US3635769A - Nickel-chromium eutectic alloy - Google Patents

Abstract

A nickel-chromium alloy consisting of about 50 percent of nickel and 50 percent of chromium and having an oriented lamellar microstructure which is obtained by directional solidification.

Description

United States Patent 1 1 3,635,769 Shaw 1451 Jan, 18, 1972 NICKEL-CHROMIUM EUTECTIC [56] References Cited ALLOY UNITED STATES PATENTS i1 11 k, [m Invent J acme Pa 3,124,452 3/1964 Kraf ..75/134 17 Assigneer Westinghouse Electric rp n, P 2,809,139 10/1957 Bloom et a1 ..148/l33 sburgh,Pa.

- Primary Examiner-Richard 0. Dean 19 9 [22] Filed Aug 6 A1tameyF. Shapoeand Lee P. Johns 121 Appl.No.: 851,859. 1

[57] ABSTRACT 52 us. Cl.....................................148/32, 72/377, 75/171, A nicke1 ch,omium alloy consisting of about 50 percent of nickel and 50 percent of chromium an having an oriented [51] In ..C22c 19/00 lamenar microSn-uctul-e which is o tamed by directional l'dification.

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4 Claims, 7 Drawing Figures PME'NNUJMM? 3 635 769 EUTECTIC FIGG.

TTTTTTTT RS PATENTED JAN 1 8 1972 SHEET [1F 4 FULLY AS GROWN LAMELLAR I ANNEALED 5O HOURS,760C PARTIALLY 0 AS GROWN LAMELLAR oCAST Ni 42Cr 6a 009 mmmmkm Jlllll 500 TIME ,HOURS Ii m Y m AE G CT Fl Ar m M X m N u a m m s M m m G H 3 E T W 8 WD O mm m T 5 4! U 7 2 3 Mm plo Q 4 N 3 as wmo Emma Luz com 30 CHROME CONTENT, WT

NlICllfEL-CHROMIUM EIJTIECTIC ALLOY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a nickel-chromium eutectic alloy and, more particularly, it pertains to members of such an alloy having an oriented lamellar structure.

2. Description of the Prior Art Increases in the efficiency of gas turbines are directly related to the properties of the material of which the turbine blades are composed. Small increases in operating temperature at the blade surface have always resulted in improved output for a given size of turbine. Thus the turbine blades are both the key to higher efficiency as well as a serious impediment to future improvements. That is true whether the turbine is designed for use in an aircraft, or in surface-based gas turbines which latter are frequently used to generate domestic electric power during peak demand hours or for standby use.

In those applications efficiency is not the only parameter of importance. An economically valid test of power-generating turbine systems is the service-free lifetime and the number of hours the unit may be operated on an intermittent duty cycle without reblading. For aircraft engines the turbine blades are used for up to 2,000 hours as compared with surface-based turbines which operate up to 10,000 hours without a service shutdown.

Related to the foregoing is the problem of corrosion re sistance which governs the service life of turbine blades and particularly land-based turbines where a wide variety of corroding ambients are prevalent. Various so-called super alloys, such as nickeland cobalt-base alloys, have practically dominated all turbine blade applications heretofore. However;

the chemical complexity of the alloys has resulted in many reactions which are not understood and are difficult to control. Overaging, for one reason, causes deterioration of mechanical properties after exposure at high temperatures and stresses. Precipitation of undesirable properties such as the sigma phase and grain boundary carbides is frequency detrimental to alloy performance and to longtime dimensional stability. Moreover, the metallurgical art which has optimized the mechanical performance of the alloys does not provide any base upon which to gauge corrosion resistance.

In the past it has been found that at least 12 percent chromium in nickel-chromium alloys was necessary to give fair corrosion resistance at high temperatures. Although the mechanical properties of this alloy are improved, as the percentage of chromium is reduced the corrosion resistance is lessened.

None of the prior existing alloys used for gas turbine blades have completely satisfied the threefold problems of thermal stability, strength, and corrosion resistance at elevated temperatures of operation.

Lamellar eutectic alloys as such are known: see US. Pat. No. 3,124,452 and an article entitled Coarsening of Eutectic Microstructures at Elevated Temperatures" by L. D. Graham and R. W. Kraft on pages 94 to 101 of Vol. 236, Jan. 1966, issue of Transactions of the Metallurgical Society of AIME."

