US20090214379A1

US20090214379A1 - Ni-based single crystal alloy

Info

Publication number: US20090214379A1
Application number: US12/389,633
Authority: US
Inventors: Akira Yoshinari; Hideki Tamaki
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 2007-11-07
Filing date: 2009-02-20
Publication date: 2009-08-27
Also published as: JP5235383B2; JP2009114501A

Abstract

Disclosed is a single crystal alloy consisting essentially of, by weight, 0.06-0.09% carbon, 0.016-0.035% B, 0.2-0.4% Hf, 0-0.02% Zr, 6.5-8.5% Cr, 0.4-1.0% Mo, 5.5-9.5% W, 1.2-3.1% Re, 8-10% Ta, 0.3-1.0% Nb, 0-0.4% Ti, 4.7-5.4% Al, 0.5-5.0% Co, 0.1-5% Fe, and the balance of Ni and unavoidable impurities. The alloy is free from solidification cracks during casting a large-sized blade of gas turbines, has grain boundary strength sufficient for assuring the reliability during operation, and further has excellent oxidation resistance to a high combustion gas temperature while having excellent high-temperature strength comparable to that of a conventional single crystal alloy.

Description

BACKGROUND OF THE INVENTION

The present invention relates to a novel Ni-based single crystal alloy used for high-temperature parts such as rotor blades and stator blades in high-temperature apparatuses such as gas turbines, more particularly to a Ni-based super alloy most suitable for members made of a single crystal alloy and used at a high temperature, which large-sized members have a complicated form and are excellent in high-temperature strength and high-temperature oxidation resistance.

