Method for analyzing influence of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics

Abstract

The invention relates to the technical field of microcosmic acoustic nonlinear effect simulation, and particularly discloses a method for analyzing the influence of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics, which comprises the following steps: firstly, constructing a pearlite undamaged model meeting a Bagaryatskii orientation relationship and a plurality of spheroidization damage models representing different spheroidization damage degrees; carrying out molecular dynamics simulation on the eight constructed models, and carrying out energy minimization and relaxation treatment to obtain a spheroidized model with a stable structure; applying ultrasonic excitation signals V1 and V2 with opposite phases to each spheroidizing damage model to obtain displacement signals after ultrasonic propagation in different areas; and finally, processing the displacement signal by adopting a phase inversion method, extracting second harmonics, analyzing the change rule of the amplitude of the second harmonics, and establishing an incidence relation between the pearlite spheroidizing damage and the ultrasonic nonlinear effect. Through molecular dynamics simulation and a phase reversal method, the influence of pearlite spheroidization on ultrasonic harmonic waves is disclosed, and accurate theoretical support and method reference are provided for nondestructive testing of the pearlite spheroidization.

Description

Technical Field

[0001] This invention relates to the field of microscopic acoustic nonlinear effect simulation technology, and in particular to a method for analyzing the effect of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics analysis. Background Technology

[0002] Low-carbon, low-alloy ferritic-pearlitic steels are widely used in various industries, such as power generation, petroleum, and chemicals, due to their excellent mechanical properties and low cost. For example, most power plant water-cooled wall tubes are made of SA106B (ASME SA-106), and superheaters are made of P11 (ASTM A335 / A335M) steel. The ferrite and pearlite phases provide ductility and strength to ferritic-pearlitic steels, respectively. Pearlite is mainly composed of alternating layers of layered ferrite and layered cementite. The plasticity at the ferrite / cementite interface is considered to strongly influence the mechanical properties of pearlitic steels.

[0003] However, when used at high temperatures, the cementite lamellae or plates in lamellar pearlite enlarge and break into smaller cementite particles. Furthermore, due to a further decrease in interfacial energy, these cementites become fewer but more widely spaced, forming spherical particles. Finally, these spherical particles dissolve. This phenomenon is called pearlite spheroidization. These changes lead to a decrease in mechanical properties such as hardness, yield strength, and tensile strength, making them unsuitable for continued use in high-temperature environments. Therefore, non-destructive testing and assessment of spheroidization damage in pearlitic steel are crucial for enabling health monitoring and remaining service life prediction of in-service components.

[0004] Currently, the analysis of spheroidization damage is typically performed using optical or electron microscopy, methods that require cutting and grinding components, which can compromise their integrity. Therefore, non-destructive testing (NDT) of spheroidization damage in pearlitic steel is gaining increasing attention. Current NDT methods commonly employ electromagnetic and magnetic Barkhausen methods, ultrasonic backscattering signals, and nonlinear ultrasonic testing to assess spheroidization damage in pearlitic steel. Electromagnetic-based testing can be affected by strong electric and magnetic fields in industrial environments, leading to errors. Furthermore, its weak penetration capability limits its ability to detect near-surface damage, restricting its practical application. Ultrasonic backscattering signals exhibit nonlinear and non-stationary characteristics, making effective signal extraction and identification extremely challenging. In recent years, nonlinear ultrasonic NDT technology, with its sensitivity to material microstructure characteristics, has been used for the NDT assessment of early-stage damage in service components. Studies have shown that nonlinear ultrasonic testing methods can effectively characterize fatigue, creep, high-temperature aging, and microcracks. In particular, phase-reversal excitation methods can eliminate the fundamental wave signal, allowing for more accurate extraction of low-amplitude second harmonic signals.

[0005] Experimental methods can obtain information about the microstructure of materials along the ultrasonic wave propagation path. This includes a wealth of microscopic information such as grain size, grain orientation, ferrite, pearlite, and carbides. However, they cannot accurately extract the influence of microstructural features such as microdamage, fracture, and spheroidization in pearlite on acoustic nonlinear effects. Simulation methods have significant advantages in studying the influence of single factors on ultrasonic nonlinear effects. Molecular dynamics methods have been used to study the plastic deformation mechanism, fracture damage mechanism, and ferrite-cementite interface structure of pearlite. Therefore, it is feasible to use molecular dynamics methods to establish the lamellar structure of pearlite and study the influence of its spheroidization damage on ultrasonic nonlinear effects.

[0006] Molecular dynamics methods can be used to establish atomic-scale acoustic models, which can intuitively reflect the interaction between local features of cementite lamellars and acoustic signals, revealing the generation efficiency of ultrasonic second harmonics induced by material interface damage. This information is difficult to obtain experimentally. Studying the second harmonic transformation law between ultrasound and ferrite / cementite interface damage at the microscopic level, and establishing the correlation between interface spheroidization damage characteristics and ultrasonic nonlinear response, is a key and important approach to understanding nonlinear ultrasonic detection results and improving detection accuracy.

