US3129069A

US3129069A - Oxidation-resistant turbine blades

Info

Publication number: US3129069A
Application number: US860147A
Authority: US
Inventors: Dean K Hanink; Erwin R Price
Original assignee: General Motors Corp
Current assignee: Motors Liquidation Co
Priority date: 1956-10-11
Filing date: 1959-12-17
Publication date: 1964-04-14
Anticipated expiration: 1981-04-14

Description

A ril 14, 1964 D. K. HANINK ETAL 3,129,059 OXIDATION-RESISTANT TURBINE BLADES Original Filed Oct. 11, 1956 s Sheets-Sheet 1 WIDE AFFECTED ION! E M/CMSTITl/ENT "(K/(MESS OF L UM.

NICKEL ALLOY LAYER AL PM ILLOY LAYER INVENWPS 8485 Menu AT TOPNE Y Apnl 14, 1964 D. K. HANINK ETAL 3,129,069

OXIDATION-RESISTANT TURBINE BLADES orlginal Filed Oct. 11, 1956 3 Sheets-Sheet 2 AL. RICH ALL 0) LAYEI? BASE ME TA.

AL RICH ALLOY LAYER 84 SE METAL 01/05 DEPTH 0F ALLOY pgpry DEPLETION AND -1 MICROSTRIATMQE CHMVE BASE METAL INVENTORS AT TGPNE Y 97 gypz w l w 6 E T a ,4, k a

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April 14, 1964 D. K. HANINK ETAL 3,129,059

OXIDATION-RESISTANT TURBINE BLADES Original Filed Oct. 11, 1956 3 Sheets-Sheet 3 Suki-ACE OXIDE OEPTH OF ALL OY PENETRATION BASE METAL 400 600 800 I000 I200 I400 I600 W0 0 200 {@299 77ME /-/auRs 0 I0 I0 7D PIP TIME -$ECOND$ 0.6 0.8 1.0 /.2 14 6 /.a 20 2.2 2.4 z THICKNESS OFALLOY INM/l. fgz'gfl A TTOPNE Y United States Patent Office 3,129,069 Patented Apr. 14, 1964 3,129,069 OXIDATION-RESISTANT TURBINE BLADES Dean K. Hanink, Indianapolis, Ind., and Erwin R. Price,

Detroit, Mich, assignors to General Motors Corporation, Detroit, Mich a corporation of Delaware Original application Oct. 11, 1956, Ser. No. 615,417, now Patent No. 3,000,755, dated Sept. 19, 1961. Divided and this application Dec. 17, 1959, Ser. No. 860,147 6 Claims. (Cl. 29-183.5)

This invention relates to oxidation-resistant turbine buckets and nozzle guide vanes for gas turbine engines. More particularly, the invention is concerned with nickel base alloy and cobalt base alloy turbine buckets having surfaces provided with a protective layer of an alloy of the base metal with aluminum. The present application is a division of our co-pending patent application Serial No. 615,417, now Patent No. 3,000,755, which was filed on October 11, 1956 as a continuation-in-part of patent application Serial No. 506,201, filed on May 5, 1955, and now abandoned.

High-temperature alloy components, such as turbine buckets and nozzle guide vanes, of gas turbine engines are subjected to extended periods of service at elevated temperatures under variable stress conditions. When such components are formed of certain high-temperature nickel base alloys and cobalt base alloys, they possess excellent strength under most high-temperature service conditions. However, durability of these turbine buckets and nozzle guide vanes is materially reduced because of inadequate resistance to oxide penetration. As a result, maximum use cannot be made of such high strength alloys, despite their outstanding capacity to sustain high stresses at elevated temperatures under nonoxidizing conditions.

Moreover, as the operating temperatures of gas turbines of the type used in turbojet and turboprop engines are increased, the problem of oxidation of gas turbine buckets becomes more acute. Today turbine engine manufacturers are designing gas turbine engines having operating temperatures as high as approximately 1900 F. At this very high temperature, oxidation of the nickel base and cobalt base alloys of which turbine buckets and nozzle guide vanes are conventionally formed seriously restricts the operating life of gas turbine engines. Nevertheless, such high-operating temperatures are desirable in order to obtain maximum thrust from the engines.

A principal object of the present invention, therefore, is to provide a turbine blade which will withstand higher operating temperatures than those presently being used. A further object of the invention is to eliminate oxidation of the base metal constituents, particularly along grain boundaries or preferred crystallographic planes, of nickel base alloy and cobalt base alloy gas turbine buckets and nozzle guide vanes. A still further object of the subject invention is to provide such turbine engine components with this protection without adversely affecting the loadcarrying ability of the base alloy.