TECHNICAL BACKGROUND

The combustion gas temperature of gas turbines tend to increase year by year in order to improve the thermal efficiency thereof, so that a material having excellent high-temperature strength is required for components of the gas turbines being exposed to a high temperature. Therefore, the material for a rotor blade, which is exposed to the harshest environment among the components of the gas turbine being exposed to a high temperature, has changed from an ordinary casting material of a Ni-based supper alloy to a columnar crystal material of the Ni-based supper alloy. Further, a single crystal material having further excellent high-temperature strength has been practically used in many gas turbines for aircraft engines. Also in gas turbines for power generation, single crystal blades have been used in some turbine types because an operational temperature of such gas turbines has increased significantly in order to improve the efficiency of the gas turbines.
The columnar crystal material and the single crystal material are one type of directionally solidified material, and both of them are cast by so called a unidirectional solidification method.
With regard to the columnar crystal material, crystal grains are caused to grow elongationally in a single direction by such a method as disclosed in U.S. Pat. No. 3,260,505, for example, so as to make grain boundaries perpendicular to an action direction of primary stress as little as possible whereby improving the high-temperature strength.
With regard to the single crystal material, an entire casting is made to have a single crystal by such a method as disclosed in U.S. Pat. No. 3,494,709, for example, whereby making it possible to further improve high-temperature strength of the material.
Further, in order to improve high-temperature strength of the Ni-based supper alloy, solution heat treatment is effective, according to which a precipitation hardening phase of γ′ is uniformly and finely precipitated. Namely, while the Ni-based supper alloy can be strengthened by precipitation of the γ′ phase primarily consisting of Ni₃(Al, Ti, Nb, Ta), preferably the γ′ phase is precipitated uniformly and finely.
In the Ni-based supper alloy as solidified, there exists a coarse γ′ phase, which consists of a γ′ phase precipitated and coarsened during cooling after solidified, and of an eutectic γ′ phase coarsely crystallized in a finally solidified region. Thus, it is possible to improve the Ni-based supper alloy in temperature strength by once heating it to a high temperature to dissolve the γ′ phase into a matrix γ phase, followed by rapid cooling (i.e. such a heat treatment is referred to as solution heat treatment), and subsequently subjecting it to aging treatment to cause γ′ phase to uniformly and finely precipitate. The solution heat treatment is preferably carried out at a temperature as high as possible that is not lower than a dissolution temperature of the γ′ phase and not higher than an initial melting temperature.
The reason for this is that as the heat treatment temperature becomes high, regions in which the γ′ phase is made uniform and fine increases, and further, as the regions in which the γ′ phase is made uniform and fine increase, the high-temperature strength is improved. Another reason why the high-temperature strength of the single crystal material is excellent is that a solution heat treatment temperature can be increased by using a single crystal alloy containing grain boundary strengthening elements, which considerably decrease a initial melting temperature of the alloy, in only a trace amount as small as the impurity level, whereby it is possible to make almost all the γ′ phase coarsely precipitated after solidification uniform and fine.
As set forth above, the single crystal material of the Ni-based supper alloy is most excellent in high-temperature strength as a material for the rotor blade of gas turbines in the state of the art. Under such a technical background, single crystal alloys such as CMSX-4 (refer to U.S. Pat. No. 4,643,782), PWA1484 (refer to U.S. Pat. No. 4,719,080), and Rene'N5 (refer to JP-A-5-59474) have been developed and applied to the rotor blades of gas turbines in aircraft engines.
However, as stated above, all of those single crystal alloys contain grain boundary strengthening elements such as C, B and Hf in only a trace amount as small as the impurity level. Therefore, if there exist crystal grain boundaries in the rotor blade produced by casting such a single crystal alloy, the strength of the rotor blade decreases extremely, and in some cases, longitudinal cracks occur along the crystal grain boundaries during solidification.
Therefore, in order to use a rotor blade produced by casting the single crystal alloy in a gas turbine, it is necessary to make the entirety of the rotor blade to have a complete single crystal structure. Since the overall length of the rotor blade of the gas turbines in aircraft engines is about 100 mm even at maximum, the occurrence probability of crystal grain boundaries when casting is low, so that it is possible to produce the rotor blades made of the single crystal alloy in some degree of yield.
However, in the case of the rotor blade of the gas turbines for power generation, since the overall length of the rotor blade is approximately 150 to 450 mm, it is very difficult to make the entirety of the rotor blade to have a complete single crystal structure. Therefore, in the state of the art, it is difficult to produce the rotor blade of gas turbines for power generation in a high yield with use of the single crystal alloy.
On the other hand, the development of an alloy for the columnar crystal material having excellent high temperature strength has been advanced in order to improve high-temperature strength of a large-sized rotor blade which cannot be produced from the single crystal alloy because of its low casting yield. As a result, columnar crystal alloys such as CM186LC (U.S. Pat. No. 5,069,873) and Rene'142 (U.S. Pat. No. 5,173,255) have been developed. These alloys contain grain boundary strengthening elements in an amount sufficient to ensure reliable operation, and have high-temperature strength comparable to that of a first-generation single crystal alloy.
However, in the large sized blade, there have been recognized problems that solidification cracks are liable to occur during casting, and longitudinal cracks are liable to occur along crystal grain boundaries during solidification due to increased thermal stress caused by the rise in combustion gas temperature.
With regard to such problems, for the purpose of obtaining an alloy composition compatibly having high-temperature strength and grain boundary strength, a JP patent application has been filed (refer to JP-A-9-272933) according to which four types of grain boundary strengthening elements of C, B, Hf and Zr are added to a single crystal alloy in various combinations of the additive elements whereby an examination has been conducted with regard to relationships among additive amounts of the grain boundary strengthening elements, high-temperature strength, grain boundary strength and solution heat treatment.
Further, a JP patent application (JP-A-2002-146460) has been filed according to which 0.1% Si is added to a single crystal alloy in order to improve oxidation resistance property thereof.
However, since this alloy does not contain C, B and Hf, and is subjected to complete solution heat treatment, it is unnecessary to consider deterioration of creep strength due to additive Si, whereas in the case of such an alloy shown in JP-A-9-272933 which contains C, B and Hf and is subjected to partial solution heat treatment, there is a problem that Si can not be simply added to the alloy.
Further, there has been proposed a method of remarkably improving the oxidation resistance property of a single crystal alloy, according to which several tens ppm of a rare earth element is added to the single crystal alloy (refer to JP-A-2004-197216).
