US20090308507A1

US20090308507A1 - Ni-BASED COMPOUND SUPERALLOY HAVING EXCELLENT OXIDATION RESISTANCE, METHOD FOR MANUFACTURING THE SAME, AND HEAT-RESISTANT STRUCTURAL MATERIAL

Info

Publication number: US20090308507A1
Application number: US12/442,038
Authority: US
Inventors: Kazuyoshi Chikugo; Takayuki Takasugi; Yasuyuki Kaneno
Original assignee: IHI Corp; Osaka Prefecture University
Current assignee: IHI Corp
Priority date: 2006-09-26
Filing date: 2007-09-26
Publication date: 2009-12-17
Also published as: JP5224246B2; WO2008041592A1; EP2078763A1; EP2078763A4; JPWO2008041592A1

Abstract

The present invention is characterized in including Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, and having a multi-phase microstructure including a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure.

Description

TECHNICAL FIELD

The present invention relates to a Ni-based compound superalloy having excellent oxidation resistance, which has a multi-phase microstructure including a primary L1₂phase and an eutectoid microstructure (L1₂phase+D0_xphase (including D0₂₂phase, D0₂₄phase, or D0_aphase)). The present invention further relates to a method for manufacturing the aforementioned Ni-based compound superalloy.
This application claims priority from Japanese Patent Application No. 2006-261569, filed on Sep. 26, 2006, the content of which is incorporated herein by reference.

BACKGROUND ART

Nowadays, most high-temperature structural materials for turbine components of jet engines or gas turbines are Ni-based superalloys. Because at least approximately 35 vol % or more of the constituent phases of Ni-based superalloy are metal phases (γ), there are limitations in melting point and high-temperature creep strength of Ni-based superalloys. As candidates for high-temperature structural materials that surpass the Ni-based superalloys, high-temperature structural materials including intermetallic compounds in which the yield stress shows positive temperature dependence can be raised. However, single-phase materials have drawbacks of poor ductility at room temperature and low creep strength at high temperature. As to multi-phase materials compared with single-phase materials, because any of Ni₃X type intermetallic compounds has a GCP (Geometrically Close Packed) crystal structure, there is a possibility that some of these intermetallic compounds may be combined with high coherency. Since there are a number of Ni₃X type intermetallic compounds that have superior properties, by forming Ni₃X type intermetallic compounds in the form of a multi-phase material, a new type of multi-phase intermetallic compounds, that is, multi intermetallics, having further excellent properties and a high freedom for microstructural control are expected to be produced.
It was previously reported that an attempt has been made to develop a multi-phase compound using a Ni₃Al(L1₂)-Ni₃Ti(D0₂₄)-Ni₃Nb(D0_a) system, and an alloy having superior properties could be developed (see Non-Patent Document 1). (Non-Patent Document 1)
K. Tomihisa, Y. Kaneno, T. Takasugi, Intermetallics, 10 (2002) 247

DISCLOSURE OF THE INVENTION

Problems To Be Resolved by the Invention

The aforementioned Ni-based superalloys are employed as structural materials for engines and the like where high-temperature heat resistance is required. In engines where this type of material is applied, the engine efficiency is influenced by the operating temperature and the engine weight. The density of the aforementioned Ni-based superalloy is 8.0 to 9.0 g/cm³, which is relatively heavy. Accordingly, there has been progress in the development of a Ni-based compound superalloy that has a slightly lighter specific gravity than that of the aforementioned Ni-based superalloy.
With this in mind, the present inventors carried out research and development with the goal of developing a superalloy having even more superior properties than these conventional Ni-based superalloys. As one aspect of these efforts, the present inventors carried out research and development of a Ni-based compound superalloy which includes Al in the amount of 5 to 13 at %, V in the amount of 9.5 to 17.5 at %, Ti in the amount of 0 to 3.5 at %, B in the amount of 1000 ppm (weight) or less, and Ni as the remainder, and has a dual multi-phase microstructure including a primary L1₂phase and an (L1₂phase+D0₂₂phase) eutectoid microstructure.
The density of this Ni-based compound superalloy is in the range of 7.5 to 8.5 g/cm³, and is lighter in weight than the previously mentioned Ni-based superalloy. This Ni-based compound superalloy also has roughly the same high-temperature strength at temperatures up to around 1 000° C. as the aforemented Ni-based superalloy.
However, the aforementioned Ni-based compound superalloy is problematic in that its oxidation resistance is inferior.
In order to solve the aforementioned problems, the present invention aims to provide a Ni-based compound superalloy that is lighter in weight than the Ni-based superalloy, has roughly the same high-temperature strength at temperatures up to around 1000° C. as the Ni-based superalloy, and, moreover, has superior resistance to oxidation.

Means to Resolve the Problems

The present invention employs the following design to achieve the above aims.

(1) One aspect of the Ni-based compound superalloy having excellent oxidation resistance according to the present invention includes: Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, and has a multi-phase microstructure including a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure.
(2) The Ni-based compound superalloy having excellent oxidation resistance according to the present invention may further include Nb: 3 at % or more to 9.5 at % or less, and the amount of V may be not less than the amount of Nb.
(3) Another aspect of the Ni-based compound superalloy having excellent oxidation resistance according to the present invention, has a multi-phase microstructure including a primary L1₂phase and an (L1₂phase +D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure, which has a composition within the limits which link point A (Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at %, Ni: 75 at %), point B (Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %, Ni: 75 at %), point C (Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %, Ni: 75 at %), point D (Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %, Ni: 75 at %), and point E (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %, Ni: 75 at %), in the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2.
(4) The Ni-based compound superalloy having excellent oxidation resistance according to the present invention may further include at least one or more of Co: 15 at % or less and Cr: 5 at % or less.
(5) The Ni-based compound superalloy having excellent oxidation resistance in any one of (1), (2) and (4) according to the present invention may further include B: 1000 ppm (weight) or less.
(6) The Ni-based compound superalloy having excellent oxidation resistance according to the present invention may have a dual multi-phase microstructure including the primary L1₂phase and the (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure.
(7) The heat-resistant structural material having excellent oxidation resistance according to the present invention includes the Ni-based compound superalloy according to any one of (1) to (6).
(8) One aspect of the method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to the present invention, includes: subjecting an alloy material containing Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, to a first heat treatment at a temperature at which a primary L1₂phase and an Al phase coexist; and thereafter cooling the alloy material to a temperature at which the primary L1₂phase and a D0₂₂phase and/or a D0₂₄phase and/or a D0_aphase coexist, or further subjecting the alloy material to a second heat treatment at this temperature, thereby converting the Al phase to an (L1₂phase+D0₂₂phase and/or D0₂₄phase and/or D0_aphase) eutectoid microstructure to form a multi-phase microstructure.
(9) In the method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to the present invention, the alloy material may further include Nb: 3 at % or more to 9.5 at % or less, and the amount of V may be not less than the amount of Nb.
(10) Another aspect of the method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to the present invention, includes: subjecting an alloy material having a composition within the limits which link point A (Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at %, Ni: 75 at %), point B (Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %, Ni: 75 at %), point C (Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %, Ni: 75 at %), point D (Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %, Ni: 75 at %), and point E (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %, Ni: 75 at %), in the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2, to a first heat treatment at a temperature at which a primary L1₂phase and an Al phase coexist; and thereafter cooling the alloy material to a temperature at which the primary L1₂phase and a D0₂₂phase and/or a D0₂₄phase and/or a D0_aphase coexist, or further subjecting the alloy material to a second heat treatment at this temperature, thereby converting the Al phase to an (L1₂phase+D0₂₂phase and/or D0₂₄phase and/or D0_aphase) eutectoid microstructure to form a multi-phase microstructure.
(11) In the method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to the present invention, the alloy material may further include at least one or more of Co: 15 at % or less, and Cr: 5 at % or less.
(12) In the method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to the present invention, the alloy material may further include B: 1000 ppm or less.
(13) In the method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to the present invention, the first heat treatment may be carried out at a temperature at which the alloy material is in a first state shown in FIG. 1.
(14) In the method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to the present invention, the second heat treatment may be carried out at 1173 K to 1273 K.

