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I.

INTRODUCTION

D U R I N G the irradiation of an alloy or steel, the different atomic species in the material are redistributed, with some species segregating toward point defect sinks and others migrating away. This phenomenon of nonequilibrium segregation during irradiation was predicted by Anthony tlj and has since been observed in a wide variety of alloys and in steels used as components in nuclear reactors, j21 It has important implications for the behavior of irradiated materials; for example, steels irradiated in advanced gas-cooled reactors suffer from increased intergranular corrosion as a result of the segregation of chromium away from grain boundaries, t31 Nonequilibrium segregation in irradiated metals or alloys is a consequence o f the high concentrations of vacancies and interstitials produced by interactions of high-energy neutrons or ions with the atoms of the material. These vacancies and interstitials exchange more readily with the atoms of some of the elements than with atoms of other elements, and this produces redistribution of the different atomic species in regions close to sinks where the concentration gradients of the vacancies and interstitials are large. In some cases, the solute atoms can be bound by vacancies and interstitials, and the solute atom then migrates as part of a point defect-solute pair. This can contribute strongly to the segregation of the solute atoms. In general, it appears that the concentration of undersized atoms rises in regions close to sinks and falls elsewhere, while the concentration of oversized atoms falls in regions close to sinks and rises elsewhere. Strong segregation of solute atoms toward point defect sinks can lead to precipitation at point defect sinks. For example, Barbu and Ardell t41 observe Ni3Si precipitates around the periphery of dislocation loops in an Ni-2 pct Si alloy. In addition to this redistribution of alloying elements around point defect sinks, irradiation-induced instabilities can produce oscillations in composition in irradiated alloys even when there appear to be very few

sinks, rSl These fluctuations in composition can lead to the formation of precipitates in undersaturated solid solution alloys; for example, Cauvin and Martin I6~ investigated the formation of zinc precipitates in an A1-1.9 pct Zn alloy and found good agreement between their experimental results and the results of their theoretical calculations of irradiation-induced instabilities. Fluctuations in composition induced by irradiation also occur in more complex materials such as Fe-Cr-Ni alloys. 17,sl Brager and Garner t71 suggest that the fluctuations in composition which they observe in alloys containing approximately 35 pct nickel are the result of a spinodal decomposition which is accelerated by the irradiation. Very similar oscillations in composition were observed by Williams e t al. tS~ in alloys containing only 15 pet nickel. However, these alloys showed no sign of decomposition after thermal aging for 13,300 hours at 645 ~ and therefore, it seems unlikely th