However, since rare earth elements are active and they reacts with a mold or a core during casting the single crystal alloy, there are problems that it is difficult to control residual amounts of the rare earth elements in the alloy by the reasons that since rare earth elements are active and they reacts with a mold or a core during casting the single crystal alloy, hetero-crystals are liable to occur whereby not only making it very difficult to cast a large-sized rotor-blade made of a single crystal alloy but also it is difficult to control residual amounts of the rare earth elements in the single crystal alloy because the additive amounts of the elements are fully consumed by the reaction.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a Ni-based single crystal alloy which is free from solidification cracks during casting a large-sized blade of gas turbines, has grain boundary strength sufficient for assuring the reliability during operation, and further has excellent oxidation resistance to a high combustion gas temperature while having excellent high-temperature strength comparable to that of a conventional single crystal alloy.
A key aspect of the invention Ni-based single crystal alloy resides in that the invention alloy is based on a single crystal alloy containing three types of grain boundary strengthening elements of C, B and Hf, to which single crystal alloy Fe and Si are added in place of reactive rare earth elements for the purpose of obtaining excellent properties of high-temperature strength, grain boundary strength and oxidation resistance which have been considered to be inconsistent with one another according to the prior art, whereby significantly improving corrosion resistance property while maintaining mechanical strength in a single crystal state, or in a state having crystal grain boundaries.
According to a first feature of the present invention, the Ni-based single crystal alloy has excellent properties of high-temperature strength, grain boundary strength, and oxidation resistance, and consists essentially of, by weight, from not less than 0.06% to not more than 0.09% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.4% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.5% to not more than 8.5% Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than 5.5% to not more than 9.5% W, from not less than 1.2% to not more than 3.1% Re, from not less than 8% to not more than 10% Ta, from not less than 0.3% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not less than 0.5% to not more than 5.0% Co, from not less than 0.1% to not more than 5% Fe, and the balance of Ni and unavoidable impurities.
According to a second feature of the present invention, the Ni-based single crystal alloy has excellent properties of high-temperature strength, grain boundary strength, and consists essentially of, by weight, from not less than 0.06% to not more than 0.08% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.3% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.9% to not more than 7.3% Cr, from not less than 0.7% to not more than 1.0% Mo, from not less than 7.0% to not more than 9.0% W, from not less than 1.2% to not more than 1.6% Re, from not less than 8.5% to not more than 9.5% Ta, from not less than 0.6% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.9% to not more than 5.2% Al, from not less than 0.8% to not more than 1.2% Co, from not less than 0.1% to not more than 5% Fe, and the balance of Ni and unavoidable impurities.
According to a third feature of the present invention, the Ni-based single crystal alloy has excellent properties of high-temperature strength, grain boundary strength, and oxidation resistance, and consists essentially of, by weight, from not less than 0.06% to not more than 0.09% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.4% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.5% to not more than 8.5% Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than 5.5% to not more than 9.5% W, from not less than 1.2% to not more than 3.1% Re, from not less than 8% to not more than 10% Ta, from not less than 0.3% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not less than 0.5% to not more than 5.0% Co, from not less than 0.5% to not more than 3% Fe, and the balance of Ni and unavoidable impurities.
According to a fourth feature of the present invention, the Ni-based single crystal alloy has excellent properties of high-temperature strength, grain boundary strength, and oxidation resistance, and consists essentially of, by weight, from not less than 0.06% to not more than 0.08% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.3% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.9% to not more than 7.3% Cr, from not less than 0.7% to not more than 1.0% Mo, from not less than 7.0% to not more than 9.0% W, from not less than 1.2% to not more than 1.6% Re, from not less than 8.5% to not more than 9.5% Ta, from not less than 0.6% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.9% to not more than 5.2% Al, from not less than 0.8% to not more than 1.2% Co, from not less than 0.5% to not more than 3% Fe, and the balance of Ni and unavoidable impurities.
According to a fifth feature of the present invention, the Ni-based single crystal alloy has excellent properties of high-temperature strength, grain boundary strength, and oxidation resistance, and consists essentially of, by weight, from not less than 0.06% to not more than 0.09% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.4% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.5% to not more than 8.5% Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than 5.5% to not more than 9.5% W, from not less than 1.2% to not more than 3.1% Re, from not less than 8% to not more than 10% Ta, from not less than 0.3% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not less than 0.5% to not more than 5.0% Co, from not less than 1% to not more than 3% Fe, and the balance of Ni and unavoidable impurities.
According to a sixth feature of the present invention, the Ni-based single crystal alloy having excellent properties of high-temperature strength, grain boundary strength, and oxidation resistance in accordance with the present invention consists essentially of, by weight, from not less than 0.06% to not more than 0.09% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.4% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.5% to not more than 8.5% Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than 5.5% to not more than 9.5% W, from not less than 1.2% to not more than 3.1% Re, from not less than 8% to not more than 10% Ta, from not less than 0.3% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not less than 0.5% to not more than 5.0% Co, from not less than 0.1% to not more than 5% Fe, from not less than 0.01% to not more than 0.2% Si, and the balance of Ni and unavoidable impurities.
According to a seventh feature of the present invention, the Ni-based single crystal alloy has excellent properties of high-temperature strength, grain boundary strength, and oxidation resistance, and consists essentially of, by weight, from not less than 0.06% to not more than 0.09% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.4% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.5% to not more than 8.5% Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than 5.5% to not more than 9.5% W, from not less than 1.2% to not more than 3.1% Re, from not less than 8% to not more than 10% Ta, from not less than 0.3% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not less than 0.5% to not more than 5.0% Co, from not less than 0.1% to not more than 5% Fe, from not less than 0.05% to not more than 0.15% Si, and the balance of Ni and unavoidable impurities.
According to another aspect of the present invention, there is provided a casting made of a Ni-based single crystal alloy having excellent properties of high-temperature strength, grain boundary strength, and oxidation resistance, wherein the Ni-based single crystal alloy consists essentially of, by weight, from not less than 0.06% to not more than 0.09% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.4% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.5% to not more than 8.5% Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than 5.5% to not more than 9.5% W, from not less than 1.2% to not more than 3.1% Re, from not less than 8% to not more than 10% Ta, from not less than 0.3% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not less than 0.5% to not more than 5.0% Co, from not less than 0.1% to not more than 5% Fe, and the balance of Ni and unavoidable impurities.

