CN110904376A

CN110904376A - High-entropy alloy and preparation method thereof

Info

Publication number: CN110904376A
Application number: CN201911234296.2A
Authority: CN
Inventors: 孙兆磊; 王守仁; 杨学锋; 杨丽颖; 刘文涛; 夏佃秀; 吴戍戌; 郭宇; 时晓宇
Original assignee: University of Jinan
Current assignee: University of Jinan
Priority date: 2019-12-05
Filing date: 2019-12-05
Publication date: 2020-03-24

Abstract

In the embodiment of the invention, ball milling is adopted to fully mix high-entropy alloy raw materials, anaerobic protection is carried out during ball milling, then a high-entropy metallurgy blank is formed by a die, sintering forming is carried out in an anaerobic environment during combustion, and pressurization is carried out during sintering, so that high-entropy metallurgy with compactness and less raw material loss is obtained.

Description

High-entropy alloy and preparation method thereof

Technical Field

The invention relates to a High-entropy alloy (HEA) and a preparation method thereof.

Background

Entropy was proposed in 1865 by the german physicist clausius, whereas high entropy alloys were not prepared until 2004 and were not put into use in 2010. High entropy alloys refer to alloys formed from five or more equal (equimolar ratios) or about equal amounts of metals. With the development of the technology, second-generation high-entropy alloys, in which the constituent elements can be in non-equimolar ratio and four or more constituent elements are present, and high-entropy ceramics have also appeared.

The main components of the traditional alloy are only one or two metals, and the main components are taken as a matrix and then trace other trace elements are added to improve the performance of the alloy. As mentioned above, the high-entropy alloy has a plurality of main components with approximately equal molar ratios, and according to the conventional concept, the alloy is more brittle if the more kinds of metals are added into the alloy, but the high-entropy alloy is different from the conventional alloy, and the high-entropy alloy is not brittle due to approximately equal amount of solid solution of the metals.

Typically, chinese patent document CN108149117A discloses a MoCrFeMnNi high-entropy alloy and a preparation method thereof, the high-entropy alloy adopts a mode of equal molar ratio of components, the preparation method comprises the steps of mixing the components in powder form, pressurizing to prepare a round cake, putting the round cake into a reaction kettle, placing a combustion initiator on the round cake for consuming oxygen, and finally preparing the final propene-shaped MoCrFeMnNi high-entropy alloy through thermite reaction. In addition, the preparation process is relatively simple, mainly the mixture is uniformly mixed and then is pressed and formed, and the bonding performance among metal element particles is relatively poor.

While the chinese patent document CN108642362A discloses a high-entropy alloy with unequal molar ratios among components, the main components are Cr, Fe, Co, Ni and Ta, the core step of the preparation method is smelting, and the inventors believe that smelting does not start with the preparation of the high-entropy alloy because unlike steel smelting, the main component of steel is Fe, and the content of other components is much lower than that of Fe, while the high-entropy alloy with unequal molar ratios has at least four main components, the melting points of the various components are different, the tapping temperature is not easy to control, and the chemical components are unstable. In particular, when two or more metals are mixed, the structural defects are very easily caused, and when more metals are mixed, the structural defects are more difficult to control.

Disclosure of Invention

The invention aims to provide a high-entropy alloy preparation method with relatively easily controlled quality, and also relates to a high-entropy alloy prepared by the high-entropy alloy preparation method.

In an embodiment of the present invention, there is provided a high-entropy alloy formed of an alloy of Fe, Cr, Co, Ni, Al, Ti, and Mo, wherein Al and Ti are in an equal atomic ratio, the remaining metals are in an equal atomic ratio, and the difference in at% between Al and Fe is not more than 0.05% and not less than 0.01%.

Optionally, Al is 14.30% at% and Fe is 14.28% in the high entropy alloy.

