CN108385010B - A cobalt-based superalloy with low density and high microstructure stability and preparation method thereof - Google Patents

Abstract

A novel gamma' phase strengthened cobalt-based high-temperature alloy with low density and high structure stability and a preparation method thereof belong to the technical field of design and development of new materials. The chemical components are as follows by atomic percent: 5-11% of Al, 0.01-3% of W, 20-35% of Ni, 8-18% of Cr, 1-6% of Mo, 0.01-1% (optional one of Y/Ce/La), 0.01-1% of Si, 0.01-1% of B, 0.01-1% of C, 0.01-1% of Zr, 0.01-1% of Hf, 0-2% of Ta, 0-4% of Ti, 0-4% of Fe, 0-4% of Nb and the balance of Co. The designed alloy components reasonably optimize W, Mo, Si and Y/La/Ce elements, so that the alloy density can be obviously reduced, and the high-temperature performance of the alloy is improved; the developed smelting process can avoid the phenomena of burning loss of low-melting-point elements and uneven melting of high-melting-point elements, and improve the accuracy and uniformity of chemical components of the cast ingot; compared with the similar deformation cobalt-based high-temperature alloy, the developed alloy has lower density and higher stability of medium-temperature structure performance, and is an excellent deformation cobalt-based high-temperature structural material.

Description

A cobalt-based superalloy with low density and high microstructure stability and preparation method thereof

Technical field:

The invention belongs to the field of new material design and development, and particularly provides a low-density, high-structure-stability γ' phase-strengthened cobalt-based superalloy composition and a preparation method thereof.

Background technique:

Compared with nickel-based superalloys, cobalt-based superalloys have excellent thermal corrosion resistance, thermal fatigue resistance and welding performance. In particular, the discovery of γ′ phase-strengthened Co-Al-W-based alloys opens a new path for the development of new cobalt-based superalloys [Sato J, Omori T, Oikawa K, et al. Cobalt-base high-temperature alloys[J ]. Science, 2006, 312(5770):90-1].

M.Novel wroughtγ/γ′cobalt base superalloyswith high strength and improved oxidation resistance[J].Scripta Materialia,2015,109:104-107]；Dye等人采用粉末冶金的方法制备了新型形变钴基高温合金，该合金与MarM247有着相似的屈服强度，在温度高于750℃后甚至有着更高的强度[Knop M,MulveyP,Ismail F,et al.A New Polycrystalline Co-Ni Superalloy[J].JOM,2014,66(12):2495-2501.]。The research and development time of new γ' phase-strengthened cobalt-based superalloys is relatively short, and the relevant research is still very limited. Cobalt-based superalloys, so far only the German Neumeier research group and the British Dye research group have carried out related research. Neumeier et al. prepared a new high-performance deformed cobalt-based superalloy by the casting-rolling method. Compared with the refractory nickel-based superalloy U720Li, the alloy has a larger hot working range, and the yield strength is higher than that at temperatures above 800 °C. U720Li alloy [Neumeier S, Freund LP,

M.Novel wroughtγ/γ′cobalt base superalloys with high strength and improved oxidation resistance[J].Scripta Materialia, 2015, 109:104-107]; Dye et al. prepared a new type of deformed cobalt-based superalloy by powder metallurgy. The alloy has a similar yield strength to MarM247, and even higher strength at temperatures higher than 750 °C [Knop M, MulveyP, Ismail F, et al.A New Polycrystalline Co-Ni Superalloy[J].JOM,2014,66 (12):2495-2501.].

However, the currently developed new Co-Al-W-based alloys contain 5 at% to 10 at% of W elements, resulting in a high alloy density, even higher than 9.0 g/cm ³ , while the density of the traditional deformed nickel-based superalloy Wasploy is only 8.2 g/cm ³ . The high density of the new cobalt-based superalloy will become one of the important factors limiting its application in the hot-end components of aviation and rocket engines. Therefore, reducing the density of new cobalt-based superalloys is one of the keys to realize the practical application of such alloys. Under the premise of ensuring the stability of the alloy structure and properties, reasonable substitution of W element is the most effective way to reduce the density of cobalt-based superalloys.

