Patent Application
20230304111
The following description relates to a high-entropy alloy and a method of heat-treating the same, and more particularly to a high-entropy alloy with an improved structure and process, and a method of heat-treating the high-entropy alloy.
Generally, a high-entropy alloy is an alloy with a face-centered cubic structure (FCC) and high mixing entropy in which a plurality of elements are contained in a predetermined amount or above, or a single-phase alloy with a body-centered cubic structure (BCC). The high-entropy alloy having the FCC mainly contains titanium (Ti), vanadium (V), Zirconium, Niobium (Nb), Molybdenum (Mo), hafnium (Hf), Tantalum (Ta), tungsten (W), and the like, and has excellent high-temperature strength, and thus is used in aerospace fields, an engine gas turbine, and the like. The high-entropy alloy having the BCC mainly contains iron (Fe), manganese (Mn), Chromium (Cr), Nickel (Ni), Cobalt (Co), and the like, and has excellent cryogenic strength and toughness, and low heat conductivity, and thus is used in a liquefied natural gas (LNG) storage tank, structures in polar regions, and the like.
The high-entropy alloy having the BCC has high toughness as well as high strength, such that there have been attempts to use the alloy in various applications, but the alloy is expensive compared to common alloy materials. For this reason, research has been conducted on a high-entropy alloy containing copper (Cu) and aluminum (Al) instead of nickel and cobalt which are high in unit price. Aluminum is generally a lightweight element, and when used in various applications, may improve efficiency of equipment. Metastable liquid phase separation (MLPS) occurs in copper and iron, such that a Cu-rich phase and a Fe-rich phase are mixed in a matrix, and unique characteristics of individual elements are mixed in each phase, such that by using excellent mechanical characteristics of iron and excellent ductility of copper, both of excellent strength and excellent elongation properties may be obtained. However, abrasion resistance is reduced due to the effect of copper, such that a treatment for improving abrasion resistance is required.
Meanwhile, when an oxide film or a nitride film is formed on the surface of the alloy by subjecting a multi-composition alloy, such as a high-entropy alloy and the like, to oxidation or nitriding heat treatment, problems occur in that surface roughness increases, and the oxide film or the nitride film may be easily separated. Accordingly, an existing oxidation or nitriding heat treatment was generally carried out for a single-phase alloy, and the entire surface or the entire inner portion of the single-phase alloy was subjected to the heat-treatment to form an oxide or a nitride on the entire surface or the entire inner portion thereof. In this method, it was difficult to selectively improve hardness or abrasion resistance while maintaining ductility. Furthermore, a change in the entire surface or the inner portion of the alloy may cause a large variation, such that twisting and the like may occur, making it difficult to be applied to cast components that require precision.
It is an objective of the present disclosure to provide a high-entropy alloy having excellent ductility, strength, hardness, and abrasion resistance, and a method of heat-treating the high-entropy alloy.
More specifically, it is another objective of the present disclosure to provide a high-entropy alloy and a method of heat-treating the same, in which a reinforcement compound is formed selectively in a first phase having relatively high strength, thereby improving strength, hardness, and abrasion resistance while maintaining excellent ductility in a second phase.
Particularly, it is yet another objective of the present disclosure to provide a high-entropy alloy having a Fe-rich phase and a Cu-rich phase, in which a reinforcement compound is formed selectively in the Fe-rich phase, such that the high-entropy alloy may have excellent ductility, strength, hardness, and abrasion resistance, and a method of heat-treating the high-entropy alloy.
A high-entropy alloy according to an embodiment of the present disclosure includes a first phase and a second phase respectively containing iron and copper, and iron and a first metal other than copper, and having different compositions, wherein a reinforcement compound formed by chemical bonding of the first metal and a non-metal is selectively included in the first phase.
The reinforcement compound may be formed as internal precipitates present within the first phase.
The reinforcement compound may include a first reinforcement precipitate formed near a surface of the first phase and having a first size, and a second reinforcement precipitate formed within the first phase and having a size larger than the first size.
Strength or hardness of the first phase may be greater than strength or hardness of the second phase, and the reinforcement compound may not be formed in the second phase. For example, the hardness of the first phase may be twice or greater than the hardness of the second phase.
The first phase may be a Fe-rich phase, and the second phase may be a Cu-rich phase.
The first metal may include aluminum.
The non-metal may have a higher solubility in the first phase than in the second phase, and may have a higher diffusion rate in the first phase than in the second phase. The non-metal may include nitrogen or oxygen.
The reinforcement compound may include aluminum nitride (AlN).
A method of heat-treating a high-entropy alloy according to an embodiment of the present disclosure includes: a preparation step of preparing a high-entropy alloy material comprising a first phase and a second phase respectively containing iron and copper, and iron and a first metal other than copper, and having different compositions; and a selective reinforcement step of forming a reinforcement compound in the first phase by performing heat-treatment using a non-metal for a chemical reaction between the first metal, contained in the first phase, and the non-metal.
The first metal may include aluminum.
The non-metal may have a higher solubility in the first phase than in the second phase, and may have a higher diffusion rate in the first phase than in the second phase. The non-metal may include nitrogen or oxygen, such that the selective reinforcement step may be carried out by nitriding treatment or oxidation treatment.
