Abstract:
A structure comprising a main body formed of a nickel base alloy, and a NiAl coating applied to the main body, wherein the NiAl coating includes a nickel rich NiAl layer and an aluminum rich NiAl layer and the thickness ratio of the nickel rich NiAl layer to the whole NiAl coating is in a range between 20 and 40%. A method of applying an aluminum coating to a nickel base alloy comprising the steps of forming an aluminide layer of Ni 2  Al 3  on the surface portion of the nickel base alloy by the diffusion and penetration of aluminum, and subjecting the alumide layer to heat treatment to change the aluminide layer to a monoaluminide layer and making the thickness ratio of a nickel rich NiAl layer to the whole monoaluminide layer be in the range between 20 and 40%.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a nickel base alloy structure provided with an aluminum coating and methods of producing same, and more particularly it is concerned with a blade and a nozzle of a gas turbine both of which are made of a nickel base alloy and are provided with an NiAl coating on its surface having excellent corrosion and thermal shock resistant properties and a method of producing same. 
     Nickel base alloys are often used for forming parts of gas turbines, jet engines, etc., that are exposed to heat of high temperature. Particularly, the blade and nozzle of a gas turbine are often exposed to corrosion and oxidation inducing combustion gas atmosphere containing sulfur, chlorine, vanadium, sodium and lead etc. Thus the nickel alloys are required to have not only high mechanical strength at elevated temperatures but also corrosion and oxidation resistant properties against combustion gas. To impart corrosion and oxidation resistant properties to the nickel alloys, it is necessary to increase the contents of chromium, aluminum and other alloying elements in the alloys. However, an increase in the contents of chromium, aluminum and other alloying elements in excess of acceptable levels would cause a reduction in the ductility of the nickel alloys and other defects, with the result that there are naturally limits to the contents of these alloying elements that could be added to the nickel alloys. 
     Under these circumstances, it has hitherto been common practice to protect the nickel alloys from combustion gas by applying surface treatment to the alloy, such as the provision of a coating layer on their surfaces. Typical treatment for applying coating on the nickel alloys is an aluminum coating applying treatment, in which Al coating is applied by such various means as the immersion of an alloy in a molten salt, sputtering, vaporization deposition, a CVD process and a pack cementation process. In these methods, the pack cementation process is most popular which, as described in Japanese Patent Publication No. 3729/1973 (corresponding to U.S. Ser. No. 796,906 of Feb. 5, 1969), includes the steps of embedding a nickel base alloy in a powder mixture of aluminum powder, inert refractory material powder (alumina, for example) and halogenated active agent powder (NH 4  Cl, NH 4  F, NaCl, NaF, AlCl 3 , etc.) and maintaining the alloy at a predetermined high temperature to cause the aluminum to be diffused into the nickel alloy to form on its surface a coating high in aluminum content. The coating applied to the surface of a nickel base alloy by the pack cementation method is composed of intermetallic compounds, such as Ni 2  Al 3 , NiAl, etc., formed by the combination of aluminum and the nickel in the nickel base alloy that is treated. When the treatment by the pack cementation method is carried out at low temperature below 850° C. to specifically increase the corrosion resistant property of the alloy by increasing the amount of chromium in the coating layer, the coating layer becomes composed mainly of Ni 2  Al 3  low in mechanical strength. This makes it necessary to apply heat treatment to the alloy following the pack cementation treatment so as to change the coating to another coating composed mainly of NiAl having increased ductility. After such heat treatment, the coating is composed of a first layer of NiAl rich in aluminum and a second layer of NiAl rich in nickel. The prior art pack cementation method has suffered the disadvantages that, since the heat treatment for changing the composition of the coating has been carried out at high temperature for a prolonged time, the layer of NiAl rich in nickel accounts for the major part of the coating, with the result that the coating provided is not necessarily high in corrosion resistance. 
