1 Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China 3 China Nuclear Power Technology Research Institute Co. Ltd., Shenzhen 518031, China
Cite this article:
MENG Zikai, MENG Zhichao, GAO Changyuan, GUO Hui, CHEN Hansen, CHEN Liutao, XU Dongsheng, YANG Rui. Molecular Dynamics Simulation of Creep Mechanism in Nanocrystalline α-Zirconium Under Various Conditions. Acta Metall Sin, 2024, 60(5): 699-712.
Zirconium alloys have been widely used because of their good mechanical properties, corrosion resistance, and suitable neutron absorption cross section. However, with the development of engineering technology, the requirements on the service performance, safety, and microstructure stability of zirconium structural materials are increasingly high. Creep refers to the phenomenon where the strain of the material increases with time under the action of stress lower than the yield strength, which will lead to the deformation of the structural material and eventually its failure. It is also an important problem faced by zirconium cladding materials in nuclear reactors. Nanocrystalline zirconium is a strengthening and toughening method, which may also improve other properties of nuclear materials. However, there are very few investigations on the microscopic creep mechanism in the atomic scale of nanocrystalline zirconium. Improving the creep resistance of zirconium materials plays an important role in the safety of components. Therefore, studying the creep behavior of nanocrystalline zirconium and its atomic mechanism is conducive to its better application in industries. In this study, the tensile creep behavior of nanocrystalline α-Zr under different conditions was investigated via molecular dynamics simulation. The main influencing factors of the creep process were analyzed and the influence of polycrystalline structure evolution and deformation mechanism during the steady-state creep process was studied. The effect of irradiation on the creep process was preliminarily explored. Results showed that temperature, stress, irradiation, and grain size affect the creep behavior of nanocrystalline α-Zr. Increasing the temperature and stress level and refining the grains can promote the creep process. The microstructure of the system changed significantly after the creep. Some grains grew up with the creep deformation process, while others gradually shrank, or even disappeared. During deformation, the lattice distortion gradually propagated from the grain boundary to the grains, partly reducing the degree of the hcp order. Simulation results showed that grain boundary migration is the main deformation mechanism of nanocrystalline α-Zr during steady-state creep. Increasing the temperature and stress level will thicken the grain boundary and promote the microstructure evolution. After the cascade collision due to neutrons of different energy levels, a large number of point defects were produced in the system and their diffusion contributed to the creep; these defects were finally collected at the grain boundary, improving the grain boundary mobility. Increasing the irradiation energy can increase the number and size of irradiation defects, promoting the creep process of the nanocrystalline system.
Table 1 Grain numbers (n) and grain sizes (d) of initial model
Fig.1 Tensile stress-strain curves for each model
Fig.2 Schematic of tensile creep simulation (Different colors represent different grains, F—tensile force)
Fig.3 Strain-time curves for systems under different stresses and temperatures (a-c), and for systems with different grain sizes under different temperatures (d-f) (t—time)
Fig.4 Radial distribution function (g(r)) curves of different temperature-M3-5 GPa systems at the final state of creep process (r—distarce between two atoms)
Fig.5 Atomic snapshots of different temperature-M3-5 GPa systems at the final state of creep process (300 ps) at 373 K (a), 573 K (b), and 773 K (c) (Colored by centrosymmetry parameter)
Fig.6 Microstructure evolution for 773 K-M3-5 GPa system during creep (Blue, yellow, green, and red atoms represent fcc, bcc, hcp structures, and unknown structures, respectively. The same in Figs.10, 11, and 16) (a) 0 ps (b) 20 ps (c) 80 ps (d) 300 ps
Fig.7 Grain boundary atom fraction evolution for systems under different conditions (a) 773 K-M3-stress systems (b) 773 K-grain size-5 GPa systems
Fig.8 Dislocation density curves for systems under different stresses and temperatures (a-c), and for systems with different grain sizes under different temperatures (d-f)
Fig.9 Dislocation structure evolutions of 573 K-M4-5 GPa system during creep process (Gray areas and colored lines represent grain boundaries and dislocation lines, respectively) (a) 0 ps (b) 15 ps (c) 75 ps (d) 135 ps (e) 200 ps (f) 300 ps
Fig.10 Atomic snapshots of 773 K-M3-different stress systems at the beginning and final states of creep process (a) before creep (b) 1 GPa-300 ps (c) 3 GPa-300 ps (d) 5 GPa-300 ps
Fig.11 Atomic snapshots of different temperature-M4-5 GPa systems at the beginning and final states of creep process (a) before creep (b) 373 K-300 ps (c) 573 K-300 ps (d) 773 K-300 ps
Fig.12 Microstructure evolutions and atomic displacement analyses of 573 K-M4-5 GPa system during creep (The grey and red atoms show the original and crept grain boundary, respectively; yellow arrows in Figs.12f-h represent the displacements of the atoms) (a-d) crystal structures at 0 ps (a), 50 ps (b), 100 ps (c), and 150 ps (d), respectively (e, f) atomic displacement analyses at 30 ps (e) and 40 ps (f), respectively (g, h) partial enlarged views of the areas in Figs.12f
Fig.13 Growthes of the largest grains for M3 under different creep conditions
Fig.14 Atomic snapshots of M4 systems after irradiation with different PKA (colored by centrosymmetry parameter; PKA—primary knock-on atom) (a) before irradiation (b) 5 keV irradiation (c) 10 keV irradiation (d) partial enlarged view of Fig.14c
Fig.15 Strain-time curves for various irradiated systems under different temperatures (a) and different stresses (b), and for systems with different grain sizes (c)
Fig.16 Crystal structures of the M4-5 GPa cascaded systems at the final state of creep process (300 ps) (a) 373 K-5 keV (b) 573 K-5 keV (c) 773 K-5 keV (d) 373 K-10 keV (e) 573 K-10 keV (f) 773 K-10 keV
Fig.17 Mean square displacement (MSD) curves for various irradiated systems under different temperatures (a) and different stresses (b), and for systems with different grain sizes (c)
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