US20230416877A1

US20230416877A1 - Super-heat-resistant alloy

Info

Publication number: US20230416877A1
Application number: US18/035,898
Authority: US
Inventors: Jeong Hyeon DO; In Soo Kim; Joong Eun JUNG; Baig Gyu Choi; Woo Young SEOK; Yu Hwa Lee
Original assignee: Korea Institute of Materials Science KIMS
Current assignee: Korea Institute of Materials Science KIMS
Priority date: 2020-11-09
Filing date: 2021-11-09
Publication date: 2023-12-28
Also published as: WO2022098206A1

Abstract

Provided is a super-heat-resistant alloy consisting of aluminum (Al): 4.0 wt % to 5.2 wt %, cobalt (Co): 1.0 wt % to 10.0 wt %, chromium (Cr): 5.0 wt % to 8.0 wt %, molybdenum (Mo): 0.5 wt % to 2.0 wt %, tantalum (Ta): 7.0 wt % to 10.0 wt %, titanium (Ti): 0 wt %<Ti≤1.5 wt %, tungsten (W): 7.0 wt % to 10.5 wt %, and nickel (Ni): balance, and not containing rhenium (Re).

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a National Stage of International Application No. PCT/KR2021/016234, filed on Nov. 9, 2021, which designates the United States and was published in Korea, and which is based upon and claims priority to Korean Patent Application No. 10-2020-0148849 filed on Nov. 9, 2020 and Korean Patent Application No. 10-2021-0152524 filed on Nov. 8, 2021, in the Korea Patent Office. All of the aforementioned applications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to an alloy, and more particularly, to a super-heat-resistant alloy.

BACKGROUND ART

Single-crystal super-heat-resistant alloys are mostly used for high-temperature machinery such as gas turbine blades/vanes. Turbine inlet temperatures are increased to improve the efficiency of gas turbines and thus the single-crystal super-heat-resistant alloys are required to endure higher temperatures. To endure high temperature, a high-priced special element such as rhenium (Re) or ruthenium (Ru) has been added but the price has increased rapidly.

RELATED ART DOCUMENT

Patent Document: Korean Patent Application No. 10-2005-0131561

DETAILED DESCRIPTION OF THE INVENTION

Technical Problem

The present invention provides a super-heat-resistant alloy excluding a high-priced element such as rhenium (Re) and having excellent properties.
However, the above description is merely an example, and the scope of the present invention is not limited thereto.

