JP5295474B2 - Niobium-based alloy heat-resistant material - Google Patents

Abstract

A heat-resistant material for a niobium base alloy which comprises a substrate of a niobium base alloy, a first alloy coating film comprising Re and at least two other metal elements formed on the surface of the substrate and, formed on the surface of the first alloy coating film, a second alloy coating film comprising one element of Al and Si and at least one metal element except these elements. The above combination of coating films exhibits excellement performance in the interception of oxygen and is less susceptible to deterioration owing to diffusion of an element. Suitable compositions of the first alloy coating film and the second alloy coating film are disclosed for each case wherein the second alloy coating film comprises Al or Si, and preferred compositions of alloy coating films for some compositions of the niobium base alloy of the substrate are also disclosed.

Description

  The present invention relates to a heat-resistant member used in a gas turbine, a jet engine, and the like, and more particularly to a niobium-based alloy heat-resistant member in which a coating for suppressing high-temperature oxidation is formed on the surface of a niobium-based alloy substrate.

  In recent years, there has been a demand for a further increase in the operating temperature of a gas turbine for power generation, and a new heat-resistant material having a higher operating temperature limit is required than Ni-based alloys that have been widely used as turbine members. One of such materials is a niobium (Nb) -based heat-resistant material such as a solid solution strengthened or precipitation strengthened Nb alloy or Nb-Al intermetallic compound (in the present invention, these are called niobium-based alloys). ) Is attracting attention.

  Although these niobium-based alloys are excellent in high-temperature strength, they are all easily oxidized in a high-temperature range, for example, a temperature range of 800 ° C. or higher, and are difficult to use as they are in a high-temperature oxidizing atmosphere such as a gas turbine. Various studies have been made on coatings aimed at oxidation resistance.

Conventionally, a method of forming a diffusion layer of Cr or Al or a method of ceramic coating has been studied as a heat-resistant / oxidation-resistant coating for a metal member used in a high-temperature oxidizing atmosphere. In particular, in Ni-based alloys, a method called thermal barrier coating (TBC) has become mainstream. This is formed by laminating a metal bond layer on the substrate surface and a ceramic heat shield layer on the surface. The metal bond layer is MCrAlY alloy (M = Ni, Co, etc.), and the heat shield layer is ZrO. Ceramics mainly composed of ₂ are often used.

As the oxidation-resistant coating of the niobium-based alloy, an Ir surface coating layer or an Ir surface coating layer and a diffusion prevention layer mainly composed of one or more of Ta, Re, and W are formed below the Ir surface coating layer. An Nb alloy heat-resistant member is disclosed (Patent Document 1 below). In addition, Ir is vacuum-deposited on the surface of the substrate and simultaneously irradiated with Al ions.
A method for producing an oxidation resistant coating layer for forming a coating layer made of an r-Al alloy is disclosed (Patent Document 2 below).

Japanese Patent Laid-Open No. 10-14333 Japanese Patent Laid-Open No. 10-140347

  In general, a ceramic film is insufficient in its toughness and adhesiveness to a base material, and thus often cracks or peels off due to thermal stress, leaving a problem in durability. Also in the above-described TBC, oxygen is blocked mainly in the metal bonding layer. Therefore, the coating for the purpose of oxidation resistance is an alloy coating with high adhesion to the substrate, and has the same ability to block non-metallic components such as oxygen and nitrogen as the above-mentioned metal bonding layer. Is desirable.

  Furthermore, the Nb-based alloy that is the subject of the present invention is intended to be used at a considerably higher operating temperature than that of the Ni-based alloy, for example, exceeding 1400 ° C. In such a high temperature range, the diffusion of elements between the film and the substrate is unavoidable, so the film is often altered in a relatively short time and loses its original function. Therefore, in order to ensure the durability of the oxidation-resistant film, it is necessary to suppress the diffusion as much as possible, and to form a coating structure with a slight alteration of the film even if there is some diffusion.