In the embodiment of the invention, the invention also provides a preparation method of the high-entropy alloy before preparation, and according to the proportion of the high-entropy alloy, given amount of powdery Fe, Cr, Co, Ni, Al, Ti and Mo are weighed and taken as ball grinding materials to be ball-milled under the anaerobic condition; during ball milling, the ball grinding materials are cooled in real time or intermittently;

filling the ball-milled material into a given die for cold press forming to form a blank;

and sintering the blank in vacuum to form the high-entropy alloy.

Optionally, after the intermittent cooling is performed for the first time of ball milling, stopping ball milling for a second time, and performing cyclic reciprocating;

the method for determining the first time is as follows: the longest time of ball milling materials from a first set temperature to a second set temperature during ball milling; wherein the second set temperature is 10-30 ℃ lower than or equal to the lowest melting point of the medium-melting-point and low-melting-point metals in the high-entropy alloy;

the first set temperature is equal to or higher than 75 ℃.

Optionally, after ball milling for 38 hours, spraying absolute ethyl alcohol into the ball milling material, and then continuing ball milling for 2 hours;

the ball grinding material is separated from the absolute ethyl alcohol before cold pressing and forming.

Alternatively, the absolute ethyl alcohol is separated out of the ball grinding material by placing the ball grinding material mixed with the absolute ethyl alcohol into a drying oven to be dried until the absolute ethyl alcohol is completely volatilized.

Optionally, a mold release material is placed on the mold cavity during cold press forming.

Optionally, the release material is graphite paper.

Optionally, the blank is placed in a sintering device, and then sequentially heated to intermediate temperatures until the blank reaches the sintering temperature, wherein each liter of temperature reaches an intermediate temperature, and the blank is required to be kept for a given time.

Optionally, when cooling is performed after sintering, heating is turned off, the workpiece is cooled to 200 ℃ along with the furnace, then the sintering furnace is turned off, and the workpiece is cooled to room temperature along with the furnace.

In the embodiment of the invention, the high-entropy alloy made of 7 metal components is adopted to prepare raw materials, wherein five components in one group have equal atomic ratio, two components in the other group have equal atomic ratio, the atomic number of single metal components between the two groups is approximately equal, and the metal elements are basically molecules with single atoms, so the atomic ratio is also the molar ratio. Because metals as raw materials are generally lost in the preparation process, the equal atomic ratio among the raw materials does not indicate that the strict atomic ratio among the metal elements in the prepared high-entropy alloy can be kept, and the prepared high-entropy alloy is classified as the high-entropy alloy with the equal molar ratio under the condition that the atomic ratio among the metal elements is not different. Furthermore, the loss amount in the preparation process is also one of indexes for evaluating the high-entropy preparation method, and the prepared high-entropy alloy has poor performance due to excessive loss of certain metal components and less loss of certain metal components. In the embodiment of the invention, the powdery metal raw material mixture is uniformly mixed in an oxygen-free ball milling mode, then the uniformly mixed metal powder is subjected to compression molding and further is subjected to sintering molding under a vacuum condition, the loss of the metal raw materials is small in the whole process, and the atomic ratio among metal components in the prepared high-entropy alloy is easy to control. In addition, the high-entropy alloy prepared by the method has the advantages that the metal components are not easy to oxidize and lose, so that the prepared high-entropy alloy has higher hardness and better wear resistance.

Drawings

FIG. 1 is an exploded view of a high-entropy alloy sample preparation mold.

FIG. 2 is a white light interference three-dimensional topographer image of the high entropy alloy sample prepared in one embodiment.

FIG. 3 is an electron microscope image of the high-entropy alloy sample prepared in one embodiment after frictional wear.

FIG. 4 is a friction coefficient chart of the high-entropy alloy sample prepared in one embodiment after being rubbed by a friction coefficient detector for 1800 times.

In the figure: 1. the device comprises a base, 2 parts of a gasket, 3 parts of an outer sleeve, 4 parts of an inner sleeve and 5 parts of a compression column.

