CN111663070B

CN111663070B - AlCoCrFeNiSiY high-entropy alloy resistant to high-temperature oxidation and preparation method thereof

Info

Publication number: CN111663070B
Application number: CN202010492403.8A
Authority: CN
Inventors: 古立建; 李翔; 李强
Original assignee: University of Shanghai for Science and Technology
Current assignee: University of Shanghai for Science and Technology
Priority date: 2020-06-03
Filing date: 2020-06-03
Publication date: 2021-11-19
Anticipated expiration: 2040-06-03
Also published as: CN111663070A

Abstract

The invention provides an AlCoCrFeNiSiY high-entropy alloy resistant to high-temperature oxidation and a preparation method thereof. Compared with the traditional NiCoCrAlY, the AlCoCrFeNiSiY high-entropy alloy shows more excellent high-temperature oxidation weight gain resistance under the condition of 1150 ℃ cyclic oxidation. The oxide film is uniform and compact, and the bonding state is good. Therefore, the high-entropy alloy has potential to be applied as a structural material in a high-temperature and oxidative environment.

Description

AlCoCrFeNiSiY high-entropy alloy resistant to high-temperature oxidation and preparation method thereof

Technical Field

The invention relates to the technical field of high-entropy alloys, in particular to a high-entropy alloy of AlCoCrFeNiSiY with high temperature oxidation resistance and a preparation method thereof.

Background

The thermal protection material has important application in the aspect of prolonging the service life of high-temperature components of industrial equipment. Taking the example of a thermal barrier coating, it is generally a two-layer coating consisting of a metallic bond coat and a ceramic layer deposited on a superalloy. The thermal barrier coating can block the heat transfer of high-temperature gas, slow down the oxidation and corrosion to the substrate and improve the service temperature of high-temperature alloy in the gas turbine. With increasing production requirements and technological advances, gas turbine pre-turbine temperatures continue to rise. The MCrAlY (M is Ni, Co) and (Ni, Pt) Al bonding layers applied to the thermal barrier coating have insufficient performance under a severe service environment, for example, the growth rate of the metal bonding layer oxide is accelerated to cause the thickness of the metal bonding layer to increase rapidly, the bonding of the metal layer and the ceramic layer interface is poor, and the spalling failure of the coating is caused. In addition, there are problems with aluminum depletion and degradation of the coating structure. Therefore, reasonable alloy components are adopted, and on the premise of keeping better mechanical property, the oxidation resistance of the alloy is improved, such as reduction of oxidation weight gain rate, and improvement of oxide flatness and bonding force, so that the method has very important significance. After the alloy composition is adjusted, the alloy can be used for protecting other types of high-temperature resistant materials.

Unlike conventional single element based alloys, multi-principal element high entropy alloys contain several high (often greater than 5% atomic fraction) levels of major alloying elements. The high entropy of mixing facilitates the generation of solid solutions rather than brittle intermetallics, expanding the range of choice and content of alloying elements. The atoms in the high-entropy alloy are often in disordered arrangement, and the structure of the high-entropy alloy is a simple face-centered cubic (FCC) or body-centered cubic (BCC) structure. The high-entropy alloy has strong solid solution strengthening and obvious grain refinement phenomena, has the characteristics of high strength, corrosion resistance, diffusion retardation and the like, and provides a new idea for improving the oxidation resistance of the alloy.

Disclosure of Invention

The invention aims to provide a high-entropy alloy of AlCoCrFeNiSiY with high-temperature oxidation resistance and a preparation method thereof, so that the high-entropy alloy of the invention has more excellent high-temperature oxidation weight gain resistance.

In order to achieve the purpose, the invention provides an AlCoCrFeNiSiY high-entropy alloy resistant to high-temperature oxidation, which is prepared by NiCoCrAlY interplanetary alloy element species Fe and Si.

Further, the AlCoCrFeNiSiY high-entropy alloy comprises the following elements in percentage by mole: 16.0-20.0% of Al16, 12.0-18.0% of Co12, 12.0-18.0% of Cr12, 14.0-20.0% of Fe14, 78-35% of Ni30, 1.0-2.0% of Si and 0.2-0.4% of Y.