     OBJECT OF THE INVENTION 
     This invention has as its object the provision of a structure of a nickel base alloy provided with a coating of aluminum-nickel intermetallic compound on its surface having both of excellent corrosion and thermal shock resistant properties, and a method of producing same. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further understood from the following detailed description and the accompanying drawings wherein: 
     FIG. 1 is a microphotograph of a microstructure of a section of a typical Ni-Al coating layer of the invention; 
     FIG. 2 is a graph showing variations in the weight percentages of the essential components of the Ni-Al coating layer alloys by EPMA: 
     FIG. 3 is a graph showing the relationship between the variations of the thickness of the Ni-Al coating layer and the corrosion and thermal shock properties; 
     FIG. 4 is a perspective view of a gas turbine blade having a Ni-Al coating layer applied thereto; and 
     FIG. 5 is a perspective view of a gas turbine nozzle having a Ni-Al coating layer applied thereto. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the present invention, a structure such as, for example, a blade of a gas turbine, a nozzle of a gas turbine, and etc. made of a Ni base alloy is subjected to a pack cementation treatment at a predetermined temperature below 850° C. to obtain diffusion and penetration of aluminum elements in the surface layer of the structure, and the structure is then subjected to heat treatment for changing the previously formed aluminide layer of Ni 2  Al 3  to a monoaluminide layer of NiAl to thereby apply a coating of NiAl having excellent properties to the nickel base alloy. The outstanding characteristic of the invention resides in the points that the heat treatment to be effected following the pack cementation treatment is carried out under the condition that the value of the parameter P o  is in the range between 50 and 52.5, which parameter varies depending on the temperature T (°K.) and time t (hr) of the heat treatment effected after the pack cementation treatment and has the following relation ##EQU1## where T i  and t i  are the temperature and time of such heat treatment respectively of the i-th times, and n is the number of time of the heat treatment effected following the pack cementation treatment, whereby the thickness ratio of the NiAl layer rich in nickel in the coating becomes in the range between 20 and 40%. If the temperature at which the pack cementation is effected exceeds 850° C., much amount of Al diffuses into the Ni base alloy, with the result that there is caused in the alloy a phase which makes the alloy brittle. 
     FIG. 1 shows a microstructure of a section of a typical NiAl coating layer embodying the present invention. FIG. 2 shows variations in weight percent of essential components constituting the NiAl coating layer when the components are analyzed by use of EPMA. In the microstructure of FIG. 1, black points in Al rich NiAl layer shows Cr particles, while σ phase in Ni rich NiAl layer is shown by black columnar shapes in contact with a base metal. In the interface between the base metal and the Ni rich NiAl layer there is a Ni 3  Al layer having several microns in thickness (not shown in FIG. 1). Ni rich NiAl layer is distinguished from Al rich NiAl layer by the ratio of nickel atom number to aluminum atom number in the NiAl layer when analyzed by the EPMA, that is, in the Ni rich NiAl layer the ratio of Ni atom number is larger than the ratio of Al atom number. The interface between the Ni rich NiAl layer and Al rich NiAl layer is shown by a broken line in FIG. 2, at the vicinity of which interface the content of chromium approaches the minimum while the increase of Ni content and decrease of Al content occur in an area from the interface to the base metal. 
     In the present invention, the ratio of the Ni rich NiAl layer thickness to the whole NiAl layer thickness is made to become within the range of 20 to 40 percents by adopting the condition of the heat treatment following the pack cementation treatment in which the value of the parameter P o  is made to be in the range between 50 and 52.5. By virtue of this feature, the NiAl coating provided by the heat treatment can be given with excellent corrosion and thermal shock resistant properties. When the ratio of the thickness of the NiAl layer rich in nickel in the coating layer exceeds 40%, the corrosion resistance of the structure is greatly reduced. The reasons for this phenomenon seems to reside in the point that the corrosion resistance of the NiAl layer rich in nickel is lower than that of the NiAl layer rich in aluminum located outside the NiAl layer rich in nickel. Conversely when the thickness ratio of the NiAl layer rich in nickel is less than 20%, the thermal impact resistance of the alloy is lessened. The reasons for this phenomenon seems to reside in the points that in the NiAl coating layer there becomes larger the ratio of the Al rich NiAl which is relatively low in ductility and that a Ni 2  Al 3  layer formed by the pack cementation treatment partly remains without changing to the NiAl layer in spite of the succeeding heat treatment. Preferably the ratio of the NiAl layer rich in nickel is in the range between 25 and 35%, more preferably in the range between 28 and 32%. The thickness of the NiAl coating layer is preferably in the range of 50 to 160 microns. 
     In FIG. 3 there is shown a relation between the variation in the ratio of the Ni rich NiAl coating layer and the corrosion and thermal shock resisting properties, wherein the amount of corrosion increases very much in case the thickness ratio of the Ni rich NiAl layer exceeds 40 percents, while cracks occur in case the thickness ratio thereof is not more than 20 percents. 