[0007] Current research on the relationship between microstructural changes in thermal damage and nonlinear ultrasonic testing results mainly relies on numerical methods such as microstructure evolution dynamics and precipitation coarsening modeling (DICTRA) to infer the variation law of defect characteristics. These are then substituted into expressions for the contribution of dislocations and precipitation to acoustic nonlinear effects and compared with actual detection results. Currently, finite element models and ultrasonic second harmonic contribution models for cementite lamellar spheroidization damage are lacking. Research on the contribution of micro-defects to acoustic nonlinearity is limited to studies of typical defects such as copper precipitation in pure iron matrices, ultrasonic wavelength effects in pure aluminum matrices, and dislocations in aluminum matrices. The influence of localized spheroidization damage of pearlite lamellars on ultrasonic second harmonic generation is currently unavailable. To investigate the influence of ferrite / cementite interface fragmentation during pearlite spheroidization on the ultrasonic second harmonic generation efficiency, this invention proposes a nonlinear ultrasonic simulation method for pearlite spheroidization damage based on molecular dynamics. Using molecular dynamics simulation and phase reversal methods, the influence of localized cementite decomposition and spheroidization on the ultrasonic second harmonic generation efficiency is studied. Summary of the Invention

[0008] The purpose of this invention is to provide a method for analyzing the influence of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics. This method addresses the lack of effective research methods on the impact of local cementite lamellar fracture and spheroidization on the generation efficiency of ultrasonic second harmonics during the spheroidization damage process of pearlite steel. Specifically, by controlling the movement of carbon atoms in the cementite lamellars to induce fracture and spheroidization, phase reversal technology is used to perform atomic-scale acoustic simulations on pearlite models with different spheroidization damage states to study the generation efficiency and local conversion law of ultrasonic second harmonics in the spheroidization damage model.

[0009] To achieve the above objectives, this invention provides a method for analyzing the influence of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics, comprising the following steps: S1. Construct an undamaged pearlite model that satisfies the Bagaryatskii orientation relationship, where the volume ratio of cementite to ferrite in the pearlite is 1:9. S2. Construct multiple spheroidization damage models to characterize different degrees of spheroidization damage; S3. Perform molecular dynamics simulations on the undamaged pearlite model constructed in S1 and the multiple spheroidized damage models constructed in S2, including the following steps: S31. Perform energy minimization and relaxation on each model to obtain a structurally stable spherical model; S32. Apply ultrasonic excitation signals V1 and V2 with opposite phases to each spherical damage model obtained in S31 to obtain displacement signals after ultrasonic wave propagation in different regions. S33. The displacement signal of S32 is processed by the phase reversal method to extract the second harmonic, analyze the variation law of the second harmonic amplitude under different degrees of spheroidization damage, and establish the correlation between pearlite spheroidization damage and ultrasonic nonlinear effect.

[0010] Preferably, in S1, the process of constructing the undamaged pearlite model is as follows: The minimum unit cells of Fe single crystals with the [1-10]

[111] [11-2] orientation and Fe3C with the

[100]

[010]

[001] orientation were obtained using the Atomsk software. Ferrite matrices and cementite lamellae of different sizes can be obtained using the duplicate command in Atomsk. Using the merge_X command in Atomsk, the ferrite matrix and cementite lamellae are combined in a preset order along the X direction to form an undamaged pearlite model.

[0011] Preferably, in S2, the carbon atoms in the selected region are migrated into the ferrite matrix using the displace_atoms command in the LAMMPS software, constructing seven spheroidization damage models characterizing different degrees of spheroidization damage, specifically: Based on the undamaged pearlite model, carbon atoms in a 4Å length region along the Y direction at the center of the cementite are selected and migrated into the ferrite to form Model 1; carbon atoms on both sides of the selected region in Model 1 are migrated to the sides of the adjacent cementite to form Model 2. Carbon atoms in six regions from 54Å to 66Å in the Y direction of cementite 1 were selected and moved. The carbon atoms in the two central regions migrated into ferrite, and the carbon atoms in the four regions on both sides migrated to the sides of the adjacent cementite to form Model 3. Carbon atoms in cementite 2 from -66Å to -54Å in the Y direction were selected and moved to form model four. Carbon atoms in the region of Y-direction coordinates from -66Å to -54Å in cementite 1 and carbon atoms in the region of Y-direction coordinates from 54Å to 66Å in cementite 2 were selected and moved to form model five. Carbon atoms in cementite 1 and 2 within the Y-direction coordinate range of -40Å to -28Å were selected and moved to form model six. Carbon atoms in cementite 1 and 2 within the Y-axis coordinate range of 27 Å to 39 Å were selected and moved to form Model 7; the movement methods of Model 4, Model 5, Model 6 and Model 7 are exactly the same as those of Model 3.