EFFECTS OF THE INVENTION

The present invention provides a Ni-based compound superalloy which includes: Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, and has a multi-phase microstructure including a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure. As a result, the Ni-based compound superalloy according to the present invention has a specific gravity that is slightly less than that of the conventional Ni-based superalloy, superior high-temperature strength at temperatures up to around 1000° C. that is on par with the Ni-based superalloy, and superior resistance to oxidation.
The manufacturing method according to the present invention enables the manufacturing of a Ni-based compound superalloy having a multi-phase microstructure including a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure, this Ni-based compound superalloy has a specific gravity that is slightly less than that of the conventional Ni-based superalloy, a superior high-temperature strength at temperatures up to around 1273 K (1000° C.) that is on par with a Ni-based superalloy, and superior resistance to oxidation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal phase diagram related to Al contents and temperatures in the case in which the Ti content is 2.5 at % for one specific example of an alloy having the composition system which serves as the base for the Ni-based compound superalloy according to the present invention.

FIG. 2 is a Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram at 1273 K which is formed from various specific examples of the Ni-based compound superalloys according to the present invention and alloys having the composition systems which serve as the base therefor.

FIG. 3 is a graph of the results of a compression test showing the relationship between temperature and yield stress for various test materials obtained from specific examples of the Ni-based compound superalloy according to the present invention.

FIG. 4 is a graph of the results of oxidation tests showing the relationship between the amount of weight increase and exposure time for various test materials obtained from specific examples of the Ni-based compound superalloy according to the present invention.

FIG. 5A are photos of microstructures of Test Materials No. 21. 22 and 23 produced in the Examples.

FIG. 5B is a photo (5000-fold magnification) of a metallographic structure of Test Material No. 21 produced in the Examples.

FIG. 6 is a photo (1000-fold magnification) of a metallographic structure of Test Material No. 28 produced in the Examples.

FIG. 7 is a photo of a metallographic structure of the same test material photographed after changing the field of view.

FIG. 8 is a photo of a metallographic structure in which a portion of the multi-phase microstructure of the same material is photographed at 2500-fold magnification.

FIG. 9 is a graph showing the results of the tests of oxidation resistance for the various test materials.

FIG. 10 is a graph of the results of oxidation tests showing the relationship between increase in mass and exposure time for Test Materials No. 41 to 48 obtained from specific examples of the Ni-based compound superalloy according to the present invention.

FIG. 11 is a graph of the results of oxidation tests showing the relationship between increase in mass and exposure time for Test Materials No. 51 to 58 obtained from specific examples of the Ni-based compound superalloy according to the present invention.

FIG. 12 is a graph of the results of oxidation tests showing the relationship between increase in mass and exposure time for Test Materials No. 63 to 67 obtained from specific examples of the Ni-based compound superalloy according to the present invention.

FIG. 13 is a graph showing the results of tensile tests for Test Materials No. 28, 41, and 65.

FIG. 14 is a photo of a microstructure of Test Material No. 41 which is photographed at 1000-fold magnification.

FIG. 15 is a photo of a microstructure of Test Material No. 41 which is photographed at 5000-fold magnification.

FIG. 16 is a photo of a microstructure of Test Material No. 47 which is photographed at 5000-fold magnification.

FIG. 17 is a photo of a microstructure of Test Material No. 48 which is photographed at 5000-fold magnification.

FIG. 18 is a photo of a microstructure of Test Material No. 52 which is photographed at 2500-fold magnification.

FIG. 19 is a photo of a microstructure of Test Material No. 57 which is photographed at 2500-fold magnification.

FIG. 20 is a photo of a microstructure of Test Material No. 65 which is photographed at 50-fold magnification.

FIG. 21 is a photo of a microstructure of Test Material No. 65 which is photographed at 1000-fold magnification.

FIG. 22 is a photo of a microstructure of Test Material No. 65 which is photographed at 5000-fold magnification.

FIG. 23 is a stress-strain diagram showing the results of tensile tests on the various tests materials obtained by adding various amounts of B to Test Material No. 65.

FIG. 24 is a photo of a microstructure of the test material obtained by subjecting a test material in which 25 ppm of B has been added to Test Material No. 65 to a homogenizing treatment at 1300° C. for 3 hours.

