Impurity-defect interactions and radiation hardening and embrittlement in BCC metals
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I.
INTRODUCTION
THE critical importance of interstitial
impurities in bcc metals has been amply illustrated over many years by the well-known behavior of carbon and nitrogen in ferritic steels, and metallurgists have long studied the mechanisms by which interstitial impurities alter the properties of ferritic steels and other bcc metals. Nevertheless, as has been found true since the practice of nuclear metallurgy began in the early days of the atomic energy enterprise, traditional ideas frequently must be reassessed when metals are subjected to the special effects of atom-displacing radiation. As concerns the role of interstitial impurities, a body of evidence is presented below that indicates that radiation-produced defects serve as trapping centers for interstitial impurities, having farreaching effects on physical and mechanical properties. II.
IMPURITY-DEFECT INTERACTIONS
A . Vanadium
A postirradiation resistivity annealing stage occurs in bcc metals at about 0.2 Tm (Tin = melting point in degrees K) or at about 170 ~ for vanadium. ]~-41As is seen in Figure 1 from Reference 3, the resistivity of vanadium increases upon neutron irradiation at 60 ~ Upon postirradiation annealing, not only does the radiationproduced increment in resistivity anneal out, but the resistivity decreases below the initial unirradiated value. Furthermore, the resistivity decrease increases with inM.S. WECHSLER, Professor, is with the Departments of Materials Science and Engineering and of Nuclear Engineering, Iowa State University, Ames, IA 50011. K.L. MURTY, Professor, is with the Departments of Nuclear Engineering and of Materials SCience and Engineering, North Carolina State University, Raleigh, NC 27695-7909. This paper is based on a presentation made in the symposium ~Irradiation-Enhanced Materials Science and Engineering" presented as part of the ASM INTERNATIONAL 75th Anniversary celebration at the 1988 World Materials Congress in Chicago, IL, September 25-29, 1988, under the auspices of the Nuclear Materials Committee of TMS-AIME and ASM-MSD. METALLURGICAL TRANSACTIONS A
creasing oxygen content. The implication, therefore, is that the annealing stage is not due to the removal of an intrinsic radiation-produced defect but is controlled instead by the motion of the interstitial impurities, particularly oxygen atoms, to the radiation-produced defects, where they are trapped and effectively removed from solid solution. Internal friction measurements have been exceedingly helpful in elucidating the mechanism of interactions between interstitial impurity atoms (IIA's) and radiationproduced defects in bcc metals. Isolated IIA's give rise to anelastic or Snoek relaxation, and the height of the relaxation peak is a direct measure of the concentration of a particular identifiable IIA in solid solution. When the irradiated bcc metal is postirradiation annealed at temperatures where the IIA is mobile, a decrease in internal friction is observed that is interpreted as being due to the trapping of the IIA's at radiation-produced defect
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