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Title: Development of Computational Tools for Predicting Thermal- and Radiation-Induced Solute Segregation at Grain Boundaries in Fe-based Alloys

Abstract

Radiation-induced segregation (RIS) has been frequently reported in structural materials such as austenitic, ferritic, and ferritic-martensitic stainless steels (SS) that have been widely used in light water reactors (LWRs). RIS has been linked to secondary degradation effects in SS including irradiation-induced stress corrosion cracking (IASCC). Earlier studies on thermal segregation in Fe-based alloys found that metalloids elements such as P, S, Si, Ge, Sn, etc., embrittle the materials when enrichment was observed at grain boundaries (GBs). RIS of Fe-Cr-Ni-based austenitic steels has been modeled in the U.S. 2015 fiscal year (FY2015), which identified the pre-enrichment due to thermal segregation can have an important role on the subsequent RIS. The goal of this work is to develop thermal segregation models for alloying elements in steels for future integration with RIS modeling.

Authors:
 [1]
  1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Nuclear Energy (NE). Light Water Reactor Sustainability (LWRS) Research and Development Effort
OSTI Identifier:
1328318
Report Number(s):
ORNL/TM-2016/460
RC0304000; NERC006; TRN: US1700339
DOE Contract Number:
AC05-00OR22725
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS; AUSTENITIC STEELS; FERRITIC STEELS; STAINLESS STEELS; MARTENSITIC STEELS; GRAIN BOUNDARIES; SEGREGATION; WATER MODERATED REACTORS; CRACKING; STRESS CORROSION; WATER COOLED REACTORS; SEMIMETALS; SOLUTES; PHYSICAL RADIATION EFFECTS; TEMPERATURE DEPENDENCE; COMPUTERIZED SIMULATION; MATHEMATICAL MODELS

Citation Formats

Yang, Ying. Development of Computational Tools for Predicting Thermal- and Radiation-Induced Solute Segregation at Grain Boundaries in Fe-based Alloys. United States: N. p., 2016. Web. doi:10.2172/1328318.
Yang, Ying. Development of Computational Tools for Predicting Thermal- and Radiation-Induced Solute Segregation at Grain Boundaries in Fe-based Alloys. United States. doi:10.2172/1328318.
Yang, Ying. 2016. "Development of Computational Tools for Predicting Thermal- and Radiation-Induced Solute Segregation at Grain Boundaries in Fe-based Alloys". United States. doi:10.2172/1328318. https://www.osti.gov/servlets/purl/1328318.
@article{osti_1328318,
title = {Development of Computational Tools for Predicting Thermal- and Radiation-Induced Solute Segregation at Grain Boundaries in Fe-based Alloys},
author = {Yang, Ying},
abstractNote = {Radiation-induced segregation (RIS) has been frequently reported in structural materials such as austenitic, ferritic, and ferritic-martensitic stainless steels (SS) that have been widely used in light water reactors (LWRs). RIS has been linked to secondary degradation effects in SS including irradiation-induced stress corrosion cracking (IASCC). Earlier studies on thermal segregation in Fe-based alloys found that metalloids elements such as P, S, Si, Ge, Sn, etc., embrittle the materials when enrichment was observed at grain boundaries (GBs). RIS of Fe-Cr-Ni-based austenitic steels has been modeled in the U.S. 2015 fiscal year (FY2015), which identified the pre-enrichment due to thermal segregation can have an important role on the subsequent RIS. The goal of this work is to develop thermal segregation models for alloying elements in steels for future integration with RIS modeling.},
doi = {10.2172/1328318},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2016,
month = 9
}

Technical Report:

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  • This work aims at developing computational tools for modeling thermal and radiation effects on solute segregation at grain boundaries (GBs) and precipitation. This report described two major efforts. One is the development of computational tools on integrated modeling of thermal equilibrium segregation (TES) and radiation-induced segregation (RIS), from which synergistic effects of thermal and radiation, pre-existing GB segregation have been taken into consideration. This integrated modeling was used in describing the Cr and Ni segregation in the Fe-Cr-Ni alloys. The other effort is thermodynamic modeling on the Fe-Cr-Ni-Mo system which includes the major alloying elements in the investigated alloys inmore » the Advanced Radiation Resistant Materials (ARRM) program. Through thermodynamic calculation, we provide baseline thermodynamic stability of the hardening phase Ni2(Cr,Mo) in selected Ni-based super alloys, and contribute knowledge on mechanistic understanding on the formation of Ni2(Cr,Mo) in the irradiated materials. The major outcomes from this work are listed in the following: 1) Under the simultaneous thermal and irradiation conditions, radiation-induced segregation played a dominant role in the GB segregation. The pre-existing GB segregation only affects the subsequent radiation-induced segregation in the short time. For the same element, the segregation tendency of Cr and Ni due to TES is opposite to it from RIS. The opposite tendency can lead to the formation of W-shape profile. These findings are consistent with literature observation of the transitory W-shape profile. 2) While TES only affects the distance of one or two atomic layers from GBs, the RIS can affect a broader distance from GB. Therefore, the W-shape due to pre-existing GB segregation is much narrower than that due to composition gradient formed during the transient state. Considering the measurement resolution of Auger or STEM analysis, the segregation tendency due to RIS should play a dominant role in the measured values. However, The GB segregation due to pre-existing GB segregation may affect the chemical potential of element at GB, and subsequently the corrosion resistance. 3) Based on the newly developed thermodynamic database of Fe-Cr-Ni-Mo, we predicted the Ni2(Cr,Mo) as a thermodynamically stable phase in all investigated low Fe-content Ni-based alloys. The calculated phase amount decreases with the increasing Fe content, being consistent with that observed in the irradiated materials. 4) The formation of the Ni2(Cr,Mo) phase in irradiated materials is due to irradiation enhanced diffusion. The calculated equilibrium Ni2(Cr,Mo) amount is more than that observed in the irradiated materials, suggesting that the amount of Ni2(Cr,Mo) is likely to increase more with further irradiation.« less
  • Thin film Fe--0.8 at. % Au bicrystals, /approximately/60 nm thick, containing (001) twist boundaries were prepared. The average compositon of the bicrystal was determined by Rutherford backscattering spectroscopy (RBS), which also showed that the Au was nonuniformly distributed along the direction normal to the interface, with a large amount concentrated at the (001) twist boundary. This observation constitutes direct evidence that the Au is segregated to the vicinity of the grain boundary. The Au distribution within the plane of the interface was analyzed in a Vacuum Generators HB-501 scanning transmission electron microscope (STEM) equipped with a field emission electron sourcemore » and a Link Analytical windowless x-ray detector. The boundary plane was oriented perpendicular to the electron beam. Digital line scans were performed across the dislocation network. Au segregates extensively to the cores of the <100>-type dislocations in small angle (001) twist boundaries in Fe--Au alloys. Our observations demonstrate that the <110> network is favored in pure Fe, which leads to the suggestion that the dislocation core energy plays an important role in determining the stable boundary structure.« less
  • The dislocation structure of small and large angle (001) twist boundaries in Fe/endash/Sb alloys is influenced by the segregation of Sb to the vicinity of the interface. Since changes in the dislocation structure of (001) twist boundaries due to solute segregation have now been observed in the Fe/endash/Au and Fe/endash/Sb systems, this behavior is believed to represent a generally occurring phenomenon in Fe alloys. 18 refs., 3 figs.
  • The studies of irradiation-induced solute segregation (IISS) and irradiation-induced precipitation (IIP) in Ni-Si and Pd-Fe alloys have been completed. Progress is reported for several other projects: irradiation damage in binary Pd-Cr, -Mn and -V alloys (15 at. %); IIP in Pd-Mo and Pd-W alloys; IIP in Pd-25 at. % Cr alloy; and irradiation damage effects in proton-bombarded metallic glasses (Ni-65 Zr, 40 Fe 40 Ni 14 P6B). 27 figures. (DLC)
  • The phenomena of radiation-induced precipitation and solute segregation effects in a W-25 at.% Re alloy have been investigated using the atom-probe field-ion microscope. This alloy is supersaturated with respect to the solvus line of the primary ..beta..-solid solution; it is in the two-phase ..beta..-plus-sigma region. The specimens had been irradiated in the EBR-II to a fast-neutron fluence of approx. 4 x 10/sup 22/ neutrons cm/sup -2/ (E > 0.1 MeV) at 575, 625, and 675/sup 0/C. This fluence corresponds to 8.6 dpa and an average displacement rate, for the two-year irradiation time, of 1.4x10/sup -7/ dpa s/sup -1/. Results showmore » significant alteration of the microstructure of this alloy as a result of neutron irradiation. Precipitates of the composition approx. WRe/sub 3/ (chi phase) have been detected. Coherent, semicoherent, and incoherent precipitates have been observed. Semicoherent and incoherent precipitates were associated with /sup 4/He atoms, i.e., heterogeneous precipitation has ocurred in this alloy. A very strong Re enrichment of grain boundaries has been observed, and no segregation to voids has occurred. Point-defect mechanisms for all of these observations are discussed.« less