SUMMARY OF THE INVENTION It has been found in accordance with this invention that the foregoing problems may be overcome by providing a nickelchromium alloy consisting of about 50 percent nickel and 50 percent chromium and having an oriented lamellar microstructure. The nickel-chromium alloy of the present invention consists of a binary eutectic alloy of nickel and chromium which is substantially devoid of any intentional additional alloy elements and which may include only incidental impurities.

Accordingly, it is a general object of this invention to satisfy the foregoing problem by providing a nickel-chromium binary eutectic alloy having oriented lamellar microstructure.

It is another object of this invention to provide members of a nickel-chromium binary eutectic alloy consisting of 50 percent nickel and 50 percent chromium and which is substantially devoid of additional elements, and has an oriented lamellar microstructure.

Finally, it is an object of this invention to satisfy the foregoing objects and desiderata in a simple and expedient manner.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the invention reference is made to the accompanying drawings, in which:

FIG. l is a phase diagram of the nickel-chromium binary system;

FIG. 2 is a photomicrograph of a transverse section of a nickel-chromium eutectic alloy having oriented lamellae produced by unidirectional solidification (X);

FIG. 3 is a graph of yield strength plotted as a function of d where d is the specimen diameter as a result of cold swaging of Ni-Cr eutectic alloy at room temperatures;

FIG. 45 is a bar chart showing the thermal stability of the Ni-Cr eutectic alloy at room temperature and at l,400 F. both in the worked and unworked condition as compared with another nickel base alloy superalloy Rene 41 FIG. 5 is a graph showing the creep-rupture behavior of Ni-Cr eutectic alloy at 760 C. in which specimens having a fully lamellar structure are compared with specimens having a partially lamellar structure;

FIG. 6 is a graph showing creep-rupture tests of the Ni-Cr eutectic alloy at l,400 F. compared with 60 percent Cr-40 percent N: (highand low-carbon) alloys; and

FIG. 7 is a graph showing the corrosion resistance of NiCr eutectic alloy as compared with other high-temperature materials which have been used previously as gas turbine blades.

DESCRIPTION OF THE PREFERRED EMBODIMENT The alloy of the present invention is a eutectic composition of nickel and chromium having a lamellar microstructure in which the lamellae are oriented in substantially the longitudinal direction of the alloy member or product, the major stress being applied in the longitudinal direction as in turbine blades, for optimum strength and high-corrosion resistance. The longitudinally oriented lamellar structure is preferably obtained by directional solidification of an alloy having a sub stantially eutectic composition of nickel and chromium. As shown in FIG. I, the eutectic composition of the nickelchromium system is located at about 50 weight percent nickel and 50 weight percent chromium. More particularly, the eutectic composition is 5011 percent of chromium and 50:] percent of nickel and may contain traces of incidental impurities such as iron. The melting temperature of the eutectic composition is approximately 1,345C.

The phase diagram of the nickel-chromium system (FIG. 1) indicates that it is amenable to unidirectional solidification. Both of the resulting phases (nickel-rich phase and chromiumrich phase) have widely spaced terminal solid solubility limits to provide for good corrosion resistance. The solubility is retrograde and second-phase precipitation is possible. When the eutectic composition solidification reaction is correlated with unidirectional solidification, there is produced a nickelchromium composition with a regular lamellar structure and a minimum number of grain boundaries, as shown in FIG. 2. In order to obtain the properties having the desired combination of high strength, thermal stability, corrosion resistance, and creep resistance at elevated temperatures, control over the lamellar spacing is necessary.

Generally, the process for obtaining these properties in the binary alloy of nickel-chromium with a lamellar eutectic structure such as shown in FIG. 2, includes the following steps:

1. Cast a melt of an alloy of eutectic composition of nickel and chromium into a container'mold having a high melting point and holding the melt at a temperature above the melting point of about l,350 C. for the alloy in a suitable furnace means such as a Bridgman furnace,

2. Lower the molten ingot through the lower end of the furnace at a constant velocity and into a quenching medium to produce a unidirectional lamellar structure; and

3. Cold work the resulting solidified ingot having a unidirectional lamellar structure until a reduction in area of from about 50 to 75 percent is achieved.