Another object of the invention is to provide nickel base alloy and cobalt base alloy turbine blades with a protective surface layer which possesses suflicient duc tility to yield with the base metal when the latter is expanding, contracting, or stretching due to creep elongation under stress at elevated temperature. A further object of this invention is to provide a turbine bucket formed of nickel base alloy or cobalt base alloy and having, at its exposed surfaces, an integral layer of aluminum with the base material which affords effective protection against high-temperature oxidation. Although in the case of a nickel base alloy this layer is referred to herein as an aluminum-nickel layer or an aluminum-nickel alloy layer, it will be understood that the layer consists essentially of aluminum in combination with all the various constituents in the nickel base alloy of which the blade is formed. Analogous terminology is similarly employed to describe the protective surface layer formed on cobalt base alloys. Examples of suitable nickel base alloys and cobalt base alloys are hereinafter set forth.

A still further object of the present invention is to provide a process for forming a thin stable layer of the aforementioned type of aluminum-nickel alloy or aluminumcobalt alloy at the surfaces of turbine buckets and nozzle diaphragms formed of nickel base alloys or cobalt base alloys, as the case may be, in such a manner that this layer will not flake or spall during the normal operating life of the engine in which these blades are installed.

The above and other objects are attained in accordance with this invention by providing a thin aluminum coating on surfaces of nickel base alloy and cobalt base alloy turbine buckets and nozzle guide vanes. On diffusion heat treatment, this aluminum coating further combines with the base alloy to form a layer of aluminum-nickel alloy or aluminum-cobalt alloy. Of course, some of the aluminum usually remains on the surface of this layer in the form of a thin overlay of aluminum oxides. The diffused aluminum-nickel alloy and aluminum-cobalt alloy layers are tough, resilient and possess good ductility. Under operating conditions, these alloy layers prevent oxide penetration of the base material, thereby improving the durability of the turbine buckets under high-temperature operating conditions. This protection is provided under static stress conditions, non-load conditions, impact load conditions, or hot working or forging conditions.

While the invention is hereinafter specifically described principally in connection with nickel base alloy turbine blades, it Will be understood that the process set forth is also applicable to cobalt base parts.

Other objects and advantages of the present invention will more fully appear from the following detailed description of preferred embodiments of the invention, reference being made to the accompanying drawings, in which:

FIGURE 1 is an elevational view, with parts broken away and in section, of a turbine bucket formed of a nickel base alloy provided with an aluminum-nickel surface layer in accordance with the invention;

FIGURE 2 is a photomicrographic view of a high temperature creep-resistant nickel base alloy, showing the metailographic structure of the alloy near its working surface after it had been exposed to cyclic heating at elevated temperatures;

FIGURE 3 is a photomicrographic view of the nickel base alloy shown in FIGURE 2 provided with a surface layer of aluminum-nickel alloy in accordance with this invention;

FIGURE 4 is a photomicrographic view of the alloy shown in FIGURE 3 having an outer aluminum-nickel layer in accordance with the invention, showing its metallographic structure after exposure to cyclic heating at elevated temperatures;

FIGURE 5 is a photomicrographic view of the coated alloy shown in FIGURE 3 which had been heat treated prior to testing, showing its metallographic structure after exposure to cyclic heating at elevated temperatures;

FIGURE 6 is a photomicrographic view of the coated alloy shown in FIGURE 3 which had been treated by a pickling process prior to heat treatment and testing, showing its metallographic structure after exposure to cyclic heat at elevated temperature;

FIGURE 7 is a photomicrographic view of the uncoated nickel base alloy shown in FIGURE 2 after being subjected to a stress-rupture test at elevated temperature;

FIGURE 8 is a photomicrographic view of the coated nickel base alloy shown in FIGURE after being subjected to a similar high-temperature stress-rupture test;

FIGURE 9 is a graph comparing time-elongation characteristics at elevated temperature of an uncoated nickel base alloy stress-rupture bar with a. stress-rupture bar of the same alloy provided with a surface layer of aluminumnickel alloy in accordance with the invention;

FIGURE 10 is a graph showing the effect of dipping temperature and time on the thickness of the diffused aluminum-nickel alloy layer; and

FIGURE 11 is a graph showing, after heat treatment, the effect of aluminum coating bath temperature and composition on the thickness of the diffused aluminumnickel alloy layer.

Tht layer of aluminum-nickel alloy or aluminum-cobalt alloy, as the case may be, may be provided at the surfaces of the nickel base or cobalt base turbine bucket or nozzle diaphragm in any desired manner. The preferred method is to apply molten aluminum or aluminum base alloy to the turbine blade under conditions such that the aluminum will form an alloy with the nickel or cobalt and result in the desired alloy layer thickness. Best results are obtained when the aluminum or aluminum base alloy is applied by any of the procedures described in United States Patent No. 2,569,097 Grange et al., owned by the assignee of the present invention.

