EP1451382A1

EP1451382A1 - Method for developing a nickel-base super alloy

Info

Publication number: EP1451382A1
Application number: EP02779820A
Authority: EP
Inventors: Mohamed Nazmy
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2001-11-09
Filing date: 2002-11-05
Publication date: 2004-09-01
Also published as: WO2003040419A1; CN100345990C; US20040261921A1; CN1585829A

Abstract

The invention relates to a method for developing a nickel-base super alloy, consisting of a gamma phase and a gamma ' phase, for the production of monocrystalline or specifically solidified substance bodies. The invention is characterized in that the properties of nickel-base super alloys, exhibiting a volume proportion of at least 50 %, are optimized after degradation at room temperature, by selecting the composition of the alloy in such a way that a high as possible positive lattice dislocation (delta) is obtained at room temperature between the gamma phase and the gamma ' phase. As a result the apparent limit of elasticity at room temperature after degradation is relatively high and only a small difference of apparent limits of elasticity occurs between the initial state and a degraded state.

Description

Process for the development of a nickel-based superalloy

Technical field

The invention relates to a method for developing a nickel-based superalloy, which is used to produce a single-crystalline or directionally solidified material body.

State of the art

A whole series of nickel-based superalloys are known from the prior art, which are used for the production of single-crystalline or directionally solidified material bodies. Such material bodies are such. B. used in power plant construction at high temperatures. By means of these single crystal components, for example, the material strength at high temperatures can be maximized, which in turn can increase the inlet temperature of gas turbines, which leads to an increase in the efficiency of the gas turbine.

So far, such alloys have been developed according to the following concepts:

- Increase in creep strength

- Increased resistance to oxidation and corrosion - Increased resistance to crack growth, especially against LCF (Low Cycle Fatigue = fatigue with low number of cycles)

- Improve the pourability and the heat treatment options

- Lower costs. Known nickel-based superalloys are, for example, the alloys CMSX-2, CMSX-4, CMSX-10, Rene N5, Rene N6, PWA 1484 and PWA 1483, the composition of which, for. B. from GL Erickson: Corrosion resistant Single Crystal Superalloys for Industrial Gas Turbine Application, International Gas & Turbine Aeroengine Congress & Exhibition, Orlando, Florida, June 2-June 5, 1997. Alloys of this type are subjected to a heat treatment after the casting process, in which, in a first solution-annealing step, the γ'-phase which has precipitated out unevenly during the casting process is completely or partially dissolved. In a second heat treatment step, this phase is eliminated in a controlled manner. In order to achieve optimum properties, this precipitation heat treatment is carried out in such a way that fine, uniformly distributed particles of the γ'-phase are formed in the γ-phase.

It is known that the lattice offset can play a decisive role in the creep rupture strength at high temperatures. Many of the known nickel-based superalloys have a positive or negative lattice offset between the γ-matrix and the γ'-phase. This lattice distortion prevents dislocations when sliding or cutting γ 'grains, which increases the short-term strength at elevated temperatures. On the one hand, in the literature for nickel-based superalloys at room temperature, a negative lattice offset with the highest possible amplitude is required (P. Caron: High γ 'solvus new generation nickel-based superalloys for Single crystal turbine blade applications. Proceedings of the 9 ^th international Symposium on Superalloys - SUPERALLOY 2000, p.737-746, Champion, USA, September 17-21, 2000), other nickel-based superalloys (see for example EP 0 914 483 B1) are designed by a corresponding choice of additional elements, that there is no lattice misalignment because it was determined that the lattice misalignment between the γ and γ 'phases in the presence of massive or deep mechanical stresses in the long term at high temperatures caused a directional grain coarsening of the γ' particles and subsequently a degradation of the γ 'Structure emerges. Presentation of the invention

The invention tries to avoid the disadvantages of the known prior art. It is based on the task of creating a method for developing nickel-based superalloys which is based on a new, simple concept.

This object is achieved according to the invention in that the properties of nickel-based superalloys with a volume fraction of γ′-phase of at least 50% are optimized after a degradation at room temperature by selecting the composition of the alloy so that at room temperature a high positive lattice offset δ between the γ phase and the γ'-phase, where δ [%] = 2 (a _γ -a _γ ) / (a _γ '+ a _γ ) and where a _{γ is} the lattice constant of the γ phase and a _γ <is the lattice constant of the γ 'phase.

