NL8204493A - NICKEL BASE SUPER ALLOY. - Google Patents

Description

-1- i *

Nickel base superalloy.

The present invention relates to nickel base superalloys which have both exceptional oxidation resistance and exceptional high temperature mechanical properties.

5 Previous researchers have worked with alloys based on the Ni-Al-Mo system. This work is covered in U.S. Pat. Nos. 2,542,962 and 3,933,483.

U.S. Patent 3,904,403 proposes the addition of 0.1-3 atomic% (total) of one or more elements from the group, which includes Cr, Ta, and W, to the Ni-Ai-Mo type alloys.

According to the present invention, a class of nickel base superalloys with significantly increased oxidation resistance is provided by the addition of coordinated amounts of Cr, Ta and Y. Improved oxidation behavior is obtained without appreciable deterioration in mechanical properties.

The wide composition range is 5.8-7.8% Al, 8-12% Mo, 4-8% W, 2-4% Cr, 1-2% Ta, 0-0.3% Hf, 0.01 -0.1% Y, balance essentially nickel. A preferred range is 6.3-7.3% Al, 8.5-11.5% Mo, 5-7% W, 2.5-3.5% Cr, 1-2% Ta, 0.05-0 .2% Hf, 0.01-0.07% Y.

Alloys within these ranges can be fabricated into useful articles using powder metallurgy techniques or by molding and then heat treating.

Accordingly, it is an object of the invention to provide high strength oxidation resistant nickel base superalloys.

This and other objects, features and advantages of the invention will now be further elucidated with reference to the following description of preferred embodiments with reference to the drawing.

Fig. 1 shows the effect of varying the yttrium content on the oxidation behavior.

Figures 2A, 2B and 2C are scanning electron micrographs, in which the oxide temperature obtained with different yttrium contents.

Fig. 3 illustrates the effect of varying the chromium content on the oxidation behavior at 1093 ° C.

Fig. 4 illustrates the effect of varying the chromium content on the oxidation behavior at 1149 ° C.

Fig. 5 illustrates the fracture load behavior of different alloys.

The invention relates to a nickel base superalloy with a specific and narrow composition range, which provides an exceptional combination of oxidation resistance and high temperature mechanical properties.

The broad and preferred composition areas 15 are given in Tables A and B. These tables are given in weight percent as well as all other percentage values in the present specification unless otherwise indicated. Table A also gives the equivalent values in atomic% -ten. The particular combination of the Ni-Al-Mo 20 constituents is similar in some respects to that described in U.S. Pat. Nos. 2,542,962, 3,655,462, 3,904,403, and 3,933,483. Ni-Al-Mo alloys are known to have exceptional mechanical properties, however, their surface stability and oxidation resistance have hitherto been unpredictable and marginal for long-term applications.

The essence of the present invention is the addition of carefully coordinated amounts of Cr, Ta, Y and optionally Hf to these Ni-Al-Mo alloys to dramatically improve oxidation resistance while preserving or improving mechanical properties.

The purpose of the Cr addition in view of the oxidation resistance is to promote the formation of 35 a ^ 2 ° 3 oxy ^ e “instead of an oxide based on NiO.

At least about 2% Cr appears to be required for this purpose. Increasing the Cr level above about 4% does not appear to provide any noticeable improvements over those obtained with about 3% Cr.

40 8204493 * * -3-

Table A

WIDE COMPOSITION

5 Eaag High (wt%) (at%) (wt%) (at%) NX (Bal (78/20) (78.65) (66.1) (66.15)

Al 5.8 12.8 7.8 17.3 10 Mo 8.0 4.7 12.0 7.8 W 4.0 1.2 8.0 2.4

Cr 2.0 2.3 4.0 4.8

Ta 1.0 0.35 2.0 1.2 Y 0.01 0.01 0.1 0.05 15 Hf 0.0 0.0 0.0 0.3

Table B

PREFERRED COMPOSITION (wt%) 20

Low High

Ni Bal. Ball.