The invention also provides a preparation method of the AlCoCrFeNiSiY high-entropy alloy, which comprises the following steps:

step 1: preparing a raw material and an intermediate alloy, and pretreating the raw material and the intermediate alloy; the raw materials comprise Al, Co, Cr, Fe and Ni; the intermediate alloy comprises FeSi and AlY;

step 2: weighing the raw materials and the intermediate alloy according to the mole percentage of the AlCoCrFeNiSiY high-entropy alloy;

and step 3: smelting the raw materials and the intermediate alloy; the method comprises the following steps:

step 3.1: sequentially placing the materials weighed in the step 2 in a water-cooled copper crucible from bottom to top according to the melting point from low to high; placing an oxygen-scavenging substance in the other crucible;

step 3.2: firstly, the water-cooled copper crucible is vacuumized to 5Pa by a mechanical pump, then a molecular pump is started, and the water-cooled copper crucible is continuously vacuumized to 5 multiplied by 10^-3Charging inert gas under Pa to make the pressure in the furnace be 0.4-0.6 atm;

step 3.3: firstly, smelting deoxidization substances for 2-3 times, removing residual oxygen in the furnace as much as possible, then smelting a target alloy, turning over an initial melting alloy ingot after each smelting, continuing to smelt again, and repeating the smelting process for 3-5 times to ensure that the components are fully homogenized;

step 3.4: cooling the alloy along with the furnace to obtain a bowl-shaped ingot;

and 4, step 4: and carrying out heat treatment on the bowl-shaped cast ingot to further make the alloy uniform, and cooling to room temperature to obtain the AlCoCrFeNiSiY high-entropy alloy.

Further, in step 1, the pretreatment comprises the steps of placing the raw material and the intermediate alloy in absolute ethyl alcohol with the concentration of 99.5% or more for carrying out an ultra-treatment for 10 minutes to remove oil contamination impurities on the surface, and then soaking the raw material and the intermediate alloy in hydrochloric acid with the mass fraction of 20% for 1min to remove surface oxides; placing pure Fe, pure Ni, pure Cr and Fe-Si and Al-Y intermediate alloy in a drying oven at 50 ℃ for drying;

the mass fractions of the elements of the FeSi intermediate alloy are as follows: 75% of Fe and 25% of Si; the mass fractions of the elements of the AlY intermediate alloy are as follows: 30% of Al and 30% of Y.

Further, in step 3, alloy smelting is carried out by using a non-consumable vacuum tungsten electrode arc furnace;

the deoxidizing substance is a titanium block; the inert gas is argon.

Further, in step 4, the heat treatment is homogenizing heat treatment of the bowl-shaped ingot at 1200 +/-30 ℃ for 6 hours.

Compared with the prior art, the invention has the advantages that: compared with the traditional NiCoCrAlY metal material, the AlCoCrFeNiSiY high-entropy alloy of the invention increases the types of alloy elements Fe and Si by using and obviously adjusts the content so as to obtain the high-entropy alloy with strong oxidation resistance. The alloy provided by the invention is an alloy with unequal atomic ratio, but the alloy components have high mixed entropy and have the basic characteristics of the high-entropy alloy. The alloy is a BCC and FCC dual-phase structure, and the BCC structure has slightly high aluminum content, oxidation resistance and better high-temperature strength; the FCC phase gives consideration to the toughness and plasticity of the alloy, and the alloy performance is relatively balanced on the whole. Si and Y can improve the oxidation resistance of NiCoCrAl series alloy. Fe can ensure that the alloy still maintains high mixed entropy when the content of Si and Y is higher, and has no obvious complex precipitated phase. Si is added in the form of Fe-Si master alloy so that Si is fully alloyed. Y is added in the form of Al-Y intermediate alloy, so that the burning loss and volatilization of Y are reduced. Compared with NiCoCrAlY, the AlCoCrFeNiSiY high-entropy alloy shows more excellent high-temperature oxidation weight gain resistance under the condition of 1150 ℃ cyclic oxidation. The oxide film is uniform and compact, and the bonding state is good. Therefore, the high-entropy alloy has potential to be applied as a structural material in a high-temperature and oxidative environment.

