RU2186144C1 - Refractory nickel alloy for single-crystal casting and product made from this alloy - Google Patents

Abstract

FIELD: nonferrous metallurgy. SUBSTANCE: alloy contains, wt %: carbon 0.001-0.05, chromium 2.0-3.0, cobalt 9.5-11.0, titanium 0.1-1.2, aluminum 5.5- 6.2, tungsten 1.0-3.0, tantalum 6.0-9.0, rhenium 8.0-11.0. niobium 0.6-2.6, boron 0.001-0.02, cerium 0.001-0.02, lanthanum 0.001-0.02, yttrium 0.001-0.02, and nickel - the balance. Alloy is used to manufacture preferably gas turbine rotor and nozzle blades operated at temperatures above 1000 C. Castings are not prone to recrystallization upon heat treatment. EFFECT: increased refractoriness level, improved workability in manufacture of blades by oriented crystallization technique, and prevented formation of surface "jet liquation"- type defects. 2 cl, 1 tbl

Description

The invention relates to the field of metallurgy, and in particular to casting heat-resistant nickel-based alloys intended for the production of directional crystallization of parts, mainly single-crystal working and nozzle blades, high-temperature gas turbines operating for a long time at temperatures exceeding 1000 ^o C.

 Heat-resistant alloys ZhS-36 (1) and CSMX-4 (2) are well known and have found application as material for single-crystal gas turbine blades. ZhS-36 and CSMX-4 alloys are carbon-free monocrystal rhenium-containing alloys of the second generation.

ZhS-36 alloy has the following chemical composition, wt.%:
Chrome - 2.5-5.5
Cobalt - 5.0-9.5
Aluminum - 5.0-6.2
Titanium - 0.7-1.5
Molybdenum - 1.0-4.0
Tungsten - 10.5-13.0
Tantalum - 0.01-4.0
Rhenium - 1.0-2.6
Niobium - 0.7-1.5
Yttrium - 0.002-0.075
Lanthanum - 0.001-0.05
Cerium - 0.001-0.05
Praseodymium - 0.002-0.01
Neodymium - 0.0002-0.005
Gadolini - 0.0002-0.005
Scandium - 0.0002-0.005
Nickel - Base
The alloy is intended for casting vanes with a single crystal structure having a predominantly crystallographic orientation [111]; in this orientation, the alloy has a high level of heat resistance (σ $\genfrac{}{}{0ex}{}{\text{1000}}{\text{100}}$ = 31 kgf / mm ² ). For blades having an internal cooling cavity, the crystallographic orientation [001] is more preferred; the level of heat resistance characteristics of this alloy with the [001] orientation is not high enough (σ $\genfrac{}{}{0ex}{}{\text{1000}}{\text{100}}$ = 25 kgf / mm ² ). In addition, alloying the alloy with a large number of elements with direct segregation (W, Re) at a relatively low content of tantalum in the alloy limits the possibility of producing single-crystal castings from this alloy on casting equipment with a low gradient at the crystallization front due to the possibility of formation of surface defects such as "jet segregation. "

A similar level of heat resistance to the ZhS-36 alloy is also possessed by the CMSX-4 alloy (σ $\genfrac{}{}{0ex}{}{\text{1000}}{\text{100}}$ = 26 kgf / mm ² ) of the following chemical composition, wt.%:
Cobalt - 9.3-10
Chrome - 6.4-6.8
Molybdenum - 0.5-0.7
Tungsten - 6.2-6.6
Tantalum - 6.3-6.7
Aluminum - 5.45-5.75
Titanium - 0.8-1.2
Hafnium - 0.02-0.12
Rhenium - 2.8-3.2
Nickel - Base
However, the CMSX-4 alloy also does not have a high level of heat resistance and exhibits phase instability associated with the release of TPU phases.

Products obtained from alloys ZhS-36 and CMSX-10 have an insufficient level of heat resistance during long-term operation in the temperature range of 900-1100 ^o C.