Technical Solution

A super-heat-resistant alloy according to an embodiment of the present invention is provided.
The super-heat-resistant alloy may consist of aluminum (Al): 4.0 wt % to 5.2 wt %, cobalt (Co): 1.0 wt % to 10.0 wt %, chromium (Cr): 5.0 wt % to 8.0 wt %, molybdenum (Mo): 0.5 wt % to 2.0 wt %, tantalum (Ta): 7.0 wt % to 10.0 wt %, titanium (Ti): 0 wt %<Ti 1.5 wt %, tungsten (W): 7.0 wt % to 10.5 wt %, and nickel (Ni): balance, and may not contain rhenium (Re). Furthermore, a stable creep resistance time of the super-heat-resistant alloy according to Equation 1 below may be 150 hours or more.
Stable creep resistance time (hours)=t _max −t _min <Equation 1>
in a period of time satisfying
[ε_t(i)−ε_t(i−1) ]/[t(i)−t(i−1)]≤0.005(%/hours)
(where ε_t(i)) denotes a creep strain of the super-heat-resistant alloy at time t(i), ε_t(i−1)denotes a creep strain of the super-heat-resistant alloy at time t(i−1), t_maxdenotes a maximum time value in the period of time, and t_mindenotes is a minimum time value in the period of time.)
A creep resistance sustainability of the super-heat-resistant alloy according to Equation 2 below may be greater than or equal to 60%.
Creep resistance sustainability=[(Stable creep resistance time)/(Total Creep Life)×100] <Equation 2>
In the super-heat-resistant alloy, the stable creep resistance time and the total creep life may be measured under conditions of 1100° C. and 137 MPa.
Super-heat-resistant alloys according to various other embodiments of the present invention are provided.
The super-heat-resistant alloy may consist of aluminum (Al): 4.0 wt % to 5.2 wt %, cobalt (Co): 1.0 wt % to 10.0 wt %, chromium (Cr): 5.0 wt % to 8.0 wt %, molybdenum (Mo): 0.5 wt % to 2.0 wt %, tantalum (Ta): 7.0 wt % to 10.0 wt %, titanium (Ti): 0 wt %<Ti≤1.5 wt %, tungsten (W): 7.0 wt % to 10.5 wt %, hafnium (Hf): 0 wt %<Hf≤1.5 wt %, and nickel (Ni): balance, and may not contain rhenium (Re). As a modified example, the super-heat-resistant alloy may further contain 0.1 wt % or less of a rare-earth element such as yttrium (Y), lanthanum (La), or cerium (Ce).
The super-heat-resistant alloy may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Ta: 7.0 wt % to 10.0 wt %, W: 7.0 wt % to 10.5 wt %, and Ni: balance, and may not contain Re. As a modified example, the super-heat-resistant alloy may further contain 0.1 wt % or less of a rare-earth element such as Y, La, or Ce.
The super-heat-resistant alloy may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Ta: 7.0 wt % to 10.0 wt %, W: 7.0 wt % to 10.5 wt %, Hf: 0 wt %<Hf≤1.5 wt %, and Ni: balance, and may not contain Re. As a modified example, the super-heat-resistant alloy may further contain 0.1 wt % or less of a rare-earth element such as Y, La, or Ce.
The super-heat-resistant alloy may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Re: 0 wt %<Re≤1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, Ti: 0 wt %<Ti 1.5 wt %, W: 7.0 wt % to 10.5 wt %, and Ni: balance. As a modified example, the super-heat-resistant alloy may further contain 0.1 wt % or less of a rare-earth element such as Y, La, or Ce.
The super-heat-resistant alloy may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Re: 0 wt %<Re≤1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, W: 7.0 wt % to 10.5 wt %, and Ni: balance. As a modified example, the super-heat-resistant alloy may further contain 0.1 wt % or less of a rare-earth element such as Y, La, or Ce.
The super-heat-resistant alloy may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Re: 0 wt %<Re≤1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, Ti: 0 wt %<Ti≤1.5 wt %, W: 7.0 wt % to 10.5 wt %, Hf: 0 wt %<Hf≤1.5 wt %, and Ni: balance. As a modified example, the super-heat-resistant alloy may further contain 0.1 wt % or less of a rare-earth element such as Y, La, or Ce.
The super-heat-resistant alloy may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Re: 0 wt %<Re≤1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, W: 7.0 wt % to 10.5 wt %, Hf: 0 wt %<Hf≤1.5 wt %, and Ni: balance. As a modified example, the super-heat-resistant alloy may further contain 0.1 wt % or less of a rare-earth element such as Y, La, or Ce.
A lattice misfit δ of the super-heat-resistant alloy according to Equation 3 below may be higher than −0.35% and lower than −0.28%.
$\begin{matrix} Lattice Misfit δ = 2 \times \frac{α_{γ'} - α_{γ}}{α_{γ′} + α_{γ}} & < Equation 3 > \end{matrix}$
(where α_γ denotes a lattice parameter of a matrix γ, and
α_γ′ denotes a lattice parameter of a precipitate γ′.)
A γ lattice parameter distribution parameter of the super-heat-resistant alloy according to Equation 8 below may be greater than 0.12.
$\begin{matrix} γ Lattice Parameter Distribution Parameter = \sum_{i} (k_{i} \times x_{i} \times V_{i}^{γ}) > 0.1 2 & < Equation 8 > \end{matrix}$
(where k_idenotes a partitioning coefficient of each alloying element and indicates x_i ^γ/x_i ^γ′, x_idenotes an atomic fraction (at. %) of each alloying element, x_i ^γ denotes an atomic fraction (at. %) of each alloying element in a matrix γ phase, x_i ^γ′ denotes an atomic fraction (at. %) of each alloying element in a precipitate γ′ phase, and V_i ^γ denotes a Vegard coefficient of each alloying element in the matrix γ phase.)
The above-described super-heat-resistant alloys according to various embodiments may not contain iron (Fe).

Advantageous Effects

According to the afore-described embodiments of the present invention, a rhenium (Re)-free single-crystal super-heat-resistant alloy with excellent creep resistance sustainability and stress rupture resistance at ultra-high temperature may be implemented. However, the scope of the present invention is not limited to the above effect.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing creep strains of super-heat-resistant alloys according to test examples of the present invention, over time.

FIG. 2 is a graph showing creep strain rates of super-heat-resistant alloys according to test examples of the present invention, over time.

FIGS. 3 to 8 are scanning electron microscope (SEM) images showing microstructures of thermally aged super-heat-resistant alloys according to test examples of the present invention.

FIG. 9 is a schematic diagram for describing a method of measuring a shape parameter η by analyzing a γ′ shape of a thermally aged super-heat-resistant alloy.