An especially advantageous method comprises preheating the turbine blade to a temperature between approximately l280 F. and 1400 F. in a fused salt bath consisting essentially of 37% to 57% KCl, 25% to 45% NaCl, 8% to 20% Na AlF and 0.5% to 12% AlF The heated turbine blade is thereafter immersed for a short time in a molten bath of aluminum or aluminum base alloy at a temperature of about 1250 F. to 1325 F. Ordinarily, the turbine blade being coated is retained in the molten aluminum or aluminum base alloy not more than approximately 10 seconds, a period between 5 and 10 seconds being preferred at present.

Subsequently, the turbine blade is removed from the aluminum bath and rinsed for a short period of time not in excess of approximately 15 seconds in the fluxing salt. Best results are obtained if the total dip time in the aluminum coating bath and the subsequent salt bath is between 10 and seconds. The excess coating material, which is still in a semi-molten or mushy condition, is then removed by rapidly vibrating the turbine blade, preferably while it is still immersed in the salt. Alternatively, an air blast may be employed to remove the surplus coating material. As thus treated, the turbine blade is provided with an extremely thin and uniform coating of aluminum bonded to the nickel base metal or cobalt base metal by an intermediate extremely thin and uniform layer of an alloy of aluminum-nickel or aluminum-cobalt.

The surfaces of the turbine blade to be coated are preferably cleaned prior to the aluminum coating and alloying operation. One satisfactory method is to clean the blade in a molten electrolytic caustic salt (such as the commercially available product called Kolene) at a temperature of about 900 F. The blade then may be washed in water and thereafter preferably further cleaned by acid pickling. A suitable acid pickling bath is an aqueous solution containing about 2% hydrofluoric acid, 7% sulfuric acid and 10% nitric acid. In some instances, the turbine blade may require only a simple degreasing treatment in a chlorinated solvent prior to the aluminum coating and alloying operation. Mechanical cleaning methods, such as grit blasting, sand blasting, hydroblasting, etc., may be employed in some cases to supplement the chemical treatment.

The steps of degreasing and pickling the turbine blade are not essential to the process, however, as heating in the fused salt prior to immersion in the aluminum or aluminum base alloy bath will provide the turbine blade 4 with clean surfaces unless it has been exceptionally contaminated initially.

After the nickel base or cobalt base turbine blade has been cleaned, any portions thereof which are not to be coated, such as the attaching portions at the base of the blade, may be treated with a suitable stop-off coating to prevent the aluminum from bonding to or alloying with the base metal at such surfaces. A suitable stop-off material for this purpose is a sodium silicate solution, such as an aqueous solution containing 20% to 50% sodium silicate.

It will be understood that variations in the aluminum coating method hereinbefore described may be made without departing from the scope of the present invention. For example, the aluminum may be applied to the turbine blade in the form of a paste or paint as described in United States Patent No. 2,885,304 Thomson et al. An example of the aluminum paste or paint which may be used is a mixture of aluminum powder with suitable amounts of a vehicle, such as low ash content lacquer or resin solution, liquid lucite or a water solution of salt flux. Thus, aluminum powder may be mixed with a suitable resinous carrier, such as vinyl or acrylic resins in appropriate organic solvents, and applied by brushing, spraying or other appropriate means. A wetting agent also may be included in the slurry. The viscosity of the paste or paint used is determined by the amount and type of the solvent employed. If a flux is mixed with the aluminum powder, it is advantageous to employ a salt flux which is capable of fluxing or cleaning the nickel base or cobalt base alloy. Hence, an aqueous solution of the salt hereinbefore described as constituting the fiuxing or heating bath may be advantageously employed as a vehicle for the aluminum powder. Moreover, a combination of resins or lacquers, salt fluxes and organic liquid vehicles may be mixed with aluminum powder to form the desired paste.

Vinyl resins and acrylic resins are among the lacquers or resins which may be used in the aluminum paste or spray. The lacquer identified as Binder Solution 13-9571, currently manufactured and sold by Pierce & Stevens, Inc., is an example of an appropriate resinous binder. This type of binder normally contains about 4% or 5% solids and solvent. The percentage of the solid resinous constituents is not as important as the volatility of the solvent, however, since high volatility is required to permit rapid drying of the paste after it has been applied to the turbine blade. Good results are obtained when approximately 30% to 50% by volume of aluminum powder, preferably between 200 and 400 mesh, is mixed with 10% to 20% by volume of binder solution and about 30% to 50% by volume of an appropriate thinner or solvent. Acetone or other conventional commercial thinners may be employed. It will be appreciated, of course, that the above ranges of the constituents in the paste composition are not critical and that a very wide variation in the composition may be used to obtain satisfactory results.

