US3793013A - Cobalt-base tantalum carbide eutectic alloys - Google Patents

Abstract

Unidirectionally solidified cobalt-base carbide-reinforced cast superalloy bodies having high strength and high resistance to creep, particularly at elevated temperatures, and having an approximate nominal (melt) composition range which consists essentially, by weight, of a trace to 26.0 percent chromium, 13.5 to 19.0 percent tantalum, a trace to 14.5 percent nickel, a trace to 6.5 percent tungsten, a trace to 1.0 percent iron and 1.2 to 1.4 percent carbon, balance essentially cobalt. A preferred composition contains the nominal (melt) percentages: 15.0 w/o chromium, 8.5 w/o nickel, 6.0 w/o tungsten, 18.67 w/o total tantalum, 1.33 w/o carbon, for a total of 20.0 w/o TaC. The resulting article, after trimming, contains from 11.5 to 12.5 wt. percent tantalum. The tantalum and carbon, primarily as an aligned dispersion of TaC fibers in the matrix of the composite structure, constitutes about 5 to 15 volume percent of the body.

Description

[ Feb. 19, 1974 COBALT-BASE. TANTALUM CARBIDE EUTECTIC ALLOYS Inventors: John L. Walter, Scotia; Harvey E.

Cline, Schenectady, both of N.Y.

Assignee: General Electric Company,

Schenectady, N.Y.

Filed: Sept. 21, 1971 Appl. No.: 182,530

Related US. Application Data [63] Continuation-impart of Ser. No. 134,235, April 15,

1971, abandoned.

[52] US. Cl. 75/171, 148/32 [51] Int. Cl. C22c 19/00 [58] Field of Search..... 75/134 F, 170, 171; 148/32, 148/325 [5 6] References Cited UNITED STATES PATENTS 3,528,808 9/1970 Lemkey et al. 75/170 3,260,505 7/l966 Ver Snyder 75/171 3,085,005 4/1963 Michael et al. 75/171 Primary Examiner-Richard 0. Dean Attorney, Agent, or FirmGerhard K. Adam; Joseph T. Cohen; Jerome C. Squillaro [5 7] ABSTRACT Unidirectionally solidified cobalt-base carbidereinforcd cast superalloy bodies having high strength and high resistance to creep, particularly at elevated temperatures, and having an approximate nominal (melt) composition range which consists essentially,- by weight, of a trace to 26.0 percentchromium, 13.5 to 19.0 percent tantalum, a trace to 14.5. percent nickel, a trace to 6.5 percent tungsten, a'trace to 1.0 percent iron and 1.2 to 1.4 percent carbon, balance essentially cobalt. A preferred composition contains the nominallmelt) percentages: 15.0 w/o chromium, 8.5 w/o nickel, 6.0 w/o tungsten, 18.67 w/o total tantalum, 1.33 w/o carbon, forla total of 20.0 w/o' TaC.

The resulting article, after trimming, contains from 11.5 to 12.5 wt. percent tantalum. The tantalum and carbon, primarily as an aligned dispersion of TaC fibers in the matrix of the composite structure, constitutes about 5 to 15 volume percent of the body.

2 Claims, 3 Drawing Figures Carbon- COBALT-BASE TANTALUM CARBIDE EUTECTIC ALLOYS CROSS-REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates in general to cobaltbase eutectic superalloy compositions, and more particularly to bodies of such compositions containing, as reinforcements of their structure, aligned fibers of a carbide.

In particular embodiments of this invention, these a1- loys have utility in gas turbine engine components.

2. Description of the Prior Art Superalloy bodies forming components in gas turbine engines are often subjected to temperatures which can approach 1,900F, under highly oxidizing conditions. Although unidirectional solidification of a number of eutectic superalloys is known to achieve increased strength, heretofore there have been no cobalt-base superalloy compositions developed which can be unidirectionally solidified to have a good combination of properties of high strength, high resistance to creep or stress-rupture at elevated temperature and good resistance to high temperature oxidation,a1l while retaining adequate ductility.

Among the commercially available cast cobalt-base superalloys used for gas turbine engine blades is the following:

Cobalt balance Chromium 25.0 Nickel 10.0 Tungsten 7.5 Iron I .0

However, the stress-rupture properties of this and similar alloys at high temperatures leave much to be desired.

SUMMARY OF THE INVENTION lidified superalloys which have excellent tensile properties, at room temperature and at temperatures up to 2,000F.

Another object of the invention is to provide cast fiber-strengthened bodies useful as components for turias 2 .24392 wb qhhave s b a o of Pr i including high yield and tensile strength combined with good creep resistance under static stress at elevated temperatures, together with good high-temperature oxidation resistance, with good engineering ductility.

