. In this regime, this rate effect is believed to be due to enhanced recombination of vacancies and interstitials at small thermally unstable vacancy clusters formed directly in displacement cascades. Enhanced recombination reduces the efficiency of radiation enhanced solute diffusion, delaying the precipitation contribution to hardening. More recently, evidence has also been found for a rate dependent regime at very low φ levels [7]. This is believed to be related to anomalous thermal contributions to solute diffusion, which increases in importance with decreasing φ below about 1014 n/m2-s at about 290°C. Understanding φ effects, and the complex interactions between metallurgical (e.g., Cu, Mn, Ni, microstructure) and irradiation variables (e.g., T and φt) requires a corresponding understanding of the embrittling features and mechanisms that control their evolution. These features are currently believed to fall into approximately three broad categories [2-4]. Copper rich precipitates (CRPs) that are alloyed with other elements (Mn, Ni) resulting from the radiation-enhanced diffusion and precipitation of supersaturated Cu are the dominant hardening feature at low to intermediate φt in sensitive s
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