Invention content:

The invention aims to reduce the density of a new type of cobalt-based superalloy, and develops a new type of γ' phase deformation cobalt-based superalloy with low density and high stability of microstructure and performance through the selection and optimization of alloy elements and the formulation of a reasonable preparation process.

The technical scheme of the present invention is:

A cobalt-based superalloy with low density and high microstructure stability, its chemical composition in atomic percentage is: 5-11% Al, 0.01-3% W, 20-35% Ni, 8-18% Cr, 1-6% %Mo, 0.01-1% (any one of Y/Ce/La), 0.01-1% Si, 0.01-1% B, 0.01-1% C, 0.01-1% Zr, 0.01-1% Hf, 0 -2% Ta, 0-4% Ti, 0-4% Fe, 0-4% Nb, balance Co.

The preparation process of the cobalt-based superalloy with low density and high microstructure stability as mentioned above includes the process of master alloy melting and alloy melting, and the main steps are as follows:

(1) Considering the high melting point elements such as W, Mo, Ta, Nb, etc., the Co-Mo-W-Ta-Nb master alloy is first smelted to reduce the melting point of the alloy and prevent the phenomenon of insufficient melting of the high melting point alloy during the smelting process. At the same time, alloy elements with less content, such as: Si, B, C, Zr, Hf, Y/Ce/La, etc., are added to the master alloy together to increase the uniformity of these low-content elements.

(2) Alloy elements other than Al and Ti elements, including Ni, Cr, Fe, etc., are put into the crucible together with the master alloy, and the easily oxidizable Al and Ti elements are put into the hopper, so that in the smelting process join in.

(3) Use a vacuum induction furnace for smelting. When the vacuum degree in the furnace is lower than 5×10 ^-2 Pa, start low-power electric heating to remove the adhering gas on the raw material, and continue to evacuate to 1×10 ^-2 Pa. Carry out high-power rapid heating to 1500℃-1600℃, hold for 10 minutes, then reduce the temperature to 1300℃-1400℃, hold for 5 minutes, add Al and Ti elements in the hopper, and then quickly heat up to 1500℃-1600℃ immediately, Pouring after 10-15 minutes of heat preservation to prepare a cobalt-based superalloy ingot.

Advantages of the present invention:

(1) In the composition design of the present invention, the comprehensive influence of each element on the cost, density, microstructure and properties of the new cobalt-based superalloy, especially the careful selection and optimization of W, Mo, Si and Y/La/Ce elements , played a significant role in the reduction of alloy density and the improvement of high temperature performance. Specific considerations are as follows:

Aluminum: Al element is a γ' phase forming element. Due to the low density of Al, the addition of Al can significantly reduce the alloy density. Too high Al element easily leads to the formation of β phase, so the content of Al element is 5-11 at%.

Tungsten: W element is a γ' phase forming element and a solid solution strengthening element. Due to the high density of W (ρ=19.3g/cm ³ ), the addition of W element will greatly increase the alloy density, so the content of W element is 0.01 -3at%.

Nickel: Ni element is a γ' phase forming element. The addition of Ni element is beneficial to expand the γ/γ' two-phase region and greatly improve the microstructure stability of cobalt-based superalloy. Therefore, the content of Ni element is 20-35at%.

Chromium: Cr element is an important anti-oxidation and anti-corrosion alloy element. The addition of Cr element can form a dense chromium-rich oxide film on the surface of the material under service conditions, preventing further oxidation of the material. Excessive Cr element will lead to instability of the γ/γ' two-phase structure, and it is easy to precipitate the σ phase at the grain boundary, so the content of Cr element is 8-18 at%.

Molybdenum: Mo element is an important solid solution strengthening element and γ' phase stabilizing element. Too high Mo element will lead to the formation of harmful TCP phase, so the content of Mo element is 1-6at%.

