Radiation-induced Precipitation Research Articles

Metallurgical achievements

Electron irradiation in a 1 MeV electron microscope has been used to study the void swelling response of several commercial austenitic stainless steels and iron-nickel based superalloys. Use of the 1 MeV microscope permits direct, continuous observation of the void development during elevated-temperature irradiations at displacement rates about 10 000 times greater then those in a fast breeder reactor. The alloys examined in this work included AISI 310, RA 330, A286, M813, Nimonic ∗ PE16, Inconel ∗ 706, Inconel ∗ 718 and Incoloy ∗ 901. Both helium preinjected specimens and uninjected specimens were studied. In all of the above alloys, swelling proceeds by formation of irradiation-induced dislocations and voids, followed by growth of the voids. The swelling rates and peak swelling temperatures vary considerably with alloy composition, heat treatment and helium preinjection.Comparisons of these results with recently reported swelling data from the same alloys after high fluence neutron irradiation in the EBR-II reactor shows good qualitative agreement in most cases. Helium preinjection of the electron irradiated specimens generally produced a poorer simulation than no helium preinjection. In one or two cases where the electron and neutron irradiation results strongly disagree, the differences appear to result from differences in irradiation-induced precipitation. Although the correlations between neutron and electron irradiation results are inadequate to obtain reliable engineering data by simulation, in-reactor swelling behavior is in general qualitatively well-represented by swelling response in the 1 MeV electron microscope.

Transmission electron microscopical examination of irradiated austenitic steel tensile specimens

Thin films prepared from irradiated [1.0−1.6 × 10 20 n/cm 2 (> 1 MeV), 3.0 − 5.7 × 10 20 n/cm 2 (thermal) at temperatures in the range 450° to 750° C] and the corresponding thermal control specimens of a 20 wt % Cr/25 wt % Ni, Nb-stabilised steel have been examined in the electron microscope after tensile testing at 650°, 700°, 750° and 800° C. The irradiated samples exhibited enhanced precipitation within the grains compared with the unirradiated specimens. Furthermore, the irradiated specimens showed little tendency to recover, as evidenced by lack of sub-grain formation, under conditions where extensive recovery occurred in the corresponding thermal control specimen. It is suggested that during elevated temperature tensile testing the irradiation-induced precipitation within the grains makes dislocation movement difficult so that deformation of the grain is limited and high stresses are placed on the grain boundaries resulting in intergranular failure and reduced ductility.

Dislocation Sources in Crystals

Lithium fluoride crystals were subjected to short stress pulses (1–10 μsec in length) in order to study dislocation nucleation in them. It was found that several heterogeneities can lead to dislocation nucleation: (a) cleavage steps, (b) dislocation loops, (c) glide bands, (d) inclusions or ``dirt'' particles, (e) precipitates (as grown), and (f) radiation-induced precipitates. Also, it was found that annealing treatments at moderate temperatures (200–600°C) followed by ``rapid'' cooling (>5°C/hr) made segments of previously pinned dislocations become mobile. These mobile segments multiplied as they moved through the crystals. Several treatments were tried that were ineffective in producing dislocation nucleation. These included: temperature changes, surface wetting, simultaneous irradiation and stressing, reflections from free surfaces, and diagonal bending. Carefully cleaned glass spheres were pressed into contact with dislocation-free regions of a LiF crystal. In this way, shear stresses that were 100 times greater than the normal yield stress of the crystal (∼G/85) were reached without causing homogeneous dislocation nucleation. It is concluded that small foreign heterogeneities cause most of the dislocation nucleation in real crystals. Homogeneous nucleation occurs only at high stresses (∼G/30). Relatively little dislocation multiplication results from classical Frank-Read sources (almost none in LiF). Most dislocation multiplication occurs as a prior dislocation moves through a crystal.