[0012] Preferably, the specific construction method of Model 1 is as follows: Based on the undamaged pearlite model, a region with a Y-coordinate of -2Å to 2Å is selected at the center of the cementite. The carbon atoms in this region are divided into two equal parts along the X-direction. The undamaged pearlite model includes two layers of cementite, namely cementite 1 and cementite 2. The selected carbon atoms in the left region of each cementite layer are moved 10Å in the negative X direction, and the carbon atoms in the right region are moved 10Å in the positive X direction, thus forming Model 1. The specific construction method of Model 2 is as follows: the carbon atoms in the selected region of Model 1 continue to migrate outward by 10 Å. The carbon atoms in the region from 2 Å to 6 Å in the Y direction of cementite 1 and cementite 2 are selected and divided into two equal parts along the X direction. The carbon atoms on the left move 10 Å in the negative X direction and 5 Å in the positive Y direction at the same time; the carbon atoms on the right move 10 Å in the positive X direction and 5 Å in the positive Y direction at the same time. Carbon atoms in the region from -6Å to -2Å in the Y direction of cementite 1 and cementite 2 are selected and divided into two equal parts along the X direction. The carbon atoms on the left move 10Å in the negative X direction and 5Å in the negative Y direction at the same time; the carbon atoms on the right move 10Å in the positive X direction and 5Å in both the positive and negative Y directions, thus forming Model 2.

[0013] Preferably, the specific construction method of Model 3 is as follows: select carbon atoms in the region of Y-direction coordinates from 54Å to 66Å in cementite 1 and move them. Divide the carbon atoms in the region of Y-direction coordinates from 58Å to 62Å into two equal parts along the X-direction. Move the carbon atoms on the left side 10Å in the negative X direction and move the carbon atoms on the right side 10Å in the positive X direction. Carbon atoms in the region of 62Å to 66Å in the Y direction are divided into two equal parts along the X direction. The carbon atoms on the left move 10Å in the negative X direction and 5Å in the positive Y direction at the same time; the carbon atoms on the right move 10Å in the positive X direction and 5Å in the positive Y direction at the same time. Carbon atoms in the region of 54 Å to 58 Å in the Y direction are divided into two equal parts along the X direction. The carbon atoms on the left move 10 Å in the negative X direction and 5 Å in the negative Y direction; the carbon atoms on the right move 10 Å in the positive X direction and 5 Å in the negative Y direction.

[0014] Preferably, in S3, when performing molecular dynamics simulation, the read_data command is used in LAMMPS software to read the undamaged pearlite model and the spheroidized damaged model. The atomic unit of the model is set to metal, the atomic type is atomic, the spatial dimension is three-dimensional, and periodic boundary conditions are used in all three directions. The simulation step size is 1fs. Atoms in the 0Å-4Å region of the X direction in the model are selected as the fixed layer for ultrasonic excitation, and atoms in the 1608Å-1713Å region of the X direction are selected as the receiving layer for ultrasonic vibration.

[0015] Preferably, S31 is as follows: Energy minimization is performed using the conjugate gradient algorithm; Use the velocity command to set the initial velocity of the atoms, set the system temperature to 300K, and use the fix_nve / limit command to limit the maximum distance the system can move in each step to 0.1Å; The initial relaxation system was run for 50,000 steps in the NVT ensemble, and the structure was optimized by annealing. The temperature was increased from 300K to 1000K in the NVT ensemble and then cooled to 0.1K. The pressure was balanced at 0 Bar. After balancing for another 50,000 steps in the NVT ensemble, a structurally stable pearlite model was obtained.

[0016] Preferably, in S32, V1 and V2 are sinusoidal ultrasonic waves, and the displacement signal is acquired as follows: The compute_displace / atom command is used to calculate the displacement of atoms in the excitation and receiving layers. The compute_reduce command is used to split the displacement vector into scalars, extracting only the displacement in the X direction. The compute_chunk / atom command is used to divide the receiving layer into several equal parts along the Y direction for receiving ultrasonic displacement signals from different regions.

[0017] Preferably, the phase reversal method used in S33 to process the displacement signal of S32 specifically includes: Read the same data file, use two simulations, one running V1 vibration and the other running V2 vibration, and obtain the displacement data of the ultrasonic receiving layer in the two simulations. Add the displacement data collected under V1 and V2 excitation to obtain the displacement signal of the second harmonic. Perform Fourier transform on the added displacement signal to obtain the amplitude of the second harmonic.

[0018] Therefore, the present invention employs the above-described method based on molecular dynamics analysis of the influence of pearlite spheroidization on ultrasonic harmonics, and the beneficial effects are as follows: (1) This invention employs a molecular dynamics simulation method, which eliminates the need for high-temperature experimental equipment and a large number of damaged samples to study the interaction between pearlitic steel spheroidization damage and ultrasonic waves to generate second harmonics, effectively reducing experimental costs and consumption. Compared with traditional experimental methods, this invention provides a clear definition of the spheroidization damage state of pearlitic steel, making the microscopic model closer to the actual microstructure of the spheroidization damage process, reducing errors caused by experimental limitations, and improving research accuracy and efficiency.

[0019] (2) Based on the spheroidization damage process of pearlitic steel under high temperature environment, the present invention uses LAMMPS software to construct a microscopic model of pearlitic steel with different degrees of spheroidization damage. The carbon atom migration method is used to ensure that the element content of the model remains unchanged, and the randomness of spheroidization damage is represented more flexibly by changing the migration of carbon atoms in local cementite positions. At the same time, the phase reversal method is used to excite ultrasonic waves to each model to efficiently extract the second harmonic, which can analyze the influence of the degree of spheroidization and local damage characteristics of pearlitic sheets on the second harmonic generation efficiency.