FIG. 25 is a photo of a microstructure of the test material obtained by subjecting a test material in which 25 ppm of B has been added to Test Material No. 65 to a homogenizing treatment at 1330° C. for 3 hours.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will now be explained using the accompanying figures. However, the present invention is not limited to the various embodiments explained below.
The Ni-based compound superalloy according to the present invention includes: Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, wherein the amount of V is not less than the amount of Nb, and the Ni-based compound superalloy has a multi-phase microstructure including a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure.
The Ni-based compound superalloy according to the present invention may include Co: 15 at % or less in addition to the above composition, and may include Cr: 5 at % or less in addition to the above composition, and also may include B: 1000 ppm (weight) or less in addition to the above composition. Further, in addition to the above composition, it is preferable that the Ni-based compound superalloy according to the present invention has a multi-phase microstructure including a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure, and it is most preferable that the Ni-based compound superalloy according to the present invention has a dual multi-phase microstructure composed of a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure.
The thus-described Ni-based compound superalloy can be manufactured by the method which includes: melting an alloy material having a composition that includes: Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, wherein the amount of V is not less than the amount of Nb; carrying out a solid solution treatment (homogenizing treatment); then carrying out a first heat treatment at a temperature at which the primary L1₂phase and an Al phase coexist; and then cooling the alloy material to a temperature at which the primary L1₂phase and a D0₂₂phase and/or a D0₂₄phase and/or a D0_aphase coexist, or further subjecting the alloy material to a second heat treatment at this temperature, thereby converting the Al phase to an (L1₂phase+D0₂₂phase and/or D0_aphase) eutectoid microstructure to form a multi-phase microstructure.
FIG. 1 is a longitudinal phase diagram of the alloy related to the composition system according to the present invention. In FIG. 1, the amount of Al (at %) is shown on the horizontal axis, and the absolute temperature (K) is shown on the vertical axis. In the phase diagram shown in FIG. 1, the amount of Ti is 2.5 at %, and the amount of V is (22.5−amount of Al) at %. FIG. 2 is a Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram at 1273 K made up from the results of various specific examples related to the composition system according to the present invention.
The phrase “carrying out a solid solution heat treatment (homogenizing heat treatment)” as used in the present embodiments means heating to and maintaining at the temperatures in the range indicated by Al in FIG. 1. In the case of Al: 5 to 10 at %, for example, this would be the temperatures between the symbols “570 ” and the symbols “Δ” in the region indicated by Al.
In the present embodiments, the alloy material may be first subjected to a solid solution heat treatment (homogenization heat treatment). The homogenization heat treatment is typically carried out at a higher temperature than that of a first heat treatment which is performed subsequently. The homogenization heat treatment is preferably carried out at a temperature in the range of 1523 to 1623 K. Here, the first heat treatment and the homogenization heat treatment may be carried out together.
In the present embodiments, the alloy is subjected to the homogenization heat treatment, and then is subjected to the first heat treatment. The first heat treatment is carried out at a temperature at which both of the primary L1₂phase and the Al phase coexist. The temperature at which the primary L1₂phase and the Al phase coexist is specifically the temperature at which the alloy is in the Al+L1₂state shown in FIG. 1, that is, the temperature between the symbols “Δ” and the symbols “∘” in the case of Al: 5 to 10 at % shown in FIG. 1.
In the present embodiments, the phrase “the first heat treatment is carried out at a temperature at which both of the primary L1₂phase and the Al phase coexist” means carrying out a heat treatment in the region described as Al+L1₂in FIG. 1. The L1₂phase is a Ni₃Al type intermetallic compound phase, and the Al phase is a fcc type Ni solid solution phase.
Due to these states, from the results of the Examples below, it is assumed that the Al phases exist in between the cuboidal or rectangular primary L1₂phases in the microstructure. This type of microstructure including the primary L1₂phases and the intervening phases can be referred to as “upper multi-phase microstructure”.
The time for carrying out this first heat treatment is not particularly restricted. However, it is desirable to carry out the first heat treatment over a time period sufficient for the entire alloy to become a microstructure including the primary L1₂phase and the Al phase. The time period for carrying out the first heat treatment is, for example, 5 to 20 hours.
The phrase “carrying out a second heat treatment in a region indicated by L1₂+D0₂₂on the alloy material which is already subjected to the first heat treatment” means carrying out a heat treatment, for example, at a temperature not more than temperatures indicated by the symbols “” in FIG. 1 in the case of Al: 5 to 10 at %. The temperatures at the “” symbols in FIG. 1 are 1281 K; however, these temperatures vary depending on the composition of the alloy. The primary L1₂phase is almost entirely unaffected by the second heat treatment. However, the Al phase decomposes into a L1₂phase and a D0₂₂phase and/or a D0₂₄phase and/or a D0_aphase. A multi-phase microstructure mainly including the L1₂phase and the D0₂₂phase and/or the D0₂₄phase and/or the D0_aphase which is provided by the decomposition of the Al phase is hereinafter referred to as “lower multi-phase microstructure”.
In the case in which the second heat treatment is carried out after the first heat treatment, cooling may be accomplished by natural cooling or forcible cooling such as water-quenching. The natural cooling may be carried out, for example, by taking out the alloy material from a heat-treatment furnace after the first heat treatment and then allowing the resulting alloy material to be put at room temperature, or by turning off a heater of the heat-treatment furnace after the first heat treatment and then allowing the resulting alloy material to be put in the heat-treatment furnace.
A temperature for the second heat treatment is, for example, about 1173 K to about 1281 K. A period for the second heat treatment is, for example, about 5 to 20 hours, for example. The Al phase may be decomposed into the L1₂phase and the D0₂₂phase by the cooling such as the simply water-quenching and the like without the second heat treatment. However, the decomposition can be more reliably achieved by the heat treatment at the relatively high temperature. After the second heat treatment, the resulting alloy material may be cooled to the room temperature by natural cooling or forcible cooling. Note that the word “to” expressing a range as used in the present specification includes the boundary values of the range unless otherwise described.
The reasons for limiting the various components of the Ni-based compound superalloy according to the present invention will now be explained below.
As is clear from the longitudinal phase diagram in FIG. 1, the phase diagram in FIG. 2, and the specific examples that follow below, the reasons for defining Al: more than 5 at % to 13 at % or less, and V: 3 at % or more to 9.5 at % or less, are that, within these ranges, the first heat treatment can be carried out at a temperature at which the primary phase L1₂and the Al phase coexist, and it is possible to cool to a temperature at which the L1₂phase and the D0₂₂phase and/or the D0₂₄phase and/or the D0_aphase coexist, or further to carry out the second heat treatment at this temperature, so that the multi-phase microstructure can be formed.
The amount of Nb may be in the range of 3 at % or more to 9.5 at % or less, and may be equal to, or less than the amount of V. To restate, the amount of V must be equal to or greater than the amount of Nb. This is because in the Ni-based compound superalloy of the present embodiments, a portion of V is substituted by Nb in order to improve the property of resistance to oxidation. Resistance to oxidation improves more as the amount of the V portion substituted with Nb increases. Note that the Ni-based compound superalloy of the present embodiments includes a smaller amount of V, includes Nb, and includes a larger amount of Al, as compared to the Ni-based compound superalloy which was researched by the present inventors and includes Al: 5 to 13 at %, V: 9.5 to 17.5 at %, Ti: 0 to 3.5 at %, and B: 1000 ppm (weight) or less, with the remainder being Ni, and has a dual multi-phase microstructure including a primary L1₂phase and an (L1₂+D0₂₂and/or D0₂₄phase and/or D0_aphase) eutectoid microstructure. These are the different features.
Co and Cr are elements that contribute to improving resistance to oxidation. Co is preferably added in the range of 0 at % or more to 15 at % or less, and Cr is preferably added in the range of 0 at % or more to 5 at % or less.
Co is an element which has complete solid solubility in Ni, so that Co is soluble in intermetallic compounds, Ni₃Al, Ni₃V, (Ni₃Ti), and the like. In order to maintain the characteristics of a Ni-based alloy, the added amount is set to be up to 15 at %.
Cr is effective of improving resistance to oxidation. However, because the solid solubility of Cr in Ni₃Al is low, there is a concern that unnecessary precipitates will be generated if Cr is added in a quantity of more than 5 at %. Accordingly, it is preferable to set the upper limit for addition of Cr to be 5 at %.
The bonding strength of V with oxygen is high, so that the surface of the alloy material readily oxidizes. Accordingly, by decreasing the amount of V, it is possible to improve resistance to oxidation. At the same time, V can be substituted with Nb which has the same valence number. Further, by increasing the amount of Al, it is possible to generate a fine oxidized film of alumina on the surface. By decreasing the amount of V, resistance to oxidation can be improved. However, if the amount of Nb exceeds the amount of V, it becomes difficult to obtain a multi-phase microstructure. Accordingly, it is necessary to increase the amount of V to be greater than the amount of Nb.
The amount of Ti is in the range of 0 at % or more to 3.5 at % or less, preferably in the range of 0.5 to 3.5 at % or less, more preferably in the range of 1 to 3.5 at %, and most preferably in the range of 2 to 3 at %. It is preferable that the Ni-based compound superalloy according to the present invention includes Ti; however, it is also acceptable not to include Ti.
The amount of Ni is preferably in the range of 73 to 77 at %, and more preferably in the range of 74 to 76 at %. This is because, in this range, the amount of Ni: the total amount of (Al, Ti, and V) approaches nearly 3:1, and therefore, a solid solution phase of Ni, Al, Ti, or V is essentially non-existent.
The amount of B is in the range of 0 ppm (weight) or more to 1000 ppm (weight) or less, preferably in the range of 1 to 1000 ppm (weight), more preferably in the range of 1 to 500 ppm (weight), and even more preferably in the range of 5 to 100 ppm (weight). It is preferable that the Ni-based compound superalloy according to the present invention includes B; however it is also acceptable that B is not included.
In addition to the various elements of the above composition, it is also acceptable to include Mo in the amount of 1 to 2 at %. Mo is an element that has the effect of improving high-temperature strength, and has complete solid solubility in V. The amount of Mo preferably satisfies V>Mo+Nb. Further, the method for strengthening the crystal grain boundary may be considered as an approach for improving ductility. For this purpose, trace quantities of elements such as C, Zr, and Hf may be added up to a maximum of 0.2 at %. It is also acceptable to include any one of elements C, Zr and Hf in a trace amount of 0.2 at % or less.
The Ni-based compound superalloy according to the present invention has a multi-phase microstructure which includes an upper multi-phase microstructure and a lower multi-phase microstructure as described above, and it is most preferable that this Ni-based compound superalloy includes a dual multi-phase microstructure including these multi-phase microstructures.
It will be demonstrated experimentally in the Examples which will follow below that the Ni-based compound superalloy according to the present invention has superior mechanical properties at high temperatures and superior resistance to oxidation. It is thought that the reason for these superior properties is because the Ni-based compound superalloy according to the present invention has the multi-phase microstructure that includes the upper multi-phase microstructure and the lower multi-phase microstructure and, the having of the aforementioned dual multi-phase microstructure of the upper multi-phase microstructure and the lower multi-phase microstructure, which is the more preferable feature, is thought to be a contributing factor to attain more superior characteristics.
Note that it is desirable that the multi-phase microstructure or the dual multi-phase microstructure forms the entire Ni-based compound superalloy according to the present invention; however, it is not necessary that the entire Ni-based compound superalloy has this microstructure. Rather, it is acceptable that at least a portion, or more preferably 50% or more, of the entire microstructure be composed of the multi-phase microstructure.
The crystal structures of the intermetallic compounds employed in the Ni-based compound superalloy according to the present invention are simple as compared to the other three constituent phases (D0₂₂phase, D0₂₄phase, and D0_aphase). As a result, it is thought that the Ni-based compound superalloy according to the present invention includes a primary phase L1₂in which dislocations are comparatively activated, and a certain degree of ductility occurs over an entire range of temperatures including a room temperature. Accordingly, this facilitates handling of the Ni-based compound superalloy.
The Ni-based compound superalloy according to the present invention has superior mechanical properties at high temperatures. Accordingly, it can be used as a heat resistant structural material. Further, among the component elements, a portion of V is substituted by Nb; thereby, improving the resistance to oxidation. Further, by adding Co and Cr in suitable quantities, resistance to oxidation is also increased.
In addition, in the case in which a composition is provided in which a portion of V is substituted by Nb, it is disadventageous to some extent from the perspective of reducing weight; however, there is a weight reduction on the order of about 0.5 g/cm³as compared to the typical Ni-based superalloy.
The above-described Ni-based compound superalloy can be effectively utilized in a temperature range that is slightly lower than 1523 K (1250° C.), for example, at high temperatures up to 1273 K to 1373 K (1000 to 1100° C.), and is suitable for low-pressure turbine blades of a turbo charger or an engine. In the case in which the high-temperature strength is high in this temperature range, the effect of achieving the same resistance to pressure at a lower weight can be realized. Thus, this is beneficial from the perspective of engine efficiency and fuel costs.
Examples of the alloy material employed to manufacture the Ni-based compound superalloy according to the present invention include a casting material, a forging material, a single crystal material, and the like. The casting material can be formed by melting (arc melting, high frequency melting, and the like) a pre-weighed raw metal, then pouring it into a casting mould, and permitting it to solidify.
The casting material is a polycrystal typically having crystal grains on the order of several hundred microns to several millimeters, and has a disadvantage of readily fracturing at boundaries between the crystal grains (crystal grain boundaries), and a disadvantage of having casting defects such as shrinkage cavities and the like. The forging material improves on these disadvantages. The forging material is formed by subjecting a casting material to a hot forging and a recrystallization annealing. These hot forging and recrystallization annealing are typically carried out at temperatures which are higher than the temperature of the first heat treatment.
The temperatures at which the hot forging and the recrystallization annealing are carried out may be the same or different. It is preferable to carry out the hot forging at around 1523 to 1623 K, and the recrystallization annealing at around 1423 to 1573 K. Prior to the first heat treatment, the alloy material may be subjected to a homogenization heat treatment. The homogenization heat treatment is typically carried out at a temperature which is higher than that of the first heat treatment. The homogenization heat treatment is preferably carried out in the range of around 1523 to 1623 K. The first heat treatment may be carried out together with the homogenization heat treatment. In the case of the forging material, the hot forging and the recrystallization annealing may be carried out together with the homogenization heat treatment. The time period for carrying out the homogenization heat treatment is not restricted; however, for example, it is on the order of 24 to 96 hours. In the case in which the alloy material is a polycrystal material (casting material, forging material, or the like), it is preferable to include B in the alloy material. The reason for this is because the crystal grain boundaries are strengthened as a result.
If a compression testing and a tensile testing are carried out to a Ni-based compound superalloy having a multi-phase microstructure which is formed by heat-treating a casting material, a forging material, or a single crystal material, it can be confirmed that the Ni-based compound superalloy has superior mechanical properties on any of these testing.