  Accordingly, the present invention provides a heat resistant material for a niobium-based alloy in which an alloy film having excellent barrier properties against non-metallic components such as oxygen and nitrogen and an alloy film that hardly undergoes alteration due to diffusion is formed on the surface of the base material of the niobium-based alloy. For the purpose.

To achieve the above object, the present invention provides:
A base material surface of a niobium-based alloy containing at least one of Mo and W based on Nb and optionally containing one or more of Cr, Si, Hf, Zr, and C. in the formula _{Re 1-ab M a R b} ( wherein, M represents Cr, Ni, one or more elements of Al, R is Nb, Mo, W, Hf, Zr, of one or more of C An alloy film of a first layer having a composition represented by the following formula: Q _1-c Al _c (wherein , a and b are atomic ratios of M and R, respectively) Q is one or more elements of Cr and Ni, c is an atomic ratio of Al), and an alloy film of a second layer having a composition represented by: The atomic ratio b is 0.01 to 0.50, a + b is 0.95 or less, and the atomic ratio c is 0.05 to 0.95. That is a heat-resistant material niobium-based alloys.

Further, in this heat-resistant material, the niobium-based alloy contains C r, before Symbol include element M is at least Cr alloy film in the first layer (in a small amount mainly more preferably element M Cr It is preferable that the element Q in the alloy film of the second layer is Cr or Cr and Ni.

The present invention makes it possible to provide a niobium-based alloy heat-resistant member in which a coating having a large effect of suppressing high-temperature oxidation is formed on the surface of a niobium-based alloy substrate. This oxidation resistant coating has a self-repairing function that maintains the action of regenerating the oxide by the oxidation of Al in the second layer film and blocking non-metallic elements such as oxygen and nitrogen in the atmosphere. In order to suppress the diffusion of elements by a single layer film, the film hardly changes even when kept in a high temperature range of 1100 ° C. or higher for a long time, and is extremely excellent in oxidation resistance and durability.

  The oxidation resistant coating of the heat resistant material of the present invention comprises a two-layer alloy film as shown in FIG. The upper second alloy film 3 is oxidized with oxygen in the atmosphere to form a dense oxide layer, and thus has a function of blocking non-metallic elements such as oxygen and nitrogen in the atmosphere. Have. At the same time, the alloy film 3 has a self-repairing function. That is, since the alloy film 3 contains a metal element that becomes an oxide, when the oxide layer generated on the surface is peeled off, the metal element is immediately oxidized and the oxide layer is formed on the surface. By being regenerated, the action of blocking oxygen, nitrogen, etc. in the atmosphere can be maintained. On the other hand, the lower first layer alloy film 2 is mainly intended to prevent the diffusion of elements between the substrate 1 and the second layer alloy film 3.

In the present invention, the metal element which becomes the oxide in the alloy film 3 of the second layer is Al. When the metal element that forms the oxide is Al (hereinafter referred to as “Al alloy coating”) , the composition of the alloy film 3 of the second layer is substantially the general formula Q _1-c Al _c (here Q is preferably one or more elements of Ni and Cr, Al is aluminum, and c is an atomic ratio of Al. As already mentioned, Al is an element necessary for forming a dense oxide layer when this heat-resistant material is oxidized in a high-temperature oxidizing atmosphere, and Q is a high temperature between Al and Al. It is an element that forms a stable phase (alloy or intermetallic compound) and is indispensable for ensuring the heat resistance and durability of the second layer film.

Further, the first layer composition of the alloy coating 2 of an Al alloy coating, substantially formula _{Re 1-ab M a R b} ( where, Re is rhenium, M consists of Cr, Ni and Al group One or more elements selected from R, R is one or more elements selected from the group consisting of Nb, Mo, W, Hf, Zr and C, and a and b are M, R is the atomic ratio of R).

  Re is an element that plays a major role in preventing diffusion. The element M is mainly contained in the first layer coating and the second layer coating (some may be included in the base material), and between the first layer coating and the second layer coating (and the first layer coating and the base layer). This is effective in reducing the diffusion between materials. The element R is mainly contained in the first layer coating and the base material (some may be included in the second layer coating), and between the first layer coating and the base material (and the first layer coating and the first layer coating). This is effective in reducing the diffusion between the two-layer coatings.

