A model describing neutron irradiation-induced segregation to grain boundaries in dilute alloys
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I.
INTRODUCTION
GRAIN boundaries are regions of discontinuity in the crystal structure of a material and, as a consequence, they are important in controlling the overall physical and mechanical properties. Some engineering problems can be encountered and associated with the chemical composition of these specific regions of the microstructure of the material. The local chemical composition of a grain boundary can result from segregation of alloying or impurity elements steming from either equilibrium or nonequilibrium processes.[1,2,3] It is these changes in composition that can have a profound effect on fracture processes. Induced changes in fracture mode from transgranular cleavage to intergranular brittle fracture have been identified in some ferritic steels and other materials as a result of neutron irradiation-induced segregation to grain boundaries.[4–7] During the past 20 years, a great deal of experimental and theoretical research has been directed towards a basic understanding of neutron irradiation-induced segregation in a series of alloys. Therefore, a good understanding of irradiation-induced segregation of alloying and impurity elements is of considerable importance to understanding the fracture processes in engineering components and structures operating in a nuclear environment.[8] Early experimental techniques made it difficult to rigorously evaluate the chemical composition of grain boundaries. However, advances in high resolution techniques, such R.G. FAULKNER, Professor of Physical Metallurgy, and SHENHUA SONG, Research Associate, are with the Institute of Polymer Technology and Materials Engineering, Loughborough University of Technology, Loughborough, Leicestershire LE11 3TU, United Kingdom. P.E.J. FLEWITT, Structural Integrity Manager, is with Berkeley Technology Centre, Nuclear Electric plc, Berkeley, Gloucestershire GL13 9PB, United Kingdom. Manuscript submitted February 6, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
as Auger electron spectroscopy (AES) and scanning transmission electron microscopy (STEM) fitted with energy dispersive and electron energy loss spectrometers, enable the segregation to be examined in detail.[9] In particular, the dedicated STEM instrument fitted with a high-brightness field emission electron source (FEGSTEM) provides a beam of ;1-nm-diameter incident on the foil specimen, thereby providing the capability to examine and quantify grain boundary segregation. Irradiation-induced segregation to grain boundaries is more difficult to detect experimentally due to the active nature of the specimens. However, progress has been made using both AES and STEM techniques by Mahon et al.,[10] Norris et al.,[11] Kameda and Bevolo,[12] Morgan et al.,[13] and Carter et al.[14] Mechanisms for irradiation-induced segregation can be classified into either inverse Kirkendall effects or solute-point defect complex effects.[15] Irradiation-induced segregation theory has received detailed attention from Johnson and Lam,[16] Lam et al.,[17] Okamoto and Wiedersich,[18] Murphy and Per
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