Closest to the proposed chemical composition and purpose, taken by the authors for the prototype, is the alloy CMSX-10 (3), having the following chemical composition, containing, wt.%:
Carbon - Up to 0.04
Chrome - 1.8-2.5
Cobalt - 1.5-2.5
Titanium - 0.1-0.5
Aluminum - 5.5-6.1
Molybdenum - 0.25-0.6
Tungsten - 3.5-6.0
Tantalum - 8.0-9.0
Rhenium - 6.2-6.8
Niobium - 0.01-0.1
Hafnium - Up to 0.04
Boron - Up to 0.01
Yttrium - Up to 0.01
Cerium - Up to 0.01
Lanthanum - Up to 0.01
Manganese - Up to 0.04
Silicon - Up to 0.05
Zirconium - Up to 0.01
Sulfur - Up to 0.001
Vanadium - Up to 0.01
Nickel - Base
The alloy is used to produce castings of blades with a single crystal structure. Single crystals of the CMSX-10 alloy of crystallographic orientation [001] for melts of various compositions within the patent have high values of long-term strength at (σ $\genfrac{}{}{0ex}{}{\text{1000}}{\text{100}}$ = 31-32 kgf / mm ² ). However, a relatively high dispersion of the durability of the samples during creep tests is noted, which indicates an insufficient balance of the chemical composition of the prototype. In addition, the tasks of creating products of new technology require a higher level of thermal performance than the achieved level of products from the CMSX-10 alloy.

The technical task of the invention was the development of an alloy with a higher level of heat resistance while maintaining technological characteristics. To solve this problem, an alloy of the following chemical composition is proposed, wt.%:
Carbon - 0.001-0.05
Chrome - 2.0-3.0
Cobalt - 9.5-11.0
Titanium - 0.1-1.2
Aluminum - 5.5-6.2
Tungsten - 1.0-3.0
Tantalum - 6.0-9.0
Rhenium - 8.0-11.0
Niobium - 0.6-2.6
Boron - 0.001-0.02
Cerium - 0.001-0.02
Lanthanum - 0.001-0.02
Yttrium - 0.001-0.02
Nickel - Other
The main differences of the proposed alloy from the known are:
1) the increased content of elements such as cobalt, rhenium, niobium; 2) low tungsten content; and 3) the absence of molybdenum and hafnium in the alloy.

 The main difficulties arising in the development of alloys with a high rhenium content are associated with the creation of alloys balanced in relation to the release of adverse phases belonging to the category of topologically close packed phases (TPU phases), which, on the one hand, embrittle the alloy and, on the other, sharply soften solid solution as a result of removal from the solid solution of elements that strengthen it.

 TPU phases are formed, as a rule, in the axes of dendrites and are plates that stand out parallel to the planes of the {111} octahedron. Discharges of this type can form both after directed crystallization and heat treatment, and during prolonged exposure to temperature and stress. The structural stability of rhenium-containing alloys relative to the formation of topologically densely packed phases is determined by the ratio of the content of elements of the 6th group — chromium, molybdenum, tungsten, and rhenium — in the alloy. These elements are solid solution hardeners, with rhenium being the most effective, which is predominantly distributed in γ-solid solution. Having the lowest diffusion coefficient in nickel, rhenium inhibits the coagulation processes of the hardening γ′-phase, thereby increasing its thermal stability.

 In developing the present invention, it was assumed that while minimizing the content of chromium and tungsten in the alloy and excluding molybdenum from the composition, the tendency to form TPU phases will be determined by the ratio of tungsten and rhenium in the alloy.

 With a high total content of rhenium and tungsten in the alloy, stable compositions will be in the region of low tungsten content and high rhenium content. Preservation of chromium in the composition of the alloy is determined by its positive effect on the resistance to gas corrosion. In alloys with a high rhenium content, the chromium content can be reduced to the level of 2.0-3.0%, since rhenium itself is one of the elements that increase the resistance to gas corrosion.

 Molybdenum is a hardener of a solid solution, however, its most significant contribution is manifested in a change in the parameter of the γ-solid solution and, as a consequence, in the morphology of the hardening secondary γ ′ phase, making it cubic and thereby providing high creep resistance of heat-resistant alloys. Compensation for the complete exclusion of molybdenum from the composition of the alloy is possible only due to a complex change in the ratio of alloying elements that change the parameters of both the γ-solid solution and the hardening γ ′ phase. An increase in the rhenium content in the alloy due to a proportional decrease in tungsten and an increase in the content of niobium in the alloy, an element mainly involved in the strengthening γ′-phase, made it possible to determine the compositions in which it was possible to ensure the cubic morphology of the strengthening γ′-phase. In addition, alloying with niobium has a positive effect on the fluidity of the alloy during casting.

 The presence of microalloying elements in the alloy composition: cerium, lanthanum, yttrium and boron has a positive effect on the formation of a single crystal structure of castings in the process of directed crystallization.

 The proposed alloy provides a high yield of castings with a single crystal structure during casting and significantly exceeds the level of heat resistance of the prototype alloy.