The selective reinforcement step may be carried out at a heat treatment temperature of 500° C. to 1500° C. For example, the selective reinforcement step may be carried out by nitriding treatment using a reactive gas containing an ammonia gas and a hydrogen gas, and the first metal including aluminum such that the reinforcement compound may include aluminum nitride (AlN). For example, the hydrogen gas may be contained in an amount of 10 vol% or less based on a total of 100 vol% of the reactive gas.
The first phase may be a Fe-rich phase, and the second phase may be a Cu-rich phase.
The reinforcement compound may be formed as internal precipitates present within the first phase.
Strength or hardness of the first phase may be greater than strength or hardness of the second phase, and the reinforcement compound may not be formed in the second phase.
According to an embodiment of the present disclosure, a first phase, having relatively high strength and hardness in a high-entropy alloy, contains a reinforcement compound, and a second phase having relatively excellent ductility contains no reinforcement compound, such that the strength of the first phase, having relatively high strength, may be greatly improved. Accordingly, strength, hardness, and abrasion resistance may be improved by the first phase while maintaining excellent ductility by the second phase. Further, the reinforcement compound is formed at a portion of the high-entropy alloy, i.e., the first phase, resulting in a small variation, such that no twisting or the like occurs, and the reinforcement compound may be applied to cast components that require precision.
In this case, in a method of heat-treating a high-entropy alloy according to an embodiment of the present disclosure, by performing a simple process of specifically defining process conditions of a selective reinforcement step, a high-entropy alloy with a reinforcement compound formed selectively in a first phase may be produced. Further, by applying the method of heat-treating a high-entropy alloy according to the embodiment of the present disclosure to various materials or compositions, a high-entropy alloy having desired characteristics may be easily produced. In this manner, by performing the simple process, the high-entropy alloy having excellent ductility, strength, hardness, and abrasion resistance may be produced.
Hereinafter, a high-entropy alloy and a method of heat-treating the same according to an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings. It will be understood that, although the terms “first,” “second,” etc., may be used herein to describe various elements, these elements should not be limited by these terms.
In this specification, the high-entropy alloy 10 is a term used to distinguish from a low-entropy alloy and may commonly refer to an alloy having entropy at a predetermined level or higher. For example, the high-entropy alloy 10 may include not only an alloy generally referred to as a high entropy alloy having entropy of 1.5R or higher, but also an alloy generally referred to as a medium entropy alloy having entropy of 1.0R or higher. That is, the high-entropy alloy 10 according to this embodiment of the present disclosure may have entropy of 1.0R or higher.
Referring to
Here, the first phase 20 and the second phase 30 may be alloys containing iron and copper, in which the first phase 20 may be a Fe-rich phase, and the second phase 30 may be a Cu-rich phase. As metastable liquid phase separation occurs in copper and iron, the first phase 20 which is the Fe-rich phase and the second phase 20 which is the Cu-rich phase are mixed in the high-entropy alloy 10. Further, the first metal contained in each of the first phase 20 and the second phase 30 may include aluminum, and the non-metal bonded to the first metal in the first phase 20 to form a reinforcement compound 22 may be nitrogen or oxygen. Accordingly, the reinforcement compound 22 may include aluminum nitride AlN or aluminum oxide AlOx., which will be described in further detail later.
In this specification, the Fe-rich phase may be a phase in which iron content (e.g., at%) is highest among a plurality of materials (e.g., elements) included therein, and the Cu-rich phase may be a phase in which copper content (e.g., at%) is highest among a plurality of materials (e.g., elements) included therein. Alternatively, in this specification, among the two phases in the alloy containing iron and copper, a phase having a relatively higher iron content and a relatively lower copper content may be the Fe-rich phase, and a phase having a relatively lower iron content and a relatively higher copper content may be the Cu-rich phase. That is, the Fe-rich phase and the Cu-rich phase may not be an absolute concept but a relative concept.
Further, in consideration of various characteristics, the high-entropy alloy according to the embodiment of the present disclosure may further contain at least one of manganese, chromium, nickel, carbon, silicon, and phosphorus. In addition, the high-entropy alloy may further include an element for lowering a melting point (material for lowering a melting point), and the element for lowering a melting point may contain carbon, silicon, phosphorus, manganese, and the like.
Iron is cheap and is excellent in strength, ductility, etc., and its strength and hardness considerably change according to a phase structure, such that iron may be easily adjusted to have characteristics desired in the high-entropy alloy. Copper has a low melting point and exhibits excellent electrical conductivity and thermal conductivity. Further, copper is not mixed with iron, allowing for a dual-phase structure with the Fe-rich phase and the Cu-rich phase, such that copper is suitable for forming the high-entropy alloy capable of improving both iron and copper characteristics.
As described above, the high-entropy alloy according to the embodiment of the present disclosure contains iron and copper which are not mixed well with each other, and thus are difficult to form an alloy unless other metal and the like are included. Accordingly, in order to prevent phase separation between iron and copper, the alloy may be formed with aluminum, manganese, nickel, and the like which have a predetermined solid solubility or higher. Accordingly, the high-entropy alloy has the Fe-rich phase and the Cu-rich phase, in which a ratio between the Fe-rich phase and the Cu-rich phase may vary according to amounts of iron and copper. For example, the high-entropy alloy according to the embodiment of the present disclosure has a dual phase structure with the Fe-rich phase and the Cu-rich phase, in which the Fe-rich phase is present in a higher volume% than the Cu-rich phase, such that the alloy has the Fe-rich phase as a main phase and the Cu-rich phase may be present as a part thereof. In this manner, segregation may be prevented, such that the high-entropy alloy may have a uniform composition, with high strength, processability, castability, and wettability.