[0020] (3) The analysis of the correlation between pearlite spheroidization and second harmonics in this invention directly establishes the mapping law between cementite damage characteristics and macroscopic acoustic response; this law can guide the thermal damage assessment of pearlite heat-resistant steel in high-temperature service, predict the pearlite spheroidization state through ultrasonic harmonic signals, and provide a theoretical basis for experimental thermal damage analysis and performance evaluation.

[0021] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0022] Figure 1 This is an overall flowchart of an embodiment of the method for analyzing the effect of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics of the present invention; Figure 2 These are schematic diagrams of various pearlite spheroidization damage models in embodiments of the present invention based on molecular dynamics analysis of the influence of pearlite spheroidization on ultrasonic harmonics, wherein (a) is model one, (b) is model two, (c) is model three, (d) is model four, (e) is model five, (f) is model six, and (g) is model seven; Figure 3This is a microscopic acoustic model containing pearlite sheets, which is an embodiment of the method for analyzing the influence of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics of the present invention. (a) is a displacement amplitude staining in the model to show the propagation of ultrasonic waves, (b) is two ultrasonic signals obtained by excitation using the phase reversal method, (c) is a harmonic signal obtained by adding the signals at the receiving layer obtained by the phase reversal method, and (d) is the frequency domain information obtained by performing Fourier transform on the ultrasonic signal at the receiving layer. Figure 4 The second harmonic amplitudes of a pure ferrite model, a non-damaged pearlite model, and different spheroidization damage models are embodiments of the method for analyzing the effect of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics analysis of the present invention. Figure 5 The receiving layer of each model in the embodiment of the present invention, based on molecular dynamics analysis of the influence of pearlite spheroidization on ultrasonic harmonics, is divided into 64 regions along the Y direction, and the second harmonic amplitude of each region is measured. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0024] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0025] like Figure 1 As shown, the method for analyzing the effect of pearlite spheroidization on ultrasonic harmonics based on molecular dynamics includes the following steps: S1. Construct an undamaged pearlite model that satisfies the Bagaryatskii orientation relationship. In this invention, the undamaged pearlite model is a lamellar model, and the volume ratio of cementite to ferrite in the pearlite is 1:9. The construction process is as follows: A monolayer pearlite is formed by combining 3 times the minimum cementite unit and 27 times the minimum ferrite unit in the X direction, and a combination of 27 times the minimum cementite unit and 26 times the minimum ferrite unit in the Z direction. This ratio results in the minimum lattice mismatch.

[0026] Initial modeling of the undamaged pearlite model was performed in the open-source software Atomsk: the minimum unit of Fe single crystal under the [1-10]

[111] [11-2] orientation and the minimum unit of Fe3C under the

[100]

[010]

[001] orientation were obtained using Atomsk software.

[0027] Based on the above proportions, the `duplicate` command in Atmospheric was used to obtain ferrite matrices of 27×26, 240×26, 340×26, and 640×26 sizes, as well as cementite lamellars of 3×27. The 640×26 ferrite matrix is ​​the pure ferrite model.

[0028] Using the merge_X command in Atomsk, layers are combined along the X direction in the following order: 240×26 ferrite matrix - 3×27 cementite lamellar - 27×26 ferrite matrix - 3×27 cementite lamellar - 340×26 ferrite matrix, to form an undamaged pearlite model.

[0029] S2. Using the `displace_atoms` command in the LAMMPS software, carbon atoms in the selected region are migrated into the ferrite matrix, such as... Figure 2 As shown, seven spheroidization damage models representing different degrees of spheroidization damage were constructed, specifically: (1) Model 1 is based on the undamaged pearlite model. It selects carbon atoms in a 4Å length region along the Y direction at the center of cementite to migrate into ferrite. This represents the initial damage to the pearlite structure.

[0030] The specific construction method of Model 1 is as follows: A region with a Y-coordinate of -2Å to 2Å, located at the center of the cementite, is selected. The carbon atoms within this region are then divided into two equal parts along the X-direction. The undamaged pearlite model includes two layers of cementite, cementite 1 and cementite 2. Four regions are selected. The selected carbon atoms in the left region of each cementite layer are moved 10Å in the negative X-direction, and the carbon atoms in the right region are moved 10Å in the positive X-direction, forming Model 1.

[0031] (2) In Model 2, the fracture range at the center of the cementite is further expanded to 12 Å, which represents the expansion of the local decomposition of the cementite. Model 2 is formed by migrating carbon atoms on both sides of the selected region in Model 1 to the sides of the adjacent cementite. Specifically: In Model 1, carbon atoms in the selected region continue to migrate outward by 10 Å. Carbon atoms in the region from 2 Å to 6 Å in the Y direction of cementite 1 and cementite 2 are selected and divided into two equal parts along the X direction. Carbon atoms on the left move 10 Å in the negative X direction and 5 Å in the positive Y direction at the same time; carbon atoms on the right move 10 Å in the positive X direction and 5 Å in the positive Y direction at the same time.

[0032] Carbon atoms in the region from -6Å to -2Å in the Y direction of cementite 1 and cementite 2 are selected and divided into two equal parts along the X direction. The carbon atoms on the left move 10Å in the negative X direction and 5Å in the negative Y direction at the same time; the carbon atoms on the right move 10Å in the positive X direction and 5Å in both the positive and negative Y directions to form Model 2.