DETAILED DESCRIPTION OF THE INVENTION

Hereafter, there will be provided an explanation of reasons for setting the component ranges and further preferred other limitation requirements of the invention Ni-based single crystal alloy having excellent properties of high-temperature strength, grain boundary strength, and oxidation resistance.

B: 0.016 to 0.035%

Boron is an element for securing both of strength along the solidification direction and strength along the direction perpendicular to the solidification direction, that is, both of the high-temperature strength and the grain boundary strength, but has a characteristics of considerably decreasing the initial melting temperature of alloy. In the case where a much amount of boron is added into the Ni-based single crystal alloy, it will be needed to consider an influence to lowering of the initial melting temperature of the alloy. In the invention alloy, the boron amount most suitable for both of the strength along the solidification direction and the strength along the direction perpendicular to the solidification direction is in the range from more than 0.015% to not more than 0.04%. Particularly, in the vicinity of 0.025% boron, both of the strength along the solidification direction and the strength along the direction perpendicular to the solidification direction are maximum.

C: 0.06 to 0.09%

Carbon is an important element for making both of the high-temperature strength and the grain boundary compatible with each other. Creep rupture strength along the solidification direction decreases as the carbon amount increases. In contrast, the creep rupture strength along the direction perpendicular to the solidification direction at the crystal grain boundary is improved as the carbon amount increases in the range of not more than 0.20%, preferably not more than 0.10%. Therefore, it is believed that the mount range of carbon most suitable for securing both of the high-temperature strength and the grain boundary strength is from 0.06% to less than 0.09%. If the carbon amount is not more than 0.05%, although the high-temperature strength is excellent, the grain boundary strength is low, so that it is impossible to prevent solidification cracks during casting and ensure reliable operation. On the other hand, if the carbon amount is 0.1% or more, the high-temperature strength decreases considerably. Therefore, the carbon amount should be in the range of 0.06 to 0.09%, preferably 0.06 to 0.08%.
Zr: not more than 0.02%
Zirconium considerably lowers the initial melting temperature of alloy whereby making the solution heat treatment at a high temperature impossible resulting in that creep rupture strength along the solidification direction of the alloy is deteriorated. This element does not contribute to improvement of the creep rupture strength along the transverse direction and the oxidation resistance property. For these reasons, preferably the Zr amount is less than 0.02%, and more preferably Zr should not be substantially added.

Hf: 0.2 to 0.3%

Like as Zr, hafnium considerably lowers the initial melting temperature of alloy whereby making the solution heat treatment at a high temperature impossible resulting in that the creep rupture strength along the solidification direction and the creep rupture strength along the transverse direction of alloy are deteriorated.
However, Hf improves tensile ductility along the transverse direction. Further, the Hf amount of about 0.25% improves both of the creep rupture strength and the tensile strength along the transverse direction although somewhat decreasing the creep rupture strength along the solidification direction. Thus, the optimum Hf amount is from 0.2% to 0.4%.

Ta: 8.0 to 10.0%

Tantalum is desirably added in an amount of not less than 8.0% in order to improve the high-temperature strength. On the other hand, a lot of additive Ta increases the solutioning temperature of the γ′ phase. Thus, if Ta is added excessively, a difference between the initial melting temperature of alloy and the solutioning temperature of γ′ phase becomes small, so that a temperature zone in which the γ′ phase can be solutioned without occurrence of initial melting decreases, and an amount of precipitates for precipitation strengthening of the alloy decreases. Thus, the additive amount of Ta exceeding 12% has no effect of improving high-temperature strength of the alloy, so that the upper limit of the additive amount of Ta is preferably not more than 10%, and the optimum value of the additive amount of Ta for improving high-temperature strength of the alloy is in the range of 8.5 to 9.5%.

Co: 0.5 to 5%

Cobalt decreases high-temperature strength of the alloy as the Co amount increases. Therefore, considering the high-temperature strength, the Co amount should be not more than 5%, preferably in the range of 0.5% to 1.2%. An additive Co amount of 0.8% to 1.2% is effective in improvement of corrosion resistance property without decreasing high-temperature strength of the alloy.

W: 5.5 to 9.5%

Tungsten is effective in improvement of high-temperature strength of the alloy by solid solution strengthening. A desirable additive amount of tungsten is not less than 5.5%. In the case where importance is attached to high-temperature strength of the alloy, preferably the additive tungsten amount is not less than 7.0%. On the other hand, the effect of additive tungsten saturates at a certain additive amount, and an excessive amount tungsten deteriorates high-temperature strength of the alloy. This is because if tungsten is added excessively exceeding a soluble limit, acicular or tabular precipitates consisting primarily of tungsten occur. Therefore, the upper limit of additive tungsten should be 9.5%, preferably 9.0%.

Re: 1.2 to 3.1%

Like as tungsten, rhenium is effective for improvement of the alloy in high-temperature strength by solid solution strengthening. A desirable additive amount of Re is not less than 1.2%. On the other hand, the effect of Re saturates at a certain additive amount, and if it is added excessively, the alloy will be deteriorated in high-temperature strength. This is because if Re is added excessively exceeding the soluble limit, acicular or tabular precipitates consisting of Re occur. Therefore, the upper limit of additive Re should be 3.1%, preferably 1.6%.
Since W and Re exhibit almost the same behavior in the alloy, the optimum additive amount of those is preferably thought as those total amount of W and Re. High-temperature strength of the alloy is maximum in the range of 9.5 to 12% of “W+Re” in total. In contrast, if the “W+Re” amount is less than 9.5%, the high-temperature strength will be deteriorated because the solid solution strengthening effect is insufficient. Also, if the “W+Re” amount exceeds 12%, a lot of the aforementioned precipitates will occur resulting in that creep strength of the alloy is considerably deteriorated at a temperature of not lower than 1000° C.