In addition, aluminum is a lightweight element (lightweight material) and a low melting point element (low melting point material) and is mixed with iron to form a body-centered cubic structure. Aluminum may improve hardness, abrasion resistance, strength, and the like while reducing ductility. When mixed with iron, manganese may improve both strength and ductility. Furthermore, as described above, the reinforcement compound 22 may be formed selectively in the first phase 20 by selective nitration or oxidation in the first phase, which will be described in further detail later. Manganese has a lower melting point than iron and may act as an element for reducing a melting point of the high-entropy alloy, thereby improving fluidity and castability of the high-entropy alloy. When mixed with iron, chromium may form a chromium oxide film on iron or the Fe-rich phase, thereby further improving corrosion resistance. Chromium may or may not be contained in the high-entropy alloy. Nickel may increase solid solubility of copper in the Fe-rich phase, thereby reducing an amount of copper in the total amount of the high-entropy alloy. Accordingly, material costs may be reduced by reducing an amount of copper, which is relatively expensive, and increasing an amount of iron which is relatively cheap. Further, a melting temperature may be reduced, and corrosion resistance may be improved in a manufacturing process of the high-entropy alloy.
Further, silicon, carbon, phosphorus, manganese, and the like may reduce a melting point of the high-entropy alloy, thereby imparting excellent fluidity, wettability, and low high-temperature viscosity during a manufacturing process of the high-entropy alloy. Accordingly, castability may be improved. Here, if silicon is contained as a low melting point element, castability may be improved, and an oxide may be formed to improve corrosion resistance. If carbon is contained as a low melting point element, a melting point may be reduced effectively. If phosphorus is contained as a low melting point element, a melting point may be reduced effectively even by using a small amount of phosphorus.
For example, high-entropy alloy materials in the high-entropy alloy 10, except a non-metal (nitrogen or oxygen) contained in the reinforcement compound 22, may include 15 at% to 80 at% of iron, 1 at% to 30 at% of copper, 5 at% to 20 at% of aluminum, 0 at% to 20 at% of nickel, 0 at% to 30 at% (e.g., 0.1 at% to 30 at%, for example, 5 at% to 30 at%) of manganese, 0 at% to 15 at% (e.g., 2 at% to 15 at%) of chromium, 0 at% to 5 at% (e.g., 3 at% to 5 at%) of carbon, 0 at% to 2 at% (e.g., 1 at% to 2 at%) of silicon, 0 at% to 2 at% (e.g., 0 at% to 1 at%) of phosphorus, and other elements or unavoidable impurities. These ranges are defined in order to improve ductility, strength, hardness, corrosion resistance, castability, and the like of the high-entropy alloy. However, the present disclosure is not limited to the above elements and amounts. Accordingly, elements or materials other than the above elements or materials may be further included, and amounts of each element or material may vary in consideration of characteristics of the high-entropy alloy.
Here, the first phase 20 is formed as the Fe-rich phase containing more iron than copper, thereby exhibiting a greater strength, hardness, and abrasion resistance than the second phase 30, and the second phase 30 is formed as the Cu-rich phase containing more copper than iron, thereby exhibiting excellent ductility. In this case, the first phase 20, having a relatively greater strength or hardness, may further contain the reinforcement compound 22 selectively, for selectively reinforcing or strengthening the first phase 20 having a greater strength or hardness. For example, the first phase 20 may have a hardness value twice as high as that of the second phase 30. Accordingly, strength, hardness, and abrasion resistance may be effectively improved by the first phase 20.
Further, the reinforcement compound 22 may not be included in the second phase 30. Accordingly, while maintaining excellent ductility of the second phase 30, reduction in strength and abrasion resistance of the second phase 30 may be prevented by selectively reinforcing the first phase 20. In another example, the reinforcement compound 22 may be included in the second phase 30, and even in this case, the reinforcement compound 22 included in the second phase 30 may be present in a smaller amount, actually a very small amount, than an amount of the reinforcement compound 22 present in the first phase 20.
In this case, the reason the reinforcement compound 22 is selectively contained in the first phase 20 is because heat-treatment of the high-entropy alloy 10 is carried out under a specific condition by considering a difference in characteristics of the first phase 20 and the second phase 30, which will be described in further detail later in a heat-treatment method of the high-entropy alloy 10.
As described above, aluminum may be used as the first metal contained in the reinforcement compound 22, and nitrogen or oxygen may be used as a non-metal. This is because aluminum is an element that allows chemical treatment (e.g., nitriding treatment or oxidation treatment) to selectively occur in an Ellingham diagram, without causing an undesirable change in characteristics. That is, aluminum is an element which is selectively nitrified or oxidized in the first phase 10 formed as the Fe-rich phase, without being nitrified or oxidized in the second phase 20 formed as the Cu-rich phase.
Further, as the non-metal, an element may be used which has a higher solubility in the first phase 20 than in the second phase 30 and has a higher diffusion rate in the first phase 20 than in the second phase 30, and nitrogen or oxygen is an element that satisfies such characteristics. Accordingly, nitrogen or oxygen is present in a larger amount and diffuses faster in the first phase 20, thereby allowing the reinforcement compound 22 to be formed easily in the first phase 20.
