GB2110240A - Nickel base superalloy - Google Patents

Abstract

Nickel base superalloys containing Ni, Al, Mo and Ware provided with enhanced oxidation resistance through the addition of coordinated amounts of Cr, Ta and Y. The resultant alloys have oxidation resistance and high temperature mechanical properties which are superior to those of other superalloys. The alloys specifically contain in wt % Al 5.8-7.8 Mo 8-12 W 4-8 Cr 2-4 Ta 1-2 Hf 0-0.3 Y 0.01-0.1 Ni balance o

Description

SPECIFICATION Nickel base superalloy This invention relates to the field of nickel base superalloys which have both exceptional resistance to oxidation and exceptional high temperature mechanical properties.

Previous investigators have worked with alloys based on the Ni-Al-Mo system. This work is typified by U.S. Patents 2 542 962; and 3 933483.

U.S. Patent 3 904403 suggests the addition of 0.1-3 atomic percent (total) of one or more elements from a group which includes Cr, Ta, and W to the Ni-Al-Mo type of alloys.

According to the present invention a class of nickel base superalloys is provided with substantially enhanced oxidation resistance through the addition of coordinated quantities of Cr, Ta and Y. Improved oxidation behavior is obtained without significant detriment to mechanical properties.

The broad composition range is 5.8-7.8% Al, 8-12% Mo, 48% 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 may be fabricated into useful articles using powder metallurgy techniques or may be cast to size and then heat treated.

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

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings.

Figure 1 illustrates the effect of varying the yttrium level on oxidation behavior.

Figures 2A, 28 and 2C are scanning electron micrographs illustrating the oxide morphology obtained with various yttrium levels.

Figure 3 illustrates the effect of varying the chromium level on oxidation behavior at 2000"F (1093"C).

Figure 4 illustrates the effect of varying the chromium level on oxidation behavior at 21 000F (11 490C).

Figure 5 illustrates the stress rupture behavior of several alloys.

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

The broad and preferred composition ranges are set forth in Tables 1 and 2. The tables are in weight percent as are all other percentage values in this application unless otherwise specified. Table 1 also contains the equivalent values in atomic percent. The particular combination of the Ni-Al-Mo constituents is similar in some respects to that described in U.S. Patents 2542 962; 3 655 462; 3 904403 and 3 933 483.

Ni-Al-Mo alloys are known to have exceptional mechanical properties, however heretofore, their surface stability and oxidation resistance have been unpredictable and marginal for long term applications.

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

Cr is added for oxidation resistance by promoting the formation of an Al203 oxide rather than an oxide based on NiO. For this purpose at least about 2% Cr appears to be necessary. Increasing the Cr level above about 4% does not appear to provide substantial improvements over those obtained with about 3% Cr.

TABLE 1 BROAD COMPOSITION Low High (wt %) (at %) (wt %) (at %) Ni (Bal) (78.20) (78.65) (66.1) (66.15) Al 5.8 12.8 7.8 17.3 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 Hf 0.0 0.0 0.0 0.3 TABLE 2 PREFERRED COMPOSITION (wt %) Low High Ni Bal. Bal.

Al 6.3 7.3 Mo 8.3 11.5 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 Since Cr concurrently reduces the mechanical properties, Cr additions in excess of about 4% are undesirable.

Ta is added to stabilize the microstructure and Ta in the levels indicated overcomes the mechanical property deficit which results from the Cr additions. Thus, the Cr and Ta levels are to a certain extent related and optimum alloy performance will be obtained by coordinating the Ta and Cr levels such that for high Cr levels, high Ta levels are employed and for low Cr levels, low Ta levels are employed.

At least one material selected from the group consisting of Y and Hf must also be added. Such elements improve the adherence of the surface oxide to superalloys, thereby reducing spallation and minimized weight loss due to oxidation. It appears that 0.1 to 0.3 (total) weight of these elements will perform the required function with the preferred range being 0.02 - 0.2% (total) and Y preferably being present in an amount of at least 0.01-0.07%.

Figures 1, 2 and 3 will help to illustrate the previously set forth element effects. The figures list the alloy compositions tested and show the weight change during oxidation testing. It will be appreciated that when an alloy oxidizes, it initially gains weight as a result of the formation of an oxide layer. Subsequently, if this oxide layer spalls off, a weight loss will result and the oxide layer will reform. Oxide spallation and resultant weight loss are undesirable since this results in the depletion of the oxide forming elements in the underlying substrate. Oxide spallation can proceed to the point where the alloy is unable to reform the desired protective oxide layer to that what forms is a non-protective oxide layer. At this point, oxidation becomes increasingly rapid and uncontrolled and eventually the specimen will be destroyed.As most alloys derive their oxidation resistance from the formation of a protective oxide layer, the desirable 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 yttrium additions are illustrated in Figure 1. This figure shows the weight loss experienced by several alloys with differing ytrrium levels, after cyclic testing at 2200"F (1 204 C) for 50 one hour cycles. It is apparent that for the base alloy tested (10% Mo, 6.7% Al, 6% W, 3% Cr, 1.5% Ta, 1% Hf, bal Ni) additions of from about 0.01 to about 0.06% Y produce a remarkable improvement in oxidation behavior. Although it has been previously observed that Y can improve the oxidation performance of coatings (U.S. Patents 3 676 085 and 3 754 903) and alloys (U.S. Patent 3 754 903) it has never before to our knowledge been shown that Y levels in excess of about 0.1% were harmful.