Yttrium/Lanthanum/Cerium: Y/La/Ce elements can purify the matrix and grain boundaries, and significantly improve the mechanical and oxidation resistance of the alloy. Excessive addition of the above elements will form harmful phases and deteriorate the performance, so Y, La, Ce elements are selected among them An addition, the content is 0.01-1at%.

Silicon: The addition of Si element can reduce the thickness of the oxide layer, promote the formation of chromium-rich oxide film, and improve the oxidation resistance. The addition of a large amount of Si element will reduce the stability of the alloy γ/γ′ two-phase structure, so the content of Si element is 0.01-1at%.

Boron: B element is an important grain boundary strengthening element, segregating at the grain boundary to enhance the grain boundary strength, but excessive B element will form excess borides, which will weaken the bonding force of the grain boundary, so the content of B element is 0.01-1at %.

Carbon: C element is also an important grain boundary strengthening element. Excessive C element will lead to the formation of film-like carbides at the grain boundary and deteriorate its mechanical properties, so the content of C element is 0.01-1at%.

Zirconium: Zr element is an important grain boundary strengthening element, which plays an important role in removing harmful impurities such as sulfur and phosphorus, but excessive Zr element will deteriorate its mechanical properties, so the content of Zr element is 0.01-1at%.

Hafnium: Hf element is a γ' phase forming element. Hf can also play an important role in purifying grain boundaries, but Hf element is expensive, so the content of Hf element is 0.01-1at%.

Tantalum: Ta element is an important γ' phase forming element, which can effectively improve the mechanical properties of cobalt-based superalloys. Considering factors such as density and cost comprehensively, the content of Ta element is 0-2at%.

Titanium: Ti element is an important γ' phase forming element, which can effectively improve the mechanical properties of cobalt-based superalloys. Too high Ti element will lead to the formation of β phase and the significant reduction of the hot working range, which will deteriorate the hot working performance. Therefore, the content of Ti element is 0-4 at%.

Iron: The addition of Fe element can effectively reduce the cost and density of the alloy, but excessive Fe element will reduce the stability of the γ/γ' two-phase structure and the precipitation of TCP phase, so the content of Fe element is 0-4at%.

Niobium: Nb element is a γ' phase forming element. The addition of Nb element can reduce the density, but excessive Nb element will lead to the precipitation of TCP phase, so the content of Nb element is 0-4at%.

(2) The smelting process raw materials of the present invention are melted according to the melting point sequence, which can avoid problems such as burning loss of low-melting-point metals and insufficient melting of high-melting-point elements, and can effectively avoid the occurrence of uneven composition and macroscopic appearance in cobalt-based superalloy ingots. Serious segregation and other defects improve the accuracy and uniformity of the chemical composition of the ingot.

(3) Compared with the similar γ' phase strengthened deformed cobalt-based/nickel-based superalloys reported recently, the density of the alloy of the present invention is reduced by 0.2-0.5 g/cm ³ , the hot working range is increased by 100-150° C. After aging at 700-900℃ for 1000-3000h, it still maintains a stable γ/γ′ two-phase structure. It can be seen that compared with similar deformed cobalt-based superalloys, the invented alloy has lower density and higher stability of microstructure and properties at medium temperature, and is an excellent deformed cobalt-based high-temperature structural material.

Description of drawings:

Figure 1 shows the γ/γ' two-phase structure of the LAMP-2 cobalt-based deformed superalloy of the present invention after solution-aging treatment

Figure 2 shows the γ/γ' two-phase structure of the LAMP-2 cobalt-based deformed superalloy of the present invention after solution-aging treatment and thermal exposure at 750°C for 2000 hours

Detailed ways

The technical solutions of the present invention will be further described below with reference to the accompanying drawings and embodiments.

