Mechanisms of Neutron Irradiation Hardening in Impurity-Doped Ferritic Alloys
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INTRODUCTION
NEUTRON irradiation embrittlement leading to an increase in the ductile-brittle transition temperature (DBTT) proceeds in aging nuclear reactor pressure vessel (RPV) steels and impurity-doped ferritic alloys, due to hardening[1–4] and intergranular phosphorus (P) segregation.[3,5–8] Irradiation hardening, which facilitates the growth of a brittle crack by suppressing dislocation activities near the crack tip, is a common problem for a variety of RPV steels, and P segregation weakens grain boundaries in RPV steels such as the C-Mn steel and the 2 14Cr-1Mo steel, but does not produce much grain-boundary weakening in the A533B steel. High-purity (HP) A533B steels, neutron irradiated to a high fluence of 1.1 · 1024 n/m2, reveal weak hardening caused by defect clustering.[4] Meanwhile, strong interactions of neutron-irradiation-induced defects with metalloids and metallic impurities lead to nonequilibrium phase transformation, that is, the formation of nanosized precipitates[2,3] the dispersion of which causes irradiation hardening. In neutron-irradiated RPV steels, copper (Cu) becomes the nuclei of the ultrafine precipY. NISHIYAMA, Research Scientist, is with the Japan Atomic Energy Agency, Tokai, Ibaraki 319, Japan. X.Y. LIU, Research Associate, is with the Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia. J. KAMEDA, Senior Scientist, Structural Integrity Associates, Inc., Annapolis, MD 21401, is Senior Fellow, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. Contact e-mail: [email protected] Manuscript submitted September 17, 2007. Article published online March 21, 2008 1118—VOLUME 39A, MAY 2008
itates, combined with alloying elements of manganese (Mn), nickel (Ni), and silicon (Si).[3] Increasing the irradiation fluence results in an increased density of Cu-rich precipitates and suppresses the coarsening that usually occurs under prolonged thermal aging. Atomistic molecular dynamic simulation[9] has demonstrated that peculiar precipitation behavior under neutron irradiation is ascribed to the dragging effect of copper and vacancy fluxes. Likewise, the presence of phosphorus is found to promote irradiation hardening in ferritic alloys and RPV steels, although the mechanism is not sufficiently known.[6,10–14] The DBTT shift in neutron-irradiated ferritic alloys is related to the temperature dependence of the hardening behavior. However, irradiation hardening behavior has generally been studied at ambient temperatures as a function of irradiation fluence rather than of irradiation temperature. As shown in Figure 1, two mechanisms controlling dislocation motion through a field of discrete obstacles (DOs), i.e., solid solution or precipitation and lattice resistance (LR), dominate in high- and lowtemperature ranges, respectively.[15–17] Therefore, an investigation into the effect of neutron irradiation on DO- and LR-controlled hardening is necessary in alloys with precipitated and dissolved impurities. To gain a better u
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