KR102220219B1 - Refractory high entropy superalloy with bcc dual phase and manufacturing method for the same - Google Patents

Abstract

The present invention relates to a heat-resistant high-entropy superalloy having a BCC two-phase composite structure and a method for manufacturing the same, (A2 base: irregular phase with excellent ductility)-(B2 precipitate: regular phase with excellent strength) The present invention relates to a heat-resistant high-entropy superalloy having a BCC A2 phase and B2 phase composite structure and a manufacturing method thereof implemented in a BCC high entropy alloy composed of. In the present invention, the separation of the two BCC phases is induced by using the mixed enthalpy relationship between elements, and the formation of intermetallic compounds other than the BCC phase is suppressed through a high entropy effect, thereby having a BCC two-phase complex structure consisting of only the A2 and B2 phases. A high entropy superalloy was developed, and excellent ductility and strength can be secured at the same time by forming a rule B2 phase with excellent strength as a precipitate on an irregular A2 phase base with excellent ductility through precise control of the composition and process. In particular, when the precipitated B2 phase, which has a crystal structure similar to that of the matrix, is controlled to a nano size, it exhibits excellent creep characteristics as in the Ni-based superalloy.

Description

Heat-resistant high-entropy superalloy with BCC two-phase composite structure and its manufacturing method {REFRACTORY HIGH ENTROPY SUPERALLOY WITH BCC DUAL PHASE AND MANUFACTURING METHOD FOR THE SAME}

The present invention relates to a heat-resistant high-entropy superalloy having a BCC two-phase composite structure and a method of manufacturing the same.

In general, materials used in a complex extreme environment of a low-temperature cycle and high pressure such as a gas turbine blade require excellent mechanical properties at high temperatures. As such an extreme environmental material, Ni-based superalloy is mainly used due to its excellent yield strength at high temperature. This Ni-based superalloy has a complex FCC structure and is based on γ-phase with excellent ductility and has excellent strength. It is characterized by having excellent mechanical properties because it has a _{Ni 3 (Al,Cr) γ'regular phase as a precipitate.} However, due to the relatively low melting point of Ni-based superalloys, softening occurs at temperatures above 800℃, resulting in rapid deterioration of mechanical properties, limiting its use, and development of high-temperature structural materials that can be stably utilized at ultra-high temperatures of 1000℃ is required. .

Recently, high-entropy alloys composed of transition metals of groups 4 to 6 and having a body centered cubic crystal structure are known to have superior high-temperature mechanical properties than superalloys used at high temperatures of 1000°C or higher, and various studies are being conducted. However, refractory metals such as Nb, Mo, Ta, and W are vulnerable to oxidation in a high temperature environment, and high entropy alloys composed of them also exhibit poor oxidation resistance. Alloying elements such as Al, Cr, and Si is effective as a method of improving the oxidation resistance of metal materials, but when alloying Al, Cr, and Si to an alloy composed of refractory metals such as Nb, Mo, Ta, and W Due to the formation of intermetallic compounds, there is a problem that the ductility of the material is greatly reduced, resulting in brittleness. In particular, the high entropy alloy composed of refractory metal elements has an A2 structure in which elements are irregularly distributed inside the BCC lattice.If Al is added to improve oxidation resistance, it has two sublattices inside the BCC. It is known that the transition to the regular B2 structure occurs, and in the case of having such a B2 structure, strength increases but brittleness appears, making it difficult to use as a structural material at room temperature.

Acta Materialia (2016, Vol. 122, pp. 448-511) JOURNAL OF MATERIALS SCIENCE (2003, Vol. 38, pp. 3995-4002) Journal of Phase Equilibria and Diffusion (2018, Vol. 39.5 pp. 549-561.)

The present invention is to solve the problems of the prior art described above, and the composite microstructure of the (base: irregular phase with excellent ductility)-(precipitate: regular phase with excellent strength) represented by the Ni-based superalloy is formed of a refractory metal. An object thereof is to provide a heat-resistant high entropy superalloy having a BCC two-phase composite structure composed of an A2 phase and a B2 structure by implementing a high entropy alloy and a method of manufacturing the same.