In the case of Al alloy coating , the reason why the first layer alloy film is composed of a ternary or higher composition is that not only the elements in the second layer film but also the elements in the base material in advance in the first layer film. This is because the chemical potential in each phase is made equal for each component to prevent diffusion. Thereby, decomposition | disassembly and quality change of an oxidation-resistant coating can be suppressed, and durability of a film | membrane can be improved significantly.

Further, the elements M and R in the Al alloy coating are preferably elements that form a stable phase with Re at a high temperature, and the addition of such elements is effective in suppressing the decomposition and alteration of the first layer film. . For example, an intermetallic compound phase such as a Re—Cr—Ni sigma phase, a Re— (Nb, Mo, W) sigma phase, or a chi phase is suitable. Since these phases themselves have a high melting point, the first layer film can be prevented from decomposing or diffusing and disappearing, and since the diffusion coefficient of other elements is small, Demonstrate the function.

In addition, the alloy film of a 1st layer and a 2nd layer should just have said composition, and may contain an unavoidable impurity element.
FIG. 2 is a schematic cross-sectional view showing changes in the coating after the heat-resistant member of the present invention is exposed to high-temperature air. As can be seen in the figure, a dense oxide layer 4a is formed on the surface of the alloy film 3 of the second layer. The oxide layer 4a is mainly and Al ₂ O ₃ or lines cover, even with a small thickness, blocking the ability of the element is large. When continuously used in this state, the first layer film 2 is a very stable phase at a high temperature containing Re and has a great effect of suppressing diffusion. Therefore, decomposition / degeneration of the second layer coating 3 can be prevented, and even if the outermost oxide layer 4a is cracked or separated, an oxide layer is formed again on the surface of the second layer coating 3. Has self-healing properties. Thus, the durability of the oxidation resistant coating is ensured.

  In the case of Al alloy coating, the atomic ratio a of the element M in the alloy film of the first layer is preferably 0.01 or more. If it is less than this, the diffusion of the element Q from the second layer coating to the first layer coating increases. The atomic ratio b of the element R is preferably 0.01 to 0.50. If b is less than 0.01, the purpose of suppressing the diffusion of element R from the base material to the first layer film cannot be achieved. If b exceeds 0.50, the Re This is because the content of M and M is not preferable.

  Further, a + b is preferably 0.95 or less. If this value is exceeded, the amount of Re is too small, and the diffusion preventing function becomes insufficient. Further, the atomic ratio c of the element Al in the alloy film of the second layer is preferably 0.05 to 0.95. If this is less than 0.05, the function of forming a dense oxide film becomes insufficient. If this exceeds 0.95, the amount of element Q is relatively small, and a stable phase is formed at high temperatures. It is because it becomes impossible.

  The present inventors have studied the mechanical properties of niobium-based alloys, and Nb—Mo or Nb—W binary alloys and Nb—Mo—W ternary alloys are excellent in high-temperature strength and toughness, and are turbine members. As a result, it was found that it is preferable. Appropriate ranges for the alloy element content are 1 to 30 at% for Mo and 1 to 15 at% for W.

The present inventors have variously studied oxidation coating of these binary or ternary alloy, in the context of the composition of the niobium-based alloy of the base material, that it is preferable to select Al alloy target covered I found out. First, in the Al alloy coating, it has been found that the second layer film is made of a Cr—Al-based alloy and that a very small amount of Cr is added to the base material to exhibit extremely excellent oxidation resistance. That is, in this heat-resistant material, the base material is Nb— (one or more of Mo and W) —Cr alloy, the first layer alloy film contains Re and Cr, and the second layer alloy film is substantially the same. In particular, it is made of a Cr—Al or Cr—Ni—Al alloy. More preferable alloy film of the first layer is mainly composed of Re and Cr, and includes one or more of a small amount of (Ni, Al) and one or more of (Mo, W, Nb). It is a waste. The base material may contain one or more of Si, Hf, Zr, and C as necessary.