MODE OF THE INVENTION

Hereinafter, the present invention will be described in detail by explaining embodiments of the invention with reference to the attached drawings. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to one of ordinary skill in the art. In the drawings, the sizes of at least some elements may be exaggerated or reduced for convenience of explanation. Like reference numerals in the drawings denote like elements.
The present invention relates to a nickel (Ni)-based single-crystal super-heat-resistant alloy, and more particularly, to a Ni-based single-crystal super-heat-resistant alloy with improved high-temperature creep properties.
In general, Ni-based super-heat-resistant alloys refer to high-strength heat-resistant alloys in which various alloying elements such as aluminum (Al), titanium (Ti), chromium (Cr), cobalt (Co), molybdenum (Mo), tungsten (W), tantalum (Ta), hafnium (Hf), rhenium (Re), and carbon (C) are included in a Ni matrix, and are widely used as materials for turbine blades and turbine disks which are core parts of aircraft engines and industrial gas turbines.
Herein, the super-heat-resistant alloys include all superalloys, oxide dispersion strengthened alloys, and crystal-controlled alloys, and may be divided into single-crystal super-heat-resistant alloys and polycrystalline super-heat-resistant alloys depending on crystal structures thereof. The single-crystal super-heat-resistant alloys satisfy extreme heat-resistant environments and are widely used for, for example, single-crystal turbine blades which are core parts of gas turbine engines. Meanwhile, due to a higher melting point than that of the polycrystalline super-heat-resistant alloys, the single-crystal super-heat-resistant alloys may be thermally homogenized to dissolve coarse precipitates produced during casting into a matrix. Once the precipitates are completely dissolved into the matrix due to the homogenization, the precipitates may be precipitated in an ideal shape through subsequent thermal aging and thus the single-crystal super-heat-resistant alloys achieve excellent mechanical properties at high temperature.
(Description of Lattice Misfit)
The microstructure and high-temperature properties of a single-crystal super-heat-resistant alloy depend on an interfacial coherence between a precipitate γ′ and a matrix γ, and a parameter that quantifies the coherence is a lattice misfit. The lattice misfit is expressed as in Equation 3 below.
$\begin{matrix} Lattice Misfit δ = 2 \times \frac{α_{γ'} - α_{γ}}{α_{γ′} + α_{γ}} & < Equation 3 > \end{matrix}$
α_γ denotes a lattice parameter of the matrix γ, and α_γ′ denotes a lattice parameter of the precipitate phase γ′. According to a general single-crystal super-heat-resistant alloy, the lattice misfit δ has a negative (−) value because the lattice parameter of the matrix γ is larger than the lattice parameter of the precipitate phase γ′, and the precipitate phase γ′ changes from a spherical shape to a cuboidal shape when an absolute lattice misfit |δ| is increased. However, because the precipitate phase exhibits a curved shape when the absolute lattice misfit |δ| is excessively increased, the absolute lattice misfit |δ| needs to have an appropriate value that is neither too large nor too small to form the precipitate in the most perfect cuboidal shape and achieve ideal high-temperature properties. Precise measurement of the lattice parameters in the scale of 10⁻¹³mm or less is required to measure the lattice misfit accurately, and thus the lattice misfit is mostly calculated using a thermodynamic calculation method.
(Description of Lattice Misfit Calculation Method)
The most widely known method for thermodynamically calculating lattice parameters and lattice misfits of single-crystal super-heat-resistant alloys is the Vegard's law calculation method and may be expressed as in Equations 4 and 5 below.
$\begin{matrix} α_{γ} = α_{γ}^{0} + \sum_{i} V_{i}^{γ} x_{i}^{γ} & < Equation 4 > \end{matrix}$ $\begin{matrix} α_{γ′} = α_{γ′}^{0} + \sum_{i} V_{i}^{γ′} x_{i}^{γ′} & < Equation 5 > \end{matrix}$
α_γ ⁰denotes a lattice parameter value of pure Ni, α_γ′ ⁰denotes a lattice parameter value of Ni₃Al, V_i ^γ denotes a Vegard coefficient of each alloying element in the matrix γ phase, V_i ^γ′ denotes a Vegard coefficient of each alloying element in the precipitate γ′ phase, x_i ^γ denotes an atomic fraction (at. %) of each alloying element in the matrix γ phase, and x_i ^γ denotes an atomic fraction (at. %) of each alloying element in the precipitate γ′ phase. The Vegard coefficients of the matrix γ and the precipitate γ′ are coefficients measured through tests conducted in Ni—X and Ni₃Al—X alloy systems, and research results thereof are disclosed.
The Vegard's law calculation method is easy and useful when alloying element fractions in each phase are known but may not accurately calculate lattice misfits and lattice parameters at high temperature because the thermal expansion effect based on temperature is not appropriately reflected.
A lattice parameter and lattice misfit calculation method considering thermal expansion based on temperature, which is proposed by Dr. Young-Kwang Kim (Inter. J. of Plasticity, 79 (2016) 153-175), may be expressed as in Equations 6 and 7 below.
α_γ(T)=α_γ ⁰(T)(1+Δα_γ) <Equation 6>
α_γ′(T)=α_γ′ ⁰(T)(1+Δα_γ′) <Equation 7>
α_γ(T) denotes a lattice parameter value of the matrix γ at an absolute temperature T(K), α_γ(T) denotes a lattice parameter value of γ′ at the absolute temperature T(K), α_γ ⁰(T) denotes a lattice parameter value of pure Ni at the absolute temperature T(K), α_γ ⁰(T) denotes a lattice parameter value of Ni₃Al at the absolute temperature T(K), Δa_γ denotes a thermal expansion lattice parameter difference calculated between the matrix γ phase and pure Ni by using x_i ^γ, and Δa_γ′ denotes a thermal expansion lattice parameter difference calculated between the precipitate γ′ phase and Ni₃Al by using x_i ^γ′. Parameters for calculating the lattice parameters are disclosed in the paper.
Nevertheless, calculation errors unavoidably occur because x_i ^γ and x_i ^γ′, which are significant variables in the thermodynamic calculation method, are assumed as being in a steady state. Therefore, a more practical method capable of estimating high-temperature mechanical properties of single-crystal super-heat-resistant alloys by inferring lattice parameters and lattice misfits is required.
Single-crystal super-heat-resistant alloys are classified into generations depending on the content of Re. First-generation single-crystal super-heat-resistant alloys do not contain Re, and examples thereof include SRR99 developed by Rolls-Royce and PWA 1480 developed by Pratt & Whitney. However, creep properties of these super-heat-resistant alloys deteriorate at high temperature. Second-generation single-crystal super-heat-resistant alloys contain about 3% of Re by weight, and representative examples thereof include CMSX-4 developed by Cannon-Muskegon and Rene N5 developed by General Electric (GE). Lastly, third-generation single-crystal super-heat-resistant alloys developed by Cannon-Muskegon and Pratt & Whitney contain about 6% of Re by weight. However, the second-generation and third-generation super-heat-resistant alloys necessarily contain Re which is a high-priced special element, and thus production costs are increased.
The present invention provides a super-heat-resistant alloy excluding or minimally containing a high-priced element such as Re and having excellent creep resistance sustainability and stress rupture resistance at ultra-high temperature.