Al: 4.7 to 5.4%

Aluminum is an indispensable element in order to form the γ′ phase which is a factor of strengthening the Ni-based supper alloy. Also, aluminum contributes to improvement of oxidation and corrosion resistance properties by forming an Al₂O₃film on the surface of the alloy. Therefore, the Al amount is preferably not less than 4.7% at the minimum. However, if Al is added excessively exceeding 6.5%, a quantity of eutectic γ′ phase n the alloy increases. The invention alloy has been so designed that that it can exhibit excellent high-temperature strength by optimizing the additive amount of elements effective for solid solution strengthening, even if the alloy is in the state of incomplete solution heat treatment. Thus, even if the eutectic γ′ phase exists in the alloy, it has excellent high-temperature strength. However, in the case of creep damage, the eutectic γ′ phase finally becomes a start point of occurrence of crack to put forward a breakage time of the alloy material, so that preferably a quantity of the eutectic γ′ phase is small. Therefore, the additive Al amount is preferably made not more than 5.4%, more preferably in the range of 4.9 to 5.2%.

Cr: 6.5 to 8.5%

Chromium forms a Cr₂O₃film on the surface of the alloy to improve the alloy in corrosion and oxidation resistance properties. Thus, desirably the Cr amount is 6.5% at the minimum. However, if Cr is added excessively, the formation of the aforementioned precipitates of W and Re will be promoted, so that there arises a need for decreasing the additive amount of W or Re effective for ensuring high-temperature strength of the alloy. Thus, in the case where importance is attached to the high-temperature strength, preferably the Cr amount is not more than 8.5%, more preferably in the range of 6.9 to 7.3%.

Mo: 0.4 to 1.0%

Molybdenum exhibits the same effect as that of W and Re, but considerably deteriorates oxidation resistance property of the alloy in a high-temperature atmosphere. Thus, in the case where importance is attached to the oxidation resistance property, it is desirable to restrict the Mo amount to not more than 1%. In the case where the alloy requires the corrosion resistance property in some degree, the additive Mo amount is preferably 0.7 to 1%.

Nb: 0.3 to 1.0%

Niobium is one element belonging to a group including Ta, and has almost the same effect as that of Ta in improvement of high-temperature strength of the alloy. 0.3 to 4% Nb can be contained in the alloy. Since Nb is liable to form sulfides in an environment in which a fuel containing a much amount of sulfur is used, it has an effect of delaying sulfur invasion into the alloy whereby improving corrosion resistance property of the alloy. In the present invention, however, it has been revealed that, in the case where a certain or larger amount of Nb and B exists in the alloy, a low melting point phase consisting primarily of Nb and B is formed in an eutectic region whereby considerably lowering the initial melting temperature of the alloy. The low melting point phase is formed by segregation. Depending on a casting condition, the low melting point phase may be or may not be formed. However, in the case where the low melting point phase is formed, the solution heat treatment at a high temperature cannot be carried out, so that high-temperature strength of the alloy cannot be improved. Also, in the case where a result of the preliminary study about solution heat treatment on a specimen alloy cast under a condition, in which the low melting point phase is not formed, is applied to another specimen alloy which has the same composition as the above specimen but is cast under a condition in which the low melting point phase is formed, a low melting point region of the alloy melts partially whereby high-temperature strength of the alloy is considerably deteriorated. From the above, in the present invention, the additive Nb amount is preferably 0.3 to 1%, more preferably 0.6 to 1.0%.
Ti: not more than 0.4%
Like as Nb, titanium is liable to form sulfides, and has an effect of improving the alloy in corrosion resistance property in an environment in which a fuel containing a much amount of sulfur is used. However, since Ti lowers the melting point of an eutectic region like as Nb, in the present invention, the additive Ti amount is set to be not more than 0.4%. Ti should not be intentionally added except for the case of impurities. If an amount of Ti contained as an impurity is not more than 0.2%, the alloy is not affected in alloy properties. Therefore, preferably the Ti amount is not more than 0.2%.

Fe: 0.1 to 5.0%

Iron is an element with which Ni is easily replaceable, and has been believed to be an element which deteriorates creep strength of the Ni-based alloy. Also, since oxidation resistance property of Fe itself is poor, Fe contained in the Ni-based alloy deteriorates the alloy in oxidation resistance property. Thus, in conventional single crystal alloys, Fe has been regarded as an impurity, so that in general the Fe amount has been limited to be not more than 0.02%.
In the present invention, a new effect of Fe was first discovered. The present invention has overturned common sense, which reveals that several percent of additive iron does not deteriorate creep strength of the Ni-based alloy, and on the contrary improves oxidation resistance property of the alloy at a high temperature.
With regard to high-temperature strength of the alloy in the case where Ni is replaced with Fe, if an excessive amount of more than 5% Fe is added in the alloy, high-temperature strength of the alloy is deteriorated, so that the Fe amount is preferably limited to not more than 5%. On the other hand, Fe improves oxidation resistance property of the alloy when the Fe amount is not less than 0.1%. Thus, in the case where importance is attached to the oxidation resistance at a high temperature, not less than 0.1% Fe is preferably added in the alloy. In order to obtain both the effects in the present invention, preferably the Fe amount is 0.1 to 5%, more preferably 0.5 to 3%, and still more preferably 1 to 3%.