Still another object of the invention is to provide a cast article having a microstructure which includes a relatively high volume percent of aligned reinforcing fibers in the matrix of the alloy.

Other objects and advantages of the invention will become apparent from the following description and appended claims.

In accordance with the objects of the invention, a range of cobalt-base superalloy compositions is provided which form a composite structure comprising a matrix reinforced with aligned fibers essentially of tantalum carbide. The range of nominal (melt) compositions of the unidirectionally solidified castings, according to the invention is approximately, in weight percent, a trace to 26.0 percent chromium, 13.5 to 19.0 percent tantalum, a trace to 10.0 percent nickel, a trace to 6.5 percent tungsten, a trace to 1.0 percent iron and 1.2 to 1.5 percent carbon, balance essentially cobalt.

DESCRIPTION OF PREFERRED EMBODIMENTS form TaC, which makes up about 12 volume percent of V the alloy.

It is therefore an object of the present invention to' The resulting articles after trimming off redundant portions not having the uniformly aligned TaC fibers,

have the preferred composition range: 14.0 to 18.0 w/o Cr, 11.5 to 12.5 w/o Ta, 8.0 to 10.0 w/o nickel, 6.0 to 8.5 w/o W, and 0.8 to 1.5 w/o carbon, balance substantially cobalt.

In order to obtain the most desirable metallographic structures and properties disclosed herein, the alloys were unidirectionally cast at a solidification rate of A inch/hr. in a temperature gradient of about 250 per inch.

The alloys of the present invention contain carbide fibers, essentially of tantalum carbide in amounts of from 5 to 15 volume percent of the alloy, with the preferred composition containing at least about 12 volume percent. The tantalum carbide fibers are preferably dispersed throughout the composite structure, but beyond about 12 volume percent the carbide tends to precipitate out as particles at the bottom of the cast body.

Typical compositions according to the invention are sho n bel w in Table I.

TABLE I Cobalt Base Carbide Alloys Composition -wt.

Designation Co Cr Ni W Ta C Fe Ta(M)* A) TaC-C045 I 45.0 25.5 8.5 6.0 13.8 |.2 B) 'luC-Co50 50.5 20.0 11.5 6.0 3.8 1.2 c) TaC-C056 56.5 20.0 14.5 13. 1.2

I I Continued i Cobalt Base Carbide Alloys Composition wt.

Designation Co Cr Ni W Ta C Fe Ta(M)' D) TaC-C051 51.5 20.0 8.5 18.67 1.33 E) TaC'Co55 55.5 15.0 8.5 6.0 13.8 1.2 F) TaC-CoSOB 50.5 15.0 8.5 6.0 18.67 1.33 G) TaC-CoSUTa 50.0 17.5 8.5 6.0 13.8 1.2 3 H) TaC-C049 49.04 20.0 8.5 6.4 13.8 1.2 0.85 I) TaC-C050Ni 50.5 15.0 14.5 0.0 18.67 1.33 F-l) (Article 54.8 16.4 9.3 8.2 12.2 0.8

portion) of TaC.

The composition designated in Table I as TaC-CoSOB, corresponding to melt composition F and article portion composition F-l (after trimming) is a preferred embodiment of the invention because of its excellent properties, as shown in Tables 11, Ill and IV. The tantalum carbide in the melt composition of this alloy is 20.0 wt. percent.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following description, taken in conjunction with the ac companying drawings, wherein:

FIG. 1 is a photomicrograph (250X) of a longitudinal section of a specimen of composition designated TaC- 50B, according to a preferred embodiment of the invention;

FIG. 2 is a photomicrograph (250X) of a transverse section of a specimen corresponding to that of FIG. 1; and

FIG. 3 is a chart illustrating stress-rupture properties under stress at high temperature, as compared to a known superalloy, Ren 80. plotting the life at temperature as the Larson-Miller parameter vs. stress.

In order to illustrate more completely the substantial 'Ta(M) refers to the wt. percent of tantalum in the matrix, as distinguished from and in addition to the tantalum present in stoichiometric proportions with carbon as fibers improvements in physical properties which are achieved by the alloys of the present invention, Tables 11 and III below respectively list the tensile properties at room temperature and at elevated temperature of 1,832F (1,000C). Table IV lists the high temperature stress-rupture properties of the alloys at the various designated temperatures.

