Nickel-base superalloy

Details Nickel-base superalloy Nickel-base superalloy

2000-05-18

2002-07-16

King, Roy (Department: 1742)

Metal treatment

Stock

Directionally solidified

C148S428000, C420S448000

Reexamination Certificate

active

06419763

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the field of materials engineering. It relates to a nickel-base superalloy, in particular for the production of single-crystal components (SX alloy) or components with a directionally solidified microstructure (DS alloy), such as for example blades for gas turbines.
BACKGROUND OF THE INVENTION
Such components made from nickel-base superalloys exhibit very good strength properties at high temperatures. It is thus possible to increase the inlet temperature of gas turbines, so that the efficiency of the gas turbine rises.
However, a perfect, relatively large, directionally solidified single-crystal component made from a nickel-base superalloy is extremely difficult to cast, because most such components exhibit flaws, for example grain boundaries, freckles (i.e. defects caused by a chain of identically oriented grains with a high eutectic content), equiaxial scatter limits, microporosity and the like. These flaws weaken the components at elevated temperatures, so that the desired service life and/or the operating temperature of the turbine are not achieved. However, since a perfectly cast single-crystal component is extremely expensive, the industry tends to permit as many defects as possible without impairing the service life or the operating temperature.
Grain boundaries constitute one of the most common forms of flaws, and are particularly damaging to the high-temperature properties of the single-crystal articles.
Grain boundaries are regions with a high local disorder in the crystal lattice, since in these regions neighboring grains butt against one another, resulting in a certain misorientation between the crystal lattices. The greater the misorientation, the greater the disorder, i.e. the greater the number of dislocations in the grain boundaries which are required for the two grains to fit together. This disorder is directly related to the performance of the material at high temperatures. It weakens the material if the temperature rises beyond the equicohesive temperature (=0.5×melting point in ° K).
This effect is known from GB 2,234,521 A. For example, in a conventional nickel-base single-crystal alloy, at a test temperature of 871° C. the breaking strength falls extremely quickly if the misorientation of the grains is greater than 6°. This was also observed in single-crystal components with a directionally solidified microstructure, so that consequently it has become the accepted view that misorientation of greater than 6° should not be permitted.
It is also known from the abovementioned GB 2,234,521 A that enriching nickel-base superalloys with boron or carbon combined with directional solidification produces microstructures which have an equiaxial or prismatic grain structure. Carbon and boron strength the grain boundaries, since C and B cause the precipitation of carbides and borides at the grain boundaries, compounds which are stable at high temperatures. Moreover, the presence of these elements reduces the diffusion process in and along the grain boundaries, which is a primary cause of the weakness of the grain boundaries. It is therefore possible to increase the misorientation to 12° while nevertheless achieving good materials properties at high temperatures if the carbon content is made higher than in conventional single-crystal alloys (250 to 770 ppm) but lower than in previous DS alloys (700 to 1600 ppm). An upper limit is defined by the growing carbide size, which has an adverse effect on the low cycle fatigue (LcF).
The latest SX alloys have a carbon content of 500 ppm. This level is regarded as optimum in terms of the defect tolerance (tolerance with regard to small-angle grain boundaries) (“Rene N4: A First Generation Single Crystal Turbine Airfoil Alloy”, Superalloys, pp. 19-26, and “Rene N6: Third Generation Single Crystal Superalloy”, pp. 27-34, The Minerals Metals and Materials Society, 1996).