In order to solve the above-described problem, the heat-resistant high-entropy superalloy having a BCC two-phase composite structure according to the present invention is (Nb,Ta,Mo,W) _x Al _y (Ti,Zr,Hf) _100-xy (however, 30≤ x≤50, 10≤y≤20 at.%), the content of Ti is at least 15 at.%, the sum of the contents of Zr and Hf is at least 15 at.%, and the It is characterized in that the sum of the Mo and W content is 10 at.% or less.

The heat-resistant high-entropy superalloy having a BCC two-phase composite structure according to the present invention has a B2 phase (regular BCC) distributed as a precipitated phase in the A2 phase (irregular BCC) base, and the precipitated phase has an average particle size of 0.01 to 10 μm. It features.

In addition, the high-entropy heat-resistant superalloy having a BCC two-phase nanostructure according to the present invention contains 5 at.% or less of at least one element in the element group consisting of (Cr, Si) having a significantly greater affinity for oxygen relative to the elemental element. It is possible to further improve oxidation resistance by adding.

The heat-resistant high-entropy superalloy having a BCC two-phase composite structure according to the present invention can secure excellent ductility and strength at the same time by forming a rule B2 phase having excellent strength as a precipitate based on an irregular A2 phase having excellent ductility. In particular, when the rule B2 precipitate is controlled on a nanoscale, there is an effect of showing more excellent creep characteristics.

1 is a diagram showing the binary mixing enthalpy between elements constituting the alloy of the present invention.
Figure 2 shows the isothermal phase equilibrium (Isothermal section) at 1200 °C of a pseudo-ternary _{alloy system having Ti 50} Zr ₅₀ , Nb, and Al as the axes of the alloy system of the present invention.
3 is a differential scanning microscope (SEM) image of a heat-resistant high entropy superalloy according to Examples 1 to 8 of the present invention.
4 is a differential scanning microscope (SEM) image of an alloy according to (a) Comparative Example 4 and (b) Comparative Example 5 of the present invention.
5 shows a stress-strain curve obtained through a room temperature tensile test of Example 1 and Comparative Example 3 of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those of ordinary skill in the art can easily implement the embodiments. The present invention may be implemented in various different forms, and is not limited to the embodiments described herein. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and the same reference numerals are used for the same or similar components throughout the specification. Also, in the case of well-known technologies, detailed descriptions thereof will be omitted.

It has been reported that the B2 phase formed by bonding of a transition metal element from Group 4 to Group 6 with Al exists in a ternary alloy system such as Ti-Al-Nb and Ti-Al-Ta, and such a ternary alloy system is Ti, Al And it is composed of one element selected from the element group consisting of (Nb, Ta). However, in the ternary alloy system, since the B2 phase occurs through the regular-irregular transformation of the A2 phase present at high temperature, all of the A2 phases are transferred to B2, and thus a microstructure in which the A2 phase and the B2 phase coexist cannot be formed. Because the transformation speed is very fast, transformation cannot be suppressed even through the quenching process. (JOURNAL OF MATERIALS SCIENCE 38 (2003) 3995-4002.) In addition, Ti-Al intermetallic compounds and (Nb, Ta)-Al intermetallic compounds are stabilized near the compositional region where B2 is formed in the ternary alloy. As it exists, intermetallic compounds are formed in the manufacturing process of the alloy, causing brittleness of the material. (Journal of Phase Equilibria and Diffusion 39.5 (2018) 549-561.) Summarizing the results reported so far, in the ternary alloy systems, a two-phase region in which the A2 and B2 phases are in equilibrium exists in the phase equilibrium diagram. Because it does not, it is not possible to implement a microstructure of a (base: irregular phase with excellent ductility)-(precipitate: a regular phase with excellent strength) such as Ni-based superalloys.

In this study, in the Ti-Al-(Nb, Ta) ternary alloy system described above, the A2 phase and the B2 phase were separated by using the mixing enthalpy of the amount between elements, and the A2 phase and the A2 phase were By suppressing the formation of intermetallic compounds other than the B2 phase, an alloy system was developed in which the A2 phase and the B2 phase having a BCC crystal structure are in two-phase equilibrium. In particular, by precisely controlling the two-phase equilibrium region of the developed A2 and B2 phases (A2 base: irregular phase with excellent ductility)-(B2 precipitate: regular phase with excellent strength), it has a unique BCC two-phase composite structure. Entropy superalloy was developed.