[0033] (3) In Model 3, a fracture occurs at a new location in cementite 1, forming a short rod-shaped cementite with the fracture location of cementite in Model 2. When constructing Model 3, carbon atoms in six regions are selected for migration. Carbon atoms in the two central regions migrate into ferrite, and carbon atoms in the four lateral regions migrate to the sides of the adjacent cementite to form Model 3. The specific construction method of Model 3 is as follows: Carbon atoms in the Y-direction coordinate region from 54 Å to 66 Å in cementite 1 are selected and moved. The carbon atoms in the Y-direction coordinate region from 58 Å to 62 Å are divided into two equal parts along the X-direction. The carbon atoms on the left are moved 10 Å in the negative X direction, and the carbon atoms on the right are moved 10 Å in the positive X direction.

[0034] Carbon atoms in the region of 62Å to 66Å in the Y direction are divided into two equal parts along the X direction. The carbon atoms on the left move 10Å in the negative X direction and 5Å in the positive Y direction at the same time; the carbon atoms on the right move 10Å in the positive X direction and 5Å in the positive Y direction at the same time.

[0035] Carbon atoms within the Y-axis coordinate region of 54 Å to 58 Å are divided into two equal parts along the X-axis. Carbon atoms on the left move 10 Å towards the negative X-axis and 5 Å towards the negative Y-axis; carbon atoms on the right move 10 Å towards the positive X-axis and 5 Å towards the negative Y-axis. Model 3 is established by moving carbon atoms within these six regions.

[0036] (4) In Model 4, the cementite 2 fractures at a new location, forming two short rod-shaped cementites. The specific construction method is as follows: select carbon atoms in the Y-direction coordinate region from -66Å to -54Å in cementite 2 and move them. The movement method is exactly the same as in Model 3. After the movement is completed, Model 4 is formed.

[0037] (5) In Model 5, cementite 1 and cementite 2 break at new locations to form six short rod-shaped cementite regions. The specific construction method is as follows: carbon atoms in the Y-direction coordinate region from -66Å to -54Å in cementite 1 and carbon atoms in the Y-direction coordinate region from 54Å to 66Å in cementite 2 are selected and moved. The movement method is exactly the same as in Model 3. After the movement is completed, Model 5 is formed.

[0038] (6) In Model 6, cementite 1 and 2 break at their new locations. The two short rod-shaped cementites are decomposed into four elliptical cementites. At this time, the model consists of four rod-shaped cementites and four elliptical cementites. The specific construction method is as follows: carbon atoms in the Y-direction coordinate range of -40Å to -28Å in cementite 1 and 2 are selected and moved. The movement method is exactly the same as in Model 3. After the movement is completed, Model 6 is formed.

[0039] (7) In Model 7, cementite 1 and cementite 2 break at their new locations. The two short rod-shaped cementites are decomposed into four elliptical cementites. At this time, the model consists of two rod-shaped cementites and eight elliptical cementites. The specific construction method is as follows: carbon atoms in the Y-direction coordinates from 27Å to 39Å in cementite 1 and 2 are selected and moved. The movement method is exactly the same as that in Model 3. After the movement is completed, Model 7 is formed.

[0040] S3. Molecular dynamics simulations were performed on the undamaged pearlite model constructed in S1 and the multiple spheroidized damaged models constructed in S2. During the molecular dynamics simulations, the `read_data` command was first used in LAMMPS software to read eight models, including the undamaged pearlite model and the spheroidized damaged model. The atomic unit was set to `metal`, the atom type to `atomic`, and the spatial dimension to three dimensions. Periodic boundary conditions were used in all three directions, and the simulation step size was 1 fs. Atoms in the 0 Å-4 Å region of the X-direction were selected as the fixed layer for ultrasonic excitation, and atoms in the 1608 Å-1713 Å region of the X-direction were selected as the receiving layer for ultrasonic vibration. Then, molecular dynamics simulations were performed according to the following steps: S31. Perform energy minimization and relaxation on each model to obtain a structurally stable spherical model, specifically as follows: The conjugate gradient algorithm is used to minimize energy.

[0041] Use the velocity command to set the initial velocity of the atoms and set the system temperature to 300K. Then use the fix_nve / limit command to limit the maximum distance the system can move in each step to 0.1Å. Run 10,000 steps to prevent the system from collapsing due to atomic overlap.

[0042] The initial relaxation system was run for 50,000 steps in the NVT ensemble, and the structure was optimized by annealing. The temperature was increased from 300K to 1000K at a rate of 7K / ps in the NVT ensemble; it was held at 1000K for 50,000 steps, and then cooled from 1000K to 1K at a rate of 100K / ps in the NVT ensemble. The temperature was then controlled to decrease from 1K to 0.1K in the NPT ensemble, and the pressure was balanced at 0 Bar. After balancing for another 50,000 steps in the NVT ensemble, the relaxation of the model was completed. The temperature control during this process was performed using a Nosé-Hoover thermostat. After relaxation, a structurally stable pearlite model was obtained.

