A corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure and a preparation method thereof

Abstract

The present invention belongs to the technical field of alloy preparation, and specifically discloses a corrosion-resistant high-entropy alloy containing Sn element with non-equimolar ratio and having a duplex structure and a preparation method thereof. Sn element is added to the CoCrFeNi multi-principal element alloy with fcc and hcp duplex phases to form a new CoCrFeNiSn 0.3 high-entropy alloy. During the corrosion process, the Sn element induces the formation of a passivation film structure on the surface of the high-entropy alloy, and under the polarization conditions of 0V Ag / AgCl , the passivation film structure is the most uniform and stable, which can hinder the occurrence of further corrosion reactions and achieve the improvement of corrosion resistance.

Description

Technical Field

[0001] The invention relates to the technical field of alloy preparation, in particular to a corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure and a preparation method thereof. Background Art

[0002] In current research, high entropy alloys have attracted widespread attention due to their excellent corrosion resistance caused by the "core effect". The high entropy effect and cocktail effect brought about by the multi-principal element characteristics ensure the phase stability of the alloy and the synergy between elements, which is considered to be the source of high performance. Due to the broad composition design space, high entropy alloys show excellent performance in various corrosive environments and show the potential for further optimization.

[0003] In the composition design of high entropy alloys, the effects of alloying elements and phase composition on corrosion resistance are inseparable. It is generally believed that a single-phase microstructure can avoid galvanic corrosion between different phases and therefore has higher corrosion resistance. The addition of a variety of passivating elements that can improve corrosion resistance, such as Al and Ti, may lead to the formation of a second phase, in which the performance gain brought by element alloying and the performance loss brought by phase separation are difficult to quantify. However, blindly denying the multiphase structure model will inevitably limit the diversity of high entropy alloy composition design. Therefore, it is necessary to study multiphase high entropy alloys, especially dual-phase high entropy alloys. CoCrFeNi is a typical face-centered cubic high entropy alloy, which is often selected as the matrix for introducing new alloying elements. For example, in Al x In the CoCrFeNi (x = 0.3, 0.5, 0.7) system, the addition of Al element induces the formation of fcc matrix and Cr-poor body-centered cubic dual-phase microstructure, which transforms the corrosion process from pitting corrosion to boundary corrosion, impairing the corrosion resistance in 3.5wt.% NaCl solution. In addition, alloying elements can also affect the passivation behavior of the matrix, such as the nucleation and growth of the passivation film, as well as the thickness and density of the film structure, which is reflected in the polarization curve as the width of the passivation zone, breakdown potential or Flade potential. Previous studies used the dissolution-diffusion deposition model to explain the promotion of the N element on the formation of the passivation film in the non-passivated CoCrFeMnNi high entropy alloy. The selection of some passivating elements, such as Cr, Cu, Sn, etc., can improve the corrosion resistance by accelerating the growth of the passivation film.

[0004] Previous studies have confirmed the passivation zone widening effect caused by Sn alloying. However, excessive addition of Sn will lead to the formation of particle precipitation phases other than the Sn-Ni second phase, which will have unpredictable effects on the properties of the alloy, for example, it may become the location of pitting nucleation. In order to prepare a dual-phase high entropy alloy that can form a stable passivation film on the corroded surface, the present invention introduces Sn to design CoCrFeNiSn 0.3High entropy alloy. Summary of the invention

[0005] The purpose of the present invention is to provide a corrosion-resistant high-entropy alloy containing non-equimolar ratio Sn elements and having a dual-phase structure and a preparation method thereof. The Sn element is introduced into the high-entropy alloy CoCrFeNi system. Due to the promotion effect of the Sn element on the formation of the passivation film, the addition of a trace amount of non-equimolar ratio avoids the formation of particle precipitation phase induced by excessive Sn elements, making the passivation film structure generated on the alloy surface during the corrosion process more uniform and stable, so the alloy has excellent corrosion resistance. And at 0V Ag / AgCl After constant potential polarization under certain conditions, the stability of the surface passivation film is optimal, and the overall corrosion resistance of the alloy can be further improved in this state.