Examples of implementation
In the vacuum-induction furnace VIAM16-35, 4 alloy melts in chemical composition within the scope of the present invention and 2 out-of-melting melts were melted (table 1). Single-crystal blanks with a diameter of 16 mm and a length of 180 mm of orientation <001> with a deviation from orientation not exceeding 5–7 ^° were obtained by directional crystallization using UVNK-8P units with liquid metal cooling. For all studied compositions, the yield of sample blanks suitable for single-crystal structure was> 90%.

The single-crystal sample blanks were subjected to high-temperature homogenization at a temperature above the dissolution of the secondary hardening γ′-phase and below the solidus temperature. Heating to the final homogenization temperature (≈1340 ^o ) was carried out with intermediate stepwise isothermal extracts, which avoided the appearance of a local reflow structure. From the homogenization temperature, the preforms were cooled at a rate of ~ 60 ^o / min and subjected to two-stage aging: 1) 1150 ^o C and 2) low-temperature at 900 ^o C. After the final heat treatment of the preforms in the individual interdendritic areas, the presence of incompletely dissolved primary γ′-eutectic phase, the amount of which was ≈2%. In the axes of dendrites and interdendritic sites, the secondary γ′-phase precipitates were cubic in size 0.25–0.35 μm in the axes of dendrites and ≈0.4 μm in the interdendritic regions.

 Lamellar precipitates of TPU phases were observed in the axes of dendrites for melting samples 7, which contained a prohibitive content of tungsten (4%) and rhenium (11.5%).

Assessment of the level of heat resistance of the proposed compositions was carried out at test temperatures of 900, 1000 and 1100 ^o C.

 The test results are presented in table 2. For comparison, the table shows the test results of the prototype alloy, taken from the patent (3).

Alloys of compositions 2-5, located in the alloying region of the proposed alloy, have a significantly higher level of properties than the alloy of the prototype. Microstructural analysis of the destroyed samples did not reveal the appearance of lamellar precipitates of TPU phases. Long-term annealing (500 h), specially carried out at compositions 3 and 4, at temperatures of 1000 and 1100 ^o C did not reveal the appearance of lamellar precipitates in the axes of dendrites.

 Extremely alloyed with chromium (4%), composition 6 with a reduced content of tungsten (0.5%) and rhenium (7.5%) had lower durability values than the proposed alloy.

 From the totality of the results it follows that the proposed alloy provides the level of heat resistance, significantly superior to the heat resistance of the prototype alloy. The alloy is technologically advanced in the production of single-crystal castings by directional crystallization and is not prone to the formation of surface defects such as "jet segregation". Castings are not subject to recrystallization during heat treatment. Alloy products are made by directional crystallization methods on various types of plants with both a low and a high temperature gradient at the crystallization front and have a high level of properties that can be used in new generation engines.

Literature
1. RF patent 1513934, MKI C 22 C 19/05.

 2. USA, Patent Number 4,643,782, Feb., 17, 1987. Int. Cl. C 22 C 19/05.

 3. USA, Patent Number 5,540,790, Jul., 30, 1996, Int. Cl. C 22 C 19/05.

Claims (1)

1. Nickel heat-resistant alloy for single crystal casting containing carbon, chromium, titanium, cobalt, aluminum, tungsten, tantalum, rhenium, niobium, boron, cerium, lanthanum and yttrium, characterized in that it has the following chemical composition, with a ratio of components, wt. %:
Carbon - 0.001-0.05
Chrome - 2.0-3.0
Cobalt - 9.5-11.0
Titanium - 0.1-1.2
Aluminum - 5.5-6.2
Tungsten - 1.0-3.0
Tantalum - 6.0-9.0
Rhenium - 8.0-11.0
Niobium - 0.6-2.6
Boron - 0.001-0.02
Cerium - 0.001-0.02
Lanthanum - 0.001-0.02
Yttrium - 0.001-0.02
Nickel - Other
2. A product from a heat-resistant casting alloy based on nickel, characterized in that it is made of an alloy of the following chemical composition, wt. %:
Carbon - 0.001-0.05
Chrome - 2.0-3.0
Cobalt - 9.5-11.0
Titanium - 0.1-1.2
Aluminum - 5.5-6.2
Tungsten - 1.0-3.0
Tantalum - 6.0-9.0
Rhenium - 8.0-11.0
Niobium - 0.6-2.6
Boron - 0.001-0.02
Cerium - 0.001-0.02
Lanthanum - 0.001-0.02
Yttrium - 0.001-0.02
Nickel - Other

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