Effect of Bulk Phosphorus Content on Hardening, Non-Equilibrium Segregation and Embrittlement in Neutron Irradiated Iron
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Effect of Bulk Phosphorus Content on Hardening, Non-Equilibrium Segregation and Embrittlement in Neutron Irradiated Iron-Based Alloys Y. Nishiyama1, T. E. Bloomer2,3 and J. Kameda2,4 Japan Atomic Energy Research Institute, Ibaraki, Japan 2 Ames Laboratory, Iowa State University, Ames IA, USA 3 Presently, U.S. Nuclear Regulatory Commission, Washington DC 4 Office of Naval Research International Field Office, Tokyo, Japan 1
ABSTRACT The effect of bulk P contents on hardening, non-equilibrium intergranular segregation and embrittlement has been studied in Mn-doped ferritic alloys subjected to neutron irradiation (E>0.1MeV: fluence of 1 x 1025 n/m2 at 711K for 2120 h) or irradiation-equivalent thermal aging. Neutron irradiation-induced intergranular P segregation became more prominent with decreasing bulk P content. Thermal aging slightly enhanced the amount of segregated P independent of the bulk P content. Intergranular C segregation in all the alloys was suppressed by the irradiation. An alloy with low bulk P content showed only moderate irradiation-induced hardening. The ductile-brittle transition temperature (DBTT) in alloys with low and intermediate amounts of P increased by the same shift during the irradiation but not at all during the thermal aging. Doping high bulk P led to a high DBTT in the as-heat-treated alloy while the irradiation decreased the DBTT. The irradiation effect on the DBTT in the model ferritic alloys containing the different levels of P is discussed in light of embrittling or toughening effects caused by the changes in the P or C segregation, and hardness. INTRODUCTION Neutron irradiation induces hardening, thereby increasing the ductile-brittle transition temperature (DBTT) in ferritic alloys. Fundamental and applied studies on irradiation-induced hardening and embrittlement have been made in model ferritic alloys and nuclear reactor pressure vessel steels [1]. Intergranular P segregation in ferritic alloys, leading to grain boundary failure, is promoted by neutron irradiation. This is related to coupled fluxes of impurity and point defects near grain boundaries generated by irradiation [2,3]. However, the mechanism of irradiation-induced non-equilibrium segregation and embrittlement has not been fully understood. Hardening and intergranular segregation behavior induced by neutron irradiation (E > 0.1 MeV: 9.4 x 1022 n/m2 at 668 K for 127 h) have been investigated in Mn-free ferritic model alloys doped with P and/or Cu containing residual S [4,5]. It has been shown that the neutron irradiation induced S segregation in alloys without P doping. In P-doped alloys, grain boundary P segregation was enhanced by the neutron irradiation while the S segregation was reduced. A recent kinetic study during post-irradiation annealing (PIA) [6] has revealed that P-interstitial complexes promote the P segregation at lower PIA temperatures whereas vacancy-enhanced diffusion controls the S segregation, which occurs over a wider PIA temperature range. Increasing PIA temperature resulted in P desegregation as
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