Nanocrystalline metal creep property prediction method based on molecular dynamics simulation

Abstract

The invention discloses a nanocrystalline metal creep property prediction method based on molecular dynamics simulation. The nanocrystalline metal creep property prediction method comprises the following steps: S1, establishing an initial atomic model of nanocrystalline metal by adopting Atomsk software; s2, simulating a nanocrystalline relaxation process by adopting molecular dynamics software Lammpps to obtain an equilibrium configuration at a specified temperature; s3, adopting molecular dynamics software Lammpps to simulate the nanoindentation; s4, simulating the nano indentation creep by adopting molecular dynamics software Lammpps; s5, a creep displacement-creep time curve is extracted; and S6, analyzing a creep mechanism. According to the method, rapid prediction of the creep property of the nanocrystalline metal at any temperature and explaining of a creep mechanism are realized. Particularly, the dilemma that the creep property of the extremely-small-size nanocrystalline cannot be tested and predicted due to the limitation of a nanocrystalline preparation method is broken through, and a technical support is provided for the design of a nanocrystalline metal material and the continuous improvement of the creep property.

Description

Technical Field

[0001] The invention relates to the field of metal materials, and in particular to a method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation. Background Art

[0002] Metal materials are widely used in aerospace, automobile manufacturing, digital 3C products, biomedical equipment and other fields due to their excellent mechanical strength, good electrical and thermal conductivity, and relatively high ductility and durability. However, with the development of society, higher requirements are placed on the strength of materials. For this reason, grain refinement has become an effective solution. By reducing the grain size to the nanoscale, the strength and hardness of the material can be significantly improved.

[0003] Although nanocrystalline metals have significant performance advantages, they still experience slow deformation, i.e. creep, under high temperature or long-term load. Traditional creep testing methods usually rely on large-scale experimental equipment, with long test cycles and cumbersome operations, making it difficult to effectively characterize creep behavior at the micro-nano scale. For this reason, creep testing methods based on nanoindentation have emerged. This method quickly evaluates the creep properties of materials by applying pressure at the nanoscale. Compared with traditional methods, the experimental setup is simpler and the testing efficiency is higher. However, it is usually difficult to explain the mechanism of nanoindentation creep phenomenon of nanocrystalline metals through experiments; furthermore, due to the limitations of current nanocrystal preparation technology, small-scale nanocrystals are still difficult to prepare, and their creep properties cannot be obtained.

[0004] Molecular dynamics (MD) simulation can analyze the time evolution of the internal structure of materials at the atomic or molecular level, and thus becomes an important tool for predicting the creep properties of nanocrystalline metals and explaining the creep mechanism. Therefore, it is urgent to establish the molecular dynamics simulation technology of nanocrystalline indentation creep process to quickly evaluate the creep properties of different types of nanocrystalline metals, especially to obtain the creep performance data of extremely small-scale nanocrystalline metals that have not been obtained under current experimental conditions. Summary of the invention

[0005] In view of the problems existing in the prior art, the present invention proposes a method for predicting the creep performance of nanocrystalline metals based on molecular dynamics simulation.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation comprises the following steps:

[0008] S1. Use Atomsk software to establish the initial atomic model of nanocrystalline metal.

[0009] S2. Molecular dynamics software Lammps is used to simulate the relaxation process of nanocrystals and obtain the equilibrium configuration at a specified temperature.

[0010] S3. Use the molecular dynamics software Lammps to simulate nanoindentation.

[0011] S4. Molecular dynamics software Lammps is used to simulate nanoindentation creep.

[0012] S5. Extract creep displacement-creep time curve: Extract creep displacement-creep time curve from Lammps simulation results, and analyze and evaluate the creep properties of nanocrystalline metal materials based on it.

[0013] S6. Analyze the creep mechanism: Use coordination number, central symmetry parameter, common neighborhood analysis and other methods to identify defect atoms and color them, and then use Ovito, Atomeye and other software for visualization to analyze the evolution of defects inside nanocrystals during creep and identify the creep mechanism.

[0014] Step S1 includes the following sub-steps:

[0015] S11. According to the type of metal being simulated, set the atomic type, crystal structure and lattice constant, and generate a single crystal unit cell file.

