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    An Analysis of Small Clusters Formed in Thermally Aged and Irradiated FeCu and FeCuNi Model Alloys

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    The pressure vessel of a pressurized water reactor, containing residual levels of copper, can be embrittled by the radiation-enhanced formation of copper-rich clusters during operation at 290°C.

    It is important to study the effect of irradiation and materials variables, such as nickel content, on the nature and development of these clusters to increase our understanding of the underlying mechanisms and to improve the theoretical models used to predict embrittlement trends. To achieve this, it is necessary to employ a variety of analytical techniques and to investigate simple model alloys, as well as complex, low-alloy steels.

    In this paper, atom-probe field-ion microscopy, small-angle neutron scattering and transmission electron microscopy have been used to study the precipitation in iron copper (FeCu) and iron-copper-nickel (FeCuNi) alloys thermally aged for up to 10 h at 550°C, or irradiated to doses up to 6.95×1019ncm2 (E>1MeV) at 290°C.

    The results on precipitate composition, number density, size distribution, and matrix composition obtained using the three techniques are compared and contrasted. In general, their agreement is good.

    For the thermally aged alloys, the most notable results include the clear observation of copper (Cu)-rich precipitates (with a diameter < 6 nm), evidence for the retention of a significant copper fraction in solid solution at the peak of hardening, for a bimodal distribution of particle sizes in the overaged condition, which was enhanced by the presence of nickel (Ni), and for a Ni “shell” around the precipitates in the ternary alloy. The precipitates were essentially pure copper (Cu>95%).

    For the irradiated alloys, there was some evidence of a difference in the matrix copper contents between FeCu and FeCuNi. Irradiation virtually depleted the FeCu matrix of copper at the lowest dose, 5×1018ncm2(E>1MeV), but copper was retained up to the highest dose, 6.95×1019ncm2 (E>1MeV), in the FeCuNi. The amount of Ni associated with the irradiation-produced precipitates was significantly higher than for those produced thermally; however, it has not been established whether this was within the particles or in the form of a “shell.”

    The microstructural results are discussed in light of the observed changes in hardness properties after aging or irradiation.


    iron alloys, thermally aged alloys, irradiation, hardness properties, radiation embrittlement

    Author Information:

    Buswell, John T.
    Research officer, CEGB Berkeley Nuclear Laboratory, Berkeley,

    English, Colin A.
    Group leader, Harwell Laboratory, Didcot,

    Hetherington, Mark G.
    Research fellow, Oxford University, Oxford,

    Phythian, William J.
    Senior scientific officer, Harwell Laboratory, Didcot,

    Smith, George D. W.
    Lecturer, Oxford University, Oxford,

    Worrall, Geoffrey M.
    Senior scientific officer, Risley Laboratory, Risley,

    Committee/Subcommittee: E10.07

    DOI: 10.1520/STP49447S