Alternatively, the aluminum may be hot sprayed onto surfaces of the nickel base or cobalt base turbine blade, this method commonly being referred to as metallizing.

Whether the aluminum is applied in the form of a paste or paint or as a hot spray, proper bonding of the aluminum coating material to the nickel base alloy or cobalt base alloy may be effected by subsequent heating, such as by immersion in the aforementioned salt bath. The paste should be allowed to dry before immersing the turbine blade in the salt so as to avoid introducing volatile matter into the hot salt. This bath provides proper fluxing of the nickel base alloy or cobalt base alloy and simultaneously melts the coating metal or keeps it in a molten state so as to distribute the aluminum thinly and evenly over the turbine blade. The molten salt thus prevents the formation of detrimental oxides which might otherwise adversely affect the resultant bond at the interface of the aluminum-nickel alloy or aluminum-cobalt alloy and the base metal.

The aluminum or aluminum base alloy coating material should contain approximately 80% or more aluminum in order to provide nickel base and cobalt base listed in the following Table II, the composition again being given in percentages by weight.

turbine blades with effective high-temperature oxidation Table I1 resistance. Hence, the word aluminum, when used in 5 the claims to refer to the coating material, is intended to Example 1 Ex-ampkz Example 3 Example 4 include not only pure aluminum and commercially pure aluminum, but also aluminum base alloys containing at gfigg g ;8 %3 813 21% least approxlmately 80% alumlnum. As hereinafter ex- Silicon--- 0.60 Max plained, an alloy consisting essentially of approximately 5 818 m 155 2% iron and the balance aluminum provides excellent Nickel 1.50- 3.50 900-1200 18.00 22.00 19.00-21.00 Its Molybdenum- 4.50- 6.50 2.5 3.25 3.50- 4. 50 resu Tungsten 6.00- 9.00 2.00- 3.00 3.50- 5.00

Turblne rotor buckets, nozzle guide vanes and stator Oolumbium 75- 3.00- 4.50 blades are all forms of turbine blades which are exposed 555mg; fig 555 015; to hlgh operatlng temperatures in gas turblne engines, Cobalt Balance Balance 18.00-22.00 400044.00 particularly of the axial flow type. All of these parts may be formed of high-temperature creep-resistant nickel base Referring more particularly to the drawings, in FIG- alloys and cobalt base alloys provided with an aluminum- URE 1 is shown a turbine bucket 10 for a gas turbine of ni l r l m n mlt surface y r in c r n the axial flow type. In accordance with the invention this with the present invention. Accordingly, the term turturbine bucket is formed of a nickel base alloy 12 pr0- bine blades is employed herein as encompassing these vided with a surface layer 14 of aluminum-nickel alloy. various types of gas turbine engine components. For purposes of description the thickness of this alloy The aluminum-nickel alley Preteeiive ayer should in layer is considerably exaggerated in FIGURE 1, the actual every instance be extremely thin. In general, the layer of thickness being in the order of only one or two thouthis alloy Should have a thickness of from approximately sandths of an inch, as hereinbefore explained. It usually 0.0005 inch to 0.0025 inch. An aluminum-nickel lay r is unnecessary to provide the aluminum-nickel layer over 0.0012 inch to 0.0020 inch thick is preferred, however, the fastening portion 16 of the turbine bucket. With a layer thiekHeSS of about 00015 inch being Con- In order to fully understand the beneficial results prosidered optimum. Similar thicknesses are appropriate vided by the surface layer of aluminum-nickel, the metaliI1 the ease of the aluminum-Cobalt Protective y lographic structure of nickel base alloys having this layer The thickness of the outer aluminum layer initially formed should be compared with similar alloys which do not Should not be in excess of approximately (1004 inch, and have this layer. Thus, reference is made to the photoit is presently preferred that this layer have a thickness micrograph of FIGURE 2, showing the surface of a tip less than about 0.0015 inch. of a wedge specimen formed of an uncoated nickel base The following Table I contains examples of suitable alloy 18 having the preferred composition set forth above high-temperature, creep-resistant nickel base alloys which which has been exposed to 197 hours of cyclic heating to y be satisffletefily Provided With a thin, protective sur- 1800 F. This heating has resulted in oxide penetration face layer of aluminum-nickel in accordance with the to the approximate depth indicated by the reference nupresent invention, the compositions being listed in permeral 20 in FIGURE 2. The oxide afiected zone does not sent by weight: have the same desirable high-temperature properties that Table 1 Example 1 Example 2 Example 3 Example 4 Example 5 Carbon 0.15 Max. 0.35-0.45 0.07 Max 0.15 0.08 Max. Manganese 1 Max. 2-3 1 Max. 1 Max 0.3-1 Chromium 15.5-17.5 23. 52e.5 19.5 19 Cobalt 2 5 Max l0-15 13. 5 10 Molybdcnum 18 2-4 4. 25 10 Tungsten. 5 75-5. 25 6-8 Iron 7 5Max 2Max. 5Max Vanadium 0 20.6 Tit'lm'nm 2. 5 2. 5 Aluminum 1.25 0.87 Silicon 1 Max. 1 Max. 0. 75 Max 0. 65 Max. Sulfur 0 03 Max Copper. Nickel Balance Balance Balance Balance Balance However, the nickel base alloy disclosed in United the nickel base alloy initially possessed. Moreover, asso- States Patent No. 2,688,536 Webbere et al. appears to ciated with the oxide penetration is a zone 22 of a needlebe the most outstanding turbine bucket material currently like microconstituent beneath the oxide layer. This zone, available with respect to stress-rupture properties, creep which is located at a progressively greater depth as the resistance, ductility and high-temperature corrosion redepth of oxide penetration is increased, is extremely sistance when provided with a surface layer of aluminumbrittle and possesses low stress-rupture properties. The nickel in accordance with this invention. This alloy comformation of this microconstituent is accompanied by disprises approximately 0.06% to 0.25% carbon, 13% to appearance of the normal intermetallic network found in 17% chromium, 4% to 6% molybdenum, 8% to 12% all as-cast components and is believed to be formed by a iron, 1.5% to 3% titanium, 1% to 4% aluminum, 0.01% combination of degeneration of the intermetallic network to 0.5% boron and the balance substantially all nickel. and precipitation from the matrix. Dissolved oxygen For some applications the aluminum content may be inand nitrogen gases are believed to be a factor in the formacreased to approximately 6% and the iron content may tion of the needle-like microconstituent. be as low as 0.1% or as high as 35%. The alloy usually The microstructure of a similar nickel base alloy 24 should not contain more than 20% iron, however. Norhaving a surface layer of aluminum-nickel alloy 26 in mally manganese and silicon not in excess of 1% each are also included in the alloy.

