Preparation method of a high-hardness high-entropy alloy thin film with a grain size gradient

Abstract

The present invention discloses a preparation method of a high-hardness high-entropy alloy thin film with a grain size gradient. A thin film with structural heterogeneity in the thickness direction is prepared by magnetron sputtering, and then crystallization is regulated by low-temperature annealing to obtain a high-hardness thin film with a grain size gradient. Samples with a gradient change in the nanocrystalline grain size along the thickness direction of the high-entropy alloy thin film are obtained in the present invention. The structural design with a gradient change in the grain size enables the hardness of the CrMnFeCoNi-based high-entropy alloy with an FCC single-phase structure to reach 13.52 GPa.

Description

Technical Field

[0001] The present invention belongs to the field of alloy materials, and particularly relates to a method for preparing a high-hardness and high-entropy alloy thin film with a grain size gradient. Background Art

[0002] In 2004, once the concept of high-entropy alloys was proposed, it attracted the attention of many scholars. It breaks the design limitations of traditional alloys, abandons the distinction between solvents and solutes in alloys, and has the structural characteristics of multi-principal elements. Among them, the CrMnFeCoNi-based high-entropy alloy with a stable FCC single-phase structure characteristic has been widely studied due to its excellent comprehensive mechanical properties such as excellent low-temperature performance and high creep resistance, but it still inevitably has a strong plasticity conflict, that is, the characteristics of high plasticity but low strength. For example, the ductility of CrMnFeCoNi is 60-70%, and the fracture toughness exceeds 200 MPa / m. However, its strength at room temperature needs to be further improved.

[0003] For FCC single-phase metals, if one wants to improve their strength without losing their ductility, or even synchronously improve the ductility to achieve the synergistic effect of strength and ductility, it is necessary to carry out structural design. Reasonable structural design is an effective means to break the strong plasticity conflict of FCC single-phase. Among them, the grain size gradient is considered to be an effective structural design, and reasonable gradient structure design can take into account both strength and plasticity. For bulk materials, various surface mechanical treatments can be used, such as shot peening, high-pressure torsion, etc., to make the surface structure nanocrystalline, and then obtain a grain size gradient structure that transitions from surface nanocrystals to internal coarse grains. The good combination of nanocrystalline strength and coarse-grain plasticity can improve the strength of FCC-phase structure materials without premature failure. However, the thickness of thin film system materials is generally in the micron range, and the grain size is mostly in the nanometer range. The structure is hard and brittle, and it is difficult to carry out structural gradient design through mechanical treatment. Therefore, the research on thin film systems mostly focuses on structural composites (multilayer films) or composition design (element addition), but there are still difficulties in gradient structure design. How to achieve a feasible grain size gradient structure design in the high-entropy alloy thin film system and effectively improve the strength of FCC single-phase alloys is challenging. Summary of the Invention

[0004] Based on the above discussion, the present invention aims to propose a method for preparing a high-hardness and high-entropy alloy thin film with a grain size gradient.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] A method for preparing a high-hardness and high-entropy alloy thin film with a grain size gradient, which uses magnetron sputtering co-sputtering to obtain a thin film and regulates it through low-temperature annealing to obtain a high-entropy alloy thin film with continuously varying grain size along the thickness direction.

[0007] The method adopted by the present invention specifically includes the following steps:

[0008] 1) After ultrasonically cleaning a single-sided polished monocrystalline silicon substrate in alcohol for 5 - 10 minutes, it is diagonally fixed to a bottom support and then horizontally inserted (with the polished surface facing up) into an ultra-high vacuum magnetron sputtering device;

[0009] 2) Fix the alloy target and the pure metal target to be sputtered on the radio frequency and direct current power supplies respectively;

[0010] 3) After pumping to the required high vacuum condition using a combination of a mechanical pump and a molecular pump, high-purity argon gas is introduced as the main ionization gas for glow discharge;

[0011] 4) After confirming the gas introduction, adjust the two power controllers to the required power and then turn on the power to start pre-sputtering for 20 - 30 minutes to remove impurities on the target surface;

[0012] 5) After the pre-sputtering is completed, use a control rod to flip the substrate (with the polished surface facing down) to start formal co-sputtering, and turn on the 360° rotation control switch to ensure sputtering uniformity;

[0013] 6) Place the deposited alloy film in a vacuum annealing furnace, keep it at 395 - 405 °C for 60 minutes, and then turn off the heating to cool the sample.