Description of Concept of Invention

Re is a solid solution strengthening element which is severely segregated in a matrix γ phase, and greatly contributes to the improvement of creep properties because of its large atomic radius and very low diffusion rate. Due to the large atomic radius of Re, in an alloy containing Re, a lattice parameter of the matrix γ may be increased and thus a lattice misfit may also be increased.
According to the present invention, to develop an alloy excluding Re and having creep properties comparable to those of second generation single-crystal super-heat-resistant alloys, a lattice parameter of a matrix γ, which is reduced due to the exclusion of Re compared to other alloys containing Re, may be increased using a combination of other alloying elements and thus resistance to creep deformation as well as a creep life, which is essential for use at high temperature, may be greatly improved.
A super-heat-resistant alloy according to a first embodiment of the present invention consists of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Ta: 7.0 wt % to 10.0 wt %, Ti: 0 wt %<Ti≤1.5 wt %, W: 7.0 wt % to 10.5 wt %, and Ni: balance, and does not contain Re.
A super-heat-resistant alloy according to a second embodiment of the present invention may be characterized in that it consists of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Ta: 7.0 wt % to 10.0 wt %, Ti: 0 wt %<Ti≤1.5 wt %, W: 7.0 wt % to 10.5 wt %, Hf: 0 wt %<Hf≤1.5 wt %, and Ni: balance, and does not contain Re.
A super-heat-resistant alloy according to a third embodiment of the present invention may be characterized in that it consists of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Ta: 7.0 wt % to 10.0 wt %, W: 7.0 wt % to 10.5 wt %, and Ni: balance, and does not contain Re.
A super-heat-resistant alloy according to a fourth embodiment of the present invention may be characterized in that it consists of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Ta: 7.0 wt % to 10.0 wt %, W: 7.0 wt % to 10.5 wt %, Hf: 0 wt %<Hf≤1.5 wt %, and Ni: balance, and does not contain Re.
A super-heat-resistant alloy according to a fifth embodiment of the present invention may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Re: 0 wt %<Re≤1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, Ti: 0 wt %<Ti≤1.5 wt %, W: 7.0 wt % to 10.5 wt %, and Ni: balance.
A super-heat-resistant alloy according to a sixth embodiment of the present invention may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Re: 0 wt %<Re≤1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, W: 7.0 wt % to 10.5 wt %, and Ni: balance.
A super-heat-resistant alloy according to a seventh embodiment of the present invention may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Re: 0 wt %<Re≤1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, Ti: 0 wt %<Ti≤1.5 wt %, W: 7.0 wt % to 10.5 wt %, Hf: 0 wt %<Hf≤1.5 wt %, and Ni: balance.
A super-heat-resistant alloy according to an eighth embodiment of the present invention may consist of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, Cr: 5.0 wt % to 8.0 wt %, Mo: 0.5 wt % to 2.0 wt %, Re: 0 wt %<Re≤1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, W: 7.0 wt % to 10.5 wt %, Hf: 0 wt %<Hf≤1.5 wt %, and Ni: balance.
When an alloy ‘consists of’ elements, it means that the alloy is composed of only the listed elements and other elements do not participate as constituent elements of the alloy except for unavoidable impurities during production. For example, the super-heat-resistant alloys according to the afore-described first to eighth embodiments may be characterized in that they do not contain iron (Fe) in a significant amount.
Meanwhile, as a modified example, the super-heat-resistant alloys according to the first to eighth embodiments of the present invention may further contain 0.1 wt % or less of a rare-earth element such as yttrium (Y), lanthanum (La), or cerium (Ce).