Drawings

FIG. 1 is an XRD pattern of a high-entropy alloy prepared by smelting in step three of example 1;

FIG. 2 is a scanning electron microscope microstructure of a high-entropy alloy prepared by melting in step three of example 1;

FIG. 3 is a graph of the rate of weight gain of the sample of example 3 under cyclic oxidation conditions.

FIG. 4 is a cross-sectional view taken by a scanning electron microscope microstructure after oxidation of the high-entropy alloy after the fourth treatment of example 3;

FIG. 5 is a surface XRD pattern of the high-entropy alloy prepared by melting in step three of example 3 after cyclic oxidation;

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention will be further described below.

Example 1:

the embodiment is a preparation method of an oxidation-resistant high-entropy alloy, which consists of Al, Co, Cr, Fe, Ni, Si and Y elements, wherein the mole percentages of the Al, Co, Cr, Fe, Ni, Si and Y elements are respectively 18.0% of Al, 15.0% of Co15.0%, 15.0% of Cr, 16.8% of Fe16.9%, 32.9% of Ni32%, 2.0% of Si and 0.3% of Y. The mass of each material is shown in the following table:

TABLE 1 raw material ratio for smelting

An AlCoCrFeNiSiY high-entropy alloy resistant to high-temperature oxidation and a preparation method thereof comprise the following steps: step one, pretreatment: the method comprises the following steps of putting raw materials of Al, Co, Cr, Fe, Ni (with the purity of 99.9% or more and mass fraction) and Al-Y intermediate alloy into 95% absolute ethyl alcohol for carrying out super treatment for 10 minutes, removing oil contamination impurities on the surface, and then soaking the surface in 20% mass fraction hydrochloric acid for 1min to remove surface oxides. The raw materials are dried in a drying oven at 50 ℃.

Step two, weighing and batching: according to the mole percentage of the elements of the alloy, Al 18.0%, Co15.0%, Cr15.0%, Fe16.8%, Ni32.9%, Si2.0% and Y0.3%, and an electronic balance with the precision of 0.001g is used for weighing the intermediate alloy of Al, Co, Cr, Fe, Ni, Fe75Si25 and Al70Y30 after pretreatment in the step one.

Step three, smelting: alloy melting is carried out by using a vacuum tungsten electrode arc furnace.

Putting the materials weighed in the step two into a water-cooled copper crucible from bottom to top in sequence according to the melting point from low to high. Placing the titanium block for removing oxygen in another crucible;

secondly, firstly using a mechanical pump to pump vacuum to 5Pa, then starting a molecular pump, and continuously pumping vacuum to 5 multiplied by 10^-3Pa, filling high-purity argon to ensure that the pressure in the furnace is 0.6 atm;

melting the titanium block for deoxidizing for 2 times, removing residual oxygen in the furnace as much as possible, and then melting the target alloy. After each smelting, turning over the primary alloy ingot, continuously smelting again, and repeating the smelting process for 4 times to ensure that the components are fully homogenized;

cooling the alloy along with the furnace to obtain the bowl-shaped cast ingot.

Step four, heat treatment: and (3) carrying out homogenization heat treatment on the ingots obtained in the step three for 6 hours at 1200 +/-30 ℃ in sequence, further homogenizing the alloy components in the process, and then air-cooling to room temperature.

And (5) carrying out performance detection on the obtained product.

FIG. 1 is the XRD pattern of the high-entropy alloy prepared in step three, which can be easily obtained, and the material has FCC and BCC dual-phase structures.

FIG. 2 is a scanning electron microscopic gold microstructure of the high-entropy alloy prepared in step three, wherein the alloy is basically in an equiaxial shape. It can be seen that the light color BCC phase inside the crystal grains grows in a dendritic manner, the volume ratio is high, and the texture of dark FCC is relatively low.