The resulting member has good strength up to about 1,400 F. When properly worked the alloy has a room temperature yield strength of about 300,000 p.s.i.

The following example is further illustrative of the present invention:

EXAMPLE Specimens of a melt of a eutectic composition of nickelchromium having 50 percent nickel and 50 percent chromium were cast into an alumina tubing having a one-quarter inch diameter and were held at temperature above the melting point of about l,350 C. The particular type of furnace involved for this purpose was a Bridgman furnace. Other heating means such as the Czochralski furnace, molten zone passing, and directional casting, are satisfactory.

Each specimen was then lowered out of the molten zone of the furnace at a rate of about 0.39 inch per hour and into a body of water located below the furnace in order to quench the specimen into a solid rod, thereby causing unidirectional solidification. This technique gave a controlled linear movement of the liquid-solid interface and a controlled temperature gradient which result in the lamellar structure as shown in H0. 2.

Compression and tensilespecimens were prepared from the as-grown material and tested over a range of temperatures. The results of the tensile and compression tests are shown in tables I and ii.

ing would then be reduced from the as-grown spacing of 6 l0"inches to approximately 5X10 inch.

The Ni-Cr eutectic made for the above series of specimens was grown at a relatively slow rate of 0.39 inch per hour and the lamellae have fairly wide spacing. However, finer lamellae are obtainable by growing at a faster velocity. Thus, the yield strength would be enhanced and the amount of cold work required to reach a given strength is reduced. As shown in tables l and ll the high-temperature tensile and compression yield strengths (about k.s.i.) for the as-grown material are high enough for commercial applications such as turbine blades. Moreover, the mechanical properties at high temperatures are enhanced by cold work such as shown in compression specimen No. 3. (table II).

Evidence indicates that the lamellar structures are stable up to within a few degrees below the melting point. However, coarsening (thickening of the lamellae) occurs at temperatures close to the melting point. The lamellar structure is highly stable up to 100 or 200 below the melting point, and the mechanical properties are retained even after extended heat treatment.

The Nickel-Rich Phase The microstructure of the specimens resulting from the unidirectional growth is indicated in FIG. 2; that is, they have directionally solidified structure. The microstructure includes a continuous matrix which is the light-colored background and is composed of the nickel-riclphase. The dark, needlelike or thinner lamellae are the chromium-rich phase. Inasmuch as a two component system is involved, which rules out ternary hardeners for the nickel, an increase in the inherent strength of the matrix is obtained only through residual strain introduced by plastic deformations sucl'as swaging or rolling.

In the directionally solidified eutectic the lamellar spacing is TABLE I.TENSILE TEST DATA ON AS-GROWN Ni-Clr EUTECTIC Gage Test strength Ultimate strength Elongation Reduction of Area Diameter length T0mp.,

(inches) (inches) F. Lbs. P.s.i. Lbs. P.s.i. Inches Percent Inches Percent Spec. Number:

TABLE IL-OOMPRESSION TEST DATA ON Ni-Ci' EUIEC'IIO The material of the specimens was ductile enough to be cold worked and was swaged down to give rods with approximately 50 and 75 percent reduction in area. Compression specimens were made from the swaged rod.

Specimens were also tested for yield strength for the asgrown material as well as swaged rods and the results have been plotted (FIG. 3) as a function of d', where d is the specimen diameter in inches. An extrapolation of the results shows that a potential yield strength of 1,000,000 p.s.i. may be possible upon refinement of the materials and the casting and working procedures of this invention. Extrapolating a bar having a cast diameter of 2.5 inches would have to be cold reduced to 0.2 inch to achieve this effect. The lamellae spacpercent aluminum 1.35 percent iron, 0.07 percent carbon,

and the balance nickel which is usually sold under the trademark, Rene 4 l. in H6. 4 the dependence of 0.2 percent yield on cold work is shown for. the Ni-Cr eutectic alloy and the Rene 41 alloy at room temperature and at l,400 F. Both specimens were held for 1 hour at temperature prior to testing. The room temperature strength of the Rene 41 alloy increases by K p.s.i. whenrolled to 40 percent reduction in area. However, at l,400 F. the strength of the worked alloy returns to the room temperature strength of the unworked alloy, indicating that the strength of the worked alloy is primarily due to an increase in dislocation density which is removed by annealing at l,400 F. The 1,400 F. performance would be even worse if it did not have dispersed particles of lamellae generally of a nature similar to that of Ni-Cr alloy. Without this both the worked and unworked Rene 41 alloy would have far lower strength at l,400 F.