Example 1: Co-Ni-Cr-Al-W-Mo-Ti-Ta-based deformed superalloy

Referring to the influence of various elements on the microstructure and properties of the new γ' phase-strengthened cobalt-based superalloy, a new Co-Ni-Cr-Al-W-Mo-Ti-Ta-based deformed superalloy was developed in this paper. The specific components are shown in Table 1. . In addition to the components in the table, the two alloys of LAMP-1 and LAMP-2 also contain Y: 0.05 at%, Si: 0.2 at%, Zr: 0.06 at%, and Hf: 0.1 at%.

The preparation process of Co-Ni-Cr-Al-W-Mo-Ti-Ta based deformed superalloy is as follows:

(1) Considering the high melting point elements such as W, Mo, Ta, etc., the Co-Mo-W-Ta master alloy is first smelted to reduce the melting point of the alloy and prevent uneven melting of the high melting point alloy during the smelting process. The alloying elements, such as: Si, B, C, Zr, Hf, Y, etc., are added to the master alloy together to increase the uniformity of these low-content elements.

(2) Alloy elements other than Al and Ti elements, including Ni, Cr, etc., are put into the crucible together with the master alloy, and the easily oxidizable Al and Ti elements are put into the hopper so as to be added during the smelting process.

(3) Use a vacuum induction furnace for smelting. When the vacuum degree in the furnace is lower than 5×10 ^-2 Pa, start low-power electric heating to remove the adhering gas on the raw material, and continue to evacuate to 1×10 ^-2 Pa. Carry out high-power rapid heating to 1600 °C, hold for 10 minutes, then reduce the temperature to 1400 °C, hold for 5 minutes, add Al and Ti elements in the hopper, then immediately heat up to 1600 °C rapidly, hold for 10 minutes, pour, and prepare cobalt Base superalloy ingots.

The alloy was solution treated at 1230°C for 24h and then air-cooled, and after 8h primary aging at 900°C and 12h secondary ageing at 750°C, a stable γ/γ' two-phase structure was obtained, and the volume fraction of γ' phase was About 55% to 65% (as shown in Figure 1), all of which are cubic or nearly cubic, and are uniformly distributed in the γ phase. After the alloy was exposed to heat at 750℃ for 2000h, it still maintained a stable γ/γ′ two-phase structure (as shown in Figure 2). Compared with the recently reported γ' phase-strengthened deformed cobalt-based superalloy (CoWAlloy2), the density of the alloy is reduced by 0.2-0.5 g/cm ³ , and the hot working range is increased compared with that of the hard-to-deform nickel-based superalloy (U720Li). At 110-150℃, the room temperature hardness is comparable to that of U720Li alloy (as shown in Table 2), and it is an excellent deformed cobalt-based high-temperature structural material.

Table 1 Atomic percentage and density of each element in Co-Ni-Cr-Al-W-Mo-Ti-Ta based alloy

[Data from CoWAlloy2 alloy: Freund L P, Giese S, Schwimmer D, et al. High temperature properties and fatigue strength of novel wroughtγ/γ′Co-base superalloys [J]. Journal of Materials Research, 2017: 1-8.]

Table 2 Room temperature hardness and hot working range of Co-Ni-Cr-Al-W-Mo-Ti-Ta based alloys

[U720Li alloy data from: Zhou L Z, Lupinc V, Guo J T.Evolution of Microstructure and Mechanical Property during Long-Term Aging in Udimet720Li[J].Journal of Materials Science&Technology,2001,17(6):633-637.]

Example 2: Co-Ni-Cr-Al-W-Mo-Ti-Nb-based deformed superalloy

Referring to the influence of various elements on the microstructure and properties of the new cobalt-based superalloy, a new Co-Ni-Cr-Al-W-Mo-Nb-based deformation superalloy was developed in this paper. The specific alloy composition is shown in Table 3. In addition to the composition in the table, the 3# and 4# alloys also contain La: 0.05at%, Si: 0.2at%, B: 0.04at%, C: 0.05at%, Zr: 0.06at%, Hf: 0.1at%.

