Proton-irradiated Stainless Steel Research Articles

Evaluation of irradiation hardening of proton irradiated stainless steels by nanoindentation

Ion irradiation experiments are useful for investigating irradiation damage. However, estimating the irradiation hardening of ion-irradiated materials is challenging because of the shallow damage induced region. Therefore, the purpose of this study is to prove usefulness of nanoindentation technique for estimation of irradiation hardening for ion-irradiated materials. SUS316L austenitic stainless steel was used and it was irradiated by 1MeV H+ ions to a nominal displacement damage of 0.1, 0.3, 1, and 8dpa at 573K. The irradiation hardness of the irradiated specimens were measured and analyzed by Nix–Gao model. The indentation size effect was observed in both unirradiated and irradiated specimens. The hardness of the irradiated specimens changed significantly at certain indentation depths. The depth at which the hardness varied indicated that the region deformed by the indenter had reached the boundary between the irradiated and unirradiated regions. The hardness of the irradiated region was proportional to the inverse of the indentation depth in the Nix–Gao plot. The bulk hardness of the irradiated region, H0, estimated by the Nix–Gao plot and Vickers hardness were found to be related to each other, and the relationship could be described by the equation, HV=0.76H0. Thus, the nanoindentation technique demonstrated in this study is valuable for measuring irradiation hardening in ion-irradiated materials.

44Ti, 26Al and 53Mn samples for nuclear astrophysics: the needs, the possibilities and the sources

Exploration of the physics involved in the production of cosmogenic radionuclides requires experiments using the same rare, radioactive nuclei in sufficient quantities. For this work, such exotic radionuclides have been extracted from previously proton-irradiated stainless steel samples using wet chemistry separation techniques. The irradiated construction material has arisen from an extended material research programme at the Paul Scherrer Institute, called STIP (SINQ Target Irradiation Program), where several thousand samples of different materials were irradiated with protons and neutrons of energies up to 570 MeV. In total, 8 × 1017 atoms of 44Ti, ∼1016 atoms of 26Al and ∼1019 atoms of 53Mn are available from selected samples. These materials may now be used to produce targets or radioactive beams for nuclear reaction studies with protons, neutrons and α-particles. The work is part of the ERAWAST initiative (Exotic Radionuclides from Accelerator Waste for Science and Technology), aimed at facilitating new collaborations between the isotope producers and users from different scientific fields including nuclear astrophysics.

Deformation microstructure of proton-irradiated stainless steels

The deformation microstructure of proton-irradiated stainless steels may play a key role in explaining their irradiation-assisted stress corrosion cracking (IASCC) susceptibility. In the present study, three model alloys (UHP-304, 304+Si, 304+Cr+Ni) with different stacking fault energies (SFEs) were irradiated with 3.2MeV protons at 360°C to 1.0 and 5.5dpa and then strained in 288°C Ar atmosphere. The deformation microstructure of the strained samples was investigated using scanning electron microscopy and transmission electron microscopy. The results showed that the slip lines interacted with grain boundaries by grain-to-grain transmission, grain boundary sliding or deformation ledge formation at grain boundaries. Expanded channels, which were formed at locations where dislocation channels intersected the grain boundaries or other channels, were found predominately in the low SFE alloys UHP-304 and 304+Si. The steps and shear strain at grain boundaries caused by channel expansion may increase the IASCC susceptibility in low SFE stainless steels by producing strain concentrations and inducing cracks in the oxide film.

High Spatial Resolution X-ray Microanalysis of Radiation-Induced Segregation in Proton-Irradiated Stainless Steels

AbstractThe spatial redistribution of alloying elements and impurities near grain boundaries in several stainless steel alloys arising from non-equilibrium processes have been measured by analytical electron microscopy (AEM) in a field emission scanning transmission electron microscope. Radiation-induced segregation (RIS) has been shown to result in significant compositional changes at point defects sinks, such as grain boundaries. The influence of irradiation dose and temperature, alloy composition, prior heat treatment, and post-irradiation annealing on the grain boundary composition profiles have been investigated. Understanding the importance of these microchemical changes relative to the radiation-induced microstructural change in irradiation-assisted stress corrosion cracking (IASCC) of the irradiated materials is the primary goal of this study.

Post-Irradiation Annealing of Dislocation Microstructure and Radiation-Induced Segregation in Proton-Irradiated Stainless Steels

AbstractBoth microstructural effects (radiation hardening) and microcompositional effects (radiationinduced segregation (RIS)) have been identified as potential contributors to irradiation-assisted stress corrosion cracking (IASCC). However, the importance of each in IASCC is unclear. In an effort to isolate their roles, post-irradiation annealing of proton-irradiated 304L stainless steel samples has been studied. Model simulations of post-irradiation annealing at intermediate temperatures (400-650°C) indicate that microstructural features such as dislocation loops are removed faster than RIS. Simulations also predict that there exist time-temperature combinations that will significantly reduce the dislocation loop population while leaving the grain boundary segregation essentially unaffected. Ultra-high purity (UHP) 304L stainless steel samples have been irradiated with 3.2 MeV protons at 360°C to 1.0 dpa and then annealed in-situ as thinned TEM disks in the temperature range of 500°C- 625°C for times between 20 and 60 minutes. RIS and dislocation loops were characterized before and after annealing in the same areas of each specimen. Post-irradiation anneals at 500TC for 45 minutes or 60 minutes resulted in no appreciable change in dislocation loop population or RIS. Annealing at 600°C for 20 minutes., or 40 minutes and at 625°C for 40 minutes, resulted in decreasing dislocation loop densities and increasing loop size with increasing annealing time or temperature, while the amount of RIS did not change significantly from the pre-annealed condition.

AEM and AES of radiation-induced segregation in proton-irradiated stainless steels

Irradiation-assisted stress corrosion cracking (IASCC) of stainless steels has been attributed in part to radiation-induced segregation (RIS) of both major alloying and impurity elements at grain boundaries. There are phenomenological similarities observed between IASCC and intergranular stress corrosion cracking (IGSCC) of thermally-sensitized stainless steels. One concern for both IGSCC and IASCC is the localized loss of corrosion resistance associated with chromium depletion at grain boundaries. In order to avoid complications related to the long-term, induced radioactivity of neutron-irradiated specimens, four type 304L alloys were irradiated to 1 dpa (displacements per atom) with 3.4 MeV protons at 400°C. Both analytical electron microscopy (AEM) in a Philips EM400T/FEG and Auger electron spectrometry (AES) in a Perkin Elmer (PHI) 660 were employed to measure composition at or near grain boundaries in unirradiated and irradiated specimens of four controlled purity alloys [ultra-high purity (UHP), UHP+S (0.03 at.%), UHP+P (0.08 at.%), and UHP+Si (0.87 at.%)]. A sufficient number of boundaries were analyzed via AEM and AES to result in the standard deviation of the mean boundary composition of less than -0.5 at.%. Further experimental details are presented elsewhere.