Examples of high-temperature cobalt base alloys which may be provided with a thin, protective surface layer of aluminum-cobalt by the process described herein are accordance with the invention is shown in FIGURE 3. When a tip of a wedge specimen having such a layer is subjected to the type of cyclic heating hereinbefore described, the oxide penetration into the base metal is completely eliminated. Instead, as can be seen in FIGURE 4, the surface of a nickel base alloy 28, which has been aluminum coated in the above-described manner, has a relatively deep aluminum-rich alloy layer 30 which protects the base metal from the oxidizing gases. The specimen shown was exposed to 200 hours cyclic heating to 1800 F. Thus, nickel base alloys having the surface layer of aluminum-nickel not only are completely protected against the aforementioned oxide penetration but, after exposure to cyclic heating, also show no evidence of the needle-like microconstituent associated with oxide penetration. This desirable result may be attributed to the formation of a protective oxide film upon the surface of the nickel base alloy and resistance to the diffusion of gases into the matrix by the diffused aluminum atoms in solid solution with the nickel base metal.

This surface oxide layer appears to the naked eye to be similar to the surface oxide layers on uncoated specimens which have been exposed to the same high-temperature test conditions. However, microexamination reveals that the surface oxide layer on the coated specimens does not penetrate through the alloy layer into the nickel base metal.

Diffusion heat treatment of the turbine blades after the aluminum coating operation may be beneficially employed to reduce the aluminum concentration in the surface alloy layer. Such a heat treatment is highly desirable to maintain high-temperature properties of the nickel base turbine blades, and it does not adversely affect the surface protection afforded by the aluminum coating. In general, a diffusion heat treatment at approximately 1700 F. to 2350 F. for one to six hours has proved to be effective, while a diffusion period of three to six hours at a temperature between 1800 F. and 2100 F. is preferred at present. Highly satisfactory results have been obtained by a fivehour diffusion heat treatment at 1800 F., followed by air cooling. A oneto three-hour heat treatment at a temperature of 2000" F. to 2150 F. is also very effective. It is desirable to subsequently vapor blast the turbine blades for inspection purposes. The photomicrograph of FIGURE shows a wedge specimen of a nickel base alloy 32 which had been subjected to the aforementioned type of diffusion heat treatment and thereafter exposed to 200 hours of cyclic heating to 1800 F. The resultant differences in the microstructure of this specimen, particularly the increased depth of the aluminum-rich alloy layer 34, as compared with the thickness of the layer 30 in FIGURE 4, is readily apparent.