[0014] The high vacuum mentioned above means that the vacuum degree needs to be as low as below 7x10 -4 Pa.

[0015] The purity of the high-purity argon gas is ≥99.999%.

[0016] During the sputtering process, the power of the radio frequency power supply is fixed (>100 W), while the power of the direct current power supply changes from large to small (0 - 30 W).

[0017] Further, during the annealing process in step 6), the timing starts when the temperature display reaches 400 °C. In the early stage of cooling, the cooling rate needs to be controlled at 10 °C / min, and then it starts to cool with the furnace after reaching 200 °C.

[0018] The present invention has the following advantages:

[0019] 1. Samples with a gradient change in the nano-crystalline grain size along the thickness direction of the high-entropy alloy film are obtained by using this method.

[0020] 2. The structural design with a gradient change in the grain size enables the hardness of the CrMnFeCoNi-based high-entropy alloy with an FCC single-phase structure to reach 13.52 GPa. Description of the Drawings

[0021] Figure 1It is the cross-sectional TEM structure diagram of the thin film;

[0022] (a) Overall cross-sectional view (thin region) of the TEM sample.

[0023] (b) High-resolution microstructural diagram at "1" in Figure (a)

[0024] (c) High-resolution microstructural diagram at "2" in Figure (a)

[0025] (d) High-resolution microstructural diagram at "3" in Figure (a)

[0026] Figure 2 It is the TEM selected area diffraction pattern and calibration result of the high-entropy alloy thin film.

[0027] Figure 3 It is the hardness comparison diagram of the FCC phase nanocrystals in the CrMnFeCoNi system. Specific implementation manners

[0028] The present invention will be described in detail below in combination with specific examples and drawings, but it is not intended to limit the present invention.

[0029] Refer to Figure 1 As shown, Figure (a) shows the overall cross-sectional view of the thin region after TEM sample preparation of the high-entropy alloy thin film, showing that the thickness of the thin film is about 1.8 microns. Figures (b-d) show the magnified microstructural diagrams of different regions. The dashed line shows the change in grain size, that is, the grain size transition from greater than 30 nm to about 5 nm. "1" is at the near-surface.

[0030] Refer to Figure 2 As shown, the diffraction pattern shows that the phase structure of the thin film is still calibrated as a single-phase FCC nanocrystalline.

[0031] Refer to Figure 3 As shown, when comparing the hardness of the thin film with that of other FCC single-phase nanocrystals in the CrMnFeCoNi system, the hardness of this thin film has obvious advantages.

[0032] Example 1

[0033] A 5-element non-equiatomic ratio high-entropy alloy thin film with the composition of CrMnFeCoNi is prepared by magnetron co-sputtering. A near-equiatomic ratio CrMnFeCoNi high-entropy alloy target and a pure manganese metal target are co-sputtered, and are respectively placed on the radio frequency power supply and the direct current power supply. The radio frequency power supply power is stabilized at 120 W, and the direct current power supply varies uniformly from 30 W to 0 W. The total sputtering duration is 4 hours. Then, it is annealed at 400 °C for 60 min in a vacuum annealing furnace. After the heat preservation is completed, the cooling rate is controlled at 10 °C / min.

[0034] The TEM characterization results of the obtained high-entropy alloy thin film are as Figure 1As shown, the results indicate that a nano-crystalline grain size gradient appears in the structure of the thin film. The surface grain size is greater than 30 nm, and as the thickness increases, the grain size gradually transitions to approximately 5 nm inside. The phase structure remains a single FCC phase (as Figure 2 shown). Compared with other high-entropy alloy thin films, the thin film prepared in the present invention has outstanding high hardness (13.52 GPa), which is prominent compared to the hardness of other FCC single-phase nano-crystalline thin films with a single size.