Al 6.3 7.3

Mo 8.3 11.5 25 W 5.0 7.0

Cr 2.5 3.5

Ta 1.0 2.0

Hf 0.0 0.2 'Y 0.01 0.7 30

Since Cr simultaneously reduces the mechanical properties, additions of Cr in excess of about 4% are undesirable. Ta is added to stabilize a microstructure, and Ta at levels, as indicated, overcomes the deterioration of mechanical properties resulting from the Cr additions. Thus, the Cr and Ta contents are related to a certain degree, and optimal alloy quality can be obtained by coordinating the Ta and Cr contents to use high Ta contents for high Cr 8204493-4 contents and low Ta contents for low Cr contents. .

At least one material selected from the group consisting of Y and Hf must also be added. Such elements improve the adhesion of the surface oxide to superalloys, reducing chipping, and minimizing weight loss due to oxidation. It appears that 0.1 to 0.3 (total) wt% of these elements provides the required function with the preferred range being 0.02% (total), while Y is preferably present in an amount of at least 0.01-0.07%.

Figures 1, 2 and 3 may help to illustrate the effects of the elements set forth above. The figures show the alloy compositions tested and show the weight change during the oxidation tests. It will be understood that when an alloy is oxidized it initially gains weight due to the formation of an oxide layer. Then, if this oxide layer peels off, a weight loss will result, and the oxide layer will reform. Oxide exfoliation and resulting weight loss are undesirable as this results in depletion of the oxide forming elements in the underlying substrate. Oxide-25 exfoliation can proceed to the point where the alloy is no longer able to reform the desired protective oxide layer to that point where the one formed is a non-protective oxide layer. At this point, the oxidation is increasingly fast and out of control and eventually the specimen will be destroyed. Since most alloys derive their oxide resistance from the formation of a protective oxide layer, the desired weight change behavior is an initial slight increase in weight, indicating the formation of a protective oxide layer followed by essentially no weight change (or a very slight increase).

The critical and unexpected result of the additions of yttrium are shown in Figure 1. This figure shows the weight loss experienced by different 40 alloys with different yttrium contents after cyclic 8204493-5. J <u *. ~> - -ϊ. ». · * '-' -»? «* - '-" * »; _; ·.? ·

a

test at 1204 ° C for 50 SSn-ums cycles. It is clear that for the base alloy tested (10% Mo, 6.7% AX, 6% W, 3% Cr, 1.5% Ta, 1% Hf, ball Ni), additions of about 0, 01 to about 0.06% Y give a remarkable improvement in oxidation behavior. While it has previously been observed that Y can improve the oxidation performance of coatings (U.S. Pat. Nos. 3,676,085 and 3,754,903) and alloys (U.S. Pat. about 10% of the damage were about 0.1%.

The results, shown in Figure 1, can be explained with reference to Figures 2A, 2B and 2C, which are SEM photographs (3000x) of the oxidized surface of three samples. The nominal sample composition 15 is that shown in Figure 2. Figure 2A is from a sample containing 0.1% Hf and less than 0.002% Y. Figure 2B is from a sample containing 0.1% Hf and 0.029% Y. Figure 2C is of a sample / containing 0.1% Hf and 0.073% Y.

Figures 2A and 2C both show a rough, irregular oxide morphology and provide evidence of oxide addition, while Figure 2B provides evidence of an adhering oxide morphology. Thus, FIGS. 1, 2A, 2B and 2C clearly demonstrate that a limited critical amount Y gives a significant improvement in the oxidation behavior.

Figures 3 and 4 illustrate that a critical chromium content is required for optimum oxidation resistance.

Figure 3 shows the effect of varying chromium content on the oxidation behavior of a base alloy, containing only 10% Mo, 7.4% AX, 6% W, 1.5% Ta, 0.1% Y, ball Ni. It can be seen that under the test conditions (500 Sen. hours cyclic of oven oxidation at 1093 ° C) the desired minimum weight change is obtained with Cr levels of about 3%.

