JP5854497B2 - Nb-Si heat resistant alloy - Google Patents

Description

The present invention relates to an Nb—Si heat-resistant alloy that is excellent in high-temperature strength and toughness and used as a high-temperature heat-resistant material.

In the fields of energy, aerospace, steel industry, chemical industry, etc., development of component materials for equipment members that can be used for a long time in a severe environment of high temperatures, that is, heat-resistant materials is expected. In particular, thermal power generation is required to improve thermal efficiency from the viewpoint of global environmental conservation and fossil fuel resource protection. Therefore, it is an urgent task to further increase the operating temperature of the gas turbine, and to further improve the characteristics such as heat resistance and durability of the constituent materials of the gas turbine.

Currently, nickel (Ni) based alloys are widely used as heat resistant materials. However, the upper limit of the usable temperature of the Ni-based alloy is about 1100 ° C., and the improvement of the heat resistance has almost reached the limit. Therefore, a new heat-resistant material that replaces the Ni-based alloy is required.

As a new heat-resistant material, a higher melting point than Ni is an essential condition, and so-called refractory metals can be considered to satisfy this condition. Among these, niobium (Nb) is particularly promising as a heat-resistant material because it has a melting point higher than that of Ni by 1000 ° C. and is lightweight. However, in order to put niobium as a heat-resistant material into practical use, it is necessary to enhance the high temperature strength and the room temperature toughness.

At present, Nb-Si alloys are expected as one of alternative materials for Ni-base superalloys that are heat-resistant materials. Patent Document 1 describes a two-phase alloy for a heat-resistant structural material containing 2 to 10 at% Mo and 18.7 to 26 at% Si in Nb, in which niobium silicide is dispersed in the Nb solid solution phase. . The inventors of the present invention relate to a method for obtaining an Nb-based composite having a new structure by first melting and solidifying Nb, Si, and an appropriate additive (for example, Mg) and then heat-treating. Patent application for the invention (Patent Document 2).

Patent Document 3 includes a metal Nb base phase and at least one metal silicide phase, and includes about 9 atomic% to about 25 atomic% Si, about 5 atomic% to about 25 atomic% Ti, about 1 Atomic percent to about 30 atomic percent Re, about 1 atomic percent to about 25 atomic percent Cr, about 1 atomic percent to about 20 atomic percent Al, up to about 20 atomic percent Hf, up to about 30 atomic percent A refractory metal intermetallic composite comprising Ru, at least one metal selected from up to about 30 atomic% W, Ta and Mo, and the balance Nb is described.

Non-Patent Document 1 discloses a composite in which Nb ₅ Si ₃ is dispersed in an Nb solid solution (base material phase) by adding Si to Nb and precipitating (eutectic) niobium silicide (Nb ₅ Si ₃ ). It is described that a tissue (Nb—Nb ₅ Si ₃ ), that is, a niobium group complex is formed. Non-Patent Document 2 describes that Nb solid solution is further strengthened by adding Mo and W to improve high-temperature strength (solid solution strengthening). The present inventors have reported on the control of the microstructure in the Nb-18.1at% Si-1.5at% Zr-100ppmMg alloy (Non-Patent Documents 3 and 4) and Nb ₃ by adding Ta to the Nb-Si-Mo alloy. It has been reported that Si is stabilized (Non-patent Document 5).

Conventionally, Ni-based alloy manufacturing methods aim to manufacture stronger Ni-based heat-resistant alloys, from ordinary cast (CC) alloys, unidirectionally solidified (DS) alloys, single crystals (SC). : Single Crystal) alloys, and oxide dispersion strengthening (ODS) alloys have been developed.

Currently, in the Ni-based alloys that are heat-resistant materials used under the harshest conditions, SC alloys are actively developed with the aim of achieving higher performance alloys. That is, high strength is realized by dispersing the compound in coarse Ni crystal grains.

Currently, DS is a possible method for producing Nb-based alloys. As with the Ni-based alloy, it is desired to further improve the SC. However, when Nb is used, a plurality of phase transformations occur at a high temperature, so that the existing methods of precision casting and unidirectional solidification cannot be applied. Therefore, it is considered that the Nb-based alloy cannot be advanced to SC by the technique used in the current metal engineering, that is, direct formation by solidification from a molten state. If the amount of Si is reduced by an order of magnitude, it is possible to produce SC. However, if the amount of the compound is reduced in this way, the strength becomes significantly smaller than that of the currently used Ni-based alloys, and it is practical. Can not be manufactured.