Si: 0.01 to 0.2%

Silicon is a replaceable element with Al, and enters into the γ′ phase of the Ni-based alloy. Si in the γ′ phase changes the lattice constant of the γ′ phase, and deteriorates creep strength. On the other hand, it has been known that Si improves oxidation resistance property of the alloy. In conventional single crystal alloys, since importance has been attached to the creep strength, Si has been regarded as an impurity element, so that in general the Si amount has been limited to not more than 0.01%.
The present invention is characterized by a combined effect by means of Fe and Si. The present inventors found a new effect that when Si is added in an alloy containing several percent of Fe, oxidation resistance property of the alloy is improved without deterioration of creep strength of the alloy. The effect of Si of improving the oxidation resistance property is obtained when the Si amount is not less than 0.01%. Thus, in the case where importance is attached to the oxidation resistance property at a high temperature, not less than 0.01% Si is preferably added in the alloy. In order to prevent the creep strength from deterioration, the upper limit of the Si amount is preferably 0.2%. In order to achieve both the effects in the present invention, the Si amount is preferably 0.01 to 0.2%, more preferably 0.05 to 0.15%.

ADVANTAGES OF THE INVENTION

The present invention relates to the Ni-based single crystal alloy which is free from solidification cracks during casting a large-sized blade of gas turbines, has grain boundary strength sufficient for ensuring the reliability during operation, and has both of excellent properties of high-temperature strength and oxidation resistance. With utilization of components made of the invention alloy in gas turbines, which components are exposed to a high temperature, advantageously it is possible to raise combustion temperature of the gas turbines, and improve a power generation efficiency of gas turbines for power generation.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship between creep rupture time and Fe amount in the present invention;

FIG. 2 is a diagram showing the relationship between weight change and Fe amount in an oxidation test in the present invention;

FIG. 3 is a diagram showing the relationship between creep rupture time and Si amount in the present invention; and

FIG. 4 is a diagram showing the relationship between weight change and Si amount in an oxidation test in the present invention.

EXAMPLE

On the basis of a single crystal alloy containing three grain boundary strengthening elements of C, B and Hf, a Ni-based single crystal alloy was achieved by adding Fe and Si into the base single crystal alloy. The thus achieved Ni-based single crystal alloy is free from solidification cracks during casting a large-sized blade of gas turbines, has grain boundary strength sufficient for ensuring the reliability during operation, and has excellent oxidation resistance to a high combustion gas temperature while having excellent high-temperature strength comparable to that of conventional single crystal alloys.
Table 1 shows chemical compositions of the present invention alloys, which were produced by vacuum induction melting with utilization of a base alloy disclosed in JP-A-9-272933. Particularly a master ingot was first produced. Next, a single crystal specimen, having a diameter of 15 mm and a length of 180 mm, was cast with use of the master ingot in a unidirectional solidification furnace. The casting temperature was set at 1540° C., and the solidification rate was set at 20 cm/h. After casting, the specimens were subjected to multi-stage solution heat treatment, according to which the heat treatment temperature was elevated from 1250° C. to 1280° C. incrementally by 10° C., wherein the specimens were held for 4 hours at each temperature stage from 1250° C. to 1280° C. After the multi-stage solution heat treatment, the specimens were cooled in air. After the solution heat treatment, the specimens were subjected to aging heat treatment was such that the specimens were held at 1080° C. for 4 hours followed by air cooling at first, and next they were held at 871° C. for 20 hours followed by air cooling. Thereafter, the specimens were machined, and subjected to a creep rupture test and an oxidation test.

TABLE 1

(wt %)

Specimen

No.