Fig. 4 shows this screen using cyclic oxidation data generated at 1149 ° C. The figure shows a change in weight as a function of the examination time. Four curves are plotted for a base alloy, which contains 10% Mo, 6.6% AX, 1.5% Ta, 0.1% Y, baX Ni (with varying Cr contents), The effect of increasing of the Cr is that the curves are rotated upward to the horizontal (or weight change).

8204493 ft -6-

Figures 3 and 4 illustrate that a chromium content of about 3% is required to give good oxidation behavior in this class of alloys.

The mechanical properties of the Al-Mo alloys 5 have proven superior in most respects to that of conventional superalloys in previous studies. The present invention, essentially balanced additions of Cr, Ta, Y and / or Hf, achieves a considerably improved oxidation behavior in combination with mechanical properties, which are at least equivalent and in some cases even superior to the properties of the baseline Al -Mo-Ni alloys. This is a marked contrast to typical alloys, in which the improvements in property are, without exception, accompanied by a decrease in other properties.

Figure 5 graphs the breaking load for various alloys, including the previously described known superalloy MAR-M200 and an alloy which is within the scope of the invention. The data in Fig. 5 have been tested for fracture load properties of the various compositions in monocrystal form in the <111> orientation. As can be seen in the figure, the modified Ni-Al-Mo composition has an improved fracture load life compared to the other alloys investigated. It appears that the modified alloy has a temperature improvement of about 105 ° C compared to the known superalloys. This means that, under equivalent load conditions, the alloy of the invention can operate at a 105 ° C higher temperature and still achieve the same life. This high temperature could result from the higher engine operating temperature or reduced air cooling flow if the engine temperature were not to change. Each of these alternatives gives an increase in costs. Another possibility is to maintain the operating conditions including temperature at the same level and to obtain a significantly increased service life for the part. Finally, one could maintain the same temperature, but by increasing the workload, obtain an increased performance 40 for the same fuel consumption and part life.

The compositions described above can be used in cast SS crystal form or alternatively be fabricated into parts using metallurgical powder techniques, followed by directional recrystallization to obtain a directional crystal structure, which in the limiting case provides an eq. can be crystal.

In the event that the cast crystal path is followed, it is necessary that the cast part be homogenized and heat treated as set forth in U.S. Patent Application 177,047. If the part is to be manufactured by the metallurgical powder method, the sauce count can be powdered by various techniques, although a technique resulting in a rapid stiffening graft is desired because of the resulting increased homogeneity. Such a process is described in U.S. Pat. Nos. 4,025,249, 4,053,264, and 4,078,873. The resulting powder 20 is then solidified and crystallized in a targeted manner to produce a desired structure. Targeted recrystallization is described in U.S. Pat. No. 3,975,219, and the specific methods for achieving different crystallographic orientations in the final structure are described in copending U.S. Patent Application 325,248 of the same filing date.

The resulting products find particular application in gas turbine engines. If the casting method is followed, a casting can be made directly to the desired size. If, however, the metallurgical powder method is followed, the vane manufacturing technique described in U.S. Pat. No. 3,872,563 can advantageously be followed to arrive at a vane with maximum cooling capacity. While the compositions described herein are exceptionally oxidation resistant, they will undoubtedly be used in coated form, and such coatings may include the aluminum coatings or the MCrAlY type coatings.

Although the invention has been described above and illustrated with reference to a preferred embodiment, 8204493-8 it will be apparent to those skilled in the art that numerous changes in shape and detail are possible without departing from the scope of the invention.

5 conclusions 8204493

Claims (2)

Nickel base superalloy with high strength and oxidation resistance, characterized in that it consists essentially of: 5.8 - 7.8% Al 8 -12% Mo 4 - 8% W 2 - 4% Cr 10 1 - 2% Ta 0 - 0.3% Hf 0.01 - 0.1% Y balance Ni. 15 2. Oxidation resistant nickel base super alloy according to ·. claim 1, characterized in that it essentially consists of: 6.3 - 7.3% Al 8.5 -11.5% Mo 20 5 - 7% W 2.5 - 3.5% Cr 1-2% Ta 0 - 0.2% Hf 0.01 - 0.07% Y 25 balance Ni. 8204493