Stress-rupture tests were conducted on both untreated nickel base alloy test specimens and similar specimens provided with a surface layer of aluminum-nickel in accordance with the present invention. Microexamination was conducted on specimens sectioned from test bars which were stress-rupture tested at 1700 F. and 10,000 pounds per square inch. The total time at 1700 F. included a twenty-hour pre-heat period before application of the load. All test bars had been investment cast in the same mold, and results were obtained on both uncoated and aluminum coated bars for direct comparison of the coating upon high-temperature properties.

When a nickel base alloy specimen was successively dipped in molten aluminum and the salt flux for a total period of 30 seconds with no diffusion heat treatment being employed and subsequently tested at 1500 F., an aluminum-rich alloy layer having a thickness of 0.0045 inch to 0.005 inch was obtained. This alloy layer was the most brittle of the various types of aluminum-nickel layers formed on the specimens tested. This brittleness is due to the high aluminum concentration in the outer portion of the alloy layer produced at the low testing temperature of 1500" F. As a result of the alloying action of the aluminum coating, the diameter of the test specimen was increased approximately 0.002 inch to 0.006 inch. Stress-rupture properties were slightly decreased as a result of the aluminum coating which, of course, improved the high-temperature oxidation resistance of the nickel base specimen. The microstructure u of this alloy after 200 hours of cyclic heating to 1800 F. is shown in FIGURE 4.

A test bar similar to that described in the aforementioned example was aluminum coated in the same manner and subsequently subjected to a five-hour diffusion heat treatment at 1800 F. followed by air cooling. This procedure increased the thickness of the aluminum-rich alloy layer between 0.005 inch and 0.0056 inch. Moreover, this alloy layer, which is shown in FIGURE 5 after the specimen was exposed to 200 hours of cyclic heating to 1800 F., was more ductile than the layer shown in the photomicrograph of FIGURE 4. The surface of this heat treated specimen possessed less pronounced aluminum concentration at the surface than the specimen shown in FIGURE 4 due to the higher diffusion temperature. However, the thick aluminum-rich alloy layer was subject to some spalling. The diameter of the diffusion heat treated aluminum coated test bar was approximately 0.002 inch to 0.004 inch larger than the diameter of the bar prior to the application of aluminum. These combined steps of aluminum coating the nickel base alloy and subsequent diffusion heat treatment resulted in slightly decreasing the stress-rupture life of the test bar and slightly increasing its elongation under tensile stress.

When similar aluminum coated nickel base test specimens were subjected to a diffusion heat treatment of five hours at 2000 F. followed by air cooling, the aluminumrich alloy layer increased in thickness to between 0.0056 inch and 0.0061 inch on the average. This layer was fairly ductile because of the low aluminum concentration at the surface due to the 2000" F. diffusion temperature. However, the thick layer was susceptible to some spalling. The increase in diameter of the test bars resulting from this aluminum coating and diffusion treatment averaged approximately 0.002 inch to 0.005 inch. These bars showed some decrease in stress-rupture life and an appreciable decrease in elongation under tensile stress.

Other nickel base specimens of the same composition were provided with a thin surface layer of aluminumnickel alloy by means of a procedure which included successively pickling the aluminum coated specimens in acid and further diffusing the aluminum into the base metal by heat treatment. Test bars processed in this manner exhibited excellent stress-rupture properties. For example, it was found that the physical properties of nickel base alloy test bars were greatly improved when these bars were coated by means of a 10-second aluminum dip followed by a 10-second salt rinse and thereafter subjected to a treatment comprising a pickling period of 25 minutes in a 10% hydrochloric acid solution, five hours diffusion at 1800 F. and air cooling. A photomicrograph of a section of a test bar so treated is shown in FIGURE 6 after the specimen was exposed to 200 hours of cyclic heating to 1800 F. It will be noted that the aluminumnickel layer 36 on the surface of the base metal 38 is relatively thin, the alloy layer being only about 0.0017 inch to 0.0019 inch thick. Of the various procedures which may be used to aluminum coat and heat treat nickel base alloys in accordance with the subject invention, this latter process produces the most ductile aluminum-rich alloy layer. Moreover, there is no indication that this layer tends to spall. Although a thinner layer of aluminum-rich alloy is thereby provided on the surface of nickel base alloys, this layer possesses a low aluminum concentration at the surface. This combination of a thin aluminum-rich layer and a low aluminum concentration at its surface provides the treated alloy with optimum physical properties with respect to stress-rupture characteristics and high-temperature oxidation resistance. Moreover, the above-described treatment does not measurably increase the diameter of a test bar.