For all these nickel-base superalloys, it is the case that the carbon content is limited by the size of the carbides which form during the solidification. Large Chinese script like carbides reduce the service life with low cycles to approximately half the service life achieved by the same alloy with small carbides in block form (Metals Handbook, 10th edition, 1990, ASM International, Vol. 1, p. 991).
It is also known that conventionally cast nickel-base superalloys (CC or equiaxial) can be provided with additions of magnesium, calcium, cerium or other rare earths, which influence the carbide morphology. The abovementioned elements have a high reactivity, so that although they are suitable for CC alloys, owing to the short contact times with the shell mold, they are unsuitable for casting of DS and SX alloys, in which the molten alloy is in contact with the shell mold at high temperatures for a long time, since these additions reduce the silicon content in the shell mold and lead to slag formation on the cast surface. Moreover, the quantitative proportions of these additions vary adversely over the height of the casting, smaller quantities being present in that part of the casting which solidified last. This is undesirable, since as a result the carbide morphology varies over the length of the casting.
Furthermore, it is known prior art to keep the nitrogen content of SX and DS nickel-base superalloys to an absolute minimum. Nitrogen is regarded as a harmful impurity which has an adverse effect on the grain region and leads to the formation of nonmetallic inclusions, for example nitrides of titanium or tantalum. Grain defects may form at these inclusions (Metals Handbook, 10th edition, 1990, ASM International, Vol. 1, p. 1000), having an adverse effect on the properties of the alloys.
SUMMARY OF THE INVENTION
The invention aims to avoid all these drawbacks. It is based on the object of providing a nickel-base superalloy (SX or DS alloy) for producing single-crystal components which, compared to the known prior art, is distinguished by a greater tolerance of small-angle grain boundaries while nevertheless exhibiting very good low cycle fatigue at high load temperatures.
Single-crystal components are to be understood as meaning articles made from single crystals and articles with a directionally solidified microstructure.
According to the invention, this is achieved by providing a nickel-base superalloy comprising (measured in % by weight): 3.0-13.0% Cr; 5.0-15.0% Co; 0-3.0% Mo; 3.5-9.5% W; 3.2-6.0% Al; 0-3.0% Ti; 2.0-10.0% Ta; 0-6.0% Re; 0.002-0.08% C; 0-0.04% B; 0-1.4% Hf; 0-0.005% Zr; and 10-60 ppm N; with the remainder of the superalloy including nickel plus impurities. Alternatively, the object of the invention may be achieved by providing a nickel-base superalloy comprising (measured in % by weight): 6.0-6.8% Cr; 8.0-10.0% Co; 0.5-0.7% Mo; 6.2-6.7% W; 5.4-5.8% Al; 0.6-1.2% Ti; 6.3-7.0% Ta; 2.7-3.2% Re; 0.02-0.04% C; 40-100 ppm B; 0.15-0.3% Hf; 15-50 ppm Mg; 0-400 ppm Y; 10-60 ppm N; with the remainder including nickel plus impurities. In a preferred embodiment of the invention, the nickel-base superalloy essentially consists of (measured in % by weight) 3.0-13.0% Cr, 5.0-15.0% Co, 0-3.0% Mo, 3.5-9.5% W, 3.2-6.0% Al, 0-3.0% Ti, 2.0-10.0% Ta, 0-6.0% Re, 0.002-0.08% C, 0-0.04% B, 0-1.4% Hf, 0-0.005% Zr, 10-60 ppm N, remainder nickel plus impurities. In another preferred embodiment, the nickel-base superalloy essentially consists of (measured in % by weight) 6.0-6.8% Cr, 8.0-10.0% Co, 0.5-0.7% Mo, 6.2-6.7% W, 5.4-5.8% Al, 0.6-1.2% Ti, 6.3-7.0% Ta, 2.7-3.2% Re, 0.02-0.04% C, 40-100 ppm B, 0.15-0.3% Hf, 15-50 ppm Mg, 0-400 ppm Y, 10-60 ppm N, remainder nickel plus impurities.
The advantages of the invention are, inter alia, that the carbides have a favorable block morphology due to the controlled addition of small quantities of nitrogen to DS or SX nickel-base superalloys. As a result, it is possible to increase the carbon content compared to the known prior art without this resulting in deterioration in the low cycle fatigue at high temperatures. The higher carbon content has a beneficial effect on the sma

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