PNNL

Abstract

A classic example of radiation-induced phase instability and degraded mechanical properties occurs in g'-g" (gamma prime - gamma double prime) -strengthened alloy 718. During neutron irradiation at 288 degrees C, the Ni3Nb g" particles at grain boundaries and in the matrix disappear after a few dpa. At higher doses, the g' (present only in the matrix) also dissolves and reprecipitates. Hardness is unaffected by disappearance of the g", but decreases as the original g' particles dissolve. Fine-probe compositional measurements in a TEM show that the softening coincides with solute redistribution rather than with the phase disappearance. Compositional changes at grain boundaries included leveling of the thermally segregated Mo as well as strong Ni enrichment and loss of Nb after higher doses. Radiation-induced softening is also observed after irradiations at low temperatures (30-60 degrees C) in a mixed spectrum of protons and spallation neutrons. In this case, both g' and g" completely disappear by 0.6 dpa as the alloy becomes hardened and embrittled. Minor softening occurs at higher doses after the g' and g" have disappeared. The complex phase stability and solute redistribution behavior reflects mainly ballistic mixing at 30-70 degrees C irradiation temperatures and the influence of significant thermal diffusivities at the higher temperatures.