Method for realizing inverse phase change strengthening of light-weight high-entropy steel by ultrasonic rolling and modified light-weight high-entropy steel

Abstract

A light high-entropy steel base material with the T0 temperature lower than the room temperature is selected, heat preservation is carried out for 4 hours at the temperature of 1100 DEG C, homogenizing annealing is carried out, then water quenching is carried out after the temperature is kept for 40 minutes at the temperature of 1100 DEG C, a structure containing BCC and FCC double phases is obtained, and at the room temperature, the T0 temperature of the light high-entropy steel base material is kept at the constant temperature of 1100 DEG C for 40 minutes, and the T0 temperature of the light high-entropy steel base material is kept at the constant temperature of 1100 DEG C for 40 minutes. And performing ultrasonic rolling treatment on the surface of the sample. The method has the advantages that BCC-to-FCC martensite inverse phase transformation can be induced on the surface layer of a workpiece, the magnetic domain of the surface layer basically disappears, the strength and plasticity are synchronously improved, and the method can be widely applied to the fields of electrical equipment, MRI devices, nuclear submarine shells, aerospace, automobile key components and the like.

Description

Technical Field

[0001] This invention relates to the field of surface strengthening technology for metallic materials, and in particular to a method for achieving reverse phase transformation strengthening of lightweight high-entropy steel through ultrasonic rolling. Background Technology

[0002] With the increasing demand for high-performance non-magnetic structural materials in modern industry, especially in highly magnetically sensitive environments such as magnetic resonance imaging (MRI), superconducting magnets, power equipment, and the pressure hulls of nuclear submarines, materials must avoid coupling with strong external magnetic fields during service to ensure equipment stability and signal accuracy. In these applications, the magnetism of the material's surface region is the main factor determining magnetic field disturbances. Once ferromagnetic structures or magnetic domains appear on the surface, they will directly cause local magnetic field distortion.

[0003] Currently, common strengthening methods for non-magnetic steels include alloying design, surface treatment, and plastic deformation techniques. While these traditional techniques can improve the mechanical properties of materials to some extent, they often inevitably introduce the ferromagnetic phase BCC during the strengthening process of non-magnetic steels, limiting their application in highly magnetically sensitive environments. How to ensure the mechanical properties of materials while avoiding inducing magnetic phase transitions on the material surface has become a research hotspot in the strengthening technology of non-magnetic materials. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method that can improve the mechanical properties of lightweight high-entropy alloys while ensuring that the surface layer is non-magnetic.

[0005] To address the aforementioned technical problems, this invention provides a method for ultrasonic rolling to achieve inverse phase transformation strengthening of lightweight high-entropy steel, comprising the following steps: S1. Material Selection: Lightweight high-entropy steel substrate with a T0 temperature lower than room temperature is selected. Its composition by mass percentage is: Mn 10%-20%, C 0.5%, Al 5%-20%, V 0.8%, with the balance being Fe. S2. Heat treatment: The lightweight high-entropy steel is homogenized by holding at 1100℃ for 4 hours, followed by isothermal quenching at 1100℃ for 40 minutes and then water quenching to obtain a microstructure containing BCC and FCC dual phases. S3. Ultrasonic rolling treatment: The sample surface is ultrasonically rolled at room temperature.

[0006] Furthermore, in step S3, the diameter of the WC / Co ball head used is 8-14 mm; the frequency of the ultrasonic rolling is 20-30 kHz, the amplitude is 10-20 μm, and the constant static pressure is 1000-1500 N.

[0007] Furthermore, the diameter of the WC / Co ball head is 12mm; the frequency of the ultrasonic rolling is 20kHz, the amplitude is 15μm, and the constant static pressure is 1200 N.

[0008] Furthermore, in step S3, the workpiece to be processed rotates at 20-50 rpm, and the WC / Co ball head advances from the circumference to the center at a feed speed of 0.028-0.05 mm / r, with a coverage rate of 100%.

[0009] Furthermore, before step S3, a grinding step is included: grinding the surface of the lightweight high-entropy steel to make it smooth.

[0010] The present invention also provides a modified lightweight high-entropy steel, which is prepared by the above method.

[0011] Beneficial effects of this invention: (1) This invention enables the workpiece to undergo a BCC→FCC martensitic inverse phase transformation within a depth range of 100–500 μm on the surface, breaking through the inherent direction of traditional stress / strain induced ferromagnetic BCC→non-magnetic FCC, obtaining an austenitic surface layer of ≥95%, with the surface magnetic domains basically disappearing, thus meeting the magnetic compatibility requirements of key components such as nuclear magnetic resonance imaging, superconducting magnets, and nuclear submarines.