[0016] S12, setting the dimensions a, b, c of the simulation area in the x, y, z directions; generating a polycrystalline node file;

[0017] S13. Set the number of created grains n and add it to the polycrystalline node file. Based on the size of the simulation area and the number of grains, the grain size d can be calculated:

[0018]

[0019] S14, using the open source software Atomsk to build the atomic model of the nanocrystal, and output the atomic model file in cfg format, and use Ovitio and Atomeye for visualization;

[0020] S15. Modify the nanocrystal atomic model cfg file format into a data file that can be directly read by the molecular dynamics software Lammps.

[0021] Step S2 includes the following sub-steps:

[0022] S21. According to the type of metal material being simulated, select the appropriate potential function and time step, and give priority to using the EAM potential;

[0023] S22, setting the boundary conditions in the indentation direction as free boundaries and the other boundaries as periodic boundaries;

[0024] S23, using the NPT system, slowly raising the temperature of the atomic model to a specified temperature under near zero pressure conditions;

[0025] S24, using the NPT system, relax the atomic model at a specified temperature for a period of time under near zero pressure conditions;

[0026] S25. Output the balanced configuration to a cfg format file and modify it to a data file;

[0027] Step S3 includes the following sub-steps:

[0028] S31, using Lammps software to read the atomic model of the nanocrystal after equilibrium;

[0029] S32. Create an indenter of a spherical or conical shape. The indenter atom is set as a diamond rigid body. The initial position is located on the surface of the nanocrystal and a small distance is maintained. 0.5 nm is preferred. Morse potential is preferred between the indenter and the substrate.

[0030] S33, set the bottom two layers of atoms in the pressing direction as fixed layers to maintain the stability of the overall structure. The remaining boundaries are set as periodic boundaries;

[0031] S34, restricting all degrees of freedom of the indenter except the indentation direction, applying a load that increases linearly with time to the center of mass of the indenter until a specified load is reached, thereby simulating the nanoindentation process.

[0032] Step S4 includes the following sub-steps:

[0033] S41, applying a specified constant load on the indenter to simulate the creep process of nanocrystals;

[0034] S42, during the simulation process, collecting the evolution data of the pressure head displacement over time and outputting it to a file;

[0035] S43. During the simulation process, nanocrystal atomic trajectory files are output at certain time step intervals.

[0036] Step S6 includes the following sub-steps:

[0037] S61, using common neighborhood analysis and other methods to identify defective atoms;

[0038] S62, visualization using Ovito software;

[0039] S63. Analyze the evolution of internal defects of nanocrystals during creep and identify the creep mechanism.

[0040] The beneficial effects of the present invention are:

[0041] The present invention proposes a method for predicting the creep performance of nanocrystalline metals based on molecular dynamics simulation, which realizes the rapid prediction of the creep performance of nanocrystalline metals at any temperature and the interpretation of the creep mechanism. In particular, it breaks through the dilemma that the creep performance of extremely small nanocrystallines cannot be tested and predicted due to the limitations of the nanocrystalline preparation method, and provides technical support for the design of nanocrystalline metal materials and the continuous improvement of creep performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments are briefly introduced below. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0043] Figure 1 Schematic diagram of the process of predicting the creep properties of nanocrystalline metals based on molecular dynamics simulation.

[0044] Figure 2 This is the creep displacement-creep time curve.

[0045] Figure 3 (a) and (b) show the atomic configuration diagrams at the central cross section of nanocrystalline Al at creep times of 0 ns and 3 ns, respectively. DETAILED DESCRIPTION

[0046] The technical solution of the present invention is further clearly and completely described below in conjunction with the accompanying drawings and specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0047] Example

[0048] like Figure 1 As shown, a schematic flow chart of a method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation of the present invention comprises the following steps:

[0049] S1. Use Atomsk software to establish the initial atomic model of Al nanocrystalline metal.