Accordingly, the reinforcement compound 22 may be formed of an oxide or nitride. More specifically, the reinforcement compound 22 may be formed of an aluminum oxide or aluminum nitride. The reinforcement compound 22, formed of ceramic, may effectively improve strength, hardness, and abrasion resistance. For example, if the reinforcement compound 22 is formed of an aluminum nitride, nitriding treatment may be carried out to minimize an undesirable change in characteristics, while stably forming the reinforcement compound 22 selectively in the first phase 20, thereby improving strength, hardness, and abrasion resistance.
Referring to (a) of
In another example, referring to (b) of
In yet another example, referring to (c) of
In yet another example, referring to (d) of
While it is illustrated in (d) of
As described above, the reinforcement compound 22 may be formed as internal precipitates which are dispersed within the first phase 20 or on the surface thereof, or may be formed in a layer shape. The reinforcement compound 22 may have various shapes, such as a spherical shape, a flake shape, a needle shape, an amorphous shape, a layer shape, and the like. The presence of the reinforcement compound 22, the shape and size thereof, and the like may be easily detected or determined by component analysis, microscope images, and the like.
For example, the reinforcement compound 22 may have a size larger than or equal to 1 nm (e.g., average size) and a size of smaller than or equal to 1 mm (e.g., average size). If the size or thickness of the reinforcement compound 22 is smaller than 1 nm, a sufficient effect may not be obtained from the reinforcement compound 22, and if the size or thickness of the reinforcement compound 22 exceeds 1 mm, brittleness of the reinforcement compound 22 may increase. Here, in the case where the reinforcement compound 22 is formed as internal precipitates with major axis and minor axis, the size of the reinforcement compound 22 may refer to the length of the major axis. In the case where the reinforcement compound 22 has a layer shape, the size of the reinforcement compound 22 may refer to the thickness of the layer shape.
However, the present disclosure is not limited thereto, and the size, shape, density, distribution, and the like of the reinforcement compound 22 may vary.
In this embodiment, in the high-entropy alloy 10 including the first phase 20 and the second phase 30 which have different compositions or characteristics, the first phase 20 having a relatively high strength and hardness selectively contains the reinforcement compound 22, and the second phase 30 having relatively excellent ductility does not contain the reinforcement compound, such that strength, hardness, and abrasion resistance of the first phase 20 having a relatively high strength may be improved while maintaining excellent ductility in the second phase 30. In this manner, the high-entropy alloy 10 may have excellent ductility, strength, hardness, and abrasion resistance. Further, the reinforcement compound 22 is formed only at a portion (i.e., first phase 22) of the high-entropy alloy 10, resulting in a small variation, such that no twisting or the like occurs, and the reinforcement compound 22 may be applied to cast components that require precision.
The aforementioned high-entropy alloy 10 contains a plurality of metals, and is a multi-composition alloy having the first phase 20 and the second phase 30. If the entire multi-composition alloy is heat-treated under general conditions, surface roughness increases, and even when an oxide film or a nitride film is formed on the surface, a problem occurs in that the formed film may be separated. Accordingly, it is required to subject the high-entropy alloy 10 to heat-treatment (e.g., selective reinforcement heat treatment) under specific process conditions in order to selectively form a desired reinforcement compound 22 only in the first phase 20.
Hereinafter, a method of heat-treating the high-entropy alloy 10 according to the embodiment of the present disclosure, in which the reinforcement compound 22 is selectively formed only in the first phase 20, will be described in further detail with reference to
Referring to
First, as illustrated in
Subsequently, as illustrated in
In this case, the first metal may be aluminum. Further, the non-metal chemically bonded to the first metal may have a higher solubility in the first phase 20 than in the second phase 30 and may have a higher diffusion rate in the first phase 20 than in the second phase 30. Accordingly, the non-metal may diffuse faster in the first phase 20 during the heat-treatment, and thus may be easily chemically bonded to the first metal, thereby increasing the possibility that the reinforcement compound 22 may be formed only in the first phase 20. For example, the non-metal may be nitrogen or oxygen, such that the selective reinforcement step (ST20) may be performed by nitriding treatment or oxidation treatment. The following description will be given of an example in which the non-metal chemically bonded to the first metal is nitrogen.
For example, under a temperature condition of 1650° C., the solubility of nitrogen in iron is 250 ppm, which is much higher than the solubility of nitrogen in copper which is 1 to 4 ppm. Accordingly, the solubility of nitrogen in the first phase 20, containing a relatively larger amount of iron, may be higher than the solubility of nitrogen in the second phase 30 containing a relatively larger amount of copper.
Further, the frequency factor (D0) and activation energy (Q) of nitrogen, which is a non-metal in the Fe-rich phase and the Cu-rich phase, and aluminum and manganese, which are contained in the high-entropy alloy 10, are shown in Table 1, and the diffusion coefficient D, calculated based on a rate at which nitrogen is adsorbed on the surface of the high-entropy alloy 10, and the metal content (at%), is shown in Table 2. The metal content shown in Table 2 is merely an example, and the present disclosure is not limited thereto.