The results shown in Figure 1 may be explained through reference to Figures 2A, 2B, and 2C which are SEM photos (at 3000 X) of the oxidized surface of three samples. The nominal sample composition is that shown in Figure 2. Figure 2A is of a sample containing 0.1% Hf and less than 0.002% Y. Figure 2B is of 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 show evidence of oxide spallation while Figure 2B shows evidence of an adherent oxide morphology. Thus, Figures 1, 2A, 2B and 2C clearly show that a limited critical amount of Y produces a substantial improvement in oxidation behavior.

Figures 3 and 4 illustrate that a critical chromium level is necessary for optimum oxidation resistance.

Figure 3 shows the effect of varying Cr content on the oxidation behavior of a base alloy containing 10% Mo, 7.4% Al, 6% W, 1.5% Ta, 0.1% Y bal Ni. It can be seen that under the test conditions (500 one hour cycles of furnace oxidation at 2000"F (1 093"C)) the desired minimal weight change is obtaind with Cr levels of about 3%.

Figure 4 makes the same point using cyclic oxidation data generated at 2100 F (1149 C). The figure shows the change in weight as a function of time in test. Four curves are plotted for a base alloy containing 10% Mo, 6.6% Al, 1.5% Ta, 0.1% Y bal Ni (with varying Cr levels). The effect of increasing the Cr is to rotate the curves up towards the horizontal (or zero weight change).

Figures 3 and 4 illustrate that a level of Cr of about 3% is necessary to provide good oxidation behavior in this class of alloys.

The mechanical properties oftheAI-Mo alloys have been shown in priorworkto be superior, in most respects, to those of conventional superalloys. The present invention, balanced additions of Cr, Ta, Y and/or Hf achieve substantially improved oxidation behavior in combination with mechanical properties which are at least equivalent and in some cases superior to the properties of the baseline Al-Mo-Ni alloys. This is a marked contrast to typical alloys in which improvement of one property is invariably accompanied by a decrease in other properties.

Figure 5 is a stress rupture plot for several alloys including the previously described MAR-M200 conventional superalloy and an alloy falling within the scope of the present invention. The data in Figure 5 is for stress rupture properties of the various compositions tested in single crystal form in the < 111 > orientation. As can be seen in the figure, the modified Ni-Al-Mo composition has an improved stress rupture life when compared with the other alloys tested. It appears that the modified alloy has about a 190"F (105"C) temperature improvement when compared with the conventional superalloys. This means that under equivalent conditions of stress, the invention alloy could be operated at 1900F (105"C) higher temperature and still achieve the same part life.This high temperature could be the result of the higher engine operating temperature or reduced flow of cooling air if the engine temperature were unchanged. Both of these alternatives give enhanced economy. Another possibility is to maintain the operating conditions including temperature at the same level and obtain a subtantially increased part life. Finally, one could maintain the same temperature, but by increasing the operating stress obtained increased performance for the same fuel consumption and part life.

The previously described compositions may be used in cast single crystal form or alternately, can be fabricated into parts using powder metallurgical techniques followed by directionally recrystallization to achieve an aligned grain structure which in the limiting case may be a single crystal.

In the event that the cast single crystal route is pursued, it is necessary that the cast part be homogenized and heat treated as outlined in U.S. Serial No. 177 047. If the part is to be fabricated through the powder metallurgical approach, the composition may be formed into powder by several techniques although a technique resulting in a rapid solidification rate is undesirable because of the enhanced homogeneity which results. Such a process is described in U.S. Patents 4025 249,4053 264 and 4078873. The resultant powder is then consolidated and directionally recrystallized to produce the desired structure. Directional recrystallization is described in U.S.Patent 3975219 and the specific approaches to achieve various crystallographic alignments in the final structure are described in pending application Serial No.325248 by H.A. Chin filed on even date herewith.

The resultant products find particular utility in gas turbine engines. If the casting approach is pursued, a casting may be produced directly to the desired size. However, if the powder metallurgical approach is pursued, the blade fabrication technique described in U.S. Patent 3 872 563 may be advantageously followed in order to drive a blade having the maximum cooling capability. Although the compositions described herein are exceptionally oxidation resistant they will undoubtedly be used in coated form and such coatings may comprise the aluminide coating or the MCrAIY type overlay coatings.

Although this invention has been shown and described with respect to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the scope of the claimed invention.

Claims (3)

1. A nickel base superalloy having high strength and oxidation resistance consisting essentially of:

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 Ni.

2.5 -

3.5 %Cr 1 2 2 Ta 0 - 0.2 %Hf 0.01 - 0.07%Y Balance Ni

2. A high strength oxidation resistant nickel base superalloy consisting essentially of:

6.3 - 7.3 %Al

8.5 - 11.5 % Mo 5 - 7 %W