Examples

Various specific examples of the Ni-based compound superalloy according to the present invention will now be explained.
In the following examples, Ni-based compound superalloys having multi-phase microstructures were manufactured by carrying out heat treatments, and the mechanical properties thereof were investigated.
In the following examples, the heat treatment at 1373 K corresponds to the first heat treatment (primary precipitation heat treatment) at a temperature at which the primary L1₂phase and the Al phase coexist (first state), and the water-quenching carried out after performing the heat treatment at 1373 K corresponds to cooling to a temperature at which the L1₂phase and the D0₂₂phase coexist. The heat treatment at 1173 K or 1273 K carried out after performing the heat treatment at 1373 K corresponds to the second heat treatment (secondary precipitation heat treatment) at a temperature at which the L1₂phase and the D0₂₂phase coexist.

Method for Producing Casting Material

Prior to producing test materials employing the composition system according to the present invention, Ni, Al, Ti, and V raw metals (each having 99.9 wt % purity) in the proportions indicated in Nos. 1 to 20 in Table 1 were melted in an arc melting furnace for obtaining casting materials for prescribing the composition limits of alloys resembling the present invention. With regard to the atmosphere inside the arc melting furnace, the melting chamber was evacuated and then the atmosphere was replaced with an inert gas (argon gas). A non-consumable tungsten electrode was employed for the electrode, and a water-cooled copper hearth was employed for the casting mold. In the case of adding other elements in addition to the above, it is acceptable to use raw metals in which elements such as Co, Cr, Mo, B, C, Hf, and the like are added in accordance with the required alloy composition, or to add ingots of these elements separately during melting.
In the following explanation, the aforementioned casting materials will be referred to as “samples”.
For actually manufacturing the Ni-based compound superalloy according to the present invention, Ni, Al, Ti, and V raw metals were employed to obtain samples so as to produce Test Materials Nos. 1 to 20 having the various compositions shown in Table 1, in order to obtain a phase diagram of the basic composition system of the Ni-based compound superalloy according to the present invention.
From the longitudinal phase diagram in FIG. 1, it may be understood that a sample having a composition in which the amount of Al is in a range from more than 5 at % to 13 at % or less becomes to have a Ni-based superalloy microstructure which is Al+L1₂phase at 1373 K, and that cooling to a temperature not more than the eutectoid temperature (1281 K) results in the occurrence of a eutectoid reaction which is Al→L1₂+D0₂₂, D0₂₄, D0_a, and formation of a dual multi-phase microstructure including a primary L1₂phase and an (Li₂+D0₂₂, D0₂₄, D0_a) eutectoid microstructure.