The diffusion rate of nitrogen is proportional to a nitrogen concentration, such that the diffusion rate of nitrogen is faster in iron than in copper. Nitrogen having a highest diffusion rate in iron may easily diffuse in iron, but may not diffuse well in copper, since a metal (i.e., manganese) having a faster diffusion rate than nitrogen is contained in copper. Accordingly, the reinforcement compound 22 may be formed in the first phase 20, in which a large amount of iron is contained such that nitrogen diffuses easily, and the reinforcement compound 22 may not be formed in the second phase 30 or formed in a smaller amount than in the first phase 20, since the second phase 30 contains a large amount of copper such that diffusion of nitrogen is blocked.
Hereinafter, process conditions and the like in the selective reinforcement step (ST20), in which a selective nitriding heat treatment using nitrogen is performed, will be described in detail, and an oxidation heat treatment using oxygen will be described later.
The selective nitriding heat treatment may be carried out by using a nitrogen-containing gas at a heat-treatment temperature higher than a room temperature.
A temperature range suitable for the selective nitriding heat treatment will be described in further detail later with reference to
Further, a reactive gas used in the selective nitriding heat treatment may be a nitrogen-containing gas. Ammonia gas (e.g., NH3) may be used as the nitrogen-containing gas, and hydrogen gas (e.g., single hydrogen gas (H2) and/or single nitrogen gas (N2) may be further included.
The ammonia gas has a relatively low bond energy, and thus may be easily dissociated into ions, thereby stably supplying nitrogen, as well as hydrogen. The effect of hydrogen supplied by the ammonia gas is the same as or similar to the effect of hydrogen gas which will be described later.
In this embodiment, the reactive gas may include a hydrogen gas for producing a reducing atmosphere, thereby providing an atmosphere suitable for the nitriding treatment or nitriding reaction. That is, if the hydrogen or a hydrogen gas is not used during the selective nitriding heat treatment, a nitriding rate is very low even when the nitriding reaction takes place, and an iron nitride may be formed during the nitriding process. Further, a nitriding driving force is so large that a nitriding reaction with unwanted metal may occur to form an undesired surface film, thereby preventing nitrogen from penetrating thereinto. By contrast, if hydrogen or a hydrogen gas is used during the selective nitriding heat treatment, a nitriding rate may increase, and the potential of nitrogen may be adjusted, thereby preventing or minimizing the formation of an undesired nitride or a surface film. Further, by forming a condition for inhibiting decomposition of the ammonia gas in a gaseous phase and by reducing a reaction factor of the decomposition of the ammonia gas, it is possible to prevent or minimize vapor phase decomposition of the ammonia gas. Then, the ammonia gas may be decomposed only on the surface of the high-entropy alloy material 10a, and nitrogen decomposed on the surface may effectively penetrate into the high-entropy alloy material 10a. Accordingly, the reinforcement compound 22 in the form of internal precipitates may be formed deep into the material, while minimizing the formation of the surface film.
For example, the hydrogen gas may be contained in an amount of 10 vol% or less (e.g., 0.1 vol% or more, more specifically 1 vol% or more) based on a total of 100 vol% of a reactive gas. If the hydrogen gas is contained in an amount of 10 vol% or less, a nitriding rate decreases due to the lack of ammonia gas, and even worse, a nitriding rate is so slow that the reinforcement compound 22 may not be formed even after a lapse of a long period of time (e.g., 10 hours). Further, if the hydrogen gas is contained in an amount of 0.1 vol% or less (e.g., 1 vol% or less), the nitriding rate may not increase sufficiently, and a nitriding reaction with an unwanted metal may occur, thereby forming a surface film and preventing nitrogen from penetrating thereinto. However, the present disclosure is not limited thereto, and a volume ratio of the hydrogen gas may vary.
A single nitrogen gas may be provided so that the reactive gas may be supplied in a volume suitable for the selective nitriding heat treatment, and in some cases may serve to further supply nitrogen. Accordingly, the selective nitriding heat treatment may be stably carried out. For example, the single nitrogen gas may be in a larger amount than each of an amount of ammonia gas or an amount of hydrogen gas, thereby allowing for a stable process. For example, the ammonia gas may be contained in an amount of 5 vol% to 20 vol% based on a total of 100 vol% of a sum of the amounts of ammonia gas and single nitrogen gas. However, this is merely an example of a volume ratio of the ammonia gas for a stable process, and the present disclosure is not limited thereto.
In this embodiment, as for a heat-treatment temperature at which the selective nitriding heat treatment is carried out, there is a temperature range in which the reinforcement compound 22 may be stably formed according to thermodynamic calculation based on Gibbs free energy and kinetic calculation, and particularly, there is an optimal range in which nitrogen may diffuse deep into the first phase 20a such that the reinforcement compound 22 may be formed deep into the first phase 20a. That is, there is a predetermined temperature range according to the thermodynamic calculation for the selective nitriding heat treatment, and the predetermined temperature range may be used for inhibiting the formation of a surface film by calculating diffusion rates of nitrogen and metal elements, and for forming the reinforcement compound 22 as precipitates within the first phase 20a. It is required to perform the nitriding heat treatment under process conditions including temperature that satisfies all these temperature ranges, in order to prevent or minimize formation of the reinforcement compound 22 in a layer shape on the surface of the first phase 20a, and to form the reinforcement compound 33 as internal precipitates by selectively nitriding a specific element (i.e., first metal) in the first phase 20a. In this manner, strength, hardness, and abrasion resistance may be effectively improved, which will be described with reference to
Referring to
First, at the heat-treatment temperature, vapor phase decomposition should not occur in a reactive gas (e.g., ammonia gas as a nitrogen-containing gas) used during the heat-treatment, or a vapor phase decomposition rate should be low. Accordingly, the heat-treatment is preferably carried out at a temperature lower than a temperature at which the vapor phase decomposition occurs in the reactive gas (e.g., ammonia gas as a nitrogen-containing gas) used during the nitriding heat treatment. Further, the heat-treatment should be carried out at a temperature, at which a diffusion rate of a metal element is low (e.g., temperature equal to or lower than a first temperature T1), so as to prevent or delay the formation of an undesired surface film that is formed when an unwanted metal diffuses to the outside and is reacted with nitrogen.