[0012] (2) Simultaneous improvement of strength and plasticity: The compressive yield strength of the micro-column is increased by about 2 times, the ultimate compressive strength is ≥6GPa, the surface yield strength is ≥2.3GPa, and the plastic strain is maintained above 35%, which solves the bottleneck of "strength-plasticity inversion".

[0013] (3) Uniform reinforcement layer: The "static pressure + ultrasonic vibration" coupling effect of ultrasonic rolling results in a surface roughness Ra≤0.2μm, which can be directly assembled and used without subsequent polishing.

[0014] (4) Green, energy-saving, and low-cost: The reverse phase change process is completed at room temperature, without the need for heating, vacuum, or chemical reagents. There is no heat-affected zone, no oxide scale, and no pollutant emissions, which is in line with the trend of green manufacturing. Attached Figure Description

[0015] Figure 1 The X-ray diffraction (XRD) analysis results of the surface layers of the examples and comparative examples are shown. Figure 2 Electron backscattering diffraction (EBSD) patterns of the surface layers in the examples and comparative examples; Figure 3 Magnetic force microscopy (MFM) images for examples and comparative examples; Figure 4 The loading force-depth curves of the examples and comparative examples in the hardness test are shown. Figure 5The examples and comparative examples show the engineering stress-strain curves during the micropillar compression process; Figure 6 The images show the microstructure analysis of Examples 1-3 and Comparative Examples 1-3. Detailed Implementation

[0016] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0017] Example 1: The ultrasonic rolling method for achieving inverse phase transformation strengthening of lightweight high-entropy steel includes the following steps: S1. A lightweight, high-entropy steel substrate with a T0 temperature below room temperature is selected. Its composition by mass percentage is: Mn 10%, C 0.5%, Al 10%, V 0.8%, with the balance being Fe. The T0 temperature of this alloy is below room temperature, and the austenitic phase is stable at room temperature, which is beneficial for subsequent reverse phase transformation.

[0018] S2. The lightweight high-entropy steel was homogenized by holding at 1100℃ for 4 hours, followed by water quenching at 1100℃ for 40 minutes to obtain a microstructure containing BCC and FCC dual phases. The volume fraction of the BCC phase was found to be approximately 46%.

[0019] S3. Grinding: Clean the end face of the workpiece and grind it with sandpaper to remove the surface oxide layer. Clean the surface dirt and oil with alcohol and blow dry the surface.

[0020] S4. Clamp the workpiece on the lathe and use the GS-X1 ultra-high energy finishing control system to perform ultrasonic rolling on the sample surface at room temperature: workpiece rotation speed 35 rpm, WC / Co ball head diameter 12 mm, constant static pressure 1200 N, feed speed 0.035 mm / r, feed path is a spiral from the outside to the inside, ultrasonic frequency 20 kHz, amplitude 15 μm, coverage 100%. Oil lubrication is used during ultrasonic rolling to lubricate and cool the surface, controlling the surface temperature ≤40℃.

[0021] In another embodiment, compared to embodiment 1, only the diameter of the WC / Co ball head, the constant static pressure, the ultrasonic frequency, and the amplitude are changed. Specifically, the diameter of the WC / Co ball head is 8 mm, the constant static pressure is 1000 N, the ultrasonic frequency is 30 kHz, and the amplitude is 10 μm.

[0022] In another embodiment, compared to embodiment 1, only the diameter of the WC / Co ball head, the constant static pressure, and the amplitude are changed. Specifically, the diameter of the WC / Co ball head is 14 mm, the constant static pressure is 1500 N, and the amplitude is 20 μm.

[0023] In another embodiment, compared to embodiment 1, only the workpiece rotation speed and the ball head feed speed are changed. Specifically, the workpiece rotation speed is 20 or 50 rpm, and the WC / Co ball head feed speed is 0.028 or 0.05 mm / r.

[0024] Example 2 The ultrasonic rolling method for reverse phase transformation strengthening of lightweight high-entropy steel is the same as that in Example 1, except that the ultrasonic rolling amplitude is changed from 15μm to 20μm, while the other steps remain unchanged.

[0025] Example 3 The ultrasonic rolling method for reverse phase transformation strengthening of lightweight high-entropy steel differs from Example 1 only in that the coverage is adjusted from 100% to 200%, while the other steps remain unchanged.

[0026] Comparative Example 1 A surface strengthening method only performs steps S1 and S2 as described in Example 1, omitting steps S3 and S4. Specifically, it includes the following steps: S1. A lightweight, high-entropy steel substrate with a T0 temperature below room temperature is selected. Its composition by mass percentage is: Mn 10%, C 0.5%, Al 10%, V 0.8%, with the balance being Fe. The T0 temperature of this alloy is below room temperature, and the austenitic phase is stable at room temperature, which is beneficial for subsequent reverse phase transformation.