Thus it will be seen that diffusion heat treatment after the aluminum coating step is highly desirable to maintain the high-temperature properties and to provide an alloy layer which is plastic at elevated temperatures during deformation with no loss of oxidation resistance. This heat treatment produces these desirable results by reducing the aluminum concentration at the surface alloy layer.

As explained above, on the other hand, aluminum coated nickel base turbine blades which have not been subjected to diffusion heat treatment are provided with an aluminum-rich alloy layer over which there is an excess unalloyed aluminum layer. The thickness of the asdipped aluminum-rich alloy layer may be controlled by varying the length of the total dip period. The term total dip period is used herein as meaning the total exposure time of the immersed article to molten aluminum and includes the periods of immersion in both the aluminum coating bath and the salt flux. However, examination has indicated that the as-dipped aluminum-rich alloy layer does not solely control the final diffused layer thickness. The excess aluminum overlay, which forms more aluminum-rich alloy in the diffusion process, has a greater influence on the final layer thickness. Although vibration of the turbine blades in the salt bath removes some of the excess aluminum, an appreciable amount of the excess aluminum still remains on the surface of the blades.

Accordingly, to obtain optimum results by means of a thinner difiused layer of aluminum-nickel, it is advantageous to remove the excess aluminum overlay before diffusion. As indicated above, this can be effectively accomplished by pickling the as-dipped nickel base turbine blades in a dilute aqueous solution of hydrochloric acid at a temperature of about 60 F. to 90 F. for approximately 15 minutes to 45 minutes. A 10% acid solution has been found to produce excellent results. The aluminum-rich alloy layer is not materially affected by this pickling process. Subsequent diffusion produces an alloy layer less than 0.002 inch thick, as compared with a layer thickness of approximately 0.005 inch to 0.006 inch for aluminum coating nickel base specimens which are not pickled before diffusion. Hence it can be seen that when the excess aluminum overlay is removed, the thickness of the final diffused aluminum-nickel layer is controlled by the thickness of the aluminum-rich alloy which is formed during dipping. Since this thickness may be controlled by varying the total time the nickel base alloy is immersed in the aluminum and the salt bath, we have found a total dip period of 10 to 20 seconds to be highly satisfactory. In this manner an aluminum coated nickel base turbine blade may be produced which possesses excellent stress-rupture and other physical properties, as well as corrosion resistance at elevated temperatures.

The photomicrograph of FIGURE 7 shows the microstructure of a stress-rupture bar formed of a nickel base alloy having the preferred composition hereinbefore set forth after the bar has been tested for 2807 hours at a temperature of 1700 F. under a stress of 8500 pounds per square inch. It will be seen that both the maximum oxide depth 40 and the depth of alloy depletion and microstructure change are appreciable. The latter zone is indicated in FIGURE 7 by the total depth of zone 40 and zone 42. When this stress-rupture bar is compared with the stress-rupture bar, such as the one shown in the photomicrograph of FIGURE 8, having the same base metal composition but provided with a diffused surface layer of aluminum-nickel in accordance with the invention, the differences in the microstructure of the two materials are readily apparent. This stress-rupture bar likewise was tested at 1700 F. under a stress of 8500 pounds per square inch, but the test was extended for 2919 hours. At the end of this test period, as shown in FIGURE 8, the thickness of surface oxide zone 44 is negligible. The depth of the aluminum-nickel alloy layer is indicated at 46.

Results of the above and other extended stress-rupture tests at temperatures between 1500 F. and 1700 F. indicate that the stress-rupture life of nickel base alloys is increased approximately 40% by the provision of a thin surface layer of aluminum-nickel by means of the preferred procedure described above. The aluminum-nickellayer eliminates oxide penetration of the base metal, and the diffusion treatment produces beneficial aging or precipitation in its microstruction. It is under these longperiod, high-temperature conditions that oxidation plays a major part in determining the life of a nickel base turbine bucket alloy.

The nickel base alloys shown in the photomicrographs of FIGURES 2 through 8 have the same composition, i.e., the preferred composition hereinbefore described. The photomicrograph of FIGURE 7 is at 250 magnifications, while the other photomicrographs are at 500 magnifications. Marbles reagent was used as the etchant for the specimens shown in FIGURE 2, Vilellos etch was employed in the specimens of FIGURE 6, while a modified acid ferric chloride Was used as the etchant for the specimens shown in the other photomicrographs.