Further, the selective nitriding heat treatment should be carried out at a predetermined temperature or higher (e.g., temperature equal to or higher than a second temperature T2), so that a diffusion rate of nitrogen in the first phase 20 may have a value equal to or greater than a predetermined value, and the selective nitriding heat treatment should be carried out at a predetermined temperature or lower (e.g., temperature equal to or lower than a third temperature T3) at which the concentration of the reactive gas (e.g., ammonia gas as a nitrogen-containing gas) remains sufficiently constant.
In consideration of the foregoing, the heat-treatment temperature during the selective nitriding heat treatment may be in a range of 500° C. to 1500° C. (e.g., 600° C. to 1100° C.). If the heat-treatment temperature during the selective nitriding heat treatment is lower than 500° C., a decomposition rate of the nitrogen-containing gas decreases, and a nitrogen diffusion rate decreases, such that the reinforcement compound 22 may not formed with a sufficient speed or amount. If the heat-treatment temperature during the selective nitriding heat treatment exceeds 1500° C., the temperature exceeds a melting point of an alloy, such that the alloy may be melted undesirably, and the metal may increasingly diffuse to the outside, thereby making it difficult to properly form the reinforcement compound 22 as internal precipitates. Further, during the selective nitriding heat treatment, a nitride may partially agglomerate on the surface with non-uniform initial nitriding, and after the nitriding process, the reinforcement compound 22 may also be non-uniform in shape. That is, in this embodiment, the heat-treatment temperature during the selective nitriding heat treatment may be maintained at 1500° C. or lower (e.g., 1100° C.) for uniform initial nitriding, and may prevent a phenomenon, such as the formation of the nitride agglomerates on the surface during the selective nitriding heat treatment, and the like.
Particularly, if the heat-treatment temperature is equal to or higher than 650° C. or lower than 900° C. (e.g., 700° C. to 850° C., for example, 750° C. to 850° C.), the temperature satisfies a temperature condition for efficiently forming the reinforcement compound 22. That is, in the above temperature range, diffusion of the metal on the surface may be prevented, and only nitrogen may penetrate into the first phase 20a, such that the reinforcement compound 22 may be selectively formed only in the first phase 20, and may be evenly distributed deep into the first phase 20. In this case, if the process temperature is lower than 650° C. (e.g., lower than 700° C.), a diffusion rate of nitrogen decreases, such that nitrogen may not penetrate deep into the first phase, and if the temperature is equal to or higher than 900° C. (e.g., higher than 850° C.), a surface film is formed due to diffusion of metal to the outside, thereby leading to a decrease in amount, thickness, density, and the like of the reinforcement compound 22 in the form of internal precipitates formed in the first phase 20.
However, the present disclosure is not limited thereto, and the heat-treatment temperature during the selective nitriding heat treatment may vary in consideration of other process conditions and the like.
In addition, a process time for the selective nitriding heat treatment may vary according to a size and thickness of the reinforcement compound 22, an internal depth of the reinforcement compound 22, and the like. For example, the process time for the selective nitriding heat treatment may be in a range of 1 minute to 10 hours, e.g., in a range of 1 to 3 hours at a temperature of 700° C. to 850° C. If the process time for the selective nitriding heat treatment is less than the above range, the reinforcement compound 22 may not be formed sufficiently, and if the process time for the selective nitriding heat treatment is greater than the above range, production costs may increase. However, the present disclosure is not limited thereto, and the process time may vary in consideration of other process conditions and the like.
In another example, the selective reinforcement step (ST20) will be described below, in which the selective oxidation heat treatment is carried out using oxygen.
The selective oxidation heat treatment may be carried out by using an oxygen-containing gas at a heat-treatment temperature higher than a room temperature. For example, the selective oxidation heat treatment may be carried out at a heat-treatment temperature in a range of 500° C. to 1500° C. Particularly, the heat-treatment temperature may be lower than 1100° C. (e.g., 700° C. to 850° C., e.g., 750° C. to 850° C.). In this temperature range, the reinforcement compound 22 in internal precipitate form may be formed to a great depth, and may be formed with a sufficient amount, thickness, density, and the like. In this case, the heat-treatment range in the selective oxidation heat treatment is defined for substantially the same reason as the selective nitriding heat treatment, except that in the selective oxidation heat treatment using oxygen instead of nitrogen, oxide is formed instead of nitride, such that a detailed description thereof will be omitted. However, the present disclosure is not limited thereto, and the heat-treatment temperature in the selective oxidation heat treatment may vary in consideration of other process conditions and the like.
