Laser shock peening (LSP) is a promising surface strengthening technology. Since the deformation with extreme strain rates occurs in the surface material of reinforced components under the action of tensile stress waves induced by LSP, spallation defects are prone to form. In this study, a molecular dynamics model based on the "EAM+L-J" hybrid potential function was developed to investigate the variation law of tensile deformation mechanical response of nickel-based superalloy (GH4169), and the microstructure evolution of the material was also analyzed for different alloy compositions, crystal structures and strain rates. At a strain rate of 109 s-1, the Young's modulus of three different single crystal models with varying alloy compositions was found to be 240, 237, and 217 GPa, while their ultimate tensile strength was 28.3, 27.1, and 17.6 GPa, respectively. For the single crystal model with eight alloying elements, at strain rates of 108, 109, and 1010 s-1, the Young's modulus remained constant at 217 GPa, while the ultimate tensile strength increased with the strain rate, reaching 17.1, 17.6, and 18.7 GPa, respectively. In the polycrystalline model, the Young's modulus was measured as 213, 220, and 226 GPa under the different strain rate conditions, whereas the ultimate tensile strength values were 9.6, 10.3, and 11.8 GPa, respectively. The results indicated that the Young's modulus and ultimate tensile strength obtained from single crystal model with eight alloy compositions were closer to the experimental results and under identical tensile deformation conditions and specific crystal orientation, the Young's modulus of the single crystal model (217 GPa) and the polycrystalline model (220 GPa) showed minimal differences, suggesting that crystal structure had a relatively small impact on the Young's modulus. In addition, the single crystal model exhibited higher plastic deformation resistance under extreme strain rate condition through the comparison with the polycrystalline model. The polycrystalline model showed the existence of inverse Hall-Petch effect in terms of dislocation morphology and distribution during deformation due to the fact that the grain size was less than 10 nm. This effect arose from the competition between dislocation pile-up within grains and their absorption at grain boundaries during plastic deformation under extreme strain rates. As the grain size decreased, the proportion of grain boundaries increased, enhancing their ability to absorb dislocations, reducing dislocation pile-up, and lowering the probability of crack initiation within grains. Moreover, during extreme strain rate tensile deformation, the Young's modulus was insensitive to the changes in strain rates, showing an obvious strain rate strengthening phenomenon. Furthermore, dislocation multiplication and sub-grain boundaries composed of dislocation walls were observed in both single and polycrystalline models because of the extremely short dislocation slip time when the strain rate reached 1010 s-1. Additionally, in the polycrystalline model, the dislocation multiplication rate surpassed the rate of which dislocations slid to and were absorbed by grain boundary within the grains, partially weakening the impact of inverse Hall-Petch effect on the reduction of material plastic deformation resistance. This study can provide valuable theoretical references for analyzing the spallation formation mechanism induced by LSP for nickel-based superalloy.
Key words
laser shock /
nickel-based superalloy /
inverse Hall-Petch effect /
extreme strain rate /
molecular dynamics
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Funding
National Natural Science Foundation Project (51575117); The Hunan Natural Science Foundation Project (2019JJ50519); The Hunan Provincial Department of Education Project (20C1601, 20C1607)
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