Bulk processing of materials with radiation
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I.
INTRODUCTION
MUCH of
the difficulty with irradiation damage to components in nuclear systems is that the damage and its effects are produced throughout the bulk of the exposed material. This is due, primarily, to the large mean free path of energetic neutrons and their ability to produce significant numbers of atomic displacements. Similar bulk damage is possible from high-energy charged particles, and this is becoming a particular concern for high-energy particle accelerator applications. In all of these cases, radiation effects are seen to degrade materials' properties or performance and are meant to be avoided or mitigated where possible. In the present paper, the possibilities for producing beneficial bulk effects by irradiation are examined. Processing of crystalline materials has traditionally proceeded by means of thermal-mechanical treatment, where elevated temperature or mechanical working or both have been applied to obtain desired material microstructures and concomitant materials properties. There may be a context in which it is possible to consider the use of radiation, either on its own or in conjunction with the other two materials processing techniques, to obtain desired materials properties. Table I lists desirable parameters for processing. A successful radiation-enhanced processing (REP) strategy must accommodate the inherent disparities in these parameters. The first item precludes the use of certain types of radiation, particularly those which produce radioactive daughter products through capture and decay processes, that is, neutrons and high-energy particle beams. The use of low-energy particle beams has been pursued but is limited by the relatively short range of ions in materials, allowing for only surface and near-surface modification. Furthermore, large beam currents are required to implant adequate levels of ions in short times at low costs. In order to concentrate on the processing of bulk materials, the focus must be drawn to radiation with large JAMES F. STUBBINS, Professor, is with the Department of Nuclear Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801-2984. 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
mean free paths in materials. This suggests the use of gamma rays and/or energetic electrons for radiation processing. The immediate drawback of using radiation with a large mean free path is that, by definition, it does not highly interact with the material. Thus, we are limited to materials applications where the process time is extremely lengthy, the source strength is very high, the amount of processed material is small, or to applications where only low levels of interactions produce desirable effects. These are particularly
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