The effect of particle misfit on void formation under electron and neutron irradiation in gamma-prime strengthened superalloys

Abstract

A well-characterized series of nickel-based superalloys in which γ' particle misfit is varied systematically through compositional modifications has been used to study the effect of misfit on irradiation-induced microstructural development. Under electron irradiation at 535–650°C in a 1 MeV electron microscope, the void and dislocation distributions formed in helium preinjected specimens vary with misfit strain. The voids form preferentially within the γ' in the case of large negative misfits ( a γ < a γ ), next to the γ/γ' matrix interface in the case of near-zero misfit, and in the matrix at large positive misfits. Irradiation-induced Frank dislocation loops with interstitial character also form preferentially in γ' particles having large positive or negative misfit strains, but form uniformly throughout alloys with near-zero misfit. The same behavior is found in these alloys after neutron irradiation to 3 × 10 22 n/ cm 2, E < 0.1 MeV, at 600°C. Several possible explanations for the void distribution observations have been proposed based on the effect of misfit stresses. The first is that the local stresses enhance dislocation microstructure evolution and this influences void development. The second is that the stress state resulting from particle/matrix mismatch produces non-uniform distributions of helium or other residual gases which cause enhanced void nucleation. The third is that void nucleation is directly enhanced in regions of positive hydrostatic stress which arise from particle/matrix misfit. However, a complete explanation appears to require at least two of these mechanisms.