TABLE 1

Test	Sample Composition		L1₂(Ni₃Al)	D0₂₄(Ni₃Ti)
Material	(at %)	Microstructure	(at %)	(at %)

No.	Ni	Al	Ti	V	at 1273 K	Ni	Al	Ti	V	Ni	Al

1	75	2.5	17.5	5	D0₂₄	—	—	—	—	72.9	2.3
2	75	2.5	12.5	10	rho + D0₂₂	—	—	—	—	—	—
3	75	2.5	7.5	15	rho + D0₂₂	—	—	—	—	—	—
4	75	5	17.5	2.5	D0₂₄	—	—	—	—	72.8	4.2
5	75	5	12.5	7.5	D0₂₄+ D0₂₂	—	—	—	—	73.6	4.8
6	75	5	7.5	12.5	D0₂₄+ D0₂₂+ rho	—	—	—	—	ND	ND
7	75	5	2.5	17.5	L1₂+ D0₂₄+ D0₂₂	ND	ND	ND	ND	ND	ND
8	75	7.5	12.5	5	L1₂+ D0₂₄	73.9	9.0	12.6	4.5	73.5	6.6
9	75	7.5	7.5	10	D0₂₄	—	—	—	—	74.3	7.2
10	75	7.5	2.5	15	L1₂+ D0₂₄+ D0₂₂	ND	ND	ND	ND	74.7	7.8
11	75	10	7.5	7.5	L1₂+ D0₂₄	74.0	10.6	7.3	8.0	74.0	7.0
12	75	10	2.5	12.5	L1₂+ D0₂₂	ND	ND	ND	ND	—	—
13	75	1.25	11.3	12.5	rho + D0₂₂	—	—	—	—	—	—
14	75	1.25	7.5	16.25	rho + D0₂₂	—	—	—	—	—	—
15	75	1.25	2.5	21.25	D0₂₂	—	—	—	—	—	—
16	75	2.5	15	7.5	D0₂₄+ rho	—	—	—	—	73.6	2.5
17	75	2.5	5	17.5	D0₂₂	—	—	—	—	—	—
18	75	7.5	15	2.5	L1₂+ D0₂₄	73.5	8.5	14.4	3.6	73.1	6.4
19	75	7.5	5	12.5	D0₂₄	—	—	—	—	73.6	7.2
20	75	10	5	10	L1₂+ D0₂₄	ND	ND	ND	ND	ND	ND

Test	D0₂₄(Ni₃Ti)	D0₂₂(Ni₃V)	Rho(Ni₃Ti_0.7V_0.3)
Material	(at %)	(at %)	(at %)

No.	Ti	V	Ni	Al	Ti	V	Ni	Al	Ti	V

1	19.5	5.3	—	—	—	—	—	—	—	—
2	—	—	73.1	1.0	7.8	18.1	73.8	2.4	13.8	10.1
3	—	—	70.6	2.8	8.7	18.0	72.3	3.0	13.5	11.2
4	19.5	3.4	—	—	—	—	—	—	—	—
5	13.9	7.7	72.6	2.2	6.0	19.1	—	—	—	—
6	ND	ND	ND	ND	ND	ND	74.8	3.7	10.8	10.8
7	ND	ND	ND	ND	ND	ND	—	—	—	—
8	13.8	6.1	—	—	—	—	—	—	—	—
9	6.8	11.8	—	—	—	—	—	—	—	—
10	3.5	14.0	ND	ND	ND	ND	—	—	—	—
11	8.0	11.0	—	—	—	—	—	—	—	—
12	—	—	ND	ND	ND	ND	—	—	—	—
13	—	—	73	0.7	8.5	17.8	73.2	1.2	14.1	11.6
14	—	—	73.6	1.1	6.2	19.1	73.7	1.6	13.5	11.2
15	—	—	74.1	0.7	2.2	23.1	—	—	—	—
16	17.2	6.8	—	—	—	—	73.1	2.7	15.6	8.6
17	—	—	73.9	1.8	4.3	19.9	—	—	—	—
18	16.8	3.8	—	—	—	—	—	—	—	—
19	5.7	13.4	—	—	—	—	—	—	—	—
20	ND	ND	—	—	—	—	—	—	—	—

Note that “rho” represents rhombohedral.

From Table 1 and FIG. 1, it may be understood that phases other than the L1₂, the D0₂₂, the D0₂₄, the D0_aand the rhombohedral phases were not present in Test Materials Nos. 1 to 20. The amount of Ni of each phase was maintained at around 75%. Further, each phase was in an equilibrium state as a single-phase or a multi-phase. Five regions where two phases were present together, and two regions where three phases were present together were observed. The L1₂-D0₂₂-D0₂₄phase coexisting microstructure, which is present in a region of low Ti content, is of particular interest as a microstructure in which the constituent phases positioned at the three vertices of the phase diagram are directly equilibrated.
Next, the Ni₃Al—Ni3Ti—Ni₃V pseudo-ternary phase diagram at 1273 K was determined in accordance with the phase diagram shown in FIG. 1.
Test Materials Nos. 1 to 20 were vacuum-sealed in quartz tubes, and each was subjected to a heat treatment at 1273 K for 7 days, and then was subjected to a water-quenching. Next, in order to form the phase diagram at 1273 K, an observation of microstructure and an analysis of each constituent phase were performed for each of Test Materials Nos. 1 to 20. The observation of microstructure was carried out using OM (Optical Microscope), SEM, and TEM. The analysis of the various constituent phases was carried out using SEM-EPMA (Scanning Electron Microscope-Electron Probe MicroAnalyzer). The results of this observation and analysis are shown in Table 1. The Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram at 1273 K obtained from this observation and an analysis is shown in FIG. 2.
The composition range surrounded by points A, B, C, D, and E shown in FIG. 2 is the region in which the multi-phase microstructure or the dual multi-phase microstructure is obtained with certainty.
The present invention is realized by reducing the amount of V and substituting a portion of V with Nb within the above composition range. As a result, by providing a composition within the range surrounded by lines that connect point A (Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at %, Ni: 75 at %), point B (Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %, Ni: 75 at %), point C (Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %, Ni: 75 at %), point D (Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %, Ni: 75 at %), and point E (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %, Ni: 75 at %), in the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2, it is possible to obtain the targeted Ni-based compound superalloy which has a multi-phase microstructure or a dual multi-phase microstructure with certainty.
Test materials having the compositions shown in Table 2 below were prepared, and then the properties thereof were evaluated in order to investigate the composition and the microstructure of the Ni-based compound superalloy having the composition system according to the present invention, based on the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2.

TABLE 2

at %	Ni	Co	Cr	Al	Ti	V	Nb

#

21	Addition of Nb	75			12.5	2.5	7	3
#22	(V is substituted)	75			12.5	2.5	5	5
#23		75			12.5	2.5		10
#24	Addition of Cr	73.5		3	12.5	2.5	8.5
#25	(V is substituted)	72.5		5	12.5	2.5	7.5
#26	Combined	70	5		12.5	2.5	7	3
	addition of
	Nb, Co
#27	Combined	68.5	5	3	12.5	2.5	8.5
	addition of
	Cr, Co
#28	Combined	68.5	5	3	12.5	2.5	5.5	3
	addition of
	Nb, Cr, Co

Each sample having the compositions shown in Table 2 was melted and subjected to a heat treatment at 1573 K (1300° C.) for 10 hours in a vacuum furnace. This treatment corresponds to a homogenizing treatment. Next, argon gas was introduced into the furnace by means of a gas fan cooling, and stirring and cooling was performed. Next, gas fan cooling was carried out at 1373 K (1100° C.) for 10 hours (first heat treatment), and then gas fan cooling was carried out at 1273 K (1000° C.) for 10 hours (second heat treatment). Each test material was thus obtained and supplied for the following compression tests.

