EP1223229B1 - Nickel based alloy for casting single crystal components - Google Patents

Abstract

Nickel-based alloy contains alloying additions of rhenium in an amount of at least 2.3 wt.%, tungsten and further elements such as aluminum, chromium and cobalt. Preferred Features: Tungsten is present in an amount of 3.0-3.7 wt.%, aluminum in an amount of 6.2-6.8 wt.%, cobalt in an amount of 7.2-7.8 wt.%, chromium in an amount of 5.8 -6.4 wt.%, hafnium in an amount of 0.01-0.15, molybdenum in an amount of 1.7-2.3 wt.%, tantalum in an amount of 2.0-2.6 wt.% and titanium in an amount of 0.9-1.1 wt.%.

Description

The invention relates to a nickel-based alloy for the casting production of monocrystalline solidified components, according to the preamble of claim 1.

Alloys of this type belong to a group of so-called superalloys which can be used under high temperatures and under high mechanical stresses and are therefore used in particular as turbine blade materials in gas turbines.

The future generation of high bypass ratio aerospace engines and low speed, high speed turbines promises significant improvements in specific fuel consumption and emissions. The weight of the engine, its size and the maintenance costs also fall under the current value drivers.
High bypass ratio engines are equipped with a reduction gearbox connected between the fan on one side and the low pressure compressor and the low pressure turbine on the other side. The transmission allows the fan to operate in the optimum range at slow speeds, and has the potential to operate the compressor and turbine at higher speeds and therefore higher pressure than conventional turbines. However, the higher circumferential speeds also increase the mechanical loads of the blading and the discs of the low-pressure turbine.

Second and third generation Ni base alloys for single crystalline devices contain about 3% by weight and 6% by weight, respectively, of the refractory element rhenium and have better creep properties than corresponding first generation alloys without Re content. The refractory element Re has a different effect on the properties of superalloys. Re has a large atomic radius, diffuses very slowly and seeps into the matrix. In addition to the effect of Solid solution hardening of the matrix, the rhenium atoms tend to form clusters that impede a dislocation movement.
Tungsten contributes significantly to solid solution hardening. The W content affects the distribution of the Re on the matrix and the γ'-excretion phase.

The high melting point and the low diffusion coefficient of both Re and W lead to an increase in the solidus temperature of the superalloys. In addition, the morphology change of the excretion phase y 'is delayed under load.

Although the alloying element tantalum (Ta) contributes to solid solution hardening and improves the cyclic oxidation behavior, it is primarily added to W and Re-containing Ni base alloys in order to counteract the so-called Freckle formation in directional solidification.

Negative properties of Ta: strong increase in density; it promotes unwanted TCP phase formation and increases the gamma prime solution temperature.

The increase in creep strength is accompanied by a concomitant increase in density down to 9 g cm ^-3 for certain 6 wt% Re alloys. For Re-free alloys, the density can be lowered down to 8 g cm ^-3 . However, Ni-base alloys with a high specific gravity are only conditionally suitable for use in modern, high-speed aircraft turbines.

A low-density Re-free superalloy is known, for example, from US Pat. No. 4,721,540, the trade name of this material being "CMSX-6". However, apart from the mechanical advantage of a relatively low density of 7.98 g cm ^-3 , this alloy also has disadvantages such as a small heat treatment window and a high recrystallization tendency.

From the international publication WO 93/24683 single-crystal castings are known whose alloy 0 to 8 weight percent rhenium, 3 to 10 weight percent Tungsten (tungsten) and, inter alia, magnesium or calcium to increase the oxidation resistance. For a specific alloy composition, the Re content should be 2.8 to 3.2% by weight, the W content should be 5.6 to 6.2% by weight. Since rhenium and tungsten are heavy metals, a relatively high component density is to be expected here, especially; when the upper limits of 8 weight percent rhenium and 10 weight percent tungsten are used. Rhenium is also a very expensive element that has a noticeable effect on the component price. The lower limit for Re is given here as 0% by weight. Small re-anteite reduce weight and price, but also lead to a significant deterioration of important material properties.