  In the heat resistant material having the Al alloy coating described above, Re in the first layer film is preferably 10 to 60 at%, and Cr is preferably 10 to 60 at%. Moreover, it is preferable that Al in a 2nd layer membrane | film | coat is 15-75 at%.

  In the present invention, the method for forming the alloy film on the surface of the substrate is not particularly limited, and may be any of PVD, CVD, thermal spraying, electrolytic coating, and the like. It may be used. Furthermore, you may add a part of component which comprises an alloy film by a thermal diffusion method. In this case, a gradient may occur in the concentration of the component elements in the depth direction. However, in the present invention, there may be a concentration gradient applied to the alloy film. The thicknesses of the first and second layer alloy films are not particularly limited, but are usually about 1 to 100 μm. If the film thickness is too small, the functions of oxidation resistance and diffusion prevention are insufficient, and if the film thickness is excessive, the thermal stress increases. Therefore, an appropriate film thickness may be selected in consideration of these.

(Evaluation of oxidation resistance)
A high-temperature oxidation test was performed on a test piece in which a two-layered oxidation-resistant film was formed on the surface of a niobium-based alloy base material according to the present invention and a comparative test piece having a single-layered film to evaluate the oxidation-resistant characteristics.

(1) Preparation of test piece Nb-5Mo-5W-5Cr (mol%) alloy was used as the niobium-based alloy of the base material. Using an Nb, Mo, W, Cr and Si powder or granular raw material with a purity of 99.9 to 99.99%, a raw material blended in a predetermined composition is melted by an arc melting method in an Ar atmosphere to produce an ingot. Produced. This alloy ingot was subjected to a homogenization heat treatment at 1700 to 1800 ° C. for 24 hours in an Ar air flow at 1 atm. Thereafter, a test piece substrate of 30 × 20 × 2 (thickness) mm was cut out and subjected to coating treatment. .

In the test piece (two-layer coating) of the present invention, metal Re having a thickness of 5 μm was first electrodeposited from a molten chloride bath containing rhenium chloride on the surface of the base alloy. Subsequently, Cr vapor was diffused by being embedded in an alumina crucible together with ferrochrome powder and holding at 1300 ° C. for 10 hours in a vacuum of 1 × 10 ⁻³ Pa. The test piece taken out from the crucible was subsequently embedded again in the alumina crucible together with the Fe—Al alloy powder and held at 1000 ° C. in a vacuum of 1 × 10 ⁻³ Pa for 6 hours to perform Al vapor diffusion treatment.

  In addition, a comparative test piece (single-layer film) coated with an Al alloy was prepared by performing Cr vapor diffusion treatment or Al vapor deposition on a base alloy prepared by the same method as described above, without performing an electrodeposition treatment of metal Re. What carried out the diffusion process on the same conditions as the above was prepared.

  As a preliminary treatment, a diffusion / oxidation treatment in which heating was performed in a static atmosphere at 1100 ° C. for 9 hours was performed on the present invention and the test specimen for comparison subjected to the above-described coating treatment. As a result, in the test piece of the present invention, as shown in FIG. 2, the first layer film 2 and the second layer film 3 are laminated on the surface of the substrate 1, and the oxide layer (Al, O) 4a is formed on the outermost surface. A heat-resistant material in which was formed was obtained. Table 1 shows the thickness and composition of each layer of the test piece coating.

  As can be seen from the results in Table 1, the Re electrodeposited layer is formed mainly by the penetration of Cr into the Re electrodeposited layer formed on the substrate surface by the vapor diffusion treatment of Cr and the diffusion of Nb from the base material. In addition, the first layer film 2 made of a ternary system of Re—Cr—Nb was changed. Further, the second layer film 3 mainly composed of Cr—Al is formed by the Al vapor diffusion treatment, and the oxide layer 4a is formed by the oxidation treatment.

