Studies of solute precipitation and precipitate phase stability in nuclear structural materials under concurrent irradiation and heat often lead to contradictory results due to the complex nature of the phenomena which is far from well understood. Here, we present a comprehensive atomistically based continuum model for the copper precipitation and re-dissolution kinetics in an ion irradiated and thermally annealed Fe-0.78 at. % Cu alloy. Our model incorporates thermal and irradiation enhanced diffusion of atomic Cu, clustering of Cu into sub-nanometer and nanometer sized precipitates, thermal dissociation of the precipitates and, in particular, a cascade re-dissolution parameter that has been made available by recent molecular dynamics simulations. Our model suggests that the Cu precipitates may form, re-dissolve, or coarsen under different irradiation and thermal conditions depending on the competition between the thermal and the ballistic effects. The quantitative predictions of our model are compared with available experiments including limited atom probe tomography data acquired in this study. The work highlights the importance of combining thermal and ballistic effects in the understanding of phase stability in extreme nuclear environments.

This paper provides an overview of advanced scanning transmission electron microscopy (STEM) techniques used for characterization of irradiated BCC Fe-based alloys. Advanced STEM methods provide the high-resolution imaging and chemical analysis necessary to understand the irradiation response of BCC Fe-based alloys. The use of STEM with energy dispersive x-ray spectroscopy (EDX) for measurement of radiation-induced segregation (RIS) is described, with an illustrated example of RIS in proton- and self-ion irradiated T91. Aberration-corrected STEM-EDX for nanocluster/nanoparticle imaging and chemical analysis is also discussed, and examples are provided from ion-irradiated oxide dispersion strengthened (ODS) alloys. Finally, STEM techniques for void, cavity, and dislocation loop imaging are described, with examples from various BCC Fe-based alloys.

Designing materials for performance in high-radiation fields can be accelerated through a carefully chosen combination of advanced multiscale modeling paired with appropriate experimental validation. The studies reported in this work, the combined efforts of six universities working together as the Consortium on Cladding and Structural Materials, use that approach to focus on improving the scientific basis for the response of ferritic–martensitic steels to irradiation. A combination of modern modeling techniques with controlled experimentation has specifically focused on improving the understanding of radiation-induced segregation, precipitate formation and growth under radiation, the stability of oxide nanoclusters, and the development of dislocation networks under radiation. Experimental studies use both model and commercial alloys, irradiated with both ion beams and neutrons. Transmission electron microscopy and atom probe are combined with both first-principles and rate theory approaches to advance the understanding of ferritic–martensitic steels.

The stability of Y–Ti–O nanoclusters, dislocation structure, and grain boundary segregation in 9Cr-ODS steels has been investigated following proton irradiation at 400 °C with damage levels up to 3.7 dpa. A slight coarsening and a decrease in number density of nanoclusters were observed as a result of irradiation. The composition of nanoclusters was also observed to change with a slight increase of Y and Cr concentration in the nanoclusters following irradiation. Size, density, and composition of the nanoclusters were investigated as a function of nanocluster size, specifically classified to three groups. In addition to the changes in nanoclusters, dislocation loops were observed after irradiation. Finally, radiation-induced enrichment of Cr and depletion of W were observed at grain boundaries after irradiation.

14YWT oxide dispersion strengthened (ODS) ferritic steel was irradiated with of 5 MeV Ni2+ ions, at 300 °C, 450 °C, and 600 °C to a damage level of 100 dpa. The stability of Ti–Y–O nanoclusters was investigated by applying atom probe tomography (APT) in voltage mode, of the samples before and after irradiations. The average size and number density of the nanoclusters was determined using the maximum separation method. These techniques allowed for the imaging of nanoclusters to sizes well below the resolution limit of conventional transmission electron microscopy techniques. The most significant changes were observed for samples irradiated at 300 °C where the size (average Guinier radius) and number density of nanoclusters were observed to decrease from 1.1 nm to 0.8 nm and 12 × 1023 to 3.6 × 1023, respectively. In this study, the nanoclusters are more stable at higher temperature.

Molecular dynamics (MD) simulations have been used to investigate the re-solution of copper atoms from coherent, nanometer-sized copper precipitates in a body-centered cubic iron matrix. The molecular dynamics simulations used Finnis–Sinclair type interatomic potentials to describe the Fe–Cu system. Precipitate diameters of 1, 3 and 5 nm were studied, with primary knock-on atom (PKA) from 1 to 100 keV, although the majority of the cascade simulations and analysis of solute re-solution were performed for cascades of 10 or 20 keV. The simulation results provide an assessment of the re-solution on a per-atom basis as a function of precipitate size, cascade location and energy. Smaller sized precipitates, with a larger surface to volume ratio, experienced larger re-solution on a per-atom basis than larger precipitates. Re-solution was observed to occur predominantly in the initial ballistic stages of the cascades when atomic collisions occur at high kinetic energy. A minimum PKA energy of around 1 keV was required to produce re-solution, and the amount of re-solution appears to saturate for PKA energies above approximately 10 keV, indicating that the MD results are representative of the energy range of interest. A model for prompt, cascade induced solute atom re-solution has been derived, following the approach used to describe fission gas bubble re-solution, and the parameters for describing copper atom re-solution are provided.

The SRIM (formerly TRIM) Monte Carlo simulation code is widely used to compute a number of parameters relevant to ion beam implantation and ion beam processing of materials. It also has the capability to compute a common radiation damage exposure unit known as atomic displacements per atom (dpa). Since dpa is a standard measure of primary radiation damage production, most researchers who employ ion beams as a tool for inducing radiation damage in materials use SRIM to determine the dpa associated with their irradiations. The use of SRIM for this purpose has been evaluated and comparisons have been made with an internationally-recognized standard definition of dpa, as well as more detailed atomistic simulations of atomic displacement cascades. Differences between the standard and SRIM-based dpa are discussed and recommendations for future usage of SRIM in radiation damage studies are made. In particular, it is recommended that when direct comparisons between ion and neutron data are intended, the Kinchin–Pease option of SRIM should be selected.

Nanostructured oxide dispersion strengthened (ODS) steels are considered candidates for nuclear fission and fusion applications at high temperature and dose. The complex oxide nanoclusters in these alloys provide high-temperature strength and are expected to afford better radiation resistance. Proton, heavy ion, and neutron irradiations have been performed to evaluate cluster stability in 14YWT and 9CrODS steel under a range of irradiation conditions. Energy-filtered transmission electron microscopy and atom probe tomography were used in this work to analyze the evolution of the oxide population.

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Ferritic–martensitic (F/M) alloys are expected to play an important role as cladding or structural components in Generation IV and other advanced nuclear systems operating in the temperature range 350–700°C and to doses up to 200 displacements per atom (dpa). Oxide dispersion strengthened (ODS) F/M steels have been developed to operate at higher temperatures than traditional F/M steels. These steels contain nanometer-sized Y–Ti–O nanoclusters for additional strengthening. A proton irradiation to 1dpa at 525°C has been performed on a 9Cr ODS steel to determine the nanocluster stability at low dose. The evolution of the nanocluster population and the composition at the nanocluster–matrix interface were studied using electron microscopy and atom probe tomography. The data from this study are contrasted to those from a previous study on heavy-ion irradiated 9Cr ODS steel.

Extended abstract of a paper presented at Microscopy and Microanalysis 2009 in Richmond, Virginia, USA, July 26 – July 30, 2009

This paper introduces the production and export of aluminum in China, it summarizes the important applications of aluminum. Lastly,the importance of restructuring is pointed out.

This paper sets forth the application of rare-earth element in steel and its development in our country.