The microstructural changes induced by irradiation lead to changes in mechanical properties. An article published in the Journal of Nuclear Materials discussed the changes in mechanical properties of A508-3 steel after irradiating it with iron (Fe+3) ions.
Study: Nanoindentation experiment and crystal plasticity study on the mechanical behavior of Fe-ion-irradiated A508-3 steel. Image Credit: Allexxandar/Shutterstock.com
Here, A508-3 steel samples were irradiated with Fe+3 ions at 20, 100, and 300 degrees Celsius with 0.1, 0.4, 2.0, and 5.0 displacements per atom using the crystal plasticity finite element model (CPFEM) and nanoindentation experiment. Applying Nix–Gao model helped obtain the hardness (H0) from the measured data, and the H0 of steel increased with radiation damage at all temperatures.
The dislocation loop expedited the mobile dislocation and retarded the immobile dislocation, leading to larger von Mises stresses of Fe ion irradiated steel samples with a flat shape, and the area of the von Mises stress shrunk with temperature. The present study demonstrated the deformation behavior of irradiated steel based on microstructural and experimental analysis.
Nanoindentation Method for Estimation of Mechanical Properties in A508-3 Steel
Changes in mechanical properties are the consequences of changes produced at the microstructure level. Irradiation-induced microstructural evolution results in the hardening of the steel by obstructing the movement of dislocations, thereby causing degradation of the fracture properties, such as the ductile-to-brittle transition temperature shift.
The irradiation-induced embrittlement of nuclear reactor pressure vessel’s steel is a concern for the nuclear power plant life assessment. A508-3 steel with 0.06 wt.% copper (Cu) is used for the pressure vessel of many newly built nuclear power plants in China.
The prolonged irradiation of nuclear reactor pressure vessel’s steel during nuclear fission reaction causes hardening/embrittlement due to the defects in the matrix, including dislocation loops, clusters, and the low Cu content in the nuclear reactor pressure vessel’s steel leading to vacancy-solute atom complexes.
However, the irradiation-induced embrittlement behavior and mechanism of the A508-3 steel are unclear. Thus, various methods were explored to investigate the influence of irradiation on defect evolution. Additionally, the micromechanical response of materials was investigated via micropillar compression, nanoindentation, and in situ tensile tests.
Among the optimized methods, nanoindentation was suitable for rapid estimation of mechanical properties at micro- or nanoscales. Since nanoindentation alone cannot produce sufficient information on mechanical response and microstructural evolution, it is combined with the CPFEM simulations. Thus, the nanoindentation and CPFEM combined approach overcomes the above drawbacks under varying loads.
Nanoindentation Experiment and CPFEM Study on A508-3 Steel
CPFEM and nanoindentation were extensively applied in numerous previous studies to determine various structural aspects and physical properties of metals. In the present study, Fe+3 ions of 3.5 megaelectronvolts were irradiated on A508-3 steel samples at temperatures 20, 100, and 300 degrees Celsius with dislocations of 0.1, 0.4, 2.0, and 5.0 displacements per atom to evaluate their mechanical properties via nanoindentation followed by simulations using physical CPFEM.
The nanoindentation helped obtain specific values of irradiation hardening of A508-3 steel samples. The hardness–indentation-depth and load–indentation-depth curves were obtained from CPFEM simulations.
While the elastic modulus of A508-3 steel samples remained uninfluenced by the irradiation dose, the hardness of the samples increased with increasing damage levels at all temperatures, indicating defects induced on the A508-3 steel samples by the irradiation of Fe+3 ions.
The microstructure evolution of A508-3 steel was investigated using the CPFEM. The crystal anisotropy in different directions indicated the effect of crystal orientation on stress distribution. The von Mises stress was observed along , , and  directions. Besides the relationship between experiment and simulation, this study helped derive the relationships between microscale and macroscale, promoting the study of nuclear reactor pressure vessels’ steel irradiation effects.
To summarize, the mechanical properties of A508-3 steel irradiated with Fe+3 ions at various displacements and temperatures were investigated by nanoindentation experiment and CPFEM simulation. The obtained data was input into Nix–Gao model to calculate H0 and characteristic length (h*). The results showed an increase in H0 with displacements per atom.
The elastic modulus of A508-3 steel was independent of irradiation dose. However, the hardness of the A508-3 steel increased with irradiation damage at all temperatures, indicating the significant influence of irradiation-induced defects on plastic behavior and dislocation gliding of the steel.
Mobile and immobile dislocations and partial- and full-absorption dislocation loops were four dominant hardening contributions to the classical crystal plasticity theory framework. The CPFEM helped simulate the hardness–indentation-depth and load–indentation-depth curves.
The macroscopic deformation behavior of Fe-ion-irradiated A508-3 steel was determined from microstructural evolution. Additionally, the von Mises stress was distributed along , , and  directions. The dislocation loop accelerated the increase of mobile dislocation and retarded the decrease of immobile dislocation, resulting in higher values of von Mises stress.
Lin, P., Nie, J., Liu, M. (2022) Nanoindentation experiment and crystal plasticity study on the mechanical behavior of Fe-ion-irradiated A508-3 steel. Journal of Nuclear Materials https://www.sciencedirect.com/science/article/pii/S0022311522004871?via%3Dihub
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