[Compression Test]

Test Materials Nos. 21, 22 and 28 shown in Table 2 were employed. The compression test was performed using square test pieces having dimensions of 2×2×5 mm³under conditions where the temperature is in a range of the room temperature to 1273 K, the atmosphere is vacuum, and the strain rate is 3.3×10⁻⁴s⁻¹. These results are shown in FIG. 3. FIG. 3 shows the 0.2% yield stresses (MPa) measured at the various temperatures of 298 K, 673 K, 773 K, 873 K 973 K, 1073 K, 1173 K, and 1273 K.
From the results of the compression tests shown in FIG. 3, it is clear that it is possible to obtain a value of 300 MPa for the 0.2% yield stress even at 1273 K (1000° C.), and that it is possible to obtain a yield stress value that exceeds 600 MPa in the temperature range of 300 K to 1073 K. Accordingly, as for the test material according to the present invention, superior high-temperature strength could be attained.

[Oxidation Test]

FIG. 4 shows the results of measurements of the amount of weight increase, including peeling, after Test Materials Nos. 21 to 28 (dimensions: 10×10×10 mm) were subjected to exposure for a specific time period at 1000° C. in air.
Also, in FIG. 4, the results of Test Material No. 10 (Al: 7.5%) in Table 1; Test Material CMSX-4 (trade name, manufactured by Cannon-Muskegon Corp. (United States)) (Ti: 1.0 wt %, Co: 9.0, Cr: 6.5, Mo: 0.6, Al: 5.6, Ta: 6.5, Hf: 0.10, rare earth (Re) 3.0, with the remainder being Ni); a test material containing Al: 14% (Al: 14%, Ti: 2.5%, V: 8.5%, Ni: 75%); and a test material containing Co: 5% (Co: 5%, Al: 7.5%, Ti: 2.5%, V: 15%, Ni: 75%) are shown for comparison.
In FIG. 4, there are six different time periods for exposure noted on the plot in order from the left: 24 hours, 50 hours, 100 hours, 200 hours, 400 hours, and 500 hours.
From the results shown in FIG. 4, it is clear that an increase in weight was suppressed for all of Test Materials Nos. 21 to 28 as compared to the test material containing Al: 14% and the test material containing Co: 5%. Note that Test Material CMSX-4 is a well-known Ni-based superalloy. However, the oxidation resistance properties of Test Materials Nos. 22, 23, and 28 were clearly superior to this superalloy. Moreover, the oxidation resistance of Test Material No. 21 was superior to that of the Test Material CMSX-4 in the case of time periods being 400 hours or less. Further, the oxidation resistances of Test Materials Nos. 24 and 25 were superior to that of the Test Material CMSX-4 in the case of time periods up to 200 hours.
Further, it was clear that all of the test materials had superior oxidation resistance as compared to a test material of the Ni₃Al—Ni₃Ti—Ni₃V system alloy (test material containing Al: 7.5% in FIG. 4) researched by the present inventors.

[Metallographic Structure]

FIG. 5 shows a photo of a metallographic structure of Test Material No. 21 (see FIG. 5(A)), a partially enlarged view (5000-fold magnification) of the photo of the metallographic structure of the same test material (see FIG. 5(B)), a photo of a metallographic structure of Test Material No. 22 (see FIG. 5(A)), and a photo of a metallographic structure of Test Material No. 23 (see FIG. 5(A)). The magnification of the photos of the various test materials shown in FIG. 5(A) is 100-fold, and a 100 μm white line is recorded in each photo for showing the magnification scale.
In the photo of Test Material No. 21, the contrast was poor so that it was difficult to discriminate; however, it was possible to confirm the presence of the Ni₃Al (L1₂) phase in almost the entirely of the test material. From the partially enlarged view (5000 times) of the photo of the metallographic structure of this test material, it was clear that a dual multi-phase microstructure including a primary L1₂phase and an (L1₂+D0₂₂) eutectoid microstructure was formed.
In the photos of Test Materials Nos. 22 and 23, the Ni₃Al (L1₂) phase is clearly confirmed; however, it is clear that the amount of the Ni₃Al(L1₂) phase is reduced. When the amount of the Ni₃Al (L1₂) crystal grains decreases as in the photo, formation of the multi-phase microstructure tends to become difficult. (Test Material No. 21 includes V: 7 at %, Nb 3 at % as shown in Table 2; Test Material No. 22 includes V, Nb: 5 at %; and Test Material No. 23 includes V: 0 at %, Nb: 10 at %.)
Among these metallographic structures, those that include a multi-phase microstructure or include a dual multi-phase microstructure do not readily undergo large changes in microstructure even at high temperatures. Due to this stability, a large high-temperature strength is attained. Further, it is important to form a microstructure in which these multi-phase microstructures are formed as finely and as coherently as possible for the purpose of enabling a microstructure which has superior mechanical properties at even higher temperatures.
FIGS. 6 and 7 show photos of a metallographic structure of Test Material No. 28 (1000-fold magnification). FIG. 8 shows a partially enlarged view (2500-fold magnification) of the photo of the metallographic structure of the same test material.
The fine granular portion in the photo of the metallographic structure shown in FIG. 6 is a L1₂-D0₂₄-D0_amicrostructure and occupies the majority of the microstructure in the photo. When this fine granular portion is enlarged at 2500-fold magnification, it could be confirmed that this portion becomes a microstructure in which numerous irregular Ni₃Al (L1₂) crystal grains are spread out as shown in FIG. 8. Note that it is clear that in the microstructure in which the numerous Ni₃Al (L1₂) crystal grains are spread out, L1₂-D0₂₄-D0_aphases exist at the boundary regions between the Ni₃Al (L1₂) crystal grains in the same way as the test material shown in FIG. 5.
From the above photos of microstructures, it is clear that test materials to which the combined addition of Cr and Co as well as the combined addition of V and Nb is employed, such as Test Material No. 28, also have a multi-phase microstructure.
Note that while a Ni₃Ti phase is observed in the lower left side of the photos of the metallographic structures in FIGS. 6 and 7, it is desirable that this type of coarse plate-like Ni₃Ti phase is not present.

[Measurement of Specific Gravity]

The specific gravity of Test Material No. 21 was 7.90. The specific gravity of Test Material No. 22 was 7.95. The specific gravity of Test Material No. 23 was 8.07. The specific gravity of Test Material No. 24 was 7.90. The specific gravity of Test Material No. 25 was 7.87. The specific gravity of Test Material No. 26 was 7.88. The specific gravity of Test Material No. 27 was 7.8. The specific gravity of Test Material No. 28 was 7.86. From these, it is clear that it is possible to achieve a reduction in weight as compared to the typical Ni-based superalloys such as MarM247 (registered trademark): 8.54 g/cm³and CMSX-4 (registered trademark): 8.70 g/cm³.
Next, based on the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2, test materials having the compositions shown in Table 3 below were produced and the properties of those test materials were evaluated in order to investigate the effects of the addition of Al, the effects of the addition of Nb, the effects of the addition of Cr, and the effects of the addition of Co, in a Ni-based compound superalloy having the composition system according to the present invention.