The advantages of the invention are that it is relatively easy to develop nickel-based superalloys with an optimized degradation behavior using this method.

It was found that there is a directional coarsening of the γ ^' particles, the so-called rafting, when there is mechanical stress and long-term high-temperature stress, and, at high γ ^' contents (i.e. with a γ ^' volume fraction of at least 50%), for inverting the microstructure, ie γ ^' becomes the continuous phase in which the previous γ matrix is embedded. Since the intermetallic γ ^' phase tends towards environmental embrittlement, this leads to a massive drop in the mechanical properties, especially the yield strength, at room temperature under certain load conditions. Ambient embrittlement occurs particularly when there is moisture and long holding times under tensile stress. If, according to the invention, a high positive lattice offset δ between the γ-phase and the γ'-phase is selected, the degradation of the properties is less pronounced, ie the Yield loss in the degraded state compared to the non-degraded state is only slight.

It is advantageous if the lattice constants of the γ-phase a _γ and the γ'-phase a _v - are determined according to the following known equations:

a _γ [Ä] = 3,524 + 0.0196Co + 0.110Cr + 0.478Mo + 0.444W + 0.441 Re + 0.3125Ru + 0.179AI + 0.422Ti + 0.7Ta + 0.7Nb,

the numbers in front of the element symbols indicate the relative atomic fractions of the respective elements in the γ phase and

a _γ - [Ä] = 3.57 - 0.004Cr + 0.208Mo + 0.194W + 0.262Re + 0.1335 Ru + 0.258Ti + 0.5Ta + 0.46Nb,

the numbers in front of the element symbols indicate the relative atomic fractions of the respective elements in the γ'-phase.

To characterize the creep behavior of nickel-based alloys, a degradation parameter D is now introduced, which is determined according to the following relationship:

D = (T-800) t 1 "/ 2 σ 1/5

with T = temperature in ° K, t = time in h and σ = stress in MPa. The yield strength σo, _{2 is determined} for different nickel-based superalloys at room temperature in the degraded state as a function of the degradation parameter, and those alloys are selected which have the smallest difference in the yield strengths between the initial state and the degraded state, i.e. those alloys which have the highest possible yield strength values in the degraded state. Brief description of the drawing

Exemplary embodiments of the invention are shown in the drawing.

Show it:

Fig. 1 shows the dependence of the yield strength after degradation at room temperature on the lattice offset between the γ-phase and the γ'-phase for different known nickel-based superalloys and

Fig. 2 shows the dependence of the yield strength at room temperature on the degradation parameter for different known nickel-based superalloys.

Only the features essential to the invention are shown. The same elements have the same reference symbols in different figures.

Ways of Carrying Out the Invention

The invention is explained in more detail below on the basis of exemplary embodiments and FIGS. 1 and 2.

It was found that there is a directional coarsening of the γ ^' particles, the so-called rafting, when there is mechanical stress and long-term high-temperature stress, and that there is high γ' content (ie with a γ'- Volume fraction of at least 50%) for the inversion of the microstructure, ie γ ^' becomes the continuous phase in which the former γ matrix is embedded. Since the intermetallic γ ^' phase for environmental embrittlement environmental embrittlement), this leads to a massive drop in mechanical properties, especially the yield strength, at room temperature under certain load conditions. The properties are therefore degraded. Ambient embrittlement occurs particularly when there is moisture and long holding times under tensile stress. If an alloy with a high positive lattice offset δ between the γ-phase and the γ'-phase is selected according to the invention, the degradation of the properties is less pronounced, ie the yield strength loss in the degraded state compared to the non-degraded state is only slight.

1 shows for different known nickel-based superalloys which are used to produce a single-crystalline or directionally solidified material body the dependence of the yield strength σo, ₂ after degradation at room temperature on the lattice offset δ between the γ phase and the γ'- Phase. The lattice offset δ between the γ-phase and the γ'-phase was calculated in a manner known per se as follows: δ [%] = 2 (a _γ -a _γ ) / (a _γ + a _γ ) and where a _γ is the lattice constant of the γ phase and a _γ - is the lattice constant of the γ 'phase.

Alloys with the chemical composition listed in Table 1 (data in% by weight) were used.