The preparation process of the new Co-Ni-Cr-Al-W-Mo-Ti-Nb-based deformation superalloy is as follows:

(1) Considering the high melting point elements such as W, Mo and Nb, the Co-Mo-W-Nb master alloy is first smelted to reduce the melting point of the alloy and prevent uneven melting of the high melting point alloy during the smelting process. The alloying elements, such as: Si, B, C, Zr, Hf, La, etc., are added to the master alloy together to increase the uniformity of these low-content elements.

(2) Alloy elements other than Al and Ti elements, including Ni, Cr, etc., are put into the crucible together with the master alloy, and the easily oxidizable Al and Ti elements are put into the hopper so as to be added during the smelting process.

(3) Use a vacuum induction furnace for smelting. When the vacuum degree in the furnace is lower than 5×10 ^-2 Pa, start low-power electric heating to remove the adhering gas on the raw material, and continue to evacuate to 1×10 ^-2 Pa. Carry out high-power rapid heating to 1550 °C, hold for 10 minutes, then reduce the temperature to 1350 °C, hold for 5 minutes, add Al and Ti elements in the hopper, and then quickly heat up to 1550 °C immediately, keep warm

Pouring after 10 minutes to prepare a cobalt-based superalloy ingot.

The alloy was solution treated at 1230°C for 12 hours and then air-cooled, and after 4 hours of primary aging at 900°C and 12 hours of secondary aging at 700°C, a stable γ/γ' two-phase structure can be obtained, and the volume fraction of γ' phase can be obtained. About 40%-50%, all of which are cubic or nearly cubic, and are uniformly distributed in the γ phase. After the alloy was exposed to heat at 700℃ for 2000h, it still maintained a stable γ/γ′ two-phase structure. Compared with the recently reported similar γ' phase-strengthened deformed cobalt-based superalloy (CoWAlloy2), the density of the alloy is reduced by 0.3-0.5g/cm ³ , and the hot working range is improved compared with the refractory nickel-based superalloy (U720Li). At 120-140°C, the room temperature hardness is comparable to that of U720Li alloy (as shown in Table 4), and it is an excellent deformed cobalt-based high-temperature structural material.

Table 3 Atomic percentage and density of each element in Co-Ni-Cr-Al-W-Mo-Ti-Nb-based alloy

Table 4 Room temperature hardness and hot working range of Co-Ni-Cr-Al-W-Mo-Ti-Nb based alloys

Example 3: Co-Ni-Cr-Al-W-Mo-Ti-Fe-based deformed superalloy

Referring to the influence of various elements on the microstructure and properties of the new cobalt-based superalloy, a new Co-Ni-Cr-Al-W-Mo-Fe-based deformation superalloy was developed in this paper. The specific alloy composition is shown in Table 5. In addition to the components in the table, the alloy contains Ce: 0.05at%, Si: 0.25at%, B: 0.06at%, C: 0.05at%, Zr: 0.04at%, and Hf: 0.05at%.

The preparation process of Co-Ni-Cr-Al-W-Mo-Ti-Fe based deformed superalloy is as follows:

(1) Considering the high melting point elements such as W and Mo, the Co-Mo-W master alloy is first smelted to reduce the melting point of the alloy and prevent uneven melting of the high melting point alloy during the smelting process. Such as: Si, B, C, Zr, Hf, Ce, etc., are added to the master alloy together to increase the uniformity of these low-content elements.

(2) Alloy elements other than Al and Ti elements, including Ni, Cr, Fe, etc., are put into the crucible together with the master alloy, and the easily oxidizable Al and Ti elements are put into the hopper, so that in the smelting process join in.

(3) Use a vacuum induction furnace for smelting. When the vacuum degree in the furnace is lower than 5×10 ^-2 Pa, start low-power electric heating to remove the adhering gas on the raw material, and continue to evacuate to 1×10 ^-2 Pa. Carry out high-power rapid heating to 1500 °C, hold for 10 minutes, then reduce the temperature to 1300 °C, hold for 5 minutes, add Al and Ti elements in the hopper, then quickly heat up to 1500 °C immediately, hold for 15 minutes and then pour to prepare cobalt Base superalloy ingots.