JP 2003-306736 A JP 2006-241484 A JP 2007-031837 A Won-Yong Kim, et al., "Materials Science and Engineering", A346 (2003) p.65-74 Jiangbo Sha, et al., “Materials Science and Engineering”, A364 (2004) p.151-158 S. Miura et al., "Metall. Mater. Trans. A", 36 (2005), p. 489 S. Miura et al., “Materials Science and Engineering”, A510-511 (2009), p.317-321 S. Miura et al., "Materials Science Forum", Vols. 654-656 (2010) p.444-447

As a heat-resistant material, conventionally, an alloy material mainly composed of Ni having a melting point of about 1300 ° C. is used. When used as a moving blade material for a jet engine or a gas turbine for power generation, the combustion efficiency can be further improved. There is little room. In addition, in order to increase the heat resistance, for example, cooling air is allowed to flow inside, so that a complicated process is required for manufacturing, and the cost is high.

Therefore, by using an Nb-based alloy having a melting point of 1900 ° C. or higher, it is possible to ensure heat resistance, efficiently use energy, reduce CO ₂ gas emission, and effectively use carbon-based resources such as petroleum. it is conceivable that.

The Nb-Si binary alloy is a eutectic alloy as shown in the phase diagram shown in FIG. 8, and the intermetallic compound Nb ₅ Si ₃ appearing in the phase diagram has been attracting attention as being effective in improving oxidation resistance and creep resistance. Although high ductility is expected by combining Nb rich in ductility and silicide having excellent high-temperature strength, there is a problem that room temperature toughness is significantly lowered when the Si concentration is about 10 to 15 at%. One of the problems with Nb-Si alloys is that they have high-temperature strength and room temperature toughness. There is also a need for materials that can be applied to existing methods such as precision casting and unidirectional solidification, and materials that are easy to hot work.

Although the Nb—Si binary alloy does not show large plastic deformation, the present inventors added one or two elements selected from Au and Pd, or Rh element, and produced Nb— The Si alloy can be made into a structure in which Nb ₅ Si ₃ is spheroidized and dispersed in ductile Nb by heat treatment at high temperature, and an Nb-based alloy exhibiting excellent plastic deformability can be obtained without sacrificing strength. I found.

That is, the present invention contains 9.0 to 17.5 atomic% of Si, 1 or 2 elements selected from Au or Pd , 1 atomic% or more and less than the solid solution limit, and the balance consists of inevitable impurities and niobium, A niobium-based heat-resistant alloy characterized by having a Nb / Nb ₅ Si ₃ eutectic structure in which spheroidized Nb ₅ Si ₃ particles are dispersed in an Nb base material phase. This alloy is characterized in that a displacement amount by a high temperature three-point bending test at 1200 ° C. is 1500 μm or more.

In addition, the present invention includes Nb in which Nb is spheroidized in an Nb base material phase, containing 9.0 to 17.5 atomic percent of Si, 1 atomic percent or more of Rh element, and the balance being inevitable impurities and niobium. _Five Si _Three Nb / Nb with dispersed particles _Five Si _Three A niobium-based heat-resistant alloy characterized by having a eutectic structure.

In the past, niobium-based ternary alloys that have been solid-solution strengthened by adding other refractory metal elements such as Mo and / or W have been reported. However, the manufacturing method of the present invention improves the characteristics such as conventional solid solution strengthening by adding one or two elements selected from Au and Pd or Rh element to Nb and Si as described above. The present invention provides a niobium-based heat-resistant alloy having characteristics not obtained by the method.

The heat-resistant alloy of the present invention has a microstructure in which fine spherical niobium silicide is dispersed in a niobium base crystal phase. For this reason, in addition to heat resistance, room temperature toughness and ductility are particularly excellent. Therefore, the niobium-based heat-resistant alloy of the present invention is suitable for processing such as hot forging and is very useful as a heat-resistant material.

It is a conceptual diagram which shows the manufacturing method of the alloy of this invention, and the structure | tissue obtained compared with the case where the conventional Zr is used as an additive element. It is a drawing substitute SEM photograph which shows the difference in the structure by the addition element of a Nb-Si alloy casting material. It is a drawing substitute SEM photograph which shows the structure | tissue as a cast of the Nb-15Si-3Au alloy of an Example, and the structure | tissue after heat processing. It is a graph which shows the room temperature three-point bending test result after heat processing of the alloy of the Example of this invention, and a comparative example. It is a graph which shows the high temperature three point bending test result in 1200 degreeC after the heat processing of the alloy of the Example of this invention, and a comparative example. It is a drawing substitute optical photograph of the test piece after a high temperature three-point bending test. It is a conceptual diagram of a three-point bending test. It is a phase diagram of a Nb-Si binary system alloy.