[0035] Example 2

[0036] By magnetron sputtering, the radio frequency power supply and the direct current power supply are simultaneously controlled to sputter a near-equiatomic ratio CrMnFeCoNi high-entropy alloy target and a pure manganese metal target. The power of the radio frequency power supply remains stable at 120 W, and the power of the direct current power supply varies from 4 W to 30 W. The total sputtering duration is 4 hours. Then, it is annealed at 400 °C for 60 min in a vacuum annealing furnace. After the heat preservation is completed, the cooling rate is controlled at 10 °C / min.

[0037] The obtained high-entropy alloy thin film shows a hardness of 13.37 GPa after nanoindentation hardness characterization. However, no change in grain size is shown in the structure, presenting a composite structure of crystal / amorphous phases.

[0038] Example 3

[0039] By magnetron sputtering, the radio frequency power supply and the direct current power supply are simultaneously controlled to sputter a near-equiatomic ratio CrMnFeCoNi high-entropy alloy target and a pure manganese metal target. The power of the radio frequency power supply is stable at 120 W, and the power of the direct current power supply is controlled at 10 W. The total sputtering duration is 4 hours. Then, it is annealed at 400 °C for 60 min in a vacuum annealing furnace. After the heat preservation is completed, the cooling rate is controlled at 10 °C / min.

[0040] The obtained high-entropy alloy thin film shows a hardness of 8.008 GPa after nanoindentation hardness characterization. However, no change in grain size is shown in the structure, presenting a composite structure of crystal / amorphous phases.

Claims

1. A preparation method of a high-hardness FCC single-way structured CrMnFeCoNi high-entropy alloy thin film with a grain size gradient, characterized in that: A thin film with structural heterogeneity in the thickness direction is prepared by magnetron sputtering, and then the crystallization is regulated by low-temperature annealing to obtain a high-hardness thin film with a grain size gradient. The specific steps are as follows: 1) After ultrasonically cleaning a single-side polished single-crystalline silicon substrate in alcohol for 5 - 10 min, it is diagonally fixed with a bottom bracket and horizontally inserted into an ultra-high vacuum magnetron sputtering device with the polished surface facing up. 2) The CrMnFeCoNi high-entropy alloy target and the pure Mn target to be sputtered are respectively fixed on the radio frequency and direct current power supplies. 3) After pumping to the required high vacuum condition by a combination of a mechanical pump and a molecular pump, high-purity argon is introduced as the main ionization gas for glow discharge. 4) After confirming the gas introduction, adjust the two power controllers to the required power and then turn on the power to start pre-sputtering for 20 - 30 min to remove impurities on the target surface; during the magnetron sputtering process, control the power of the direct current power supply to change uniformly from 30 W to 0 W. 5) After the pre-sputtering is completed, use a control rod to flip the substrate so that the polished surface faces down to start formal co-sputtering, and turn on the rotation control switch to ensure sputtering uniformity. 6) Place the deposited alloy thin film in a vacuum annealing furnace, keep it at 395 - 405 °C for 60 min, and then turn off the heating to cool the sample to obtain a high-hardness thin film with a grain size gradient.

2. The preparation method according to claim 1, characterized in that: High vacuum means that the degree of vacuum needs to be as low as 7x10 -4 Pa or less.

3. The preparation method according to claim 1, characterized in that: The purity of the high-purity argon is ≥99.999%.

4. The preparation method according to claim 1, characterized in that: During the co-sputtering process, the power of the radio frequency power supply is fixed >100 W, and the power of the direct current power supply changes from large to small from 30 - 0 W.

5. The preparation method according to claim 1, characterized in that: In the early stage of cooling, the cooling rate needs to be controlled at 9 - 11 °C / min, and then start furnace cooling after reaching 195 - 200 °C.

6. The preparation method according to claim 1, characterized in that: The surface grain size of the prepared high-entropy alloy thin film is greater than 30 nm, and along the thickness increase, the grain size gradually transitions to 4 - 6 nm inside, and it has a high hardness of 13.52 GPa.

7. The preparation method according to claim 1, characterized in that: In step 5), it is a 360° rotation control switch.

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