Improved measurement of void swelling

Abstract

Irradiation of crystalline materials by neutrons or other energetic particles often causes swelling by formation of internal cavities, or voids. The swelling, or total void volume within a given sample volume, can be determined from transmission electron micrographs by simple measurement of the void images. However, errors of over 100% in the calculated void volumes are possible if the voids are approximated as spheres. The problem arises because the voids are usually polyhedral, crystallographically oriented features and no single size parameter has been defined for the spherical approximation. In austenitic stainless steels and other face-centered-cubic (fCC) alloys, voids range from octahedra with {111} faces to cubes with {100} faces and exhibit all intermediate (truncated) forms. Void truncation may also vary widely within a given field of view. Voids in ferritic steels and other body-centered-cubic (bcc) materials may range from octahedra with {111} faces to cubes with {100} faces to dodecahedra with {110} faces. The relationship between void shape on electron micrographs and void volume was therefore studied with the aim of improving the accuracy of swelling determination without requiring a separate shape determination for each void. Void volumes were determined as a function of a shape parameter and related to the various ‘size’ parameters available in different crystal orientations. Procedures were then defined to minimize the error in swelling measurement.