<Composition and Structure of Niobium-Based Heat-Resistant Alloy According to the Present Invention> In general, Nb—Si binary alloys have a matrix (continuous phase) having a large ductility in a hypoeutectic region where Nb is 17.5 at% or less. In the hypereutectic region where Nb is 17.5 at% or more in contrast to the Nb phase, silicide with low ductility becomes a matrix, so that it becomes a hard and brittle material and it is difficult to ensure toughness.

The amount of silicon in the niobium-based heat-resistant alloy according to the present invention is in the range of about 9 atomic% to about 17.5 atomic% of the hypoeutectic region, more preferably in the range of 10 atomic% to about 16 atomic%. . When the amount of silicon is less than about 9 atomic%, the amount of niobium silicide is reduced and the heat resistance is lowered. The upper limit is preferably the eutectic point. Spherical intermetallic compound Nb ₅ Si ₃
In order to form, it is necessary to add one or two elements selected from Au and Pd, or an Rh element (hereinafter also referred to as “addition metal such as Au”). If the content of these elements is 1 atomic% or more, the spheroidizing effect is exhibited, and the addition amount can be selected in consideration of the relationship between the cost and the effect, but it can be added up to the solid solution limit. These added metals are dissolved in the niobium base material phase and niobium silicide, but the solid solution amount of the Nb base material phase is about 2 to 6 at%.

This niobium-based heat-resistant alloy is a composite having a base material phase containing Nb crystals and a compound phase containing niobium silicide. The compound phase is a spherical intermetallic compound Nb ₅ Si ₃ dispersed in the Nb matrix phase. The size of the compound phase is preferably smaller. This is because if the size of the compound phase is large, it will be difficult to function as plastic deformation resistance. In addition, since the crack formed when fractured by load stress is the same as the size of the compound phase, if the size of the compound phase is large, stress is easily concentrated and the toughness is lowered. Although it does not specifically limit as a magnitude | size of a compound phase, A diameter is 1 micrometer or less.

FIG. 1 is a conceptual diagram showing a method for producing an alloy of the present invention in comparison with a conventional case where Zr is used as an additive element. In a conventional Nb-15Si-1.5Zr-based alloy, for example, a melt-cast solidified body has a microstructure in which a fine niobium solid solution is precipitated in niobium silicide containing Nb ₃ Si as a main component. Since this Nb ₃ Si is brittle, defects such as cracks are introduced when the alloy is produced. On the other hand, in the alloy of the present invention, the melt-cast solidified body has a niobium and Nb ₅ Si ₃ lamella (plate-like) structure directly from the melted state or via the Nb + Nb ₃ Si eutectic. This is presumably because the added metal extremely destabilizes Nb ₃ Si, and a structure morphology considered to be Nb / Nb ₅ Si ₃ eutectic occurs in the solidification process. If Nb ₅ Si ₃ is finally spheroidized, the improvement in toughness is achieved.

Niobium silicide is excellent in high-temperature strength, but has poor room temperature toughness. In the conventional Nb-niobium silicide composite, since the niobium silicide has been connected for a long time, there is a problem that cracks in the silicide progress more and more, resulting in destruction.

However, since the niobium-based heat-resistant alloy of the present invention has the above-described structure, it can suppress the above-described progress of fracture, and has high strength and room temperature toughness. In addition, since the mechanism for suppressing the progress of destruction described above is effective regardless of the low temperature and high temperature, the niobium-based heat-resistant alloy of the present invention can be used in a wide temperature environment.

<Method for Producing Niobium-Based Heat-Resistant Alloy According to the Present Invention> The method for producing a niobium-based heat-resistant alloy according to the present invention is obtained by a melting step for melting raw materials of additive metals such as Nb, Si and Au, and the above-described melting step. A solidification step for eutectic solidification of the melt and a heat treatment step for heat-treating the solidified product obtained in the solidification step in a solid state. The melting step and the eutectic solidification step can suitably use the melting step and solidification step performed in a conventionally known alloy production method, and the specific method, conditions, etc. thereof are not limited.