The graph of FIGURE 9 compares the creep elongation of the uncoated nickel base alloy, shown by the curve 43, with the creep elongation of the same alloy provided with the above-described surface layer of alumimum-nickel. The curve 50 indicates the creep characteristics of the latter material. A tensile load of 2500 pounds per square inch and a temperature of 1500 F. were employed in these tests. The increased life of the treated bars is reflected in a decreased creep rate, indicating that the strength of the alloy can be more effectively utilized by the formation of the protective layer. This is important when ductility is measured in terms of extremely long periods of time at elevated temperature. For example, an uncoated nickel base alloy specimen, when tested at a temperature of 1700 F. under a tensile load of 8500 pounds per square inch, deformed approximately 1.1% in 2807 hours, while a similar specimen provided with the aforementioned surface layer of aluminum-nickel showed no measurable deformation in 2919 hours.

It also has been found that the thickness of the final aluminum-nickel layer formed during diffusion treatment may be controlled to some extent by both the temperature of the aluminum dip bath and the composition of the aluminum coating material. The graph of FIGURE 10 indicates the effect of temperature and the length of the total dip period on the thickness of this layer. For example, the final difiused alloy layer on samples dipped in pure aluminum for the preferred immersion period of 10 to 20 seconds increases approximately 0.00015 inch for each 10 F. rise in dipping temperature from 1300 F. to 1400" F. This is shown in FIGURE 10 wherein the

curves

52, 54 and 56 indicate aluminum bath tem peratures of 1300 F., 1350 F. and 1400" F., respec tively.

As indicated by the graph of FIGURE 11, if the total dipping time and dipping temperature remain constant, the presence of a small amount of iron in the aluminum coating bath tends to reduce the thickness of the diffused aluminum-nickellayer. In this graph the curve 58 shows the approximate thickness of this layer when commercial 28 aluminum is used as the coating material, while the curve 60 indicates the thickness of the aluminum-nickel layer when the aluminum coating material contains approximately 2% iron. When the preferred total dip time of 10 to 20 seconds is employed with a coating metal bath at a temperature of approximately 1350 F., the thickness of the diffused alloy layer on nickel base turbine blades coated with an alloy of 2% iron and 98% aluminum, for example, will be approximately 0.0005 inch less than the thickness of the aluminum-nickel layer formed by coating with pure aluminum.

While this invention has been described by means of certain specific examples, it will be understood that the scope of the invention is not to be limited thereby except as defined in the following claims.

We claim:

1. An oxidation-resistant turbine blade formed of a metal selected from the class consisting of nickel base alloys and cobalt base alloys, the Working surfaces of said blade having a thin layer of an alloy of aluminum with the base metal of said blade, said layer having a thickness of approximately 0.0005 inch to 0.0025 inch.

2. A highly oxidation-resistant turbine blade for a gas turbine engine, said blade being formed of a nickel base alloy and having at its fluid-contacting surfaces an integral layer of a diffused aluminum-nickel alloy of a thickness between approximately 0.0005 inch and 0.0025 inch.

3. A creep-resistant cast turbine blade characterized by outstanding oxidation resistance at elevated temperature formed from a nickel base alloy and having its gas-contacting surfaces provided with a relatively ductile integral layer of an aluminum-nickel alloy having a thickness of approximately 0.0012 inch to 0.002 inch.

4. A turbine blade characterized by high oxidationresistance at elevated temperature, said turbine blade being formed of a cobalt base alloy having its fluid-contacting surfaces formed of an integral layer of diffused aluminum-cobalt alloy having a thickness of about 0.0005 inch to 0.0025 inch.

5. A cast turbine blade formed from a metal consisting essentially of about 0.06% to 0.25% carbon, 13% to 17% chromium, 4% to 6% molybdenum, 1% to 6% 12 aluminum, 1.5% to 3% titanium, iron not in excess of 20%, boron not in excess of 0.5% and the balance substantially all nickel, said blade having gas-contacting surfaces provided with a thin, relatively ductile, oxidation-resistant layer of aluminum-nickel alloy.

6. A high-temperature creep-resistant cast turbine blade for a gas turbine engine formed from a metal consisting essentially of about 0.06% to 0.25% carbon, 13% to 17% chromium, 4% to 6% molybdenum, 1% to 6% aluminum, 1.5% to 3% titanium, iron not in excess of 35%, boron not in excess of 0.5% and the balance nickel, the gas-contacting surfaces of said blade being provided with a relatively ductile oxidation-resistant integral layer of aluminum-nickel alloy having a thickness of approximately 0.005 inch to 0.0025 inch.

References Cited in the file of this patent UNITED STATES PATENTS 2,664,874 Graham Jan. 5, 1954 2,682,101 Whitfield June 29, 1954 2,683,305 Goetzel July 13, 1954 2,752,567 Schaefer July 3, 1956 2,807,435 Hewlett Sept. 24, 1957 2,837,818 Storchheim June 10, 1958 FOREIGN PATENTS 449,998 Canada July 20, 1948