[en] Neutron damage can significantly alter the process of precipitation from supersaturated solid solution. This is demonstrated by a series of experiments using a precipitation strengthened superalloy, Nimonic PE16 irradiated with fast neutrons over the temperature 400 to 650⁰C. In disagreement with earlier predictions, precipitate development is found to be controlled by the competing processes of Ostwald coarsening and solute segregation due to drag by point defects to point defect sinks. Analysis of the kinetics of Ostwald coarsening reveals significant enhancement of diffusion rates due to irradiation in agreement with predictions and involving an activation energy on the order of one quarter that of thermal diffusion. Unusual precipitate morphologies were observed such as void shells, linear precipitate arrays and Archimedes' screw configurations. However, predicted temperature dependencies for solute segregation are not obeyed

[en] Neutron damage can significantly alter the process of precipitation from supersaturated solid solution. This is demonstrated by a series of experiments using a precipitation strengthened superalloy, Nimonic PE16 irradiated with fast neutrons over the temperature 400 to 650⁰C. In disagreement with earlier predictions, precipitate development is found to be controlled by the competing processes of Ostwald coarsening and solute segregation due to drag by point defects to point defect sinks. Analysis of the kinetics of Ostwald coarsening reveals significant enhancement of diffusion rates due to irradiation in agreement with predictions and involving an activation energy on the order of one quarter that of thermal diffusion. Unusual precipitate morphologies were observed such as void shells, linear precipitate arrays and Archimedes' screw configurations. However, predicted temperature dependencies for solute segregation are not obeyed. An explanation is presented involving the coupling of the Ostwald coarsening mechanism with the solute segregation process

[en] γ' precipitates in Nimonic PE16 have been examined using bright and dark field imaging techniques in an electron microscope. The validity of these techniques to determine mean precipitate diameters, precipitate density and volume fraction is discussed. It is concluded that bright field imaging techniques are sufficiently accurate to measure γ' precipitate diameters but it is essential to use dark field imaging techniques to determine γ' precipitate density or volume fraction. (author)

[en] Nimonic alloy PE 16 was subjected to a specific thermomechanical processing designed to improve its post-irradiation ductility. The precipitation behaviour of the carbides and the γ'[Ni₃(Al,Ti)] phase at each stage of the thermomechanical treatment was studied with the help of optical and transmission electron microscopy. At the end of this processing, the alloy contained a uniform dispersion of fine and equiaxed TiC particles throughout the matrix, with a relatively small number of M₂₃C₆ particles precipitated mainly along the grain boundaries. In addition, a uniform dispersion of spherical γ' particles with a duplex size distribution was obtained. Tensile properties of the thermomechanically processed alloy, evaluated at different temperatures, showed a gradual decrease in the strength properties up to 773 K followed by a rapid decline above this temperature. In addition, a ductility trough was noticed at 773 K. An attempt has been made in this paper to correlate the mechanical properties of the alloy with the microstructural features and with the deformation and facture behaviour observed at different temperatures. (orig.)

[en] Isothermal precipitation curves for small grain boundary precipitates in Nimonic PE16 have been determined experimentally. A theoretical model which describes the growth of grain boundary precipitates is summarised and its application specific to Nimonic PE16 is considered. The fit between experimental and theoretical isothermal precipitation curves and thus the usefulness of the model in predicting improved treatments for the alloy is discussed. (author)

[en] Nimonic PE-16 is a γ' hardened, austenitic alloy which has been chosen as a candidate for fuel element cladding and structural component applications in breeder reactors due to its high temperature strength properties and swelling resistance. Microstructure of the alloy was examined after neutron irradiation

[en] Solute segregation at grain boundaries has been examined in solution-treated Nimonic PE16 after irradiation in EBR-II to fluences of 6 and 10 x 10²² n/cm² (E > 0.1 MeV) in the temperature range 425 to 650⁰C. Energy-dispersive x-ray spectrometry shows grain boundary enrichment of Ni, Ti, Al, and Si and depletion of Fe and Cr in samples irradiated at temperatures above 510⁰C. Electron microscope examinations further show that a continuous 30 nm thick layer of γ' forms at grain boundaries and at incoherent twin boundaries in samples irradiated at temperatures above 510⁰C and that antiphase boundaries are clearly revealed in superlattice dark-field images of the γ'

[en] The effects of ionizing radiation on the ordering state of initially ordered precipitations in a disordered matrix have been studied in the Nimonic PE16 alloy. In this alloy, thermal aging induces an ordered γ'-phase (of the type L1₂) to develop in a cubic face-centered γ-matrix. The effects on the γ'-precipitations of irradiation with Ni⁺ heavy ions of an energy of 300 keV have been investigated in the temperature range between 293 K and 623 K. The aging rate has been found to vary between 10^-3 and 10^-2 dpa s-1. The disordering process has been observed by means of electron diffraction. At an aging rate of 10^-3 dpa s^-1 and temperatures between 293 K and 523 K, the irradiation leads to a significant decrease of intensity of the superlattice reflections after a fluence of 0.1 dpa. Increasing the fluence still further reduces the intensity to relatively low, residiual values until at a fluence of 2-3 dpa, intensities are no longer detectable. (orig.)

[en] The effects of heat treatment variables and damage-rate variations on the creep of PE16 have been studied in DFR irradiations of loaded helices at doses up to 13 dpa (n/2). Strain, at a given dose, was shown to increase with precipitate size and with decreasing damage-rate

[en] Initial experiments have begun on the effects of HFIR irradiation of PE-16, where large amounts of helium are produced in the alloy. Irradiation at 400 and 650⁰C produced cavities coated with the γ' precipitate phase. After irradiation at 400⁰C and a helium content of 370 at. ppM, the average cavity size was 16.5 nm; at 650⁰C and a helium content of 1030 at. ppM, a bimodal cavity distribution was created with average sizes of 31.3 and 124.0 nm. Faulted loops were formed at 400⁰C but not at 650⁰C. A new observation for irradiated PE-16 was the precipitation of M₂₃C₆ in the grain boundaries at 650⁰C. Experiments under way should help explain many of the microstructural changes in these exploratory PE-16 specimens