[0050] Step S1 includes the following sub-steps:

[0051] S11, set the atomic type to Al, the crystal structure to FCC, and the lattice constant to Generate the unit cell file of Al.xsf. The program language for generating Al.xsf file is: atomsk--create fcc 4.05Al Al.xsf;

[0052] S12, set the dimensions of the simulation area in the x, y, and z directions respectively Generate Al_voronoi.txt format polycrystalline node file, the program language is: echo "box 420 420 300">Al_voronoi.txt;

[0053] S13, setting the number of created grains to 808 and adding them to the Al_voronoi.txt file, the program language is: echo "random 808">>Al_voronoi.txt. According to formula 1, the size of the created nanocrystal grain is calculated to be 5 nm.

[0054] S14, using the open source software Atomsk to establish an atomic model of the nanocrystal, and output the atomic model file Al_polycrystal.cfg, the program language is: atomsk--polycrystal Al.xsf Al_voronoi.txt Al_polycrystal.cfg-wrap;

[0055] S15. Modify the nanocrystal atomic model cfg file format into a data file that can be directly read by the molecular dynamics software Lammps.

[0056] S2. Molecular dynamics software Lammps is used to simulate the relaxation process of nanocrystals and obtain the equilibrium configuration at a specified temperature.

[0057] Step S2 includes the following sub-steps:

[0058] S21. Set the EAM potential function file Al99.eam.alloy and the time step to 2fs.

[0059] S22, setting the boundary conditions in the indentation direction as free boundaries and the other boundaries as periodic boundaries;

[0060] S23, using the NPT system, slowly raise the temperature of the atomic model to 450K at 1 bar pressure, with 50,000 steps of heating process;

[0061] S24, using the NPT system, relax the atomic model at 450K for 100,000 steps at 1 bar pressure;

[0062] S25. Output the equilibrium configuration to the dump.heat100000.cfg file and modify it to the data file format.

[0063] S3. Use the molecular dynamics software Lammps to simulate nanoindentation.

[0064] Step S3 includes the following sub-steps:

[0065] S31, using Lammps software to read the atomic model of the nanocrystal after equilibrium;

[0066] S32. Create a hollow diamond spherical indenter with a radius of 10 nm and a central radius of 9.2 nm. Only half of the atoms below the central plane of the indenter are retained to reduce the amount of calculation. The indenter is set as a rigid body. The Morse potential is set between the indenter and the substrate, with parameters of 0.28 2.78 2.2.

[0067] S33, set the bottom two layers of atoms in the pressing direction as fixed layers to maintain the stability of the overall structure. The remaining boundaries are set as periodic boundaries;

[0068] S34, restrict all degrees of freedom of the indenter except the direction of indentation, and increase the load on the indenter linearly from 0 to Realize the simulation of nanoindentation process;

[0069] S4. Molecular dynamics software Lammps is used to simulate nanoindentation creep.

[0070] Step S4 includes the following sub-steps:

[0071] S41, apply a constant load on the pressure head The simulation was performed for 1.5 million steps to simulate the creep process of nanocrystals;

[0072] S42, during the simulation process, collect the evolution data of the displacement of the indenter over time, record a data point every 100 steps, and output it to a file;

[0073] S43. During the simulation, the nanocrystal atomic trajectory file is output every 2000 steps.

[0074] S5. Extract creep displacement-creep time curve: Extract creep displacement-creep time curve from Lammps simulation results, such as Figure 2 As shown in the figure, it can be seen that with the increase of time, the creep displacement first increases rapidly and then increases slowly, indicating that the nanocrystals have undergone significant creep phenomenon.

[0075] S6. Analyze creep mechanism

[0076] Step S6 includes the following sub-steps:

[0077] S61, using common neighborhood analysis and other methods to identify defective atoms;

[0078] S62, visualization using Ovito software;

[0079] S63. Analyze the evolution of internal defects of nanocrystals during creep and identify the creep mechanism. Figure 3(a) and (b) show the atomic configuration diagrams at the center cross section of nanocrystalline Al at creep times of 0 ns and 3 ns, respectively. As can be seen from the figure, with the increase of creep time, the grain size under the indenter increases, and no obvious dislocation proliferation is found. This result shows that under the current simulation conditions, grain boundary sliding is the main creep mechanism of nanocrystalline Al.