Further, a reactive gas used in the selective oxidation heat treatment may include an oxygen-containing gas. A single oxygen gas (e.g., O2) may be used as a main oxygen-containing gas for supplying oxygen, and an oxygen-containing compound gas (e.g., carbon monoxide gas (CO), carbon dioxide gas (CO2) or a mixture thereof (mixture gas of CO and CO2), and/or a hydrogen-containing gas (e.g., water (H2O), single hydrogen gas H2, or a mixture gas of H2O and H2,) may be further used as an additional gas.
If the selective oxidation heat treatment is carried out by using only the single oxygen gas, selective oxidation may occur only in a very narrow range of oxygen partial pressure. In this case, it is required to adjust the oxygen partial pressure in a narrow range for the selective oxidation, but in practice, micro adjusting during the selective oxidation heat treatment process is very difficult, such that selective oxidation may not occur. Accordingly, in this embodiment, by further using an oxygen-containing compound gas, such as carbon monoxide, carbon dioxide, etc., or water, and a hydrogen-containing gas, such as a single hydrogen gas and the like, a wider range of oxygen partial pressure may be provided for the selective oxidation. In addition, the oxygen-containing compound gas may serve as an auxiliary oxygen-containing gas for supplying oxygen. In addition, water and a hydrogen-containing gas, such as a single hydrogen gas, may function to prevent problems, such as human casualties caused by leakage of carbon monoxide, carbon dioxide, and the like. Conditions for the selective oxidation during the selective oxidation heat treatment may be identified from the Ellingham diagram or experimentally.
A process time for the selective oxidation heat treatment may vary according to a size and thickness of the reinforcement compound 22, an internal depth of the reinforcement compound 22 formed in the first phase 20, and the like. For example, if the process time for the selective oxidation heat treatment may be in a range of 1 minute to 10 hours, for example, in a range of 1 to 3 hours at a temperature of 700° C. to 850° C. If the process time for the selective oxidation heat treatment is less than the above range, the reinforcement compound 22 may not be formed sufficiently, and if the process time for the selective oxidation heat treatment is greater than the above range, the process becomes complicated and production costs may increase. However, the present disclosure is not limited thereto, and the process time may vary in consideration of other process conditions and the like.
For reference, as illustrated in (a) of
In addition, when the selective reinforcement step (ST20) is performed under process conditions of low temperature and high gas partial pressure, a diffusion rate of the metals is low such that the reinforcement compound 22 may be formed non-uniformly. Further, when the selective reinforcement step (ST20) is performed under process conditions of high temperature and low gas partial pressure, a diffusion rate of the metals is high, such that a surface film may be formed. When the surface film is formed, a diffusion rate of the non-metal is very low in the surface film, thereby inhibiting diffusion of the non-metal, such that it is required to form the reinforcement compound 22 while no surface film is formed. It is important to form the reinforcement compound 22 uniformly in the first phase 20 of the high-entropy alloy 10, but if the process conditions or the like are not controlled precisely or if the surface film is partially formed, the surface film inhibits the diffusion of the non-metal, such that the reinforcement compound 22 may be formed non-uniformly. For example, the reinforcement compound 22 may be formed with a high density near the surface of the first phase, and within the first phase, the reinforcement compound 22 may not be formed or may be formed with a lower density than on the surface.
In consideration of the foregoing, the heat-treatment temperature, type of reactive gas (particularly, reactive gas containing hydrogen gas), volume ratio, and the like are specifically defined in this embodiment, such that as illustrated in (a) of
In this embodiment, by performing a simple process of specifically defining the process conditions of the selective reinforcement step (ST20), the reinforcement compound 22 may be formed selectively in the first phase 20. Particularly, even when a dual-phase alloy or a multi-composition alloy has a composition which is slightly different from a desired composition, the high-entropy alloy 10 having desired characteristics may be easily produced by performing the method of heat-treating the high-entropy alloy 10 according to the embodiment of the present disclosure. In this case, the reinforcement compound 22 may be formed in the shape of internal precipitates or, if necessary, may have a layer shape in which the reinforcement compound 22 is formed on the surface. In this manner, by performing the simple process, the high-entropy alloy 10 having excellent ductility, strength, hardness, and abrasion resistance may be produced.
The high-entropy alloy according to the embodiment of the present disclosure may be used to manufacture various products. For example, the high-entropy alloy according to the embodiment of the present disclosure may be used to manufacture Oldham ring that prevents rotation of a scroll and allowing only a left-right orbital motion in a scroll compressor. The Oldham ring should have excellent hardness and abrasion resistance to provide long-term reliability, and an Oldham ring having excellent hardness and abrasion resistance may be manufactured according to the embodiment of the present disclosure. However, the present disclosure is not limited thereto.
In the foregoing description, the high-entropy alloy 10 having the first phase 20 formed as a Fe-rich phase and the second phase 30 formed as the Cu-rich phase is presented as an example, in which the high-entropy alloy 10 has both excellent strength provided by the Fe-rich phase and excellent ductility provided by the Cu-rich phase, while reducing material costs. However, the present disclosure is not limited thereto. Accordingly, the present disclosure may be applied to various types of high-entropy alloy 10 with the first phase 20 and the second phase 30 which have different compositions.