For example, as a melting method in the melting step, methods such as an arc melting method, an electron beam melting method, a high frequency melting method, and an optical heating method can be applied. In particular, by using a unidirectional solidification apparatus, a region where the orientations of niobium crystals are aligned can be enlarged.

Further, before the material is melted, powder sintering may be performed in order to make the powder sample into a certain shape. That is, when powder sintering is performed, the powder sintering does not result in a dissolved state, and therefore, after powder sintering, the powder is melted by the melting method described above. The melting step is preferably performed in an inert atmosphere such as argon or under vacuum conditions.

In addition, each constituent element (addition metal such as Nb, Si, Au, etc.) may be included in a single state, or in a compound or alloy state, in the material to be melted in the melting step. Also good. Further, the form of the material is not particularly limited, and a material in a state of a lump, flake, powder, or the like can be used. For example, high-purity Nb agglomerates, Si flakes, and additive metal flakes such as Au may be used as materials, and pure metal or alloy powders of additive metals such as Nb, Si, and Au are used, and these powders are powder-fired. The material may be melted by the above-described melting method after the material is made into a certain shape by processing by a sintering method or the like.

(Heat treatment step) The heat treatment step according to the present invention is a step of heat-treating the sample that has undergone the above solidification step in a solid state. The heat treatment step is preferably performed in a vacuum or in an inert atmosphere.

The niobium-based heat-resistant alloy obtained through this heat treatment step is a niobium-based composite composed of a matrix phase mainly composed of Nb crystals and a compound phase mainly composed of niobium silicide, and the niobium silicide of this niobium-based composite Most of these are Nb ₅ Si ₃ . Further, when the structure of the niobium group composite is observed, a spherical Nb ₅ Si ₃ phase is dispersed in the Nb phase (base material phase).

It is considered that the niobium group composite having such a structure is obtained by the melting process, the solidification process, and the heat treatment process described above according to the following process. By passing through the melting step and the solidification step described above, a solidified product having a Nb phase mainly composed of Nb and a lamellar structure mainly composed of flaky niobium silicide (Nb ₅ Si ₃ ) is obtained from Nb and Si in the material. Can be obtained. When the above heat treatment step is performed on the solidified product, the flaky Nb ₅ Si ₃ in the solidified product is divided into a large number of particles by the heat treatment step and spheroidized.

Such a structure cannot be obtained without the addition of an additive metal such as Au. This is considered to be because when the heat treatment step is performed without adding an additive metal such as Au, the interface energy of Nb ₅ Si ₃ is not isotropic, so that the spheronization process does not proceed. When the additive element is Mo or W, it is not spheroidized even by heat treatment and remains a lamella.

The temperature and time in the heat treatment step may be set so that the lamellar structure Nb ₅ Si ₃ is spheroidized, and the heating temperature is about 1100 to 1700 ° C, preferably about 1300 to 1700 ° C, and more preferably Is 1500-1650 ° C.

This alloy product can be processed and formed into the desired article by various methods. For example, a molten alloy product can be cast in a suitable apparatus. Various other methods (alone or in combination) can also be used to process the alloy product. Non-limiting examples include extrusion (eg, hot extrusion), forging, hot isostatic pressing and rolling. The person skilled in the art is familiar with details regarding the appropriate thermomechanical treatment of these alloys.

<Examples and Comparative Examples> The alloy composition was Nb-15at% Si-3at% X, and the additive metal X was a transition metal such as Fe, Co, Ni, Cu, Ru, Rh, Pd, or Au. Regarding Cu, the solid solution limit amount in Nb is 1.2 at%, so 1 at% was added. Bulk ingot Nb having a purity of about 99.9%, Si flakes having a purity of about 99.999%, and flakes of added metals having a purity of about 99% were arc-melted and cast to produce about 20 g of ingot. An argon arc melting furnace was used for melting.

An ingot obtained by solidifying the molten metal was cut, wet-polished from # 180 to # 2000, and mirror-finished with alumina powder (particle diameter: 0.1 μm) to obtain an observation sample. For structural observation, SEM (JXA-8900 (JEOL)) was used, and wavelength dispersion X-ray spectroscopic analysis was performed in order to identify each alloy constituent phase and measure the solid solution amount of the additive element. Further, a sample in which Ta or Ti was further added to Nb-15Si-3Au at 5 at% was also prepared, and the same observation and measurement were performed.