1 is a diagram showing the binary mixing enthalpy between elements constituting the alloy of the present invention. Since Zr has a positive mixing enthalpy with Nb, (+4 kJ/mol) has a physical relationship that tries to separate from each other. This appears as a miscibility gap in the Zr-Nb binary phase diagram, and the Zr-Nb alloy can be separated into two BCCs (A2) at a temperature of 1000 ℃ or less. When Zr having such characteristics is substituted with Ti of the Ti-Al-Nb alloy system, separation into a BCC phase with a high Zr content and a BCC phase with a high Nb content occurs. At this time, the Al content of the BCC phase containing a large amount of Zr, which has the strongest bonding strength with Al among the constituent elements of the alloy, increases, and the BCC phase of Zr-rich with a high Al content constitutes the rule B2 phase, and the Al content is The BCC phase of less Nb-rich may constitute an irregular A2 phase and cause two-phase separation.

FIG. 2 shows an isothermal section at 1200° C. of a pseudo ternary alloy system _{having Ti 50} Zr ₅₀ , Nb, and Al as axes in the alloy system of the present invention. This is a pseudo ternary phase equilibrium diagram of a quaternary alloy system in which half of the mole fraction of Ti is substituted with Zr in the Ti-Al-Nb ternary alloy system, and was constructed using the TCHEA 3 database of the Thermo-Calc program. It can be seen from the drawings that the heat-resistant high-entropy superalloy of the two-phase structure according to the present invention can prevent the formation of intermetallic compounds except for the A2 and B2 phases. This unique phase stability is the high entropy effect, which is the main characteristic of the high-entropy alloy composed of a multi-component system.In addition, the high entropy effect is the Gibbs energy stabilized by increasing the mixing entropy expressed by the following equation, allowing various elements to be dissolved. It refers to the effect of stabilizing the phases of simple crystal structures such as BCC, FCC and HCP.

는 원소 i의 첨가 원자분율을 의미한다.) (Where R is the gas constant,

Means the added atomic fraction of element i.)

Incidentally, in the present invention, the intermetallic compounds other than the A2 phase and the B2 phase are all intermetallic compounds formed by bonding with Al, and these are selectively combined with Al, which is a specific element, to form a compound (ZrAl, Zr ₂ Al). , Nb ₃ Al) On the other hand, BCC phases such as A2 and B2 phases can dissolve all Group 4 to Group 6 transition metal elements, and the high entropy alloy of the present invention forms intermetallic compounds other than B2 due to the high entropy effect. Is suppressed.

Accordingly, heat-resistant high entropy having the BCC two-phase structure of the present invention in the composition region of _{Nb x} Al _y (Ti ₅₀ Zr ₅₀ ) _{100-xy (30≤x≤50, 10≤y≤20 at.%) through FIG. 2} It can be seen that there is a possibility to manufacture superalloys. However, since the phase equilibrium simulation performed in FIG. 2 does not include information on the regular and irregular grid images of the BCC grid, the two-phase equilibrium regions of the A2 and B2 phases do not appear.

Al acts as a major element constituting the B2 phase, and if it is composed of less than 10 at.%, the alloy does not form the B2 phase and maintains the irregular A2 phase. If it is composed of more than 20 at.%, the alloy is the B2 phase. Since it is composed of only or other intermetallic compounds other than the A2 phase and the B2 phase are precipitated, it is preferably contained in an amount of 10 at.% or more and 20 at.% or less.

Since Ta has a positive mixing enthalpy with Zr and Hf, (Ta-Zr: +3 kJ/mol, Ta-Hf: +3 kJ/mol) forms an electrifying solid solution with Nb, it has the same effect as Nb above. And can replace Nb in the alloy.

Since Hf has very similar physical properties to Zr and has a positive mixing enthalpy with Nb and Ta, (Hf-Nb: +4 kJ/mol, Hf-Ta: +3 kJ/mol) can replace Zr. have.

Since Mo and W have a negative mixing enthalpy with Zr and Hf, (Mo-Zr: -6 kJ/mol, Mo-Hf: -4 kJ/mol, W-Zr: -9 kJ/mol, Mo-Hf : -6 kJ/mol) It cannot produce the same effect as Nb, but it is known as an element that forms an electrifying solid solution with Nb and Ta and improves the strength of the alloy, so Nb is substituted with a content of 10 at.% or less of the alloy. It has the effect of increasing the strength. However, if the content exceeds 10 at.%, the formation of other intermetallic compounds is caused, so it is preferable to use 10 at.% or less.