Detailed Description

Metal smelting and other processes all generate a certain amount of raw material loss, and for high-entropy metallurgy, no matter what preparation method, the raw material loss is generated. Generally, the loss amount of each component can be finally determined by a sample preparation mode, and the proportion of each metal raw material is adjusted so as to ensure that the obtained high-entropy metallurgy has ideal designed atomic ratio percentage.

The atomic ratio is measured as at%, i.e. atomic percentage, and is used to describe the atomic content, i.e. percentage, of various elements in an inorganic substance, which unit has been abolished in our country, but does not affect the use in the field and the correct understanding of those skilled in the art. Relatively speaking, molar ratios are more commonly used, especially for metal alloys, where the molecules and atoms of the metal element are equal, i.e. generally a molecule contains only one atom.

The atomic ratio relationship between the metals of the raw materials can be determined by weighing in that the atomic weight of the metal elements is known, on the basis of which the weight ratio under the condition of the atomic ratio is easily determined. In more applications, the raw material dosage is generally determined by weight percentage, so that the ingredients are convenient to prepare.

It should be noted that, in general, metals used in industrial production are not monosomes, and in the examples of the present invention, the purity of the raw material used, i.e., the raw material of each metal component, should be not less than 99.9%. Accordingly, the prepared high-entropy alloy also has inevitable impurities.

In the embodiment of the invention, the metal powder is used as a raw material, such as Fe powder, Cr powder, Al powder and the like, the finer the particle size of the metal powder is, the better the dispersion uniformity after mixing is, the more obvious the alloy phase is, but the finer the particle size is, the higher the cost is, in the embodiment of the invention, the particle size of the metal powder is 400 meshes (corresponding to the particle size of 37 μm), the metal powder belongs to fine powder (the particle size is 44-150 μm, medium powder, 10-44 μm, ultra-fine powder, 0.5-10 μm and ultra-fine powder smaller than 0.5 μm), the price is moderate, the number of atoms contained in each particle is appropriate, and under the condition of uniform mixing, the formed high-entropy alloy has relatively obvious alloy properties and relatively weak elementary substance properties.

When the total amount of the metal powder was small, the raw materials were weighed using an electronic balance, and in the examples of the present invention, in order to conveniently measure the properties of the prepared high-entropy alloy, the total weight of the raw materials weighed each time was 20g, and then prepared in the form of a cylindrical sample.

Table 1 shows at% of each metal material.

And putting the weighed metal raw material powder and grinding balls into a planetary ball milling tank together, wherein the ball material ratio is 15: 1. Wherein the material of the grinding ball is hard alloy, the radius of the grinding ball is 3mm and 6mm mixed ball material, and the quantity ratio of the two grinding balls is 2: 1.

During grinding, the ball milling tank is repeatedly vacuumized and filled with protective gas, such as common industrial protective gas argon, the purity of the argon is 99.9 percent, so as to maintain the anaerobic environment of the ball milling tank.

In order to ensure the maintenance of the oxygen-free environment, the residual oxygen is carried out by supplementing the gas and then vacuumizing during ball milling.

In some embodiments, to avoid the external air from entering, the anaerobic environment may be maintained without using a negative pressure, the pressure inside the tank may be higher than the external pressure, and argon gas may be used to maintain the pressure inside the tank.

The air pressure in the tank is slightly higher than the external air pressure, and the air pressure in the tank can adopt 1.1 standard atmospheric pressure.

The ball grinding material is not oxidized in the ball milling process by maintaining an oxygen-free environment.

During ball milling, the working rotating speed of the planetary ball milling tank is set to be 300r/min, the working rotating speed is not too high, otherwise, the temperature of ball-milled materials is increased too fast, and partial materials are melted. The working speed is not too small to maintain proper working efficiency.

In some embodiments, the ball mill employs batch ball milling in order to prevent melting of metal powder as a ball milling material due to frictional heat generation during ball milling.

For the preparation of the high entropy metallurgy of the feedstock shown in table 1, in a preferred embodiment, the ball mill is batch ball milled in a cycle of 10 minutes of operation per revolution followed by 5 minutes of rest for a total milling time of 38 hours.