Claims

1. A method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation, characterized in that: The steps include: S1. Use Atomsk software to establish the initial atomic model of nanocrystalline metal; S2, using the molecular dynamics software Lammps to simulate the relaxation process of nanocrystals and obtain the equilibrium configuration at a specified temperature; S3, using the molecular dynamics software Lammps to simulate nanoindentation; S4, using the molecular dynamics software Lammps to simulate nanoindentation creep; S5. Extract creep displacement-creep time curve; S6. Analyze the creep mechanism.

2. The method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation according to claim 1, characterized in that: In step S1, the specific process of establishing the initial atomic model of the nanocrystalline metal is: S11. According to the type of metal being simulated, set the atomic type, crystal structure and lattice constant, and generate a single crystal unit cell file. S12, setting the dimensions a, b, c of the simulation area in the x, y, z directions; generating a polycrystalline node file; S13, setting the number n of created grains, adding them to the polycrystalline node file, and calculating the grain size d according to the size of the simulation area and the number of grains; S14, using the open source software Atomsk to build the atomic model of the nanocrystal, and output the atomic model file in cfg format, and use Ovitio and Atomeye for visualization; S15. Modify the nanocrystal atomic model cfg file format into a data file that can be directly read by the molecular dynamics software Lammps.

3. The method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation according to claim 1, characterized in that: In step S2, the specific process of simulating the nanocrystal relaxation process to obtain the equilibrium configuration at a specified temperature is: S21. According to the simulated metal material, select the appropriate potential function and time step, and give priority to using the EAM potential; S22, setting the boundary conditions in the indentation direction as free boundaries and the other boundaries as periodic boundaries; S23, using the NPT system, slowly raising the system temperature to a specified temperature under near zero pressure conditions; S24, using the NPT system, under conditions close to zero pressure, the system is relaxed at a specified temperature for a period of time; S25. Output the balanced configuration to a cfg format file and modify it to a data file.

4. The method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation according to claim 1, characterized in that: In step S3, the specific process of simulating nanoindentation is as follows: S31, using Lammps software to read the atomic model of the nanocrystal after equilibrium; S32, establishing a spherical, conical, and other shaped indenter; S33, setting the bottom two layers of atoms in the pressing direction as fixed layers to maintain the stability of the overall structure, and setting the remaining boundaries as periodic boundaries; S34, restricting all degrees of freedom of the indenter except the indentation direction, applying a load that increases linearly with time to the center of mass of the indenter until a specified load is reached, thereby simulating the nanoindentation process.

5. The method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation according to claim 1, characterized in that: In step S4, the specific process of simulating nanoindentation creep is: S41, applying a specified constant load on the indenter to simulate the creep process of nanocrystals; S42, during the simulation process, collecting the evolution data of the pressure head displacement over time and outputting it to a file; S43. During the simulation process, nanocrystal atomic trajectory files are output at certain time step intervals.

6. The method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation according to claim 1, characterized in that: In step S5, a creep displacement-creep time curve is extracted from the Lammps simulation result, and the creep performance of the nanocrystalline metal material is analyzed and evaluated based on the curve.

7. The method for predicting creep properties of nanocrystalline metals based on molecular dynamics simulation according to claim 1, characterized in that: In step S6, the specific process of analyzing the creep mechanism is: S61, using coordination number, central symmetry parameter, common neighborhood analysis and other methods to identify defective atoms; S62, using Ovito, Atomeye and other software for visualization; S63. Analyze the evolution of internal defects of nanocrystals during creep and identify the creep mechanism.

8. In the step S11 of establishing the initial atomic model of nanocrystalline metal according to claim 2, it is characterized in that: The metal may be any metal.

9. The method of claim 3, wherein the step of simulating the nanocrystal relaxation process to obtain the equilibrium configuration at a specified temperature is characterized in that: The specified temperatures shown may be any temperature below the melting point of the metal.

10. In the step S32 of simulating nanoindentation according to claim 4, it is characterized in that: The shape of the indenter can be a spherical indenter, a conical indenter, a Berkovich indenter, or other indenters commonly used in nanoindentation.

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