[0006] To achieve the above object, the present invention is implemented according to the following technical solutions:

[0007] One of the purposes of the present invention is to provide a method for preparing a corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure, comprising the following steps:

[0008] S1. Take Co, Cr, Fe, Ni, and Sn single metals with a purity higher than 99.9% in a molar ratio of 1:1:1:1:0.3, clean and remove oil and impurities on the surface of each metal raw material, rinse with alcohol, and blow dry for later use;

[0009] S2. Wipe the inner wall of the electric arc furnace with gauze dipped in anhydrous ethanol, and use fine sandpaper to polish the tungsten electrode. Place each metal raw material in the sample slot in the furnace of the electric arc furnace, and put titanium ingots in the remaining sample slots to absorb oxygen and further eliminate oxygen in the furnace;

[0010] S3. After placing each metal raw material into an electric arc furnace, evacuate the furnace until the gas pressure in the furnace drops to 0.01 Pa, and the arc starting current is 300A. Then, move the tungsten electrode to the top of each metal raw material, and wait for each metal raw material to melt and form an ingot. Repeat the smelting process 5 times to ensure that the alloy composition is uniform. After cooling, a corrosion-resistant high-entropy alloy containing Sn elements in a non-equimolar ratio and having a dual-phase structure is obtained.

[0011] Furthermore, in the step S1, ultrasonic waves are used to clean the metal raw materials.

[0012] Furthermore, in the step S1, before ultrasonic cleaning of each metal raw material, the oxide scale on the surface of each metal raw material is first removed.

[0013] Furthermore, in step S2, the metal raw materials are placed in the sample slots in the furnace of the electric arc furnace in order from low to high melting points.

[0014] The second object of the present invention is to provide a corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure prepared by the above method.

[0015] Compared with the prior art, the present invention provides a corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure and a preparation method thereof, which has a novel alloy component ratio and can ensure the uniformity of the alloy and reduce oxidation during the preparation process by repeated smelting in a vacuum arc furnace. 0.3 High entropy alloys have a dendritic microstructure, in which the dendrite region is composed of a CoCrFe-rich fcc phase and the interdendritic region is composed of a SnNi-rich hcp (hexagonal close-packed structure) phase. The segregation of Ni and Sn elements is caused by the strong chemical affinity between them. When the external potential exceeds the corrosion potential, passivation behavior occurs spontaneously. The current density increases slowly with the increase of the external potential. When the external potential reaches the breakdown potential, the current density begins to increase rapidly, corresponding to the appearance of local corrosion, such as pitting or selective leaching. Continuing to increase the external potential, the corrosion enters the over-passivation region, and secondary passivation occurs in this interval. When the current density is greater than 1mA / cm 2 The scan is reversed when the voltage drops to -0.15V. The net anodic reaction continues until the applied potential drops to the Flade potential corresponding to the new equilibrium of the corrosion reaction. According to the polarization curve, three potential values ​​are selected to prepare the passive film by constant potential polarization, namely 0.2, 0 and -0.15V. Ag / AgCl , corresponding to the midpoint of the passivation region, the upper region of the Flade potential, and the lower region of the Flade potential, respectively. AgCl At a potential of -0.15 V, the current density remains stable after an initial rapid decay. Ag / AgCl When , it is negative, indicating that the rate of the cathode reaction is greater than that of the anode reaction. The net cathode current can be attributed to the volume fraction difference between the two phases in the high entropy alloy. The anodic reaction occurs in the Sn-rich interdendritic region, while the cathode reaction mainly occurs in the dendrite region. Since the volume fraction of the dendrite region is larger than that of the interdendritic region, the larger reaction area provides a higher cathode reaction current density. At 0V Ag / AgCl At a potential of 0.2V, the overpotential of the anodic reaction is greater and the reaction rate is faster, so the current density is net anodic and positive. Ag / AgClAt a potential of 0.1 V, the current density first decreases and then increases sharply to a final steady state. At this relatively high potential, anodic reactions occur in both the dendrites and interdendritic regions, forming Cr-rich and Sn-rich oxides, respectively. The formation of oxides on the corrosion surface hinders further reactions, resulting in a decrease in the current density. When the surfaces of the dendrites and interdendritic regions are completely covered with oxides, galvanic corrosion occurs due to the potential difference between the two different oxides, significantly increasing the current density. In contrast, at 0 V, the current density decreases. AgCl Under the condition of 0V, the anodic oxide film in the interdendritic region epitaxially grows to the dendrite region and forms a uniform structure, which is expected to have excellent corrosion resistance. After constant potential polarization, the corrosion potential of the alloys increased. The corrosion resistance under 0V conditions is the highest because the passivation film is uniform and complete. In addition, the phase angle under this condition is about 80°, showing impedance characteristics similar to those of a capacitor element, which also confirms the formation of a stable passivation film.