In the super-heat-resistant alloys according to the afore-described embodiments of the present invention, Al is a constituent element of γ′, which is a main strengthening phase of the Ni-based super-heat-resistant alloy, and thus is crucial to the improvement of high-temperature creep properties. In addition, Al contributes to the improvement of oxidation resistance. However, when Al is less than 4.0 wt %, the strength improvement effect based on the formation of the precipitate phase may not be easily achieved, and when Al is more than 5.2 wt %, a fraction of the γ′ phase may be excessively increased to lower a high-temperature strength, and solution heat treatment may not be easily performed.
Co serves as a solid solution strengthener dissolved in the Ni matrix to strengthen the matrix and improves creep properties at high temperature. However, when Co is increased in amount, it may be combined with other alloying elements to form an intermetallic compound to cause a reduction in strength and an increase in alloy price. Therefore, Co may have a range of 1.0 wt % to 10.0 wt %.
Cr may serve to improve corrosion resistance and oxidation resistance but form a carbide or topologically close packed (TCP) phase in the super-heat-resistant alloy. Therefore, 8.0 wt % or less and 5.0 wt % or more of Cr may be contained.
Mo is a solid solution strengthening element and serves to improve high-temperature tensile properties and creep properties of the super-heat-resistant alloy, and the solid solution strengthening effect may not be easily expected when Mo is less than 0.5 wt %. However, when more than 2.0 wt % of Mo is added, a TCP phase may be easily formed.
W is an element having an excellent solid solution strengthening effect and may have a range of 7.0 wt % to 10.5 wt %. When the content of W is less than 7.0 wt %, the high-temperature strength improvement effect is insignificant, and when the content of W is higher than 10.5 wt %, a density is increased, a TCP phase is easily formed, and thus phase stability is reduced.
Ti, like Al, is a constituent element of the γ′ phase and may be added because it aids in the improvement of a high-temperature strength and contributes to the improvement of corrosion resistance. However, because ductility may be reduced and an unnecessary phase such as eta phase may be formed when Ti is added excessively, Ti is limited to 1.5 wt % or less.
Ta is an element which not only contributes to solid solution strengthening of the matrix γ phase but also strengthens the γ′ phase by substituting for Al of the γ′ phase together with Ti. In addition, Ta, which is a high-density heat-resistant element, segregates into a liquid phase during solidification and increases the density of the liquid phase between dendrites, thereby reducing the buoyancy of the liquid phase between dendrites during unidirectional solidification or single-crystal solidification and suppressing the formation of freckle defects. Therefore, in the alloy having a high W content of 7.0 wt % to 10.5 wt %, 7.0 wt % or more of Ta may be added but the addition of more than 10.0 wt % of Ta may promote the formation of a TCP phase such as mu phase and deteriorate high-temperature mechanical properties.
Hf reduces a temperature range or time used for solution heat treatment of the alloy by reducing an initial melting point of the alloy. The addition of Hf may increase the weight of a product produced using the alloy, increase the density of the alloy, and reduce the stability of the microstructure of the alloy. However, because the castability of the alloy may be considerably reduced when more than 1.5 wt % of Hf is added, the content of Hf may be controlled to 1.5 wt % or less.
According to the present invention, alloying elements are controlled and designed to increase the lattice parameter of the matrix γ. To this end, a γ lattice parameter distribution parameter is derived and expressed as in Equation 8 below.
$\begin{matrix} γ Lattice Parameter Distribution Parameter = \sum_{i} (k_{i} \times x_{i} \times V_{i}^{γ}) > 0.1 2 & < Equation 8 > \end{matrix}$
k_idenotes a partitioning coefficient of each alloying element and indicates x_i ^γ/x_i ^γ′. x_idenotes an atomic fraction (at. %) of each alloying element, x_i ^γ denotes an atomic fraction (at. %) of each alloying element in the γ phase, and x_i ^γ′ denotes an atomic fraction (at. %) of each alloying element in the precipitate γ′ phase. V_i ^γ denotes a Vegard coefficient of each alloying element in the matrix γ phase and has values as shown in Table 1 below.