[0043] S32, such as Figure 3 As shown, ultrasonic excitation signals V1 and V2 with opposite phases were applied to each spheroidized damage model obtained in S31 to obtain displacement signals after ultrasonic wave propagation in different regions. V1 and V2 are sinusoidal ultrasonic waves. The displacement signals were obtained as follows: Reset the current time step, use the compute_displace / atom command to calculate the displacement of atoms in the excitation and receiving layers, use the compute_reduce command to split the displacement vector into scalars, and extract only the displacement in the X direction; use the compute_chunk / atom command to divide the receiving layer into 64 equal parts along the Y direction for receiving ultrasonic displacement signals from different regions.

[0044] In this invention, the variable command is used to define two sine waves to be excited. Where A is the amplitude, typically set to 0.1 Å, step is the current system time step, T is the excitation ultrasonic frequency, and dt is the current system time step size. The fix_move command applies two sinusoidal vibrations, V1 and V2, to the ultrasonic excitation layer. For a 2 THz ultrasonic wave, its single-cycle excitation time is set to 2,000 steps, and the number of vibration cycles and the excitation frequency can be flexibly adjusted according to actual needs. The entire ultrasonic wave propagation process operates under the NVE ensemble.

[0045] S33. The displacement signal of S32 is processed by the phase reversal method to extract the second harmonic, analyze the variation law of the second harmonic amplitude under different degrees of spheroidization damage, and establish the correlation between pearlite spheroidization damage and ultrasonic nonlinear effect.

[0046] Specifically, the phase reversal method for processing the displacement signal of S32 includes: The phase reversal method was used to simulate the second harmonic generation efficiency of a pearlite model without spheroidization damage. Using the same data file, two simulations were performed, one running under V1 vibration and the other under V2 vibration. Displacement data of the ultrasonic receiving layer in both simulations were acquired. The displacement data acquired under V1 and V2 excitations were summed to obtain the displacement signal of the second harmonic. The amplitude of the second harmonic was obtained by performing a Fourier transform on the summed displacement signal. Figure 4 As shown.

[0047] The phase reversal method was used to simulate the second harmonic generation efficiency of all models. The method of obtaining the overall second harmonic amplitude of the model was completely consistent with that of the pearlite model without spheroid damage. That is, by running two vibration examples, V1 and V2, the displacement data of the corresponding ultrasonic receiving layer was read and added together, and then the Fourier transform of the added displacement signal was performed.

[0048] The ultrasonic receiving layer is divided into 64 equal regions, and the ultrasonic displacement signals of each of the 64 regions can be acquired independently. For these signals, the same process of superimposing the signals from the corresponding regions under the V1 and V2 vibration models is performed. After Fourier transform, the amplitude information of the second harmonic along the cementite lamellar direction can be obtained, such as... Figure 5 As shown.

[0049] Example 1 (1) All models satisfy the Bagaryatskii orientation relationship between ferrite and cementite in pearlite: the orientation of ferrite is [1-10]

[111] [11-2], and the orientation of cementite is

[100]

[010]

[001] .

[0050] To construct a microscopic model of pearlite spheroidization under thermal damage conditions, this embodiment uses the built-in commands of the LAMMPS software to select carbon atoms in the cementite lamellars and control the gradual migration of carbon atoms in different regions to simulate the spheroidization process.

[0051] (2) The model uses metal units, the spatial dimension is set to three dimensions, and periodic boundary conditions are applied in all three directions; the simulation time step is set to 1fs, and the number of nearest neighbors of atoms is set to 2.0bin.

[0052] (3) Regarding the potential function, the MEAM potential function is selected to describe the interaction forces between Fe-Fe and Fe-C.

[0053] (4) In order to obtain a structurally stable pearlite model under different damage states, energy minimization and relaxation processing are required: energy minimization is performed by using the conjugate gradient algorithm, the system temperature is set to 300K by the velocity command, and then structural optimization is performed by annealing. After relaxation is completed, a structurally stable pearlite model can be obtained.

[0054] (5) After completing the model preparation, reset the current time step and use the fix_move command in LAMMPS to apply two sinusoidal vibrations, V1 and V2, to the ultrasonic excitation layer. For ultrasonic waves with a frequency of 2THz, set the single-cycle excitation time to 2000 steps, and adjust the number of vibration cycles and frequency according to actual needs; while the system always operates under the NVE ensemble during ultrasonic wave propagation.

[0055] (6) To monitor the simulation process in real time, statistical parameters are output via the thermo command, once every 1000 steps. The thermo_style custom output method is used, and the output physical quantities include the simulation step number (step), system temperature (temp), potential energy (pe), kinetic energy (ke), total energy (etotal), model pressure (press), x-direction length (lx), y-direction length (ly), z-direction length (lz), and model volume (vol). These parameters can reflect the rationality of the model's state during ultrasonic wave propagation in real time, providing a basis for subsequent adjustment and modification of model parameters.

[0056] (7) In terms of displacement calculation and trajectory output, the compute_displace / atom command is used to calculate the displacement of the receiving layer atoms. Then, the compute_reduce command is used to decouple the vector atom displacements into scalars to obtain the displacement values ​​in the X direction. The dump command is used to output the atomic trajectories of the model. The same custom output method is used. The output physical quantities include x, y, z coordinates, x-direction displacement and x-direction stress. The data is saved every 10,000 steps.