On the other hand, the Ni-Cr alloy having the same amount of cold work as the Rene 41 alloy has an increase of 15 K p.s.i. at room temperature which increase is attributable to the combined effects of dislocation density and to the reduction in the lamellar spacing. As evident from FIG. 4 at l,400 F. the difference between the work and unworked Ni-Cr alloy samples is about 4| K p.s.i. which is about a factorof two greater mercially cast alloys having Lo carbon" and Hi carbon" as than the unworked strength. This is comparable to the dif- Cr-40 Ni alloys, both hi" and low" carbon. lt is evident that ference between the two Rene 41 specimens at this temperathe Ni-Cr eutectic alloy has creep resistance properties that ture. It is believed therefore that the lamellar spacing in the are highly satisfactory as well as being superior to the 60 Ni-Cr alloy does form an effective barrier to moving disloca- Cr40 Ni alloys. tions. Moreover, the effectiveness of the lamellar structure 5 Corrosion R i tance would be greatly improved if the lamellar i g were in the past it has been found that at least l2 percent chromidecreased by, say, an order of magnitude. um in Ni-Cr alloys was necessary to give fair corrosion re The Chromium-Rich Phase sistance at high temperature. The mechanical properties of The lamellae are composed of a chromium-rich phase and this alloy are improved as the percentage of chromium is contribute substantially to the strength of the composite to reduced, but at the same time the corrosion resistance is leswhere the thickness of the lamellae is in the range of subsehed- The corrosion resistance of the 50-50 Percent micron to micron dimensions. A reduction in the chromium eutectic alloy is excellent and the mechanical Properties are lamellae thickness by controlled solidification results in suboutstanding as a eehseqhehce of the lamellar Structure AS t tial increases i th trength ro tie cast specimens of NiCr eutectic alloys were subjected to an A h wn i FIG, 1 th lidu slope f th h miu i h l accelerated 500 hour combustion test at L500 F. in Diesel phase is very steep and retrogrades near the eutectic isotherm, fuel oil containing 6.9 percent sulfur. The specimens were first sweeping from a composition of about 30 percent of Ni dipped in a molten eutectic salt bath of magnesium and sodidownwardly toward a very much lower value of about a few um sulfates which provide a representative fuel slag coating percent at room temperature. The chromium-rich phase is and accelerate the tests at this high-sulfur level. To maintain therefore more highly supersaturated with respect to the h Sulfa e stability. the partial pressures of SO, and 80;, were nickel-rich phase. artificially increased to 2,000 and 70 ppm, respectively. As Creep Behavior shown in FIG. 7, the corrosion resistance of the Ni-Cr eutectic In addition to the foregoing tests, specimens of Ni-Cr eutecalloy is superior to several high-temperature superalloys and tic alloys were tested for creep resistance at 1,400 F. (760 stainless steels previously used for turbine blades, such as lnco The Stress-rupture data are summarized in FIG. 5 where 713-C, lUdimet 700 and 500 alloys. The Ni-Cr alloy was also three sets of data are represented. The effect of directional compared with other known corrosion resistant alloys such as solidification is quite pronounced. An increase of close to two Hastelloy-X and type 310 stainless steel. Specimens of the orders of magnitude in time to failure for a given stress is ob- Ni-Cr eutectic alloy were examined for penetration in both tained by the lamellar alloy over the cas alloy whi h this sulfidation and also in simple oxidation tests, and these represents the nickel-rich matrix in the eutectic composite. tests showed that the chromium-rich lamellae very effectively The intermediate curve is for partially lamellar and partially bl k penetration O id i i hi h penetrated h first dendritic material, where the full benefit of aligned composite nickeprich phase were Stopped ff ti l by the fi st Chromistructure is not realized. um lamellae they encountered.