TABLE 1

	V_Al ^γ	V_Co ^γ	V_Cr ^γ	V_Mo ^γ	V_Ta ^γ	V_Ti ^γ	V_W ^γ	V_Hf ^γ

Vegard	0.179	0.0196	0.11	0.478	0.7	0.422	0.444	0.769
coefficient
(Å)

Specific test examples of super-heat-resistant alloys designed using the γ lattice parameter distribution parameter will now be described. Table 2 shows compositions (unit: wt %) and γ lattice parameter distribution parameters of super-heat-resistant alloys according to test examples of the present invention.

TABLE 2

											γ Lattice
											Parameter
											Distribution
	Al	Co	Cr	Mo	Re	Ta	Ti	W	Hf	Ni	Parameter

Test	5.6	9.0	6.4	0.6	3.0	6.5	1.0	6.4	0.1	Bal.	0.0930
Example 1
Test	4.4	8.0	4.0	1.0	0	9.0	2.0	10.0	0	Bal.	0.1191
Example 2
Test	4.7	8.0	7.0	1.0	0	9.1	1.0	10.1	0	Bal.	0.1296
Example 3
Test	5.0	8.0	7.0	1.0	0	9.0	1.1	9.0	0	Bal.	0.1232
Example 4
Test	5.4	0	9.0	0.6	0	10.0	0	7.6	0	Bal.	0.1087
Example 5
Test	4.7	8	6	1	0	9.1	1	10.1	0	Bal.	0.1228
Example 6
Test	5	8	6	1	0	9.1	1	10.1	0	Bal.	0.1207
Example 7
Test	4.97	8	7.02	1	0	9.2	1	10.4	0	Bal.	0.1302
Example 8
Test	4.7	8	7	1	0	9.1	1	10.1	0.1	Bal.	0.1299
Example 9
Test	4.7	3	7	1	0	9.1	1	10.1	0	Bal.	0.1227
Example 10
Test	4.7	8	6	0.4	0	9.1	1	10.1	0	Bal.	0.1121
Example 11
Test	4.7	8	4	1	0	9.1	1	10.1	0	Bal.	0.1103
Example 12
Test	4.7	5.5	7	1	0	9.1	1	10.1	0	Bal.	0.1263
Example 13
Test	4.7	0	7	1	0	9.1	1	10.1	0	Bal.	0.1185
Example 14
Test	5.3	8	7	1	0	9.1	0	10.1	0	Bal.	0.1188
Example 15

Referring to Table 2, Test Examples 3, 4, 6, 7, 8, 9, 10, and 13 satisfy the composition of the super-heat-resistant alloy according to any one of the afore-described first to eighth embodiments of the present invention.
Unlike this, the super-heat-resistant alloy according to Test Example 1 does not satisfy any of the compositions of the super-heat-resistant alloys according to the afore-described first to eighth embodiments of the present invention. Specifically, the ranges of Al: 4.0 wt % to 5.2 wt %, Re: 0 wt % to 1.0 wt %, Ta: 7.0 wt % to 10.0 wt %, and W: 7.0 wt % to 10.5 wt % are not satisfied.
The super-heat-resistant alloy according to Test Example 2 does not satisfy any of the compositions of the super-heat-resistant alloys according to the afore-described first to eighth embodiments of the present invention. Specifically, the ranges of Cr: 5.0 wt % to 8.0 wt % and Ti: 0 wt % to 1.5 wt % are not satisfied. The super-heat-resistant alloy according to Test Example 5 does not satisfy any of the compositions of the super-heat-resistant alloys according to the afore-described first to eighth embodiments of the present invention. Specifically, the ranges of Al: 4.0 wt % to 5.2 wt %, Co: 1.0 wt % to 10.0 wt %, and Cr: 5.0 wt % to 8.0 wt % are not satisfied.
The super-heat-resistant alloy according to Test Example 11 does not satisfy any of the compositions of the super-heat-resistant alloys according to the afore-described first to eighth embodiments of the present invention. Specifically, the range of Mo: 0.5 wt % to 2.0 wt % is not satisfied.
The super-heat-resistant alloy according to Test Example 12 does not satisfy any of the compositions of the super-heat-resistant alloys according to the afore-described first to eighth embodiments of the present invention. Specifically, the range of Cr: 5.0 wt % to 8.0 wt % is not satisfied.
The super-heat-resistant alloy according to Test Example 14 does not satisfy any of the compositions of the super-heat-resistant alloys according to the afore-described first to eighth embodiments of the present invention. Specifically, the range of Co: 1.0 wt % to 10.0 wt % is not satisfied.
The super-heat-resistant alloy according to Test Example 15 does not satisfy any of the compositions of the super-heat-resistant alloys according to the afore-described first to eighth embodiments of the present invention. Specifically, the range of Al: 4.0 wt % to 5.2 wt % is not satisfied.
Table 3 shows creep resistance continuities, stable creep resistance times, total creep lives, and γ lattice parameter distribution parameters of super-heat-resistant alloys according to test examples of the present invention.

TABLE 3

Total	Stable creep	Creep	γ Lattice
Creep	resistance	resistance	Parameter
Life	time	sustainability	Distribution
(hours)	(hours)	(%)	Parameter

Test Example 1	141	0	0.00	0.0930
Test Example 2	78	0	0.00	0.1191
Test Example 3	368	308	83.70	0.1296
Test Example 4	286	229	80.07	0.1232
Test Example 5	71	0	0.00	0.1087
Test Example 6	231	163	70.56	0.1228
Test Example 7	359	244	67.94	0.1207
Test Example 8	306	248	81.05	0.1302
Test Example 9	437	407	93.25	0.1299
Test Example 10	431	366	84.92	0.1227
Test Example 11	64	0	0	0.1121
Test Example 12	48	0	0	0.1103
Test Example 13	363	307	84.57	0.1263
Test Example 14	214	126	59.01	0.1185
Test Example 15	148	62	41.69	0.1188