The lattice offset δ between the γ-phase and the γ'-phase at room temperature is in the range of approx. - 0.24% to + 0.58% for the alloys examined. With an increase in the positive replacement, the yield strength σo _{, 2 also increases} after degradation at room temperature. Of the alloys examined, alloy PW1480 has the highest positive lattice offset δ between the γ-phase and the γ'-phase and consequently also the highest yield strength σ _0.2 after degradation at room temperature. The lattice constants of the γ-phase a _γ and the γ'-phase a _γ - were known per se according to the following (see P. Caron: High γ 'solvus new generation nickel-based superalloys for Single crystal turbine blade applications. Proceedings of the 9 ^th international Symposium on Superalloys - SUPERALLOY 2000, p.737-746, Champion, USA, September 17-21, 2000) determined:

a _γ [A] = 3,524 + 0.0196Co + 0.110Cr + 0.478MO + 0.444W + 0.441 Re + 0.3125Ru + 0.179AI + 0.422Ti + 0.7Ta + 0.7Nb,

where the numbers before the element symbols indicate the relative atomic fractions of the respective elements in the γ phase and

a _γ [A] = 3.57 - 0.004Cr + 0.208Mo + 0.194W + 0.262Re + 0.1335 Ru + 0.258TΪ + 0.5Ta + 0.46Nb,

where the numbers in front of the element symbols indicate the relative atomic fraction of the respective elements in the γ'-phase. The alloying elements B, Zr and C do not play a significant role in relation to the lattice offset, especially since they are only present in small amounts as trace elements.

The degradation behavior of the alloy can now be optimized according to the invention by setting the highest possible positive lattice offset δ between the γ-phase and the γ'-phase by varying the composition. To characterize the creep behavior, a degradation parameter D was introduced for the nickel-based alloys, which is determined according to the following relationship:

D = (T-800) f 1 / "2 σ 1/5

with T = temperature in ° K, t = time in h and σ = stress in MPa. The yield strength σo, 2 is then determined at room temperature after degradation as a function of the degradation parameter. 2, these values are plotted against one another for the alloys from Table 1. In order to optimize properties, the yield point at room temperature should be as high as possible for the various degradation parameters. The alloy PW1480, which has a lattice offset δ between the γ-phase and the γ'-phase of + 0.58%, best meets this requirement. In contrast, the alloy CMSX4, which with a lattice shift δ between the γ-phase and the γ'-phase of −0.24% is the most below the requirement of the invention, has, depending on the degradation parameter D, which is at least about 5000 KhMPa, the lowest values of the yield strength. This alloy should therefore be unsuitable in terms of degradation behavior.

LIST OF REFERENCE NUMBERS

O ^" 0.2 yield strength δ lattice offset

3γ lattice constant of the γ phase a _γ - lattice constant of the γ 'phase

D degradation parameters

T temperature t time

C

Table 1: Chemical composition of the alloys in% by weight

Claims

claims

1. Process for the development of a nickel-based superalloy consisting of γ- and γ'-phase, for the production of single-crystalline or directionally solidified material bodies, characterized in that the properties of nickel-based superalloys with a volume fraction of γ'- Phase of at least 50% after a degradation at room temperature can be optimized by selecting the composition of the alloy so that at room temperature there is as high a positive lattice offset (δ) as possible between the γ-phase and the γ'-phase, whereby and where a _{γ is} the lattice constant of the γ phase and a _γ - is the lattice constant of the γ 'phase.

2. The method according to claim 1, characterized in that the lattice constant of the γ phase (a _γ ) and the lattice constant of the γ'-phase (a _γ ) are determined according to the following known equations: a _γ [A] = 3,524 + 0.0196Co + 0.110Cr + 0.478Mo + 0.444W + 0.441 Re

+ 0.3125Ru + 0.179AI + 0.422Ti + 0.7Ta + 0.7Nb, where the numbers in front of the element symbols indicate the relative atomic proportion of the respective elements in the γ phase and a [A] = 3.57 - 0.004Cr + 0.208Mo + 0.194W + 0.262Re + 0.1335 Ru + 0.258Ti + 0.5Ta + 0.46Nb, where the numbers in front of the element symbols indicate the relative atomic proportion of the respective elements in the γ'-phase. A method according to claim 1, characterized in that a degradation parameter (D) is introduced to identify the creep behavior of nickel-based alloys, which is determined according to the following relationship:

D = (T-800) t ^1/2 σ ^1/5 with T = temperature in ° K, t = time in h and σ = stress in MPa, the yield strength (σo, ₂ ) at room temperature after degradation depending on the degradation parameter (D) is determined and this should have the highest possible values in order to optimize properties.