[0027] S2. The lightweight high-entropy steel was homogenized by holding at 1100℃ for 4 hours, followed by water quenching at 1100℃ for 40 minutes to obtain a microstructure containing BCC and FCC dual phases. The volume fraction of the BCC phase was found to be approximately 46%.

[0028] Comparative Example 2 A surface strengthening method, compared to Example 1, only reduces the constant static pressure from 1200N to 800N, while the other steps remain unchanged.

[0029] Comparative Example 3: A surface strengthening method, compared to Example 1, omits step S2, while retaining all other steps. Specifically, it includes the following steps: S1. A lightweight, high-entropy steel substrate with a T0 temperature below room temperature is selected. Its composition by mass percentage is: Mn 10%, C 0.5%, Al 10%, V 0.8%, with the balance being Fe. The T0 temperature of this alloy is below room temperature, and the austenitic phase is stable at room temperature, which is beneficial for subsequent reverse phase transformation.

[0030] S2. Grinding: Clean the end face of the workpiece and grind it with sandpaper to remove the surface oxide layer. Clean the surface dirt and oil with alcohol and blow dry the surface.

[0031] S3. Clamp the workpiece on the lathe and use the GS-X1 ultra-high energy finishing control system to perform ultrasonic rolling on the sample surface at room temperature: workpiece rotation speed 35 rpm, WC / Co ball head diameter 12 mm, constant static pressure 1200 N, feed speed 0.035 mm / r, feed path is a spiral from the outside to the inside, ultrasonic frequency 20 kHz, amplitude 15 μm, coverage 100%. Oil lubrication is used for lubrication and cooling during ultrasonic rolling, controlling the surface temperature ≤40℃.

[0032] Comparative Example 4: A surface strengthening method, compared to Example 1, differs only in the materials used in step S1. Specifically, the material composition by mass percentage is: Mn 10%, C 0.5%, Al 5%, V 0.8%, Si 5%, with the balance being Fe. Compared to the material in Example 1, the T0 temperature of the material in Comparative Example 4 is significantly increased to a level significantly higher than room temperature. Room temperature is 20~35℃.

[0033] To verify the technical effectiveness of the embodiments, the following experiments were conducted: Experiment 1: XRD Detection XRD phase analysis was performed on multiple regions of the surface of the samples prepared in Examples 1-3 and Comparative Examples 1-4. The analysis results are as follows: Figure 1 As shown.

[0034] In XRD analysis, different peaks correspond to different crystal structures, thus indicating different phases. The peak intensity reflects, to some extent, the relative abundance of different phases. Figure 1 The analysis results show that Examples 1, 2 and 3 all contain almost all of the FCC phase, while Comparative Examples 1-4 show a state of coexistence of two phases.

[0035] Experiment 2: EBSD Phase Detection To further determine the phase content, phase analysis was performed on cross-sections of samples from Examples 1-3 and Comparative Examples 1-4. EBSD detection can more intuitively display the tissue and its content, such as... Figure 2As shown in the figure. The results show that the BCC content in Example 1 was only 0.2%, the BCC content in Example 2 was 0%, the BCC phase content in Example 3 was 3.8%, the BCC content in Comparative Example 1 was 48.2%, the BCC content in Comparative Example 2 was 12.6%, the BCC content in Comparative Example 3 was 40.7%, and the BCC content in Comparative Example 4 was 22.0%. The above results corroborate the results in Experiment 1, fully demonstrating that the sample surface in the examples was almost entirely composed of the FCC phase.

[0036] Experiment 3: Magnetic Performance Testing As is well known, BCC structures are typically magnetic, while FCC structures generally exhibit non-magnetic behavior. This is mainly due to the crystal symmetry of the FCC phase, which leads to the splitting of electronic energy levels, thereby suppressing the formation of spontaneous magnetic moments and maintaining the material's non-magnetic nature. To further verify this characteristic, magnetic force microscopy (MFM) mode in atomic force microscopy (AFM) was used to detect the magnetic properties of multiple regions on the surface of the samples from Examples 1-3 and Comparative Examples 1-4. The results are as follows: Figure 3 As shown in the figure. The test results show that the comparative sample has obvious magnetic domain structure, indicating that it contains the BCC phase; while no magnetic domains were observed in the example sample, indicating that its surface is non-magnetic. Since the other comparative samples have a high content of BCC phase, only comparative example 1 is used to illustrate the magnetic characteristics of the BCC phase.