In view of these disadvantages, the object of the invention is to provide nickel Besisiegierungen for the casting production of monocrystalline solidified components that allow by optimizing the proportions of rhenium and tungsten particularly favorable material and thus component properties, such as low density, high mechanical strength including low creep as well as high temperature resistance. In addition, easy castability and favorable heat-aging properties of the alloy are required.

This object is achieved by the alloying elements specified in claim 1 and their share ranges.

According to the invention, the rhenium content should be at least 2.3 to at most 2.6 percent by weight, the weight ratio of tungsten to rhenium should be at least 1 and not more than 1.6. Thus, the respective alloy always contains more tungsten than rhenium and this in a defined ratio range.

In subclaim 2, a concrete alloy is mentioned in which the ratio W to Re is restricted in relation to claim 1, i. even more limited

This material, which internally as "L judges in E k ristall 94" is designated (LEK94), thus has the following composition in weight percent al from 6.2 to 6.8 Co from 7.2 to 7.8 Cr from 5.8 to 6.4 Hf from 0.05 to 0.15 Not a word from 1.7 to 2.3 re from 2.3 to 2.6 Ta from 2.0 to 2.6 Ti from 0.9 to 1.1 W from 3.0 to 3.7 Ni Rest including impurities

Any impurities in the form of further elements or compounds are here combined with the Ni content. Likewise, the proportions of the abovementioned elements may be e.g. two digits behind the decimal point (one hundredth of a percent) have deviations that are obvious to the person skilled in the art and have no relevant influence on the material properties.

This special material "LEK94" is a high-alloy single-crystal alloy of low density, which was developed for use in high-speed turbines. To optimize the detrimental requirements of high temperature resistance and low density, the alloy contents of the elements Re and W were varied.

• 1. verbessertes Rekristallisationsverhalten
• 2. Low Density Alloy mit Dichte ρ≈8 g/cm³
• 3. Vermeidung einer niedrig schmelzenden Diffusionszone bei Beschichtung
• 4. Verbessertes Kriechverhalten
• 5. Erfüllung allgemeiner Vergießbarkeitskriterien und ausreichendes Lösungsglühfenster
• 6. geringe Neigung zur TCP-Phasenbildung (Sprödphasen, N_v-Kriterium)

The development of the "LEK94" was carried out with the following objectives (starting point CMSX-6 according to US Patent 4,721,540):

• 1. improved recrystallization behavior
• 2. Low Density Alloy with density ρ≈8 g / cm ³
• 3. Avoidance of a low-melting diffusion zone during coating
• 4. Improved creep behavior
• 5. Fulfillment of general castability criteria and adequate solution annealing window
• 6. slight tendency to TCP phase formation (brittle phases, N _v criterion)

Approach:

Addition of W and Re
but at lower levels than in known Ni base alloys of the 2.
generation
Optimization of the W and Re content (ie minimization, but determination of a minimum value)

Improvement over the prior art

"LEK94" is a Re-containing Enkristalllegierung low density in the range from 8.1 to 8.3 g cm ^-3 and high temperature strength. Good castability and a significantly large heat treatment window characterize this material.

Claims (2)

1. Nickel-based alloy for producing, by casting, components which have solidified in single-crystal form, in particular blades for high-speed turbine stages in gas turbines, having a proportion of rhenium (Re) of at least 2.3 to at most 2.6 % by weight, a proportion of tungsten (W), the weight ratio of which to the proportion of rhenium (Re) is at least 1.1 to at most 1.6, a proportion of aluminium (Al) of at least 6.2 to at most 6.8 % by weight, a proportion of cobalt (Co) of at least 7.2 to at most 7.8 % by weight, a proportion of chromium (Cr) of at least 5.8 to at most 6.4 % by weight, a proportion of hafnium (Hf) of at least 0.05 to at most 0.15 % by weight, a proportion of molybdenum (Mo) of at least 1.7 to at most 2.3 % by weight, a proportion of tantalum (Ta) of at least 2.0 to at most 2.6 % by weight and a proportion of titanium (Ti) of at least 0.9 to at most 1.1 % by weight, the remainder being nickel and impurities.
2. A nickel-based alloy according to claim 1, having a proportion of tungsten (W) of at least 3.0 to at most 3.7 % by weight.