Ni

C

B

Co

Cr

Mo

W

Ta

Re

Al

Hf

Nb

Ti

Zr

Fe

Si

Base	Bal.	0.07	0.020	1.0	7.2	0.9	8.8	8.8	1.4	5.0	0.3	0.8	<0.1	<0.02	<0.02	<0.01
alloy
Y-10A	Bal.	0.07	0.017	1.0	7.0	0.8	8.8	8.8	1.4	5.1	0.3	0.8	<0.1	<0.02	0.5	<0.01
Y-10B	Bal.	0.07	0.018	0.9	7.1	0.8	8.9	8.8	1.4	5.1	0.3	0.8	<0.1	<0.02	1.0	<0.01
Y-10C	Bal.	0.07	0.020	1.0	7.2	0.8	8.8	8.9	1.5	5.2	0.2	0.7	<0.1	<0.02	2.0	<0.01
Y-10D	Bal.	0.07	0.017	1.0	7.2	0.8	8.7	8.9	1.4	5.2	0.2	0.8	<0.1	<0.02	3.0	<0.01
Y-10E	Bal.	0.06	0.016	1.0	7.2	0.8	8.8	8.8	1.4	5.1	0.2	0.7	<0.1	<0.02	5.0	<0.01
Y-20A	Bal.	0.06	0.020	1.1	7.2	0.9	8.9	8.9	1.4	5.1	0.2	0.7	<0.1	<0.02	<0.02	0.05
Y-20B	Bal.	0.06	0.020	1.0	7.1	0.7	8.8	8.9	1.3	5.2	0.2	0.7	<0.1	<0.02	<0.02	0.10
Y-20C	Bal.	0.07	0.017	1.0	7.2	0.8	8.9	8.8	1.5	5.2	0.3	0.7	<0.1	<0.02	<0.02	0.20
Y-20D	Bal.	0.06	0.020	1.0	7.2	0.8	8.7	8.9	1.4	5.2	0.2	0.8	<0.1	<0.02	<0.02	0.50
Y-30A	Bal.	0.07	0.018	1.0	7.1	0.9	8.9	8.9	1.4	5.1	0.2	0.8	<0.1	<0.02	1.0	0.05
Y-30B	Bal.	0.07	0.018	1.0	7.2	0.7	8.8	8.8	1.3	5.1	0.3	0.9	<0.1	<0.02	1.0	0.10
Y-30C	Bal.	0.07	0.019	0.9	7.1	0.8	8.7	8.9	1.5	5.2	0.2	0.7	<0.1	<0.02	2.0	0.10
Y-30D	Bal.	0.07	0.024	1.0	7.1	0.7	8.9	8.9	1.5	5.2	0.3	0.7	<0.1	<0.02	2.5	0.15

Table 2 shows the results of the creep rupture test and the oxidation test. The creep rupture test was carried out under stress of 14 kg/mm²at a temperature of 1040° C. The oxidation test was carried out by holding the specimens at 1150° C. for 100 hours repeatedly until a total time of 1,000 hours.

TABLE 2

			Creep test	Oxidation test
Specimen	Fe	Si	Rupture time (h)	Weight change (mg)
No.	(wt %)	(wt %)	(1040° C.-137 MPa)	(1100° C./1000 h)

Base	<0.02	<0.01	643.7	−52.6
alloy			—	−57.6
Y-10A	0.5	<0.01	643.4	−38.1
Y-10B	1.0	<0.01	513.4	−29.9
Y-10C	2.0	<0.01	641.9	−32.5
Y-10D	3.0	<0.01	599.5	−31.2
Y-10E	5.0	<0.01	370.3	−47.1
Y-20A	<0.02	0.05	601.4	−35.5
Y-20B	<0.02	0.10	411.3	−21.2
Y-20C	<0.02	0.20	333.5	−23.3
Y-20D	<0.02	0.50	206.3	−19.9
Y-30A	1.0	0.05	594.8	−22.4
Y-30B	1.0	0.10	520.4	−20.1
Y-30C	2.0	0.10	711.0	−18.4
Y-30D	2.5	0.15	612.7	−15.7

FIG. 1 shows the relationship between creep rupture time and the Fe amount. In the case where the Si amount is not more than 0.01%, the creep strength scarcely decreases when the Fe amount is not more than 3%, and decreases when the Fe amount is 5%.
On the other hand, for the alloy containing 0.05 to 0.15% Si, the creep strength becomes maximum when the Fe amount is about 2%. Seeing FIG. 2 which shows the relationship between the weight change and the Fe amount in the oxidation test, it can be understood that as the Fe amount increases, the weight change (a weight decrease due to oxidation) decreases, and the oxidation resistance is improved.
FIG. 3 shows the relationship between the creep rupture time and the Si amount. From FIG. 3, it can be seen that in the case where the Fe amount is not more than 0.02%, as the Si amount increases, the creep strength decreases.
Seeing FIG. 4 which shows the relationship between the weight change and the Si amount in the oxidation test, it can be understood that as the Si amount increases, the weight change (a weight decrease due to oxidation) decreases, and the oxidation resistance is improved.
Seeing FIGS. 3 and 4, it can be understood that in the case where the Fe amount is not more than 0.02%, both of the creep strength and oxidation resistance are compatible up to 0.1% Si, and for the alloy containing 1.0 to 2.5% Fe, both of the creep strength and the oxidation resistance are compatible up to about 0.2% Si.
The master ingots of the base alloy and Y-10C shown in Table 1 were produced by vacuum induction melting, and subsequently unidirectionally solidified flat plates, each having a size of 15 mm×100 mm×220 mm, were cast in a furnace for unidirectional solidification. These alloys were subjected to solution heat treatment and aging heat treatment under the same conditions as those of the first example, thereafter a creep rupture test was performed under the conditions of a temperature of 927° C. and stress of 32 kg/mm². Test results are shown in Table 3. As seen from the table, the rupture time of the base alloy was 34.8 hours. However, in the case where the base alloy was unidirectionally solidified, the rupture time decreased to about a half, being 14.7 hours. The reason for this that in the case of the unidirectionally solidified alloy, there exist crystal grain boundaries which strength is low. In contrast, the Y-10C alloy containing additive Fe in an amount of exhibited the rupture time of 32.1 hours, which was generally the same as the rupture time of the single crystal base alloy.