[0037] Experiment 4: Surface Roughness Testing After ultrasonic rolling, the surface of the lightweight high-entropy steel sample exhibited a significant mirror-like finish. To verify the quality and performance of the lightweight high-entropy steel surface after ultrasonic rolling treatment according to the present invention, its surface roughness was measured. A surface roughness tester was used to measure the surface roughness of samples from Examples 1-3 and Comparative Examples 1-4. During the test, multiple representative areas of each sample surface were selected for measurement, and the Sa value (arithmetic mean surface roughness) of each area was calculated. The Sa value reflects the flatness and smoothness of the sample surface. The average value of the measurement results is as follows:

[0038] Test results show that the surface roughness of the examples is significantly lower than that of comparative examples 1-2. This indicates that after ultrasonic rolling treatment, the sample surface has a very smooth and uniform microstructure, which meets the requirement that it can be directly assembled and used without subsequent polishing.

[0039] Test 5: Hardness Testing Nanoscale hardness tests were performed at multiple locations on the surfaces of samples from Examples 1-3, Comparative Examples 1-4, and commercially available non-magnetic steels 7Mn15 and Mn13. Hardness measurements were performed using a nanoindenter, and the load-depth curves during the loading-hold-unloading process were analyzed using the Oliver-Pharr method. Typical loading force-displacement curves for each sample are shown below. Figure 4 As shown. The average value of the test results from multiple points is taken, and the results are as follows:

[0040] The results show that the hardness of the samples in the examples is significantly higher than that of the comparative examples and commercial non-magnetic steel, indicating that the samples in the examples have better surface mechanical properties.

[0041] The composition of Mn13 by mass percentage is: Mn: 11.0%-14.0%, C: 1.0%-1.4%, Si: ≤0.5%, P: ≤0.07%, S: ≤0.03%, and the balance Fe.

[0042] The composition of 7Mn15 by mass percentage is as follows: Mn: 16-20%, C: 0.6-0.8%, Si: 0.5%, P: 0.028%, V: 1.0-1.6%, Cr: 3.0-4.5%, Ni: 0.9-1.2%, N: 0.2%, W: 0.2-0.5%, Mo: 0.2-0.5%, and the balance Fe.

[0043] Experiment 6: Cyclic Loading Test of Nanoindentation During surface plastic deformation strengthening, a gradient structure forms within the material, inducing back stress. Back stress not only enhances the overall mechanical properties of the material but also significantly improves its strength. Therefore, cyclic loading tests were conducted using a nanoindenter to measure the surface back stress of samples from Examples 1-3, Comparative Examples 1-4, and commercial non-magnetic steels 7Mn15 and Mn13. By analyzing the load-displacement hysteresis loops obtained during cyclic loading, back stress-related information can be extracted, and the dislocation accumulation effect caused by the gradient structure can be quantitatively characterized. The corresponding test data are shown below:

[0044] As can be seen, the back stress of Examples 1-3 is significantly higher than that of Comparative Examples 1-3, 7Mn15, and Mn13, indicating that the treatment in this application can increase the back stress on the sample surface, which will significantly improve the mechanical properties of the sample surface. Although Comparative Example 4 also has a high back stress, it does not meet the performance requirement of a non-magnetic surface due to its two-phase structure.

[0045] Experiment 7: Microcolumn Compression Test Micropillar compression tests were used to directly measure the yield strength, ultimate compressive strength, and plastic strain of local materials at the microscale, verifying the synergistic improvement in strength and plasticity after ultrasonic rolling treatment of the surface layer. Since, except for Comparative Example 2 which contained a small amount of BCC phase, the other comparative example samples exhibited a large amount of BCC phase, failing to meet the performance requirements of non-magnetic steel, only samples prepared in Examples 1-3 and Comparative Examples 1-2 were selected. For each of the five processes, three samples were taken to prepare micropillars. Focused ion beam (FIP) was used to prepare micropillars with a diameter of 2 μm and a height of 5 μm on the surface layer. All micropillar samples were subjected to a strain rate of 3 × 10⁻⁶. -3 s -1 Micropillar compression test, engineering stress-strain curve as shown Figure 5 As shown, the average yield strength, ultimate compressive strength, and strain rate corresponding to the five processes were calculated, and the specific data results are as follows:

[0046] It can be seen that the yield strength, ultimate compressive strength, and strain rate of Examples 1-3 are all significantly improved. Furthermore, Examples 1 and 3 both exhibit very high ultimate compressive strength and extremely high strain rates. The extremely high yield strength and ultimate compressive strength indicate a significant improvement in the strength of the materials in these examples, while the extremely high strain rate indicates a significant improvement in the plasticity of the materials. This simultaneous improvement in strength and plasticity overcomes the bottleneck of the "strength-plasticity inversion."