Fuflheri creep tests were P the eutectic Typical analyses for the indicated nickel-base alloys are alloy specimens prepared in accordance with this invention, li d i bl ]V TABLE IV Mo Cb Ti Al B Zr Fe .0 Bal. 0. s 0.50 0. 19-22 2.0 1.5

compared with data for a commercially cast 60 percent 50 CONCLUSION 'l f th chmmlum'm percfim mckel alloy The and ySIS 0 e com The overall thermal stability of the lamellar structure of the Ni-Cr eutectic alloy was demonstrated to be satisfactory. Thus, the lamellar structure of the alloy exhibits little change in creep resistance after exposure for 15 hours at 15 K psi. and l,500 F. The excellent lamellar structure remained unchanged. Moreover, the improvement in the yield strength of the cold-worked Ni-Cr eutectic alloy has been shown to be generally superior to prior known alloys which were conven- 60 tionally regarded as highly satisfactory. In addition the corrosion resistance of the Ni-Cr eutectic alloy is better than or well as the Ni-Cr eutectic alloy used for comparison are shown in table Ill.

TABLE Ill Chemistries of two International Nickel Co. percent Cr-40 percent Ni alloys, and of the least pure Ni-Cr eutectic (all other elements 0.01 percent) Fc Si (3 Mn equivalent to the best prior used alloys having acceptable corrosion resistance properties. ,9 0.01 Accordingly, the Ni-Cr eutectic alloy having a lamellar eu- "Hi c" 3.45 1.1 014 tectic microstructure is suitable for use in many applications Nici 0.l .0 M5 including such diverse uses as in watch springs and turbine w p r H r U blades. In the latter environment the alloy more than satisfies The creep-rupture test results are Plotted in 6, and in the life time requirement to which turbine blades are sub- View of the P y of the eutectic it demohstl'etes jected at temperatures of up to 1,400 F. and at the same time eXCelleht Performance compared with the two 60 Percent the alloy demonstrates satisfactory resistance to corrosion 40 Percent Ni alleys CF10" and carbon content as Set without sacrificing strength, thermal stability, and creep rcforth in table lll. Here again it is evident that a fine, well-coni ta properties,

r lamellar structure Provides a highly Satisfactory creep Although the best known embodiment of the invention has resistance P y in the eutectic ybeen described in detail, it is understood that the invention is Moreover, the creep-rupture tests for the Ni-Cr eutectic noiiimiied thereto orthereby, alloy are included in FIG. 6 to compare the alloy with the 60 Wh t i l im d i l. A member of a directionally solidified high-temperature alloy having high creep-rupture properties, thermal stability and resistance to corrosion and consisting of a eutectic composition of 50:1 percent nickel and 50:1 percent chromium, and the member having a metallurgical microstructure comprising oriented chrornium-rich lamellae in a nickel-rich matrix.

2. A member of the alloy of claim 1 which has been cold worked to provide a reduction in area of about from 50 to 75 percent.

3. A turbine blade comprising a directionally solidified eutectic composition of 50:1 percent nickel and 50:] percent chromium, and having a metallurgical microstructure comprising oriented chromium-rich lamellae in a nickel-rich matrix.

4. A process for producing a member comprising a eutectic composition of nickel and chromium which comprises (1) casting a melt of eutectic alloy of 5011 percent nickel and 50:1 percent chromium into a mold having a high-melting point and holding the melt at a temperature above the melting point of about l,350 C. for the alloy in a suitable furnace, (2) lowering the melt out of the furnace at a constant velocity and into a quenching medium to produce an ingot having a unidirectional lamellar structure, and (3) cold working the ingot until a reduction in area of from about 50 to 75 percent is achieved.

Claims (3)

1. 2. A member of the alloy of claim 1 which has been cold worked to provide a reduction in area of about from 50 to 75 percent.
2. 3. A turbine blade comprising a directionally solidified eutectic composition of 50 + or - 1 percent nickel and 50 + or -1 percent chromium, and having a metallurgical microstructure comprising oriented chromium-rich lamellae in a nickel-rich matrix.
3. 4. A process for producing a member comprising a eutectic composition of nickel and chromium which comprises (1) casting a melt of eutectic alloy of 50 + or - 1 percent nickel and 50 + or - 1 percent chromium into a mold having a high-melting point and holding the melt at a temperature above the melting point of about 1,350* C. for the alloy in a suitable furnace, (2) lowering the melt out of the furnace at a constant velocity and into a quenching medium to produce an ingot having a unidirectional lamellar structure, and (3) cold working the ingot until a reduction in area of from about 50 to 75 percent is achieved.

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