[0016] The present invention adds a small amount of Sn element to the fcc and hcp dual-phase CoCrFeNi alloy to form a new type of CoCrFeNiSn 0.3 High entropy alloy. During the corrosion process, the Sn element induces the formation of a passivation film structure on the surface of the high entropy alloy, and at 0V Ag / AgCl Under polarization conditions, the passive film structure is the most uniform and stable, which can hinder the occurrence of further corrosion reactions and achieve improved corrosion resistance. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 CoCrFeNiSn according to the embodiment of the present invention 0.3 Microstructure and phase structure morphology: (a) is the dual-phase dendrite microstructure; (b) is the XRD diagram; (c) is the EDS analysis result.

[0018] Figure 2 CoCrFeNiSn according to the embodiment of the present invention 0.3 Potentiodynamic polarization curve.

[0019] Figure 3 CoCrFeNiSn according to the embodiment of the present invention 0.3 The variation curve of current density during constant potential polarization.

[0020] Figure 4 CoCrFeNiSn according to the embodiment of the present invention 0.3 After constant potential polarization, the change curves of electrochemical behavior are: (a) is the potentiodynamic polarization curve; (b) is the electrochemical impedance Bode diagram.

[0021] Figure 5 CoCrFeNiSn according to the embodiment of the present invention 0.3 Morphology after corrosion. DETAILED DESCRIPTION

[0022] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments. The specific embodiments described herein are only used to explain the present invention and are not used to limit the invention.

[0023] Example 1

[0024] 1) Use an electronic balance to accurately weigh the metal and rare earth element raw materials required for alloy ingots: Co, Cr, Fe, Ni metal elements with a purity higher than 99.9% and the rare earth element Sn according to the alloy composition CoCrFeNiSn 0.3 Preparation, ultrasonic cleaning of each metal and rare earth element raw material, followed by alcohol washing, and finally drying for standby use; wherein, before cleaning each raw material, descaling of each element is performed in advance.

[0025] 2) Clean the furnace of the vacuum arc furnace: wipe the inner wall of the furnace with anhydrous ethanol, and polish the tungsten electrode until the tip is bright. Place the spare metal and rare earth element raw materials in the sample slot, and put in the titanium ingot at the same time to prevent the alloy from oxidizing during the smelting process; among them, place the raw materials in the sample slot in order from low to high melting points, and place the raw materials with low melting points at the bottom layer, which can alleviate the burning of elements during the smelting process to a certain extent.

[0026] 3) Melting: After placing each metal raw material into an electric arc furnace, vacuum is evacuated until the pressure in the furnace drops to 0.01Pa, and the arc current is 300A. Then the tungsten electrode is moved above each metal raw material to wait for each metal raw material to melt and synthesize an ingot. This smelting is repeated 5 times to ensure that the alloy composition is uniform. After cooling, a corrosion-resistant high-entropy alloy containing non-equimolar Sn elements and having a dual-phase structure is obtained.

[0027] The corrosion-resistant high-entropy alloy containing non-equimolar Sn elements and having a dual-phase structure prepared in this embodiment was tested by conventional methods in the art, and its hardness was 643.46 HV, the friction coefficient was 0.335±0.005, and the weight loss was 0.007 g; the corrosion potential of the corrosion-resistant high-entropy alloy containing non-equimolar Sn elements and having a dual-phase structure prepared in this embodiment was -206.62 mV, and the corrosion current density was 78.6 nA; after constant potential polarization at 0VAg / AgCl potential, a stable passivation film structure was generated on its surface, the corrosion potential increased to -82.95 mV, and the corrosion current density decreased to 2.56 nA.

[0028] Furthermore, the elemental composition of the corrosion-resistant high entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure prepared in this embodiment was detected, and the results are shown in Table 1.

[0029] Table 1

[0030]

[0031] Furthermore, the electrochemical parameters of the corrosion-resistant high entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure prepared in this embodiment were tested, and the results are shown in Table 2.