The cooling method belongs to natural cooling, and it can be understood that, in order to improve the ball milling efficiency, a forced cooling method can be adopted, and the forced cooling can adopt relatively mild air cooling or relatively severe water cooling.

In some embodiments, because the milling bowl is rotating, it is preferred that air cooling be used for its forced cooling.

After the ball-milled materials are ball-milled for 38 hours, a proper amount of absolute ethyl alcohol is sprayed into the ball-milling tank, and then the ball-milling tank is vacuumized to continue to grind for 2 hours, so that the powder on the inner wall of the tank is completely separated, and the process loss of the materials is reduced.

The dosage of the absolute ethyl alcohol is that the liquid level of the absolute ethyl alcohol reaches two thirds of the high position of the ball milling material.

The absolute ethyl alcohol belongs to volatile liquid, and can be completely volatilized by heating under the condition of no other impurities. And putting the mixed high-entropy alloy powder mixed with the absolute ethyl alcohol into a drying oven, setting the temperature at 70 ℃, and drying for 1-2 hours to completely volatilize the absolute ethyl alcohol.

In the embodiment of the invention, taking the preparation of a high-entropy alloy sample as an example, fig. 1 is a preparation die of a cylindrical sample, thereby it can be understood that a high-entropy metallurgy blank can be formed through the die generally.

In fig. 1, a base 1 is a stepped circular truncated cone structure, the diameter of a lower truncated cone is 110mm, and the diameter of an upper boss is 40 mm. The gasket 2 is padded on the upper table surface of the boss, the diameter of the gasket is the same as that of the boss, and the gasket 2 can be a graphite gasket. The outer sleeve 3 is a circular sleeve, the inner diameter of the pipe is the same as the diameter of the boss, the lower end of the outer sleeve 3 is sleeved on the boss, and the outer sleeve and the boss can be in interference fit or transition fit. An inner sleeve 4 is additionally provided, and the inner sleeve 4 is a graphite paper sleeve, so that the demoulding is convenient. After the mixed high-entropy metallurgical powder is filled into the inner jacket 4, the lower end of the plunger 5 is inserted into the inner jacket 4, and the plunger 5 is pressed by, for example, a press machine to form a cylindrical sample by cold press molding.

For the cold press forming of the sample, the working pressure applied to the high-entropy alloy through the pressing column 5 is 25MPa, so that the alloy powder has proper density, and the structural morphology of the sintered high-entropy alloy is favorably improved.

Obtaining a high-entropy metallurgical blank after cold pressing and forming, and further performing hot-pressing sintering on the blank, namely reheating after cold pressing, wherein hot pressing is performed in a vacuum environment, and the specific process flow of the hot-pressing sintering is as follows:

and putting the blank together with the die into a vacuum sintering furnace, and vacuumizing the sintering furnace to ensure that the vacuum degree in the furnace is not higher than 0.01 Pa.

The working temperature of the sintering is 1200 ℃, and for the temperature setting, the following steps are adopted: raising the temperature in the sintering furnace from normal temperature to 200 ℃ for 0-40 min; 40-100min, raising the temperature to 800 ℃ at 200 ℃; finally, the temperature in the sintering furnace is increased from 800 ℃ to 1200 ℃ at the temperature increase rate of 5 ℃/min; keeping the temperature at 1200 ℃ for 1 h.

The pressure of the hot pressing was set to 50MPa, and the direction of the hot pressing was the axial direction of the cylindrical test piece, i.e., the pressure applied by the pressing cylinder 5.

And after the hot-pressing sintering is finished, closing heating and pressurizing to slowly reduce the temperature, and closing the temperature control of the sintering furnace when the temperature in the sintering furnace is gradually reduced to 200 ℃ so as to reduce the temperature of the sample to room temperature along with the furnace.