TABLE 3

	Rupture time	elongation	reduction of
Alloy	(h)	(%)	area (%)

Base alloy/SC	34.8	25.6	30.4
Base alloy/DS	14.7	19.9	18.0
Y-10C/DS	32.1	17.8	25.1

Thus, it was revealed that the invention alloy containing Fe has an effect of improving not only the oxidation resistance but also the grain boundary strength. From this, it can be appreciated that in the large-size rotor blade made of a single crystal, an allowable existence range of hetero-crystals is expanded.
The invention alloy is suitable for products being directionally solidified by the unidirectional solidification method. In particular, when producing rotor blades of gas turbines it is desirable to cast those with a solidification direction according with a direction of centrifugal force to be acted on the rotor blades. Although the above description has been made on the basis of using the invention alloy in the rotor blades of gas turbines, the invention alloy can be applied to stator blades of gas turbines and other type members used at a high temperature. In the case where the invention alloy is applied to the stator blades, it is preferably cast with a solidification direction according with a direction of maximum thermal stress occurring in the stator blades. The invention alloy can be used for not only ordinary columnar rotor blades and columnar stator blades but also rotor blades in which crystal grain boundaries occur partially in the rotor blade during casting as a single crystal. Such a rotor blade has conventionally been regarded as a defective product. However, in the case where the invention alloy is used, such a rotor blade can stand up to use. As a result, the casting yield of single crystal blades can be improved significantly. Also, the invention alloy can be applied to usual single crystal rotor blades. Although there might be the case where complete single crystal rotor blades and complete single crystal stator blades, being made of conventional single crystal alloys, can be cast with a high yield, according to the invention alloy, an inspection work for determining whether crystal grain boundaries exist can be simplified, so that the production cost can be saved. Further, although the presence or absence of crystal grain boundaries in the inner surface of rotor blade has conventionally been ensured by the sampling destructive inspection, according to the invention alloy, the strength can be ensured even if crystal grain boundaries exist, so that the reliability of the rotor blades can be improved significantly.
As will be apparent from the above, the invention Ni-based single crystal alloy can be used for members exposed to a high temperature, such as rotor blades and stator blades of high-temperature equipment including a gas turbine.
It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.

Claims

1. A Ni-based single crystal alloy consisting essentially of, by weight, from not less than 0.06% to not more than 0.09% carbon, from not less than 0.016% to not more than 0.035% B, from not less than 0.2% to not more than 0.4% Hf, from inclusive zero to not more than 0.02% Zr, from not less than 6.5% to not more than 8.5% Cr, from not less than 0.4% to not more than 1.0% Mo, from not less than 5.5% to not more than 9.5% W, from not less than 1.2% to not more than 3.1% Re, from not less than 8% to not more than 10% Ta, from not less than 0.3% to not more than 1.0% Nb, from inclusive zero to not more than 0.4% Ti, from not less than 4.7% to not more than 5.4% Al, from not less than 0.5% to not more than 5.0% Co, from not less than 0.1% to not more than 5% Fe, and the balance of Ni and unavoidable impurities.

2. A Ni-based single crystal alloy according to claim 1, which consists essentially of, by weight, from not less than 0.06% to not more than 0.08% carbon, from not less than 0.2% to not more than 0.3% Hf, from not less than 6.9% to not more than 7.3% Cr, from not less than 0.7% to not more than 1.0% Mo, from not less than 7.0% to not more than 9.0% W, from not less than 1.2% to not more than 1.6% Re, from not less than 8.5% to not more than 9.5% Ta, from not less than 0.6% to not more than 1.0% Nb, from not less than 4.9% to not more than 5.2% Al, from not less than 0.8% to not more than 1.2% Co, and the balance of Ni and unavoidable impurities.

3. A Ni-based single crystal alloy according to claim 1, which contains, by weight, from not less than 0.5% to not more than 3% Fe.

4. A Ni-based single crystal alloy according to claim 1, which consists essentially of, by weight, from not less than 0.06% to not more than 0.08% carbon, from not less than 0.2% to not more than 0.3% Hf, from not less than 6.9% to not more than 7.3% Cr, from not less than 0.7% to not more than 1.0% Mo, from not less than 7.0% to not more than 9.0% W, from not less than 1.2% to not more than 1.6% Re, from not less than 8.5% to not more than 9.5% Ta, from not less than 0.6% to not more than 1.0% Nb, from not less than 4.9% to not more than 5.2% Al, from not less than 0.8% to not more than 1.2% Co, from not less than 0.5% to not more than 3% Fe, and the balance of Ni and unavoidable impurities.

5. A Ni-based single crystal alloy according to claim 1, which contains, by weight, from not less than 1% to not more than 3% Fe.

6. A Ni-based single crystal alloy according to claim 1, which further contains, by weight, from 0.01% to not more than 0.2% Si.

7. A Ni-based single crystal alloy according to claim 1, which further contains, by weight, from 0.05% to not more than 0.15% Si.

8. A casting made of a Ni-based alloy having the alloy composition as defined in claim 1.