     The blade of a gas turbine is preferably of a casting consisting essentially of, by weight, 0.1 to 0.25% C, 10 to 20% Cr, 5 to 13% Co, 1 to 3% Mo, 1 to 5% W, 0.4 to 1.5% Nb, 2 to 5% Ti, 2 to 5% Al, 0.005 to 0.1% B, 0.05 to 0.3% Zr, 1 to 3% Ta and the balance Ni. The nozzle of a gas turbine is preferably of another casting consisting essentially of, by weight, 0.1 to 0.2% C, 20 to 25% Cr, 16 to 22% Co, 1.5 to 2.5% W, 0.5 to 1.5% Nb, 1 to 2% Ta, 3 to 4.5% Ti, 1 to 3% Al, 0.05 to 0.15% Zr, 0.005  to 0.015% B, and the balance Ni. 
     One example of the invention will now be described. It is to be understood that the invention is not limited to this specific example. 
     EXAMPLE 1 
     A coating layer shown in the present invention was applied to a gas turbine blade 41 shown in FIG. 4 which is made of Ni base alloy of IN-738LC having the following components: 
     
         ______________________________________C           0.17 wt %   Nb       0.9 wt %Cr         16.1         Ti       3.3Ni         Balance      Al       3.4Co          8.6         B        0.01Mo          1.8         Zr       0.09W           2.5         Ta       1.7______________________________________ 
    
     First, a portion of the gas turbine blade 41 defined between the terminal end thereof and the center 43 of a shank 42 was embedded in a powder mixture for the pack cementation process and then is maintained at 750° C. in 4 hours in an argon gas atmosphere, whereby a Ni 2  Al 3  layer was formed on the surface of such portion of the gas turbine blade 41. The powder mixture consists of Al powder of 25%, NH 4  Cl powder of 1.5% and Al 2  O 3  powder of 73.5%. Since coating treatment is not necessary regarding another portion of the turbine blade 41 defined between the center 43 of the shank 42 and a dovetail, such another portion was embedded in Al 2  O 3  powder of 100%. After applying Ni 2  Al 3  layer on the gas turbine blade 41 by use of the pack cementation process, there was effected a heat treatment of 2 hours at 1120° C. and then another heat treatment of 24 hours at  843° C. to thereby change the Ni 2  Al 3  layer to NiAl coating layer having a thickness of 120 microns while the thickness ratio of the Ni rich NiAl layer is made to be 30 percents (as shown in a test piece No. 2 in Table 1). After the same pack cementation treatment is applied to other gas turbine blades, heat treatments shown in Table 1 were applied to change Ni 2  Al 3  layer into NiAl layer having various ratios of Ni rich NiAl layer, while the heat treatment parameter P o  was obtained regarding each heat treatment. 
     Then, the gas turbine blades having the NiAl coating layer were assembled into an actual gas turbine, while gas turbine was operated actually about 10,000 hours by use of light oil including 0.3% sulfur as a fuel so as to test the gas turbine blade as to whether high temperature corrosion and/or crack occurs, wherein the temperature of the surface of the gas turbine blade during the operation is about 800° C. As a result, there was not observed the occurrence of high temperature corrosion nor the occurrence of crack regarding the gas turbine blade test pieces Nos. 1, 2 and 3 shown in Table 1. However, the occurrence of crack was observed regarding the test pieces No. 4, while the high temperature corrosion was observed regarding the test pieces Nos. 5 and 6. 
     For testing in detail the corrosion and thermal shock resisting properties of the gas turbine blade applied provided on the surface thereof with the NiAl coating layer, other experiments were effected regarding the gas turbine blade Nos. 1 to 5 in Table 1. 
     As regards the experiment for testing corrosion resistance, the test pieces were immersed in a molten salt of 25% NaCl+75% Na 2  SO 4  and maintained at 850° C. for 120 hours in an electric furnace, and then subjected to descaling by boiling them in an aqueous solution of 18% NaOH+3% KMnO 4  and then in an aqueous solution of 10% ammonium citrate. Following the treatment, so that corrosion losses were determined to assess the corrosion resistance of the test pieces. Table 1 shows the corrosion losses determined by the test. It will be seen that the corrosion losses of specimens No. 1-3 obtained by the method according to the invention, in which the heat treatment parameter P o  has a value in the range between 50.7 and 52.4 and the thickness ratio of the nickel rich NiAl layer in the NiAl coating to the whole covering layer is in the range between 23 and 38%, were in the range between 0.43 and 0.7 mg/cm 2 . In contrast, corrosion losses of test pieces Nos. 5 and 6 prepared for comparison having the parameter P o  value of 53.2 and 80.3 respectively and the thickness ratios of nickel rich NiAl layer of 50% and 60% respectively to the whole coating layer were 6.0 mg/cm 2  and 10.7 mg/cm 2  respectively. It is apparent that the test pieces Nos. 1-3 obtained in accordance with the present invention have excellent corrosion resistance. 