As described above, in order to suppress the precipitation of intermetallic compounds through a high entropy effect, the Ti content is at least 15 at.% or more, and the sum of the Zr and Hf contents is at least 15 at.% or more. It is desirable to configure.

In addition, the high-entropy heat-resistant superalloy having a BCC two-phase nanostructure according to the present invention contains at least one element from the element group consisting of (Cr, Si) that has a remarkably high affinity with oxygen relative to the constituent elements. It is possible to further improve oxidation resistance by adding less than .%. However, when the content of (Cr, Si) exceeds 5 at.%, an additional intermetallic compound is formed, which is not preferable.

In the method of manufacturing a heat-resistant high-entropy superalloy having a BCC two-phase composite structure according to the present invention, the raw material is (Ti,Zr,Hf) _100-xy (Nb,Ta,Mo,W) _x Al _y (However, 30≦x≦50, 10≦y≦20 at.%), preparing an alloy by dissolving the raw material, and controlling the microstructure through a subsequent heat treatment process.

In the present invention, the subsequent heat treatment step can be largely performed in two ways, and first, aging heat treatment is performed for 1 to 96 hours at a heat treatment temperature in the range of 1000°C to 1300°C where the two phases A2 and B2 achieve thermodynamic equilibrium. Thus, it is possible to control the shape, size and fraction of Rule B2 deposited on the base.

In addition, after the second step of dissolving the raw material to prepare an alloy, a step of quenching after a solution treatment for 1 to 48 hours at a temperature range of 1300°C to 1600°C where a single BCC solid solution is stable May contain additionally. By adding such a heat treatment step, a homogenized A2 single phase can be obtained, and the shape, size, and fraction of the B2 precipitated phase can be more effectively and precisely controlled by sequentially performing the first heat treatment step.

Accordingly, the microstructure of the heat-resistant high-entropy superalloy having the BCC two-phase composite structure according to the present invention can be controlled according to characteristics.

Table 1 shows the microstructure according to the composition and process of the alloy expressed by the atomic ratio (at.%) of Examples 1 to 8 and Comparative Examples 1 to 5 of the present invention.

Ti Zr Hf Nb Mo Ta W Al Heat tretment Microstructure Example 1 25 25 30 20 As-cast ingot B2 in A2 Example 2 15 15 50 20 As-cast ingot B2 in A2 Example 3 27.5 27.5 30 15 As-cast ingot B2 in A2 Example 4 25 12.5 12.5 20 10 20 As-cast ingot B2 in A2 Example 5 25 12.5 12.5 20 5 5 20 As-cast ingot B2 in A2 Example 6 25 12.5 12.5 20 5 2.5 2.5 20 As-cast ingot B2 in A2 Example 7 30 15 15 20 10 10 1200℃ 72h
Quenching B2 in A2 Example 8 20 20 30 10 20 1200℃ 72h
Quenching
after
1400℃ 12h B2 in A2 Comparative Example 1 25 25 25 25 1300℃ 6h
Quenching A2 Comparative Example 2 25 25 25 25 As-cast ingot B2 Comparative Example 3 50 29 21 As-cast ingot B2 + Orthorhombic Comparative Example 4 20 10 10 20 20 20 1500℃ 6h
Quenching A2 + compounds Comparative Example 5 10 10 60 20 As-cast ingot A2 + compounds

3 is a microstructure of a high entropy heat-resistant superalloy having a BCC two-phase composite structure prepared in the compositions of Examples 1 to 8. As can be seen from the drawings, when an alloy is formed and manufactured as in Examples 1 to 8 of the present invention, a microstructure in which the B2 phase is distributed in the form of nanoprecipitates within the matrix of the A2 phase can be formed.

However, if Al is not included as in Comparative Example 1, the alloy is composed of only the A2 phase, which is an irregular BCC, and if Al is composed at the level of 25 at.% as in Comparative Example 2, the A2 phase cannot be stabilized, so that the B2 single phase alloy is Is formed. In addition, when Zr or Hf is not included as in Comparative Example 3, a Ti-Al-Nb compound having an orthorhombic structure as well as a B2 phase is formed.