[0047] Experiment 8: Friction and Wear Test Because the comparative examples contained a large amount of BCC phase, they did not meet the requirements for non-magnetic steel. Therefore, multiple friction and wear tests were conducted on the surfaces of samples from Examples 1-3, Comparative Examples 1-2, and commercial non-magnetic steels 7Mn15 and Mn13, representing seven different processes. The wear amount and wear rate in the test results were averaged, and the results are as follows:

[0048] The results show that the samples in the examples have better wear resistance on a non-magnetic surface, which is significantly higher than that of the comparative examples and commercial non-magnetic steel.

[0049] Experiment 9: Microstructural Analysis To further analyze the reasons for the improved mechanical properties, the microstructure of the embodiments was analyzed. The microstructure of the reinforcing layers in Examples 1-3 and Comparative Examples 1-3 was observed using transmission electron microscopy. The microstructure distribution is as follows: Figure 6 As shown.

[0050] Figure 6The results show that the strengthening layers of Examples 1-3 contain a large number of layered structures, which are composed of parallel stacked stacked faults and accompanied by high-density dislocation slip activity. A partial enlarged view of Example 1 shows that stacked faults are arranged in a cross pattern on mutually angled {111} slip systems, forming a network structure. Typical LC-locked structures are also observed in the intersection regions of multiple slip systems. This structure, formed by the interaction of dislocations from different slip systems, can significantly pin slip dislocations, acting as a strong barrier, and can also act as a Frank-Read source to promote dislocation multiplication. This enhances the work hardening ability of the material and improves the structural stability of the deformation zone, significantly improving the synergistic development of the material's strength and plasticity. Comparative Examples 1-3, however, do not exhibit layered structures.

[0051] The above-described tests demonstrate that the embodiments treated according to this application not only significantly improve the surface mechanical properties but also maintain a non-magnetic state. This avoids the phenomenon of traditional FCC non-magnetic steel transforming into BCC during the strengthening process, which would affect its magnetic properties. This will serve as a reference for non-magnetic structural steels used in power equipment, magnetic resonance imaging devices, pressure hulls of nuclear submarines, and large generators.

[0052] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for achieving inverse phase transformation strengthening of lightweight high-entropy steel through ultrasonic rolling, characterized in that: Includes the following steps: S1. Material Selection: Lightweight high-entropy steel substrate with a T0 temperature lower than room temperature is selected. Its composition by mass percentage is: Mn 10%-20%, C 0.5%, Al 5%-20%, V 0.8%, with the balance being Fe. S2. Heat treatment: The lightweight high-entropy steel is homogenized by holding at 1100℃ for 4 hours, followed by isothermal quenching at 1100℃ for 40 minutes and then water quenching to obtain a microstructure containing BCC and FCC dual phases. S3. Ultrasonic rolling treatment: The sample surface is ultrasonically rolled at room temperature.

2. The method for ultrasonic rolling to achieve inverse phase transformation strengthening of lightweight high-entropy steel according to claim 1, characterized in that: In step S3, the diameter of the WC / Co ball head used is 8-14 mm; the frequency of the ultrasonic rolling is 20-30 kHz, the amplitude is 10-20 μm, and the constant static pressure is 1000-1500 N.

3. The method for ultrasonic rolling to achieve inverse phase transformation strengthening of lightweight high-entropy steel according to claim 2, characterized in that: The diameter of the WC / Co ball head is 12mm; the frequency of the ultrasonic rolling is 20kHz, the amplitude is 15μm, and the constant static pressure is 1200N.

4. The method for ultrasonic rolling to achieve inverse phase transformation strengthening of lightweight high-entropy steel according to claim 2, characterized in that: In step S3, the workpiece to be processed rotates at 20-50 rpm, and the WC / Co ball head advances from the circumference to the center at a feed speed of 0.028-0.05 mm / r.

5. The method for ultrasonic rolling to achieve inverse phase transformation strengthening of lightweight high-entropy steel according to claim 1, characterized in that: The coverage of ultrasonic rolling treatment in step S3 is 100%.

6. The method for ultrasonic rolling to achieve inverse phase transformation strengthening of lightweight high-entropy steel according to claim 1, characterized in that: Before step S3, a grinding step is also included: grinding the surface of the lightweight high-entropy steel to make it smooth.

7. A modified lightweight high-entropy steel, prepared by the ultrasonic rolling method described in any one of claims 1-6 to achieve inverse phase transformation strengthening of lightweight high-entropy steel.

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