Use of small angle neutron scattering to study grain boundary cavitation

The technique of small angle neutron scattering can be used to obtain information on cavitation (e.g. void volume fractions, void size distributions, nucleation rates) which can scarcely be duplicated by other void measurement methods. Void size distribution curves for creep cavities in copper show that no voids are detectable below a cutoff radius which corresponds to the smallest stable void radius rsub(c) predicted by the equation rsub(c) = 2ν/σ. (Here ν is the surface energy and σ is the normal stress acting across the grain boundary). In contrast, most of the voids in copper fatigued under similar conditions (same temperature, stress amplitude equal to the creep stress) are far below this critical size. The average size of fatigue voids does not change appreciably with continued deformation whereas the average size of creep voids moves toward larger radii. There is no evidence for the existence of an incubation time associated with void nucleation. It is found that the void density is proportional to the creep time and therefore to the creep strain. (author)