TABLE 3

at %	Ni	Co	Cr	Al	Ti	V	Nb	B	Ta

Comparative	74.95			7.5	2.5	15		0.05
material
Al
12%	75			12	2.5	10.5
Al 13%	75			13	2.5	9.5
Al 14%	75			14	2.5	8.5
Cr 5%	70		5	7.5	2.5	15
Co 5%	70	5		7.5	2.5	15
Nb 3%	72			7.5	2.5	15	3
Nb 1%	74			7.5	2.5	15	1

Test materials having the compositions shown in Table 3 were produced in the same manner as the test materials shown in Table 2, and the oxidation resistance test was performed for each test material at a testing temperature of 1000° C. These results are shown in FIG. 9.
From the results shown in FIG. 9, it is clear that, with regard to the Ni-based compound superalloy having the composition system according to the present invention, a large improvement in the property of oxidation resistance cannot be achieved by just providing a composition system in which Co or Cr is simply added. Moreover, it is clear that the same holds true for Al. Thus, selection of specific ranges as explained above is extremely important in the present invention.
Various samples having the compositions shown in Table 4 were melted and subjected to a heat treatment at 1563 K (1290° C.) for 10 hours in a vacuum furnace. This treatment corresponds to a homogenizing treatment. Next, argon gas was introduced into the furnace by means of a gas fan cooling, and stirring and cooling was carried out. Next, a heat treatment was carried out at 1373K (1100° C.) for 10 hours, and then gas fan cooling was carried out (first heat treatment). Thereafter, a heat treatment was carried out at 1273 K (1000° C.) for 10 hours, and then gas fan cooling was carried out (second heat treatment). Each test material was thus obtained and supplied for the following tests.

TABLE 4

Test

Material

Sample Composition (at %)

No.	Ni	Co	Cr	Al	Ti	V	Nb	Zr

41	68.5	5	3	12.5	1.5	6.25	3.25
42	68.5	5	3	12.5	0.5	7	3.5
43	68.5	5	3	10	1.5	8	4
44	68.5	5	3	7.5	1.5	9.5	5
45	68.5	5	3	10	1.5	6	6
47	67.5	5	5	10	1.5	6	5
48	63.5	10	3	10	1.5	8	4
51	68.5	5	3	12.5	2	5.75	3.25
52	68.5	5	3	12.5	2	5.25	3.75
53	68.5	5	3	12.5	1.5	6	3.5
54	68.5	5	3	12.5	1.5	5.5	4
55	69	5	3	12.5	1.5	5.75	3.25
56	69	5	3	12.5	1.5	5.25	3.75
57	68.5	5	3	12	1.5	6.25	3.75
58	68.5	5	3	11.5	1.5	6.25	4.25
63	68.5	5	3	12.5	1.5	5.75	3.75
64	69	5	3	12	1.5	5.75	3.75
65	69.5	5	3	11.5	1.5	5.75	3.75
67	69.5	5	3	11.5	1.5	5.75	3.75	1.5

For the test materials shown in Table 4, the specific gravity of Test Material No. 41 was 7.94, and the specific gravity of Test Material No. 65 was 8.01. In contrast, the specific gravity of Test Material No. 10 in Table 1 was 8.00. From these, it is clear that it is possible to achieve a reduction in weight for Test Materials Nos. 41 and 65 as compared to the above-described typical Ni-based superalloys such as MarM247 (registered trademark): specific gravity is 8.54 and CMSX-4 (registered trademark): specific gravity is 8.70.
FIG. 10 shows the results of oxidation tests for Test Materials Nos. 41 to 48 shown in Table 4, which were obtained by measuring the amount of weight increase including peeling after each test material (dimensions: 10×10×10 mm) was subjected to exposure at 1000° C. for a specific time period in air. In FIG. 10, the results for Test Material No. 10 (Al: 7.5%) shown in the previous Table 1 are also shown for comparison.
From the results of the oxidation tests shown in FIG. 10, all Test Materials Nos. 41 to 48 according to the present invention demonstrated excellent oxidation resistance as compared to Test Material No. 10. More specifically, Test Materials Nos. 28, 41, 46, 42, and 47 showed, in this order, superior oxidation resistance.
The same oxidation tests were conducted for Test Materials Nos. 51 to 58 and Test Materials Nos. 63 to 67 shown in Table 4, and the results of Test Materials Nos. 51 to 58 are shown in FIG. 11, and the results of Test Materials Nos. 63 to 67 are shown in FIG. 12. In FIGS. 11 and 12, the results of Test Materials Nos. 10, 28, and 41 are also included.
From the results of the oxidation tests shown in FIGS. 11 and 12, all Test Materials Nos. 51 to 58 and Nos. 63 to 67 according to the present invention demonstrated better oxidation resistance than that of Test Material No. 10. Note that Test Material No. 67 is a test material that includes Zr in the amount of 1.5 at %, in addition to prescribed amounts of Co, Cr, Al, Ti, V, and Nb. Test Material No. 67 demonstrates oxidation resistance properties which are superior to those of Test Material No. 10. Accordingly, it became clear that a Ni-based compound superalloy having superior oxidation resistance can be obtained in the case of a composition system in which Zr is added to the composition according to the present invention.
Next, the results of tensile tests carried out on Test Materials Nos. 28, 41, and 65 shown in Tables 2 and 4 are shown in FIG. 13. The test materials used in these tensile tests are test materials in which boron (B) was added in the amount of 100 ppm for substituting Ni. From these tests, it may be understood that, while the tensile strength of Test Materials Nos. 28, 41 and 65 was slightly less than that of the Test Material No. 10 in the temperature range from the room temperature to 700° C., the rates of reduction in tensile strength for Test Materials Nos. 28, 41 and 65 were less than that of Test Material No. 10 in the temperature range from more than 700° C. to 1000° C. Further, Test Materials Nos. 28, 41 and 65 demonstrated a higher strength than that of Test Material No. 10 in the temperature range from 800 to 1000° C. Accordingly, it is clear that the Ni-based compound superalloy according to the present invention is suitable as a structural material required to have high-temperature heat resistance, such as for an engine or the like where high-temperature strength is particularly demanded.
From among the test materials shown in Table 4, photos of metallographic structures of Test Materials Nos. 41, 47, 48, 52, 57 and 65 are shown in FIGS. 14 to 22.
FIG. 14 shows a photo of a metallographic structure in which the surface of Test Material No. 41 is enlarged at 1000-fold magnification. FIG. 15 shows a photo of a metallographic structure in which the surface of the same test material is enlarged at 5000-fold magnification. As in the case of the photos of the metallographic structures of the test materials shown in FIGS. 6 and 8, the fine granular portions in the photos of the metallographic structures are the L1₂-D0₂4-D0_amicrostructures and occupy the majority (entirety) of the microstructures in the photos. When this fine granular portion is enlarged at 5000-fold magnification, it could be confirmed that that this portion becomes a microstructure in which numerous irregular Ni₃Al(L1₂) crystal grains are spread out as shown in FIG. 15. Note that it is clear that in the microstructure in which the numerous Ni₃Al (L1₂) crystal grains are spread out, L1₂-D0₂₄-D0_aphases exist at the boundary regions between the Ni₃Al(L1₂) crystal grains in the same way as the previous test material. Note that the magnification scale indicated by the 11 white points shown in FIG. 14 is 30 μm, and the magnification scale indicated by the 11 white points shown in FIG. 15 is 6 μm.
FIG. 16 shows a photo of a metallographic structure in which the surface of Test Material No. 47 is enlarged at 5000-fold magnification. FIG. 17 shows a photo of a metallographic structure in which the surface of Test Material No. 48 is enlarged at 5000-fold magnification. FIG. 18 shows a photo of a metallographic structure in which the surface of the Test Material No. 52 is enlarged at 2500-fold magnification. FIG. 19 shows a photo of a metallographic structure in which the surface of Test Material No. 57 is enlarged at 2500-fold magnification. FIG. 20 shows a photo of a metallographic structure in which the surface of Test Material No. 65 is enlarged at 50-fold magnification. FIG. 21 shows a photo of a metallographic structure in which the surface of Test Material No. 65 is enlarged at 100-fold magnification. FIG. 22 shows a photo of a metallographic structure in which the surface of Test Material No. 65 is enlarged at 5000-fold magnification. Note that the magnification scales indicated by the white lines shown in FIGS. 16 and 17 are 5 μm; the magnification scales indicated by the white lines shown in FIGS. 18 and 19 are 10 μm; the magnification scale indicated by the white line shown in FIG. 20 is 500 μm; the magnification scale indicated by the white line shown in FIG. 21 is 10 μm; and the magnification scale indicated by the white line shown in FIG. 22 is 5 μm.
From these photos of the metallographic structures, it is clear that the fine granular portion in the photo of the metallographic structure is the L1₂-D0₂₄-D0_amicrostructure and occupies the majority (entirety) of the microstructure in the photo for each of Test Materials Nos. 47, 48, 52, 57 and 65.
FIG. 23 shows the results of tensile testing at room temperature for test materials that were prepared by adding various amounts of boron to Test Material No. 65 for substituting Ni. For the test material shown in FIG. 23, there was absolutely no plastic elongation, and the tensile strength was low in the case when no (0 ppm) boron was added. In the case in which the added amount of boron was increased to 25 ppm, the elongation increased, plastic elongation was demonstrated, and the tensile strength increased. However, in the case in which boron was added in excess of the upper limit of 1000 ppm, any plastic elongation was not demonstrated again, and the fracture strength was low. From these results, it is desirable that the amount of boron added to the superalloy according to the present invention is 0 ppm or more to 1000 ppm or less, or less than 1000 ppm from the perspective of elongation.
FIG. 24 shows the photo of a metallographic structure (3000-fold magnification, white line magnification scale: 5 μm) for a test material which was obtained by adding 25 ppm of boron to Test Material No. 65 and was subjected to a homogenizing treatment at 1300° C. for 3 hours. FIG. 25 shows the photo of a metallographic structure (3000-fold magnification, white line magnification scale: 5 μm) for a test material which was obtained by adding 25 ppm of boron to Test Material No. 65 and was subjected to a homogenizing treatment at 1330° C. for 3 hours. These test materials were subjected to the homogenization treatment at 1300° C. or 1330° C. for 3 hours, and then were cooled. Thereafter, both of them were subjected to a same heat treatment which includes a process of heating including heating at 1100° C. for 10 hours and then cooling, and a process of heating including heating at 1000° C. for 10 hours and then cooling.
As is clear from comparing FIGS. 24 and 25, when the temperature of the homogenizing heat treatment performed on the test materials relating to Test Material No. 65 are increased, it is possible to make the microstructure finer. Further, it can be assumed that the effect of improving the tensile strength is attained by making the microstructure finer.