     The specimens Nos. 1-3 obtained by the method embodying the present invention were further tested regarding thermal impact resistance. For comparison, the test piece No. 4 having the heat treatment parameter P o  of 49.7 and 16% thickness ratio of the nickel rich NiAl layer to the whole NiAl layer was also tested for thermal impact resistance. In the thermal impact resistance test, the test pieces were subjected to the repetition of heating and water cooling by three times between room temperature and 800° C., one cycle of which repetition lasts for six minutes. After the test, cracks formed on the aluminum coating were observed with an optical microscope. The results show that no crack formation was observed on the surfaces of the test pieces Nos. 1-3 whole formation of cracks of a mean spacing of 200 microns was observed on the surface of the test piece No. 4. 
     EXAMPLE 2 
     A NiAl coating layer formed in accordance with the heat treatment conditions shown in the test pieces Nos. 2 and 5 in Table 1 is applied on the whole surface of a gas turbine nozzle 51 shown in FIG. 5, which gas turbine nozzle is made of a Ni base alloy having the following components: 
     
         ______________________________________C           0.15 wt %   Ta       1.4 wt %Cr         22.5         Ti       3.7Ni         Bal          Al       1.9Co         19.0         Zr       0.1W           2.0         B        0.01Nb          1.0______________________________________ 
    
     Then, the gas turbine nozzle was assembled into an actual gas turbine, which gas turbine was operated actually about 10,000 hours by use of light oil including 0.3% sulfur as a fuel so as to test the gas turbine nozzle as to whether or not high temperature corrosion and/or crack occurs. As a result, in a nozzle having a NiAl coating layer formed in accordance with the heat treatment corresponding to that of the test piece 2 shown in Table 1 there was observed no high temperature corrosion nor crack. However, in another nozzle having another NiAl coating layer formed in accordance with the heat treatment corresponding to that of the test piece No. 5 shown in Table 1 there was observed the occurrence of high temperature corrosion. 
     Thus, in the present invention, it becomes possible to obtain a structure provided on the surface thereof with a NiAl coating layer having both of excellent corrosion resistance and excellent thermal resistivity, which structure is made of a Ni base alloy. Also, in the present invention it becomes possible to obtain a method of producing such temperature superior in quality above described. 
     
                                           TABLE 1__________________________________________________________________________Number       Condition of Heat                 Heat         Thickness Ratio ofof   Condition of        Treatment after                 Treatment                       Thickness of                              Nirich NiAl to                                        Amount ofTest Pack Cementa-        Pack Cementation                 Parameter                       NiAl coating                              The Whole Coating                                        Decrease By                                               Thermal ShockPiecetion Treatment        Treatment                 Po    Layer  Layer     Corrosion                                               Resistivity__________________________________________________________________________1    750° C.        1050° C., 2 hrs                 50.7  About 120                               23%      0.43 mg/cm.sup.2                                               no crack4 hrs   → 843° C., 24 hrs                       microns2    750° C.        1120° C., 2 hrs                 52.1  About 120                              30        0.5    no crack4 hrs   → 843° C., 24 hrs                       microns3    760° C.        1120° C., 3 hrs                 52.4  About 120                              38        0.7    no crack3 hrs   → 843° C., 24 hrs                       microns4    750° C.        1000° C., 2 hrs                 49.7  About 120                              16        --     Occurrence of Crack5 hrs   → 843° C., 24 hrs                       microns                 with Mean Spacing                                               of about 200μ5    750° C.        1150° C., 4 hrs                 53.2  About 120                              50        6.0    --2 hrs   → 843° C., 24 hrs                       microns6    750° C.        1080° C., 6 hrs                 80.3  About 120                              60        10.7   --2 hrs   → 1120° C., 2 hrs                       microns        → 843° C., 24 hrs__________________________________________________________________________