Meanwhile, FIG. 1 is a graph showing creep strains of super-heat-resistant alloys according to test examples of the present invention, over time, and FIG. 2 is a graph showing creep strain rates of super-heat-resistant alloys according to test examples of the present invention, over time.
In FIG. 1 , the horizontal axis represents time (unit: hours) and the vertical axis represents a creep strain (unit: %). In FIG. 2 , the horizontal axis represents time (unit: hours) and the vertical axis represents a creep strain rate (unit: %/hr). The creep strain rate of FIG. 2 may be understood as a gradient value at a tangent point of the creep strain of FIG. 1 . That is, the creep strain rate may correspond to a differential value of the creep strain over time. Test conditions for observing the creep properties in FIGS. 1 and 2 include conditions of 1100° C. and 137 MPa. However, in the super-heat-resistant alloys according to the embodiments of the present invention, the test conditions for observing the creep properties in FIGS. 1 and 2 may be extended to ranges of 1050° C. to 1150° C. and 135 MPa to 140 MPa.
Referring to FIGS. 1 and 2 and Tables 2 and 3 together, in the super-heat-resistant alloys according to Test Examples 3, 4, 6, 7, 8, 9, 10, and 13, the γ lattice parameter distribution parameter according to Equation 8 above is greater than 0.12 and the stable creep resistance time according to Equation 1 below is 150 hours or more. On the contrary, in the super-heat-resistant alloys according to Test Examples 1, 2, 5, 11, 12, 14, and 15, the γ lattice parameter distribution parameter according to Equation 8 above is less than 0.12 and the stable creep resistance time according to Equation 1 below is less than 150 hours. As such, the super-heat-resistant alloys according to Test Examples 3, 4, 6, 7, 8, 9, 10, and 13 are remarkably superior to the super-heat-resistant alloys according to Test Examples 1, 2, 5, 11, 12, 14, and 15 in creep resistance sustainability at ultra-high temperature.
Stable creep resistance time (hours)=t _max −t _min <Equation 1>
in a period of time satisfying
[ε_t(i)−ε_t(i−1) ]/[t(i)−t(i−1)]≤0.005(%/hours)
(where ε_t(i)denotes a creep strain of the super-heat-resistant alloy at time t(i), εt_(i−1)denotes a creep strain of the super-heat-resistant alloy at time t(i−1), t_maxdenotes a maximum time value in the period of time, and t_mindenotes is a minimum time value in the period of time.)
Referring to FIGS. 1 and 2 and Table 3 together, in the super-heat-resistant alloys according to Test Examples 3, 4, 6, 7, 8, 9, 10, and 13, the creep resistance sustainability according to Equation 2 below is greater than or equal to 60%. On the contrary, in the super-heat-resistant alloys according to Test Examples 1, 2, 5, 11, 12, 14, and 15, the creep resistance sustainability according to Equation 2 below is less than 60%. As such, the super-heat-resistant alloys according to Test Examples 3, 4, 6, 7, 8, 9, 10, and 13 are remarkably superior to the super-heat-resistant alloys according to Test Examples 1, 2, 5, 11, 12, 14, and 15 in creep resistance sustainability at ultra-high temperature.
Creep resistance sustainability=[(Stable creep resistance time)/(Total Creep Life)×100] <Equation 2>
Table 4 comparatively shows results of calculating total creep lives and lattice misfits of super-heat-resistant alloys according to test examples of the present invention. The lattice misfit values are calculated according to Equation 3 above by using the calculation method considering thermal expansion, which is proposed by Dr. Young-Kwang Kim.

	TABLE 4

	Lattice Misfit δ	Total Creep Life
	(%)	(hours)

Test Example 1	−0.117	141
Test Example 2	−0.279	78
Test Example 3	−0.307	368
Test Example 4	−0.287	286
Test Example 5	−0.314	71
Test Example 6	−0.295	231
Test Example 7	−0.31	359
Test Example 8	−0.331	306
Test Example 9	−0.311	437
Test Example 10	−0.342	431
Test Example 11	−0.237	64
Test Example 12	−0.258	48
Test Example 13	−0.324	363
Test Example 14	−0.368	214
Test Example 15	−0.301	148

As mentioned above, the lattice misfit δ generally has a negative (−) value and, when the absolute lattice misfit |δ| has an appropriate value that is neither too large nor too small, the precipitate is formed in the most perfect cuboidal shape and ideal high-temperature properties are achieved. A specific lattice misfit value capable of achieving excellent high-temperature creep properties is found by comparing the total creep lives and the lattice misfit values and is expressed as in Equation 9 below.
−0.35%<δ<−0.28% <Equation 9>
In the super-heat-resistant alloys according to Test Examples 3, 4, 6, 7, 8, 9, and 13, the lattice misfit calculated according to Equation 3 above is higher than −0.35% and lower than −0.28%. On the contrary, in the super-heat-resistant alloys according to Test Examples 1, 11, and 12, the calculated lattice misfit is higher than −0.28% and, in the super-heat-resistant alloy according to Test Example 14, the calculated lattice misfit is lower than −0.35%.
Because the value of the lattice misfit is closely correlated with the shape of the precipitate γ′, the lattice misfit values may be relatively compared by analyzing the shapes of the precipitates γ′. FIGS. 3 to 8 are scanning electron microscope (SEM) images showing microstructures of thermally aged super-heat-resistant alloys according to test examples of the present invention. Shape parameters η are measured by analyzing γ′ shapes of the thermally aged super-heat-resistant alloys, and a measurement method is schematically shown in FIG. 9 and expressed as in Equation 10 below.
$\begin{matrix} Shape parameter, η = \frac{\sqrt{{(\frac{B_{1} + B_{2}}{2})}^{2}}}{\sqrt{(A_{1}^{2} + A_{2}^{2})}} & < Equation 10 > \end{matrix}$
As the shape parameter η, an average of values measured per test example by analyzing more than 1,000 precipitate particles located at the center of dendrite by using an image analysis program is shown in the following table. The shape parameter η is a variable that forms a perfect rectangle as it approaches 1, and a test example having a large shape parameter η may be determined as an alloy having a large lattice misfit. A specific shape parameter η value capable of achieving excellent high-temperature creep properties is found by comparing the total creep lives and the shape parameter η values and is expressed as in the following equation.