Hereinafter, the present disclosure will be described in further detail with reference to experimental examples. However, the experimental examples are merely for illustrative purposes, and the present disclosure is not limited thereto.
By preparing a high-entropy alloy material of Al15(FeCuMn)85 having a first phase and a second phase containing iron, copper, aluminum, and manganese in the amounts (at%) shown in Table 3, and supplying a reactive gas at a heat-treatment temperature of 700° C., the selective nitriding heat treatment was carried out to produce a high-entropy alloy. In this case, as the reactive gas, a single nitrogen gas, an ammonia gas, and a hydrogen gas were used in a volume ratio of 0.855: 0.095: 0.05.
The high-entropy alloy was produced by subjecting the high-entropy alloy material to the selective nitriding heat treatment in the same manner as in Example 1, except that the heat-treatment temperature was 800° C.
The high-entropy alloy was produced by subjecting the high-entropy alloy material to the selective nitriding heat treatment in the same manner as in Example 1, except that the heat-treatment temperature was 900° C.
Field emission scanning electron microscope (FE-SEM) images of the high-entropy alloy material used in Examples 1 to 3 are shown in
Optical microscope images of a cross-section of the high-entropy alloy in Example 1 are shown in
Referring to (a) of
Optical microscope images of a cross-section of the high-entropy alloy in Example 2 are shown in
Referring to (a) of
Optical microscope images of a cross-section of the high-entropy alloy in Example 3 are shown in
Referring to (a) of
Referring to
Referring to
Based on the above, it can be seen that as the heat-treatment temperature increases during the selective nitriding heat treatment, diffusion rates of nitrogen and manganese become similar, and manganese diffuses at a faster rate than nitrogen on the second phase. Accordingly, in the high-entropy alloy in Example 2 with the heat-treatment temperature of 800° C., manganese diffuses such that manganese nitride (MnN) was formed on the surface of the second phase, and nitriding was also carried out on the first phase. As the temperature increases, a diffusion rate of metal increases faster, such that in the high-entropy alloy in Example 3 with the heat-treatment temperature of 900° C., a diffusion rate of nitrogen is similar to a diffusion rate of metal on the first phase, such that manganese and aluminum may simultaneously form a nitride.
In addition, nitride content according to depth from the surface was measured for the high-entropy alloy material (Comparative Example), and the high-entropy alloy in Examples 1 to 3 in which the selective nitriding heat treatment was carried out for two hours, and the results are shown in
Referring to
Further, content of each element according to depth from the surface was measured for the high-entropy alloy in Examples 1 to 3 in which the selective nitriding heat treatment was carried out for two hours, and the results are shown in
Referring to
Referring to
Meanwhile, hardness was measured for the high-entropy alloy material (Comparative Example), and the high-entropy alloy in Examples 1 to 3 in which the selective nitriding heat treatment was carried out for two hours, and the results are shown in
Referring to
Further, it can be seen that hardness of the high-entropy alloy in Examples 1 to 3 greatly increased approximately two to four times after the selective nitriding heat treatment was carried out on the surface of the first phase. It can be seen that in Example 3 with the heat-treatment temperature of 900° C., a degree of increase in hardness was not higher than Examples 1 and 2. The reason that a degree of increase in hardness in Example 3 is lower than in Examples 1 and 2 is because, in Example 3, a larger amount of manganese and aluminum diffused within the alloy during the selective nitriding heat treatment, and reacted with nitrogen on the surface to form a non-uniform nitride. Furthermore, with a lower nitrogen concentration on the surface than in Example 2, nitriding was performed less than in Example 2. By contrast, hardness of the high-entropy alloy in Examples 1 to 3 was observed to slightly increase on the surface of the second phase before and after the selective nitriding heat treatment was carried out, but no significant change in hardness was observed compared to the surface of the first phase. The reason that the hardness on the surface of the second phase slightly increased is because a surface film (e.g., additional compound layer containing manganese nitride) was formed.
Accordingly, it can be seen that the selective nitriding heat treatment was carried out on the first phase in Examples 1 to 3. Particularly, the selective nitriding heat treatment was carried out to obtain excellent hardness in Example 2.
Meanwhile, an abrasion resistance test was carried out for the high-entropy alloy material (Comparative Example), and the high-entropy alloy in Examples 1 to 3 in which the selective nitriding heat treatment was carried out for two hours, and wear track images are shown as (a), (b), (c), and (d) in
Referring to Table 4, it can be seen that the base material (high-entropy alloy) and the counter material in Examples 1 to 3 have a lower rate of mass change, i.e., loss of mass, than the rate of mass change of the base material (high-entropy alloy material) and the counter material in Comparative Example 1. Further, referring to
Particularly, the high-entropy alloy in Example 2 with the heat-treatment temperature of 800° C. has a smallest wear track width, and a rate of mass change decreases by 90 % or more compared to the high-entropy alloy material in the Comparison Example, such that abrasion resistance increases 10 times or more.
Referring to
The features, structures, effects, and the like described in the embodiments are included in at least one embodiment of the present disclosure, and are not necessarily limited to only one embodiment. Furthermore, the features, structures, effects and the like illustrated in the embodiments can be combined and modified by those skilled in the art to which the embodiments pertain. Therefore, it should be understood that the combined and modified embodiments are included in the present disclosure.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2020/008188 | 6/23/2020 | WO |