The invention adopts an AM200 type non-consumable vacuum electric arc furnace of Edmund Buehler GmbH in Germany to smelt, and the electric arc furnace comprises a furnace body, a water-cooled crucible, a vacuum maintaining system, a cooling device and a power supply; a cooling device is arranged on the left side of the furnace body, and circulating water is refrigerated through a compressor; the vacuum device adopts a mechanical pump and a molecular pump, and the vacuum degree can be up to 10^-4Pa below; a tungsten head electrode is arranged right above the furnace body, and the electrode is fixed on a control handle at the top end and can rotate through the control handle. A water-cooled copper crucible is arranged under the electrode, a crucible groove is arranged on the crucible, and circulating water is arranged under the crucible, so that the crucible is not damaged in the smelting process; the smelting furnace is provided with an observation window, and dark glass of the window can protect the eyesight of an operator during smelting.

Example 2: the alloy composition 18.0Al-15.0Co-15.0Cr-16.8Fe-32.9Ni-2.0Si-0.3Y was calculated. Obtaining the atomic size difference delta, the valence electron concentration VEC and the mixed entropy delta S of the alloy_mixMixed enthalpy Δ H_mixWeighted melting point T_mAnd the parameter Ω, are listed in table 2. The alloy satisfies Delta S_mix>1.5R (R is an ideal gas constant), parameter omega>1.1，ΔH_mixIs between (-15 to 3.2) kj/mol, so that the alloy can be called high-entropy alloy.

The calculation formula of each parameter in table 2 is:

in the formula: r is an ideal gas constant; c represents the atomic fraction of the element in the alloy system; r is the atomic radius; t is a melting point; the subscript i represents the corresponding i-th element；ΔH_i,jIs the i-j binary mixed enthalpy.

TABLE 2 calculation of parameters associated with high entropy alloys

Example 3:

a cyclic oxidation test was performed in comparison to NiCoCrAlY alloy. The sample obtained by the fourth step of example 1 and a NiCoCrAlY block (composition: 40.95Ni-19.74Co-16.54Cr-22.49Al-0.28Y, atomic%) were cut into square test pieces of 15 mm. times.15 mm. times.2 mm in size, 2 pieces for each composition, using a precision cutter. And then gradually polishing to 2000# with sand paper to ensure the surface to be flat, and weighing with an electronic balance. The conditions of the cyclic oxidation experiment were: heating in air at 1150 deg.C for 1h by using a tube furnace, taking out to room temperature, air cooling for 30min, weighing with an electronic balance, repeating for 20 times, and averaging the mass increased by oxidation with the mass increased by two test pieces.

The oxidation weight gain of both is shown in FIG. 3. It can be seen that the initial 1h oxidation rate and the subsequent oxidation rate of the high-entropy alloy are both lower than those of NiCoCrAlY alloy. After cyclic oxidation for 20h, the oxidation weight gain is 2.67 multiplied by 10 respectively^-4g/cm²And 5.18X 10^-4g/cm²。

The state of the cross section of the thermally generated oxide after the cyclic oxidation is shown in FIG. 4. The thickness of the oxide film is relatively uniform, about 2-2.5 mu m, and the oxide film is tightly combined with the alloy matrix and is not separated obviously.

The XRD pattern of the alloy surface after cyclic oxidation is shown in figure 5. The analyzed main component of the oxide is alpha-Al₂O₃Possibly containing a smaller amount of NiO. alpha-Al₂O₃Has good oxygen resistance and strong oxidation resistance. The method for alloying Fe and Si is shown to improve the oxidation resistance of the alloy.

Example 4:

the surface roughness (R) of the high entropy alloy and NiCoCrAlY was measured after 2000# sanding, before and after oxidation in example 3_a). The model of the surface roughness tester is TR200, and the measurement length is 2.5 mm.Each specimen was tested 5 times by selecting a different location and the results are summarized in table 3 below.

TABLE 3 surface roughness (Ra, μm)

The data in table 3 illustrates: after the alloy is polished by 2000# abrasive paper, the alloy is convenient to unify and compare, and the surface roughness of the alloy is very low. After cyclic oxidation, the thermal generation of the high-entropy alloy generates an oxide film layer, and the surface roughness of the oxide film layer is smoother than that of an oxide film of NiCoCrAlY. This indicates that the surface of the high-entropy alloy is smoother and the oxide is generated uniformly, thereby facilitating the good combination with the matrix, relieving the mismatch of thermal stress and improving the protection effect on the matrix.