[0057] (8) In subsequent data processing, it is necessary to obtain the overall x-direction displacement of the receiving layer under the vibration conditions of V1 and V2 for each model, as well as the x-direction displacement of each region after the receiving layer is divided into 64 regions. For the receiving layer as a whole, the signals under the excitation of V1 and V2 are added together to obtain the second harmonic time domain signal, and then the Fourier transform is performed on it to obtain the second harmonic amplitude.

[0058] The second harmonic amplitudes of different models were plotted to reflect the influence of spheroidization damage on the efficiency of ultrasonic second harmonic generation. For 64 regions, the signals of each region under V1 and V2 excitation were summed and Fourier transformed to obtain the second harmonic amplitude of each region. The harmonic amplitudes of the 64 regions of each model were plotted as a curve to reflect the relationship between local cementite damage and second harmonic generation.

[0059] Furthermore, the atomic trajectory files were visualized using OVITO software. The color_coding module was used to select the x-direction displacement in the model output file, with upper and lower limits set to -0.1 Å to 0.1 Å, to obtain the real-time ultrasonic wave propagation diagram. By analyzing the above performance diagrams, the influence of interface damage in the pearlite spheroidization state on the generation law of ultrasonic second harmonics can be clearly identified.

[0060] (9) The results show that the second harmonic amplitude curve indicates that the spheroidization process of pearlite reduces the ultrasonic nonlinear effect. Through comparative verification, it is proven that the conclusions obtained by the modeling method proposed in this embodiment have good reliability and reference value; when applied to molecular dynamics simulation, it can realize an intuitive analysis of the influence of pearlite lamellar spheroidization on ultrasonic nonlinear effect.

[0061] Therefore, this invention employs the aforementioned method based on molecular dynamics analysis of the influence of pearlite spheroidization on ultrasonic harmonics. By constructing pearlite models with different degrees of spheroidization damage through molecular dynamics simulation and extracting second harmonics using the phase reversal method, it can reduce experimental costs, intuitively present the interaction process between the pearlite structure and ultrasound, reveal the influence of spheroidization damage on ultrasonic nonlinear effects, provide microscopic information and theoretical basis for macroscopic detection, improve the accuracy of thermal damage detection, and provide support for experimental analysis and performance evaluation.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for analyzing the effect of pearlite spheroidization on ultrasound harmonics based on molecular dynamics, characterized in that, The method comprises the following steps: S1, constructing a pearlite undamaged model satisfying the Bagaryatskii orientation relationship, the volume ratio of cementite to ferrite in the pearlite being 1:9; S2, constructing a plurality of spheroidization damage models representing different spheroidization damage degrees; S3, performing molecular dynamics simulation on the pearlite undamaged model constructed in S1 and the plurality of spheroidization damage models constructed in S2, comprising the following steps: S31, performing energy minimization and relaxation processing on each model to obtain a spheroidization model stable in structure; S32, applying phase-opposed ultrasonic excitation signals V1 and V2 to each spheroidization damage model obtained in S31 to obtain displacement signals after ultrasonic propagation in different regions; S33, processing the displacement signals in S32 by using a phase inversion method, extracting a second harmonic, analyzing the change rule of the second harmonic amplitude under different spheroidization damage degrees, and establishing a correlation between pearlite spheroidization damage and ultrasonic nonlinear effect.

2. The method of claim 1, wherein the molecular dynamics analysis is based on a model of a ferrite grain with a spherical shape. In S1, the construction process of the pearlite undamaged model is as follows: Using Atomsk software to obtain the minimum unit of Fe single crystal in [1-10] [111] [11-2] orientation and the minimum unit of Fe3C in [100] [010] [001] orientation; Obtaining ferrite matrix and cementite lamella of different sizes by using the duplicate command in Atomsk; Using the merge_X command in Atomsk to combine the ferrite matrix and the cementite lamella in a preset order along the X direction to form the pearlite undamaged model.

3. The method of claim 2, wherein the molecular dynamics analysis is based on a model of a ferrite grain with a spherical shape. In S2, the displace_atoms command in the LAMMPS software is used to migrate the carbon atoms in the selected region into the ferrite matrix to construct seven spheroidization damage models representing different spheroidization damage degrees, specifically as follows: On the basis of the pearlite undamaged model, the carbon atoms in the 4Å length region in the center of the cementite along the Y direction are migrated into the ferrite to form model one; the carbon atoms on both sides of the selected region of model one are migrated to the two sides of the adjacent cementite to form model two; The carbon atoms in the six regions of Y direction coordinates 54Å to 66Å in cementite 1 are moved, the carbon atoms in the center two regions are migrated into the ferrite, and the carbon atoms in the four regions on both sides are migrated to the two sides of the adjacent cementite to form model three; The carbon atoms in the region of Y direction coordinates -66Å to -54Å in cementite 2 are moved to form model four; The carbon atoms in the region of Y direction coordinates -66Å to -54Å in cementite 1 and the carbon atoms in the region of Y direction coordinates 54Å to 66Å in cementite 2 are moved to form model five; The carbon atoms in the regions of Y direction coordinates -40Å to -28Å in cementite 1 and 2 are moved to form model six; The carbon atoms in the regions of Y direction coordinates 27Å to 39Å in cementite 1 and 2 are moved to form model seven; wherein, the moving modes of model four, model five, model six and model seven are the same as those of model three.

