Radiation-Induced Phase Instabilities and Their Effects on Hardening and Solute Segregation in Precipitation-Strengthened Alloy 718

Abstract

ABSTRACTA classic example of radiation-induced phase instability and degraded mechanical properties occurs in γ′-γ″-strengthened alloy 718. During irradiation with neutrons or protons at ∼ 30 to 288°C, the Ni 3Nb γ″ particles disappear after low doses. The γ′ (present only in the matrix) also disappears after <0.6 dpa at 30 to 55°C, but at 288°C it persists to higher doses and eventually reprecipitates as new γ′ with changed composition. Hardness of the alloy is unaffected by disappearance of the γ″, but decreases appreciably at 288°C as the original γ′ particles dissolve. Fine-probe compositional measurements in a TEM showed that the softening coincides with solute redistribution and reprecipitation 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 high doses. The complex phase stability and solute redistribution behavior reflects mainly ballistic mixing at 30-70°C irradiation temperatures and the influence of significant thermal diffusivities at the higher temperatures.