[0032] Table 2

[0033]

[0034] The phase structure of the corrosion-resistant high entropy alloy containing non-equimolar Sn elements and having a dual-phase structure was detected by XRD diffraction (Shimadzu 7000, Kyoto, Japan). The microstructure of the alloy including the dendrite structure was observed under a thermal field scanning electron microscope (SU8010, Hitachi, Japan). The results are as follows: Figure 1 As shown; the corrosion and passivation behaviors were tested at room temperature by an electrochemical workstation (Chi604e). A three-electrode system was used, and the working electrode was a sample sealed in a Teflon mold with an exposed area of ​​1 cm 2 The counter electrode has a surface area of ​​15 × 15 mm 2 The reference electrode is an Ag / AgCl electrode (Ag / AgCl / Cl-, αCl-=1 mol / L, E=0.2224 V vs. SHE), and the electrolyte is a 3.5 wt.% NaCl solution. Each test was repeated at least 3 times to ensure accuracy.

[0035] Depend on Figure 1 As shown in (a), CoCrFeNiSn 0.3 The two-phase dendrite microstructure of the image shows that the dendrite area is gray and the interdendritic area is white. According to XRD and EDS analysis, Figure 1 (b) and Figure 1 As shown in (c), DR consists of a CoCrFe-rich fcc phase, and ID consists of a SnNi-rich hcp (hexagonal close-packed structure) phase. The segregation of Ni and Sn elements is caused by the strong chemical affinity between them, which can be quantified by a more negative mixing enthalpy or formation energy.

[0036] Figure 2 As-cast CoCrFeNiSn 0.3 Potentiodynamic polarization curve of high entropy alloy. Figure 2 It can be seen that -0.212V Ag / AgCl The corrosion potential of CoCrFeNi and CoCrFeNiSn is higher than that of CoCrFeNi (-257 and -252 V respectively). Ag / AgCl), which corresponds to a trend that corrosion reactions are more difficult to occur. When the external potential exceeds the corrosion potential, passivation behavior occurs spontaneously. The current density increases slowly with the increase of the external potential. When the external potential reaches the breakdown potential, the current density begins to increase rapidly, corresponding to the appearance of local corrosion, such as pitting or selective leaching. Continuing to increase the external potential, corrosion enters the over-passivation area, and secondary passivation occurs in this interval. When the current density is greater than 1mA / cm 2 The scan is reversed when the voltage drops to -0.15V. The net anodic reaction continues until the applied potential drops to the Flade potential corresponding to the new equilibrium of the corrosion reaction. According to the polarization curve, three potential values ​​are selected to prepare the passive film by constant potential polarization, namely 0.2, 0 and -0.15V. Ag / AgCl , corresponding to the midpoint of the passivation zone, the upper region of the Flade potential, and the lower region of the Flade potential, respectively.

[0037] Figure 3 CoCrFeNiSn according to the embodiment of the present invention 0.3 The variation curve of current density during the constant potential polarization process. Figure 3 It can be seen that at -0.15 and 0V AgCl At a potential of -0.15 V, the current density remains stable after an initial rapid decay. Ag / AgCl When , it is negative, indicating that the rate of the cathode reaction is greater than that of the anode reaction. The net cathode current can be attributed to the volume fraction difference between the two phases in the high entropy alloy. The anodic reaction occurs in the Sn-rich interdendritic region, while the cathode reaction mainly occurs in the dendrite region. Since the volume fraction of the dendrite region is larger than that of the interdendritic region, the larger reaction area provides a higher cathode reaction current density. At 0V Ag / AgCl At a potential of 0.2V, the overpotential of the anodic reaction is greater and the reaction rate is faster, so the current density is net anodic and positive. Ag / AgCl At a potential of 0.1 V, the current density first decreases and then increases sharply to a final steady state. At this relatively high potential, anodic reactions occur in both the dendrites and interdendritic regions, forming Cr-rich and Sn-rich oxides, respectively. The formation of oxides on the corrosion surface hinders further reactions, resulting in a decrease in the current density. When the surfaces of the dendrites and interdendritic regions are completely covered with oxides, galvanic corrosion occurs due to the potential difference between the two different oxides, significantly increasing the current density. In contrast, at 0 V, the current density decreases. AgCl Under this condition, the anodic oxide film in the interdendritic region epitaxially grows into the dendrite region and forms a uniform structure, which is expected to have excellent corrosion resistance.