In FIG. 2, the SEM observation result after heat processing of each sample is shown. The Nb-15Si-3Au alloy has a Nb / Nb ₅ Si ₃ two-phase structure of the Nb matrix in which spheroidized Nb ₅ Si ₃ particles are dispersed . It was concluded that the Nb-15Si-3 (Cu, Ru, Rh, Pd) alloy has a spheroidizing effect due to accelerated decomposition because of the eutectoid decomposition of Nb ₃ Si in the as-cast material. The Nb-15Si-3Co alloy shows a Nb / Nb ₅ Si ₃ two-phase structure of the Nb ₅ Si ₃ matrix. In Co and Ni, a phase considered to be a ternary compound phase was observed instead of Nb ₅ Si ₃ . The amount of solid solution in the Nb ₃ Si phase was as small as 1 at% or less for all the added elements, but the amount of solid solution was 2 to 6 at% in the Nb ₅ Si ₃ phase. In addition, any additive element is more solidly dissolved in the Nb phase than the compound phase. There was a tendency that the solid solution amount of the additive element in the Nb dendrite primary crystal increased as the atomic radius of the additive element approached the Nb atomic radius. Similar to previous studies [Non-Patent Document 5] that Nb ₃ Si by Ta addition to Nb-Si-Mo alloy is stabilized, Nb ₃ Si is stabilized by Ta addition even in Nb-Si-Au On the other hand, the result that Ti addition did not contribute to the stabilization of Nb ₃ Si was obtained.

Next, each sample was heat-treated at 1650 ° C. for 100 hours using a vertical ultra-high temperature furnace while flowing high-purity argon gas. The Nb-15Si-3Au alloy was heat-treated at 1300 ° C. × 100 Hr, 1500 ° C. × 100 Hr, 1650 ° C. × 100 Hr, and the structure was observed. FIG. 3 shows the as-cast structure and the heat-treated structure of the Nb-15Si-3Au alloy.

Furthermore, FIG. 4 shows a normal temperature three-point bending test result, and FIG. 5 shows a high-temperature three-point bending test result at 1200 ° C. FIG. 6 shows an optical photograph of the test pieces (Nb-15Si-3Pd and Nb-15Si-3Au) after the high-temperature three-point bending test. In the three-point bending test, as shown in FIG. 7, a rod-shaped test piece having a cross section of 1 mm × 2 mm and a length of 10 mm was used, and the distance between alumina pins was 8 mm. The sample was heated in an infrared image furnace whose temperature was controlled by an R thermocouple, and a central pin was pushed in at a speed of 1 micron / second using a piezoelectric element, and the amount of movement was measured.

4 and 5, the horizontal axis represents the pin travel distance of the three-point bending test, and the vertical axis represents the load. Large plastic deformation was not observed in the binary alloy, but the alloy specimens to which Au and Pd were added showed large plastic deformation without cracking until the end of the test. Ru is broken in the middle, but other additive materials have almost the same plastic deformability.

Nb-15Si-1Cu alloy is excellent in the normal temperature bending test result, but the high temperature bending test result is not good. The Nb-Si alloy with Au and Pd added shows excellent results in the normal temperature bending test, and the high temperature bending test shows that it does not break even when the displacement exceeds 1500 μm.

Since the alloy of the present invention has very suitable properties as a heat resistant material, it can be used as a component material for equipment members that can be used for a long time in severe environments of high temperatures in the energy, aerospace, steel industry, chemical industry, etc. Can be used. For example, by using it as a moving blade material for a jet engine or a power generation gas turbine, the gas combustion temperature can be improved and the thermal efficiency can be improved.

Claims (3)

Containing 9.0 to 17.5 atomic% of Si, 1 or 2 elements selected from Au or Pd, containing 1 atomic% or more and below the solid solution limit, the balance consisting of inevitable impurities and niobium, and in the Nb matrix phase A niobium-based heat-resistant alloy having an Nb / Nb ₅ Si ₃ eutectic structure in which spheroidized Nb ₅ Si ₃ particles are dispersed. The niobium-based heat-resistant alloy according to claim 1, wherein a displacement amount by a high-temperature three-point bending test at 1200 ° C is 1500 µm or more. Nb that contains 9.0 to 17.5 atomic percent of Si, 1 atomic percent or more of Rh element and below the solid solution limit, and the balance consists of inevitable impurities and niobium, and is spheroidized in the Nb matrix phase _Five Si _Three Nb / Nb with dispersed particles _Five Si _Three A niobium-based heat-resistant alloy characterized by having a eutectic structure.