The alloy was solution treated at 1220°C for 12 hours and then air-cooled, and after 4 hours of primary aging at 900°C and 12 hours of secondary aging at 700°C, a stable γ/γ' two-phase structure can be obtained, and the volume fraction of γ' phase can be obtained. About 35% to 45%, all of which are spherical or nearly cubic, and are uniformly distributed in the γ phase. After the alloy was exposed to heat at 700℃ for 2000h, it still maintained a stable γ/γ′ two-phase structure. Compared with the recently reported similar γ' phase-strengthened deformed cobalt-based superalloy (CoWAlloy2), the density of the alloy is reduced by about 0.4 g/cm ³ , and the hot working range is increased by 120 compared with the refractory nickel-based superalloy (U720Li). -140 °C, the room temperature hardness is comparable to that of U720Li alloy (as shown in Table 6), and it is an excellent deformed cobalt-based high-temperature structural material.

Table 5 Atomic percentage and density of each element in Co-Ni-Cr-Al-W-Mo-Ti-Fe based alloy

Table 6 Room temperature hardness and hot working range of Co-Ni-Cr-Al-W-Mo-Ti-Fe based alloys

Claims (2)

1. A cobalt-based high-temperature alloy with low density and high structure stability is characterized in that the alloy comprises the following chemical components in atomic percent: 5-11% of Al, 0.01-3% of W, 20-35% of Ni, 8-18% of Cr, 1-6% of Mo, 0.01-1% of Si, 0.01-1% of B, 0.01-1% of C, 0.01-1% of Zr, 0.01-1% of Hf, one of Y, Ce or La, 0.01-1% of Hf, 0-2% of Ta, 0-4% of Ti, 0-4% of Fe, 0-4% of Nb and the balance of Co;

the cobalt-based alloy with low density and high structure stability is a gamma '-phase reinforced cobalt-based high-temperature alloy which mainly depends on gamma' -Co which is compatible with a matrix₃Strengthening by (Al, W) phase;

after the alloy is subjected to solution treatment and aging at the temperature of 700 ℃ for 1000-3000h, a stable gamma/gamma' two-phase structure is still kept;

the alloy is of the same kindCompared with the gamma' phase strengthening deformation cobalt-based high-temperature alloy, the density is reduced by 0.2 to 0.5g/cm³Compared with the hard-deformation nickel-based high-temperature alloy U720Li, the hot working range is improved by 100-150 ℃, and the room temperature hardness is equivalent to that of the U720Li alloy.

2. The method for preparing a cobalt-based superalloy with low density and high structural stability as defined in claim 1, wherein the method comprises an intermediate alloy smelting process and an alloy smelting process, and the preparation steps are as follows:

(1) firstly, smelting Co-Mo-W-Ta-Nb intermediate alloy, reducing the melting point of the alloy, and preventing the phenomenon of insufficient melting of high-melting-point alloy in the smelting process; meanwhile, alloying elements with small content, such as Si, B, C, Zr, Hf and Y/Ce/La, are added into the intermediate alloy together to increase the uniformity of the low-content elements;

(2) putting alloy elements, namely simple substances Ni, Cr and Fe except Al and Ti, and an intermediate alloy into a crucible, and putting easily oxidized Al and Ti into a hopper so as to be added in the smelting process;

(3) smelting in a vacuum induction furnace with the vacuum degree lower than 5 × 10^-2When Pa is reached, the raw material is heated by low-power electric power transmission to remove the adhering gas, and the vacuum is continuously pumped to 1 × 10^-2And when Pa is needed, rapidly heating to 1500-1600 ℃ with high power, preserving heat for 10 minutes, then reducing the temperature to 1300-1400 ℃, preserving heat for 5 minutes, adding Al and Ti elements in a hopper, immediately and rapidly heating to 1500-1600 ℃, preserving heat for 10-15 minutes, and then pouring to prepare the cobalt-based high-temperature alloy ingot.

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