And taking out the hot-pressed sample from the die to obtain a FeCrCoNiAlTiMo high-entropy alloy sample.

And (3) polishing the sintered sample by using sand paper, then mechanically polishing the polished alloy (a polishing agent is a diamond polishing machine), then putting the polished alloy into a beaker filled with absolute ethyl alcohol for ultrasonic cleaning treatment for 5min, and removing stains on the surface.

When the alloy is subjected to XRD (X-ray diffraction) phase analysis, the specific result is shown in fig. 2, and it can be seen that the high-entropy alloy is composed of two BCC (body centered cubic lattice) phases, does not contain intermetallic compound phases, and all the elements exist in a solid solution form, so that the solid solution strengthening effect is significant.

The hardness of the alloy is tested by using a Vickers microhardness tester, 7 test points are selected to test the microhardness value of the alloy, and finally an average value is obtained, and the result is shown in Table 2, so that the average hardness of the novel high-entropy alloy can reach 883HV, which is caused by the solid solution strengthening result.

TABLE 2 microhardness test of high entropy alloys

EDS (energy dispersive spectrometer) element content analysis is carried out on the sintered high-entropy alloy, the specific content is shown in the following table 3, and the content of each element is changed but not changed greatly when compared with the content in the table 1 before sintering. Artificially, the content of each component of the high-entropy alloy does not change before and after sintering.

TABLE 3 content of each element component of the sintered alloy

As can be seen from fig. 3 and 4, after the sample is rubbed for about 300 times, the friction coefficient is relatively stable, in other words, the structure consistency of the prepared high-entropy alloy is relatively good after the boundary layer caused by the forming die is abraded.

FIG. 3 is an electron microscope image after 1800 times of rubbing, and the high-entropy alloy metal of the prepared high-entropy alloy is relatively obvious.

Claims

1. The high-entropy alloy is characterized in that the high-entropy alloy is formed by Fe, Cr, Co, Ni, Al, Ti and Mo, the Al and the Ti are in equal atomic ratio in the preparation raw materials, the rest metals are in equal atomic ratio, and the at% difference between the Al and the Fe is not more than 0.05% and not less than 0.01%.

2. A high entropy alloy as claimed in claim 1, wherein Al is present in the high entropy alloy at% 14.30% and Fe at% 14.28%.

3. A preparation method for preparing the high-entropy alloy of claim 1 or 2, characterized in that according to the proportion of the high-entropy alloy, given amounts of powdery Fe, Cr, Co, Ni, Al, Ti and Mo are weighed and taken together as ball grinding materials to be ball-milled under the anaerobic condition; during ball milling, the ball grinding materials are cooled in real time or intermittently;

and sintering the blank in vacuum to form the high-entropy alloy.

4. The preparation method of claim 3, wherein the intermittent cooling is that after the first time of ball milling, the ball milling is stopped for a second time and the circulation is repeated;

the first set temperature is equal to or higher than 75 ℃.

5. The preparation method of claim 4, wherein after ball milling for 38 hours, absolute ethyl alcohol is sprayed into the ball milled material, and then ball milling is continued for 2 hours;

6. The method according to claim 5, wherein the absolute ethanol is separated from the ball-milling material by placing the ball-milling material mixed with the absolute ethanol in a drying oven and drying until the absolute ethanol is completely volatilized.

7. The method of claim 1, wherein a mold release material is placed on the cavity of the mold during cold press forming.

8. The method of claim 7 wherein the release material is graphite paper.

9. The method of claim 1, wherein the step-by-step temperature increase is used during sintering, and the blank is placed in a sintering device and then sequentially heated to intermediate temperatures up to the sintering temperature, wherein each liter of temperature is raised to an intermediate temperature, and the blank is kept for a given time.

10. The production method according to claim 1 or 9, wherein when cooling is performed after completion of sintering, heating is turned off, and the workpiece is furnace-cooled to 200 ℃, and then the sintering furnace is turned off, and the workpiece is furnace-cooled to room temperature.