	TABLE 5

		Total Creep Life
	Shape Parameter η	(hours)

Test Example 2	0.8954	78
Test Example 3	0.9593	368
Test Example 10	0.9712	431
Test Example 12	0.8547	48
Test Example 13	0.9424	363
Test Example 14	0.9279	214

Referring to Table 5, when the shape parameter η is greater than 0.93, the total creep life may be relatively good and excellent high-temperature creep properties may be achieved. While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the following claims.

Claims

1. A super-heat-resistant alloy consisting of aluminum (Al): 4.0 wt % to wt %, cobalt (Co): 1.0 wt % to 10.0 wt %, chromium (Cr): 5.0 wt % to 8.0 wt %, molybdenum (Mo): 0.5 wt % to 2.0 wt %, tantalum (Ta): 7.0 wt % to 10.0 wt %, titanium (Ti): 0 wt %<Ti≤1.5 wt %, tungsten (W): 7.0 wt % to 10.5 wt %, and nickel (Ni): balance, and not containing rhenium (Re),

wherein a creep resistance sustainability of the super-heat-resistant alloy according to Equations 1 and 2 below is greater than or equal to 60%.

Stable creep resistance time (hours)=t _max −t _min <Equation 1>

in a period of time satisfying

[ε_t(i)−ε_t(i−1) ]/[t(i)−t(i−1)]≤0.005(%/hours)

(where ε_t(i)denotes a creep strain of the super-heat-resistant alloy at time t(i), ε_t(i−1)denotes a creep strain of the super-heat-resistant alloy at time t(i−1), t_maxdenotes a maximum time value in the period of time, and t_mindenotes is a minimum time value in the period of time.)

Creep resistance sustainability=[(Stable creep resistance time)/(Total CreepLife)×100] <Equation 2>

(where the stable creep resistance time and the total creep life are measured under conditions of 1100° C. and 137 MPa.)

2. The super-heat-resistant alloy of claim 1, wherein the time stable creep resistance time according to Equation 1 is 150 hours or more.

3. A super-heat-resistant alloy consisting of aluminum (Al): 4.0 wt % to wt %, cobalt (Co): 1.0 wt % to 10.0 wt %, chromium (Cr): 5.0 wt % to 8.0 wt %, molybdenum (Mo): 0.5 wt % to 2.0 wt %, tantalum (Ta): 7.0 wt % to 10.0 wt %, titanium (Ti): 0 wt %<Ti≤1.5 wt %, tungsten (W): 7.0 wt % to 10.5 wt %, hafnium (Hf): 0 wt %<Hf≤1.5 wt %, and nickel (Ni): balance, and not containing rhenium (Re),

Stable creep resistance time (hours)=t _max −t _min <Equation 1>

in a period of time satisfying

[ε_t(i)−ε_t(i−1) ]/[t(i)−t(i−1)]≤0.005(%/hours)

4. The super-heat-resistant alloy of claim 3, wherein the stable creep resistance time according to Equation 1 is 150 hours or more.

5. The super-heat-resistant alloy of claim 1, wherein the super-heat-resistant alloy does not contain iron (Fe).

6. The super-heat-resistant alloy of claim 1, wherein a lattice misfit δ of the super-heat-resistant alloy according to Equation 3 below is higher than −0.35% and lower than −0.28%.

\begin{matrix} Lattice Misfit δ = 2 \times \frac{α_{γ'} - α_{γ}}{α_{γ′} + α_{γ}} & [Equation 3] \end{matrix}

(where α_γ denotes a lattice parameter of a matrix γ, and α_γ′ denotes a lattice parameter of a precipitate γ′.)

7. The super-heat-resistant alloy of claim 1, wherein a γ lattice parameter distribution parameter of the super-heat-resistant alloy according to Equation 8 below is greater than 0.12.

\begin{matrix} γ Lattice Parameter Distribution Parameter = \sum_{i} (k_{i} \times x_{i} \times V_{i}^{γ}) > 0.1 2 & [Equation 8] \end{matrix}

(where k_idenotes a partitioning coefficient of each alloying element and indicates x_i ^γ/x_i ^γ′, x_idenotes an atomic fraction (at. %) of each alloying element, x_i ^γ denotes an atomic fraction (at. %) of each alloying element in a matrix γ phase, x_i ^γ′ denotes an atomic fraction (at. %) of each alloying element in a precipitate γ′ phase, and V_i ^γ denotes a Vegard coefficient of each alloying element in the matrix γ phase.)