4 is a microstructure of an alloy prepared in Comparative Example 4 and Comparative Example 5. When containing 10 at.% or more of Mo as in Comparative Example 4, a process structure by at least one or more eutectic reactions appears, and when 50 at.% or more of Nb is included as in Comparative Example 5, Nb- Al intermetallic compounds are deposited in the form of a plate.

5 is a room temperature compression test results of the alloys according to Example 1 and Comparative Example 3 of the present invention. As can be seen from the drawing, extreme brittleness occurs when only the regular B2 phase is included as in Comparative Example 3, and the B2 phase in the base of the A2 phase like the heat-resistant high-entropy superalloy having the BCC two-phase composite structure of the present invention. When distributed in the form of precipitates, a very high elongation of 30% or more can be secured while maintaining a high strength of 1400 MPa or more.

The present invention has been described above through preferred embodiments, but the above-described embodiments are only illustrative of the technical idea of the present invention, and various changes are possible within the scope not departing from the technical idea of the present invention. Those of ordinary skill will understand. Therefore, the scope of protection of the present invention should be interpreted not by specific embodiments, but by the matters described in the claims, and all technical ideas within the scope equivalent thereto should be interpreted as being included in the scope of the present invention.

Claims (6)

The total alloy composition is expressed as follows (Chemical Formula);
The content of Ti in the entire composition is at least 15 at.% or more,
The sum of the contents of Zr and Hf in the entire composition is at least 15 at.% or more,
The sum of the Mo and W content in the entire composition is 10 at.%,
In the heat treatment process, characterized in that the BCC phase separation transformation occurs through a phase separation mechanism inside the solubility gap,
Heat-resistant high-entropy superalloy with BCC two-phase composite structure.
(Chemical formula)
(Nb _1-abc Ta _a Mo _b W _c ) _x Al _y (Ti _1-de Zr _d Hf _e ) _100-xy
(However, 0≤a≤1, 0≤b≤1, 0≤c≤1, 0≤d≤1, 0≤e≤1, 0≤a+b+c≤1, 0≤d+e≤1 , 30≤x≤50, 10≤y≤20 at.%)
The method according to claim 1,
In the heat-resistant high-entropy superalloy, the B2 phase (rule BCC) is distributed as a precipitated phase in the A2 phase (irregular BCC) base;
The precipitated phase is a heat-resistant high-entropy superalloy having a BCC two-phase composite structure, characterized in that it has an average particle size of 0.01 to 10 μm.
The method according to claim 1,
A heat-resistant high-entropy superalloy having a BCC two-phase composite structure, characterized in that the oxidation resistance is improved by adding at least one or more elements in the element group consisting of (Cr, Si) by 5 at.% or less.
Prepare the parent element of the entire alloy as follows (Chemical Formula),
The content of Ti in the entire composition is at least 15 at.% or more,
The sum of the contents of Zr and Hf in the entire composition is at least 15 at.% or more,
Preparing the sum of the Mo and W contents to be 10 at.% or less in the entire composition;
Dissolving the raw material to prepare an alloy; And
Aging heat treatment for 1 hour to 96 hours at a heat treatment temperature in the range of 1000° C. to 1300° C. in which the two phases of A2 and B2 achieve thermodynamic equilibrium, thereby causing a phase separation transformation of BCC through a phase separation mechanism within the solubility gap;
Method for producing a heat-resistant high-entropy superalloy having a BCC two-phase composite structure, characterized in that produced through
(Chemical formula)
(Nb _1-abc Ta _a Mo _b W _c ) _x Al _y (Ti _1-de Zr _d Hf _e ) _100-xy
(However, 0≤a≤1, 0≤b≤1, 0≤c≤1, 0≤d≤1, 0≤e≤1, 0≤a+b+c≤1, 0≤d+e≤1 , 30≤x≤50, 10≤y≤20 at.%)
The method of claim 4,
Heat-resistant high entropy superalloy manufacturing method having a BCC two-phase composite structure, characterized in that the oxidation resistance is improved by adding at least one or more elements from the element group consisting of (Cr, Si) to the raw material by 5 at.% or less .
The method of claim 4,
After the step of preparing an alloy by dissolving the raw material, a single BCC solid solution is stable at a temperature range of 1300° C. or higher and 1600° C. for 1 hour to 48 hours, followed by quenching after a solution treatment. Characterized in, the method for producing a heat-resistant high entropy superalloy having a BCC two-phase composite structure.

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