  Further, in the comparative test piece after the pretreatment, as shown in FIG. 3, in order from the surface, an oxide layer (Al, O) 4a having a thickness of about 1.5 μm and an oxide layer (Cr having a thickness of about 2 μm) , O) 4b, and a film having a thickness of about 8 μm and a layer composed mainly of Cr and Nb. The Cr—Nb layer has a two-layer structure of an upper Cr rich layer 5b and a lower Nb rich layer 5a. Table 2 shows the thickness and composition of each layer in this test piece.

(2) High-temperature oxidation test results of test pieces coated with Al alloy High-temperature oxidation test in which the present invention and comparative test pieces prepared as described above are heated isothermally in a static atmosphere at 1100 ° C. The oxidation resistance characteristics were compared. For the test piece of the present invention, the heating time was 168 hours. Since the change in appearance of the test specimen for comparison was remarkable, it was set at 12 hours. The results are shown in Tables 3 and 4 .

In the test piece of the present invention, there was no significant change in the coating structure even after the high temperature oxidation test, and the state shown in FIG. 2 was maintained. Table 3 shows the change in the thickness of the oxide layer 4a before and after the oxidation resistance test for 168 hours and the change in the Al concentration in the second layer film 3 under the oxide layer 4a. Show. Even after the oxidation for 168 hours, the Al concentration of 14% was maintained in the second layer, and from this, the second layer of Al diffused inward (diffusion to the substrate side) in the first layer. It can be seen that there is an action of a diffusion preventing layer, that is, it is prevented from being lost.

  The oxide layer 4a was α-alumina according to X-ray diffraction. In addition, the fact that the alumina is maintained on the substrate surface without an extreme change in thickness indicates that the Al concentration of the second layer is equal to or higher than the concentration at which the alumina-forming ability in the Cr-Al alloy can be expressed. .

On the other hand, the state of the film after the 12-hour high-temperature oxidation test of the comparative test piece is schematically shown in FIG. Table 4 shows changes in the thickness of the oxide layer before and after the high temperature oxidation test. After the high-temperature oxidation test, the oxide layer (Cr, Nb, O) 4c on the surface side and the oxide layer (Nb, O) 4d on the lower side were two layers. Most of (about 150 μm) is a layer 4d made of Nb and O, indicating that the Nb-based alloy of the base material has been oxidized.

  FIG. 1 is a schematic diagram for explaining the structure of an oxidation-resistant coating of a heat-resistant material of the present invention, and FIG. 2 illustrates a change in a film after the heat-resistant material of the present invention is exposed to high-temperature air. It is a schematic diagram of the cross section for. FIG. 3 is a schematic diagram showing a cross-section of a film before a high-temperature oxidation test of a comparative test piece in the oxidation resistance evaluation. FIG. 4 is a schematic view showing a cross section of the film after the high-temperature oxidation test of this comparative test piece.

Claims (3)

A base material surface of a niobium-based alloy containing at least one of Mo and W based on Nb and optionally containing one or more of Cr, Si, Hf, Zr, and C. in the formula _{Re 1-ab M a R b} ( wherein, M represents Cr, Ni, one or more elements of Al, R is Nb, Mo, W, Hf, Zr, of one or more of C An alloy film of a first layer having a composition represented by the following formula: Q _1-c Al _c (wherein, a and b are atomic ratios of M and R, respectively) Q is Cr, 1 or more elements of Ni, c is Ri Na and alloy film of the second layer is formed having a composition represented by the atomic ratio of Al), the atomic ratio a is 0. 01 or more, the atomic ratio b is 0.01 to 0.50, a + b is 0.95 or less, and the atomic ratio c is 0.05 to 0.95. Heat resistant material that niobium-based alloys. The niobium-based alloy is an alloy containing Cr, the element M in the alloy film of the first layer contains at least Cr, and the element Q in the alloy film of the second layer is Cr or Cr and Ni The heat resistant material of the niobium-based alloy according to claim 1 . The heat-resistant material for a niobium-based alloy according to claim 2 , wherein the element M is mainly composed of Cr and contains one or more of a small amount of Al and Ni.

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