[0038] Figure 4 CoCrFeNiSn according to the embodiment of the present invention 0.3 The change curve of electrochemical behavior after constant potential polarization. Figure 4As shown in (a) in the figure, under all three polarization conditions, CoCrFeNiSn 0.3 The corrosion potential of the steel increased. Among them, the improvement under 0V condition is the greatest, corresponding to the most difficult corrosion trend. The corrosion current density at 0V and -0.15V decreases, corresponding to the improvement of corrosion resistance. However, the corrosion current density at 0.2V increases. As mentioned earlier, under 0.2V conditions, the passivation film is damaged, so its corrosion resistance is the worst, even worse than the cast state. The corrosion resistance under 0V condition is the highest because the passivation film is uniform and complete. Figure 4 As shown in (b), the passivation region at 0V is narrower than that at -0.15V because the formation of a stable passivation film hinders further oxidation reactions. The maximum modulus also indicates the highest corrosion resistance at 0V. In addition, in the low-frequency region (about 0.1 to 1000 Hz), there are significant fluctuations in the phase angles at -0.15V and 0.2V, corresponding to different responses of various complex surface reactions. In contrast, the phase angle at 0V remains at about 80°, showing impedance characteristics similar to those of a capacitor element, which also confirms the formation of a stable passivation film.

[0039] Figure 5 CoCrFeNiSn according to the embodiment of the present invention 0.3 The morphology after corrosion. Figure 5 It can be seen that pitting is the main form of corrosion. Pitting not only occurs at the interface between the two phases, but is also randomly distributed at the interface and the internal area of ​​the phase, which means that the occurrence of pitting is due to the fluctuation of the composition of the outer passive film, rather than the fluctuation of the composition of the alloy. Therefore, after the passive film is formed under polarization, CoCrFeNiSn 0.3 The corrosion resistance of the electrode depends more on the performance of the passivation film. In other words, a more stable, compact and uniform passivation film can provide higher corrosion resistance. The density of pitting corresponds to the degree of corrosion, from mild to severe at 0V, -0.15V and 0.2V, which is consistent with the results of the electrochemical test.

[0040] The present invention adds Sn element to the fcc and hcp dual-phase CoCrFeNi multi-principal alloy to form a new CoCrFeNiSn 0.3 High entropy alloy. During the corrosion process, the Sn element induces the formation of a passivation film structure on the surface of the high entropy alloy, and at 0V Ag / AgCl Under polarization conditions, the passive film structure is the most uniform and stable, which can hinder the occurrence of further corrosion reactions and achieve improved corrosion resistance.

[0041] The technical solution of the present invention is not limited to the above-mentioned specific embodiments. All technical variations made according to the technical solution of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure, characterized in that: The following steps are involved: S1. Take Co, Cr, Fe, Ni, and Sn single metals with a purity higher than 99.9% in a molar ratio of 1:1:1:1:0.3, clean and remove oil and impurities on the surface of each metal raw material, rinse with alcohol and blow dry for standby use; wherein, before cleaning each raw material, descaling each element in advance; S2. Wipe the inner wall of the electric arc furnace with gauze dipped in anhydrous ethanol, and use fine sandpaper to polish the tungsten electrode, place each metal raw material in the sample slot in the furnace of the electric arc furnace, and put titanium ingots in the remaining sample slots to absorb oxygen and further eliminate oxygen in the furnace; wherein, each raw material is placed in the sample slot in the order of the melting point from low to high; S3. After placing each metal raw material into an electric arc furnace, evacuate the furnace until the gas pressure in the furnace drops to 0.01 Pa, and the arc starting current is 300A. Then, move the tungsten electrode to the top of each metal raw material, and wait for each metal raw material to melt and form an ingot. Repeat the smelting process 5 times to ensure that the alloy composition is uniform. After cooling, a corrosion-resistant high-entropy alloy containing Sn elements in a non-equimolar ratio and having a dual-phase structure is obtained.

2. The method for preparing a corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure according to claim 1, characterized in that: In the step S1, each metal raw material is cleaned using ultrasonic waves.

3. The method for preparing a corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure according to claim 2, characterized in that: In the step S1, before ultrasonic cleaning of each metal raw material, the oxide scale on the surface of each metal raw material is removed.

4. The method for preparing a corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure according to claim 1, characterized in that: In the step S2, the metal raw materials are placed in the sample slots in the furnace of the electric arc furnace in order from low to high melting points.

5. A corrosion-resistant high-entropy alloy containing Sn elements in non-equimolar ratios and having a dual-phase structure, prepared by the method according to any one of claims 1 to 4.

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