It should be noted that the application range of the oxidation-resistant high-entropy alloy in the invention is not limited to the bonding layer in the thermal barrier coating, but also applicable to various structural members in various industrial fields needing to work under high-temperature environments, such as parts on vehicles such as aerospace, ships and automobiles.

In addition, in the examples 1 to 4, in the alcocrfeniiy high entropy alloy, the method for determining the range of the Si content thereof was: ensuring alloy Delta S_mixNot less than 1.5R (R is an ideal gas constant), delta<0.066, parameter Ω>1.1，ΔH_mixIs between (-15 to 3.2) kJ/mol, and meets the general forming conditions of the high-entropy alloy. On the premise of meeting the above conditions, the Al content is ensured to be more than 16%, and the Cr content is ensured to be more than 12% so as to ensure the oxidation resistance of the alloy. On the basis, the Ni content is calculated to be higher as much as possible so as to ensure that the alloy has a certain proportion of FCC phase and reduce the room temperature brittleness of the alloy.

When the Si content is more than 2%, the alloy meeting the conditions has low Ni content and low FCC phase content, tends to have a single BCC structure, and the mechanical properties at room temperature are influenced. Si is beneficial to improving the oxidation resistance of the alloy, and 1-2% is considered comprehensively.

The above description is only a preferred embodiment of the present invention, and does not limit the present invention in any way. It will be understood by those skilled in the art that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. An AlCoCrFeNiSiY high-entropy alloy resistant to high-temperature oxidation is characterized in that the AlCoCrFeNiSiY high-entropy alloy is prepared by adding alloy element types Fe and Si into NiCoCrAlY; the AlCoCrFeNiSiY high-entropy alloy comprises the following elements in percentage by mole: 18.0 percent of Al, 15.0 percent of Co, 15.0 percent of Cr, 16.8 percent of FeC, 32.9 percent of Ni, 2.0 percent of Si and 0.3 percent of Y.

2. A preparation method of the AlCoCrFeNiSiY high-entropy alloy is used for preparing the AlCoCrFeNiSiY high-entropy alloy as claimed in claim 1, and is characterized by comprising the following steps:

step 2: weighing the raw materials and the intermediate alloy according to the mole percentage of the AlCoCrFeNiSiY high-entropy alloy; the AlCoCrFeNiSiY high-entropy alloy comprises the following elements in percentage by mole: 18.0% of Al, 15.0% of Co, 15.0% of Cr, 16.8% of FeC, 32.9% of Ni, 2.0% of Si and 0.3% of Y;

step 3.2: firstly, vacuumizing the water-cooled copper crucible to 5Pa by using a mechanical pump, then starting a molecular pump, continuously vacuumizing to below 5 multiplied by 10 < -3 > Pa, and filling inert gas to ensure that the pressure in the furnace is 0.6 atm;

3. The preparation method of the AlCoCrFeNiSiY high-entropy alloy as claimed in claim 2, wherein in the step 1, the pretreatment comprises the steps of placing the raw material and the intermediate alloy in more than 99.5% of absolute ethyl alcohol for ultrasonic cleaning for 10 minutes, removing oil contamination impurities on the surface, and then soaking the surface in 20% of hydrochloric acid by mass fraction for 1min to remove surface oxides; putting pure Fe, pure Ni, pure Cr and Fe-Si and Al-Y intermediate alloy into a drying box at 50 ℃ for drying;

4. The method for preparing the AlCoCrFeNiSiY high-entropy alloy according to claim 2, wherein in the step 3, alloy smelting is performed by using a non-consumable vacuum tungsten electrode arc furnace;

the deoxidizing substance is a titanium block; the inert gas is argon.

5. The method for preparing the AlCoCrFeNiSiY high-entropy alloy as claimed in claim 2, wherein in the step 4, the heat treatment is homogenizing heat treatment of the bowl-shaped ingot at 1200 +/-30 ℃ for 6 hours.