Search

Formation of a high density of Mn–Ni–Si nanoscale precipitates in irradiated Cu-free and Cu-bearing reactor pressure vessel steels could lead to severe unexpected embrittlement. Models long ago predicted that these precipitates, which are not treated in current embrittlement prediction models, would emerge only at high fluence. However, the mechanisms and variables that control Mn–Ni–Si precipitate formation, and their detailed characteristics, have not been well understood. High flux irradiations of six steels with systematic variations in Cu and Ni contents were carried out at ∼295 °C to high and very high neutron fluences of ∼1.3 × 10 20 and ∼1.1 × 10 21 n cm −2 . Atom probe tomography shows that significant mole fractions of Mn–Ni–Si-dominated precipitates form in the Cu-bearing steels at ∼1.3 × 10 20 n cm −2 , while they are only beginning to develop in Cu-free steels. However, large mole fractions of these precipitates, far in excess of those found in previous studies, are observed at 1.1 × 10 21 n cm −2 at all Cu contents. At the highest fluence, the precipitate mole fractions primarily depend on the alloy Ni, rather than Cu, content. The Mn–Ni–Si precipitates lead to very large increases in measured hardness, corresponding to yield strength elevations of up to almost 700 MPa. [ABSTRACT FROM AUTHOR]

Irradiation effects are studied using accelerated-damage experiments in test reactors and charged-particle accelerator facilities. Neutrons and ions create vacancy and self-interstitial defects, in dose unit of displacements per atom (dpa). It is important to understand the effects of irradiation dose rate on microstructural evolution processes, including irradiation enhanced precipitation, to properly interpret results from experiments using accelerated-damage experiments. Here we report on precipitation in Fe-Cu and Fe-Cu-Mn model alloys irradiated at dose rates ranging from ∼10 −10 dpa/s (neutrons) to ∼10 −5 dpa/s (70 MeV Fe ions). Atom probe tomography, small angle neutron scattering, and rate theory models show that precipitation is affected by both dose rate and alloy composition. Higher dose rates: 1) decrease the efficiency of radiation enhanced diffusion, due to increased vacancy-self interstitial recombination; and, 2) increase the rate of ballistic mixing that partly dissolves the precipitate constituents. The key parameter is the rate of ballistic mixing relative to the rate that solutes are reacquired by diffusion. The Mn in the ternary alloy traps vacancies and enhances recombination, leading to very different precipitate evolution kinetics. [ABSTRACT FROM AUTHOR]