INDUSTRIAL APPLICABILITY

The Ni-based compound superalloy according to the present invention can be employed as a structural material where high-temperature heat resistance is required, such as for an engine. The Ni-based superalloy according to the present invention has a slightly lower specific gravity than those of conventional Ni-based superalloys, and has superior in oxidation resistance and excellent tensile strength at high temperatures. As a result, an improvement in engine efficiency can be attained in the engine in which the Ni-based compound superalloy according to the present invention is employed.

Claims

1. A Ni-based compound superalloy having excellent oxidation resistance, comprising: Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, and having a multi-phase microstructure comprising a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure.

2. The Ni-based compound superalloy according to claim 1, wherein the Ni-based compound superalloy further comprises Nb: 3 at % or more to 9.5 at % or less, and the amount of V is not less than the amount of Nb.

3. A Ni-based compound superalloy having excellent oxidation resistance, having a multi-phase microstructure comprising a primary L1₂phase and an (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure, which has a composition within the limits which link point A (Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at %, Ni: 75 at %), point B (Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %, Ni: 75 at %), point C (Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %, Ni: 75 at %), point D (Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %, Ni: 75 at %), and point E (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %, Ni: 75 at %), in the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2.

4. The Ni-based compound superalloy having excellent oxidation resistance according to claim 2, wherein the Ni-based compound superalloy further comprises at least one or more of Co: 15 at % or less and Cr: 5 at % or less.

5. The Ni-based compound superalloy having excellent oxidation resistance according to claim 4, wherein the Ni-based compound superalloy further comprises B: 1000 ppm (weight) or less.

6. The Ni-based compound superalloy according to claim 1, wherein the Ni-based compound superalloy has a dual multi-phase microstructure including the primary L1₂phase and the (L1₂phase+D0₂₂phase and/or D0₂₄and/or D0_aphase) eutectoid microstructure.

7. A heat-resistant structural material having excellent oxidation resistance, comprising the Ni-based compound superalloy according to claim 1.

8. A method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance,

the method comprising: subjecting an alloy material containing Al: more than 5 at % to 13 at % or less; V: 3 at % or more to 9.5 at % or less; and Ti: 0 at % or more to 3.5 at % or less, with the remainder being Ni and unavoidable impurities, to a first heat treatment at a temperature at which a primary L1₂phase and an Al phase coexist; and

thereafter cooling the alloy material to a temperature at which the primary L1₂phase and a D0₂₂phase and/or a D0₂₄phase and/or a D0_aphase coexist, or further subjecting the alloy material to a second heat treatment at this temperature, thereby converting the Al phase to an (L1₂phase+D0₂₂phase and/or D0₂₄phase and/or D0_aphase) eutectoid microstructure to form a multi-phase microstructure.

9. The method for manufacturing a Ni-based compound superalloy according to claim 8, wherein the alloy material further comprises Nb: 3 at % or more to 9.5 at % or less, and the amount of V is not less than the amount of Nb.

10. A method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance,

the method comprising: subjecting an alloy material having a composition within the limits which link point A (Al: 14.0 at %, Ti: 0 at %, (V+Nb): 11.0 at %, Ni: 75 at %), point B (Al: 12.5 at %, Ti: 2.8 at %, (V+Nb): 9.8 at %, Ni: 75 at %), point C (Al: 8.0 at %, Ti: 3.8 at %, (V+Nb): 13.3 at %, Ni: 75 at %), point D (Al: 2.3 at %, Ti: 2.0 at %, (V+Nb): 20.8 at %, Ni: 75 at %), and point E (Al: 2.0 at %, Ti: 0 at %, (V+Nb): 23.0 at %, Ni: 75 at %), in the Ni₃Al—Ni₃Ti—Ni₃V pseudo-ternary phase diagram shown in FIG. 2, to a first heat treatment at a temperature at which a primary L1₂phase and an Al phase coexist; and

11. The method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to claim 8, wherein the alloy material further comprises at least one or more of Co: 15 at % or less, and Cr: 5 at % or less.

12. The method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to claim 8, wherein the alloy material further comprises B: 1000 ppm or less.

13. The method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to claim 8, wherein the first heat treatment is carried out at a temperature at which the alloy material is in a first state shown in FIG. 1.

14. The method for manufacturing a Ni-based compound superalloy having excellent oxidation resistance according to claim 8, wherein the second heat treatment is carried out at 1173K to 1273K.