4. The method of claim 3, wherein the molecular dynamics analysis is based on a model of a ferrite grain with a spherical shape. The specific construction mode of model one is as follows: On the basis of the pearlite undamaged model, the region with Y direction coordinate of -2 Å to 2 Å in the middle position of cementite was selected, and the carbon atoms in the region were divided into two parts along the X direction. The pearlite undamaged model included two layers of cementite, namely cementite 1 and cementite 2. The carbon atoms in the left region of each layer of cementite were moved 10 Å to the negative direction of X, and the carbon atoms in the right region were moved 10 Å to the positive direction of X to form model one; The specific construction method of model two is that the carbon atoms in the selected region of model one continue to migrate 10 Å outward, and the carbon atoms in the region with Y direction coordinate of 2 Å to 6 Å in cementite 1 and cementite 2 are selected and divided into two parts along the X direction. The left carbon atoms are moved 10 Å to the negative direction of X and 5 Å to the positive direction of Y, and the right carbon atoms are moved 10 Å to the positive direction of X and 5 Å to the positive direction of Y. The carbon atoms in the region with Y direction coordinate of -6 Å to -2 Å in cementite 1 and cementite 2 are selected and divided into two parts along the X direction. The left carbon atoms are moved 10 Å to the negative direction of X and 5 Å to the negative direction of Y, and the right carbon atoms are moved 10 Å to the positive direction of X and 5 Å to the positive direction of Y to form model two.

5. The method of claim 4, wherein the molecular dynamics analysis is based on a model of a ferrite grain with a spherical shape. The specific construction method of model three is that the carbon atoms in the region with Y direction coordinate of 54 Å to 66 Å in cementite 1 are moved. The carbon atoms in the region with Y direction coordinate of 58 Å to 62 Å are divided into two parts along the X direction. The left carbon atoms are moved 10 Å to the negative direction of X, and the right carbon atoms are moved 10 Å to the positive direction of X. The carbon atoms in the region with Y direction coordinate of 62 Å to 66 Å are divided into two parts along the X direction. The left carbon atoms are moved 10 Å to the negative direction of X and 5 Å to the positive direction of Y, and the right carbon atoms are moved 10 Å to the positive direction of X and 5 Å to the positive direction of Y. The carbon atoms in the region with Y direction coordinate of 54 Å to 58 Å are divided into two parts along the X direction. The left carbon atoms are moved 10 Å to the negative direction of X and 5 Å to the negative direction of Y, and the right carbon atoms are moved 10 Å to the positive direction of X and 5 Å to the negative direction of Y.

6. The method of claim 1, wherein the molecular dynamics analysis is based on a model of a ferrite grain with a spherical shape. In S3, when performing molecular dynamics simulation, the read_data command in LAMMPS software is used to read the pearlite undamaged model and spheroidization damage model. The model atom unit is set as metal, the atom type is set as atomic, the spatial dimension is set as three-dimensional, the periodic boundary condition is used in three directions, the simulation step is set as 1 fs; The atoms in the region of 0 Å-4 Å in the X direction of the model are selected as the fixed layer excited by ultrasonic waves, and the atoms in the region of 1608 Å-1713 Å in the X direction are selected as the receiving layer of ultrasonic wave vibration.

7. The method of claim 1, wherein the molecular dynamics analysis is based on a model of a ferrite grain with a spherical shape. S31 is specifically: The conjugate gradient algorithm is used for energy minimization; The initial velocity of the atom is set by using the velocity command, the system temperature is set to 300 K, and the fix_nve / limit command is used to limit the maximum movement distance of the system to 0.1 Å per step. The system was initially relaxed for 50,000 steps in the NVT ensemble, and the structure was optimized in an annealing manner. The system was heated from 300 K to 1000 K and then cooled to 0.1 K in the NVT ensemble, and the pressure was balanced at 0 Bar. The stable structure of the pearlite model was obtained after the system was balanced for 50,000 steps in the NVT ensemble.

8. The method of claim 1, wherein the molecular dynamics analysis is based on a model of a ferrite grain with a spherical shape. In S32, V1 and V2 are sinusoidal ultrasonic waves, and the displacement signal is obtained in the following manner: The compute_displace / atom command is used to calculate the displacement of the atoms in the excitation layer and the receiving layer, the compute_reduce command is used to decompose the displacement vector into a scalar, and only the displacement in the X direction is extracted; the compute_chunk / atom command is used to divide the receiving layer into several parts along the Y direction for receiving ultrasonic displacement signals in different regions.

9. The method of claim 1, wherein the method is based on molecular dynamics analysis of the effect of pearlite spheroidization on ultrasound harmonics. In S33, the displacement signal in S32 is processed by using the phase inversion method, and the processing specifically includes: The same data file is read, two examples are run, one for V1 vibration and the other for V2 vibration, the displacement data of the ultrasonic receiving layer in the two examples are obtained, the displacement data collected under V1 and V2 excitation are added to obtain the second harmonic displacement signal; the added displacement signal is subjected to Fourier transform to obtain the amplitude of the second harmonic.

Images