EXAMPLE I lngots of the melt composition designated in Table l as F), TaC-CoB, were prepared and directionally solidified from molten condition at 1 1 inch/hr. and in a temperature gradient of about 250C/inch. After customary trimming off of redundant segregated portions. the resulting microstructure, as may be seen in FIGS. 1 and 2, comprised aligned carbide fibers essentially single crystal tantalum carbide, in a cobalt-base alloy matrix. The mechanical properties of the uniform fibrous portion of the preferred alloy TaC-CoSOB, had the article composition designated F1 in Table I. The results of specimens of each of the alloys of Table I, tested at a strain rate of 2 X l0' /min. are listed in Table II for room temperature and in Tables Ill and IV at the elevated temperature of 1,832F.

. BLE 11 Room Temperature Tensile Properties Ult. Stress Composition Description (psi) elong.

A) TaC-C045 Cobalt superalloy with TaC 1 11,300 1.0 B) TaC-C050 Cobalt superalloy with TaC 1 18,400 8.0 C) TaC-C056 Cobalt superalloy with TaC 1 15,300 7.5 D) TaC-C051 Cobalt superalloy TaC 134,700 16.8 E) TaC-C055 Cobalt superalloy TaC 108,600 4.8 F) TaC-C0508 Cobalt superalloy TaC 149,000 30.0 G) TaC-CoSOTa Cobalt superalloy Ta & TaC 96,700 5.0

TABLE III Tensile Properties at High Temperature (1832F) Ult.Stress Composition Description (psi) Elong.

B) TaC-C050 Cobalt superalloy TaC 25,300 2l.2 C) TaC-C056 Cobalt superalloy TaC 31,200 9.4 D) TaC-C051 Cobalt superalloy TaC 33,600 10.2 E) TaC-C055 Cobalt superalloy TaC 28,000 10.4 F) TaC-C0503 Cobalt superalloy TaC 54,900 7.0 TaC-CoSOTa 25,700 26.0

Cobalt superalloy Ta & TaC

TABL 1V V High Temperature Stress-Rupture Properties Stress(psi) Life(hrs) Composition Description & Temp. to rupture Elong.

A) TaC-Co45 Cobalt superalloy 17,000 276.3 18.7

with TaC (I800F) B) TaC-C050 Cobalt superalloy 17,000 5.42 22.1

with TaC (1960F) C) TaC-C056 Cobalt superalloy 17,000 5.09 10.9

with TaC (2000F) F) Cobalt superalloy 17,000 1000 5 .TaC-CoSOB with TaC (2012F) at 20,000 146 F) Cobalt superalloy 35,000 93.12 5.1 TaC-C0508 with C (1832F) Test discontinued no rupture (see explanation below).

The specimen TaCCOSOB was tested at 2,0l2F and 17,000 psi for 1,000 hours with no rupture, at which time the stress was increased to 20,000 psi. After an additional 146 hours at the higher stress, the test was terminated to preserve the specimen intact for further examirgtLn FIG. 1 is a photomicrograph (250ilof adoiigitudinal section of a cobalt-base superalloy with tantalum carbide fibers and having the nominal composition F of Table I. In FIG. 1 it may be seen that the tantalum carbide fibers are relatively long and mutually aligned, 1

thus contributing to maximum strength of the alloy in the direction of their alignment. Transmission electron microscopy results show that these fibers are cubic and single crystal.

FIG. 2 is a photomicrograph (250X) showing a transverse section of the alloy body of FIG. 1, illustrating pressed as the Larson-Miller parameter, of the same alloy as in FIGS. 1 and 2, with those of Rene 80, a common alloy with relatively high creep resistance. This well known alloy was selected for the comparison because its creepresistance properties, expressed as the Larson-Miller parameter, are known.

The Larson-Miller parameter mentioned above is discussed in the book High-Temperature Materials and Technology by Campbell and Sherwood, John Wiley and Sons, pages 857-858; and in Transactions ASME, 74, P 7

As may be seen from the curves and data of FIG. 3, the typical nominal (melt) composition F of the tantalum carbide reinforced cobalt-base alloy of the present invention has superior high temperature creep resistance, and thus is capable of achieving a longer life under stress at elevated temperatures. For example, at a temperature of l,832F (1,000C) and 35,000 psi stress, the Rene' 80 alloy of the prior art withstood the stress for only 2 hours before rupture; whereas under the same conditions the cobalt-base alloy with tantalum carbide of the invention withstood the stress for 93 hours before rupture.

At a higher temperature of 2,012F (1,100C) and 17,000 psi, with Rene alloy withstood the applied stress for only 5 hours before rupture; whereas under the same conditions the cobalt-base alloy with tantalum carbide fiber reinforcement (identified as TaC-50B) withstood the stress in excess of 1,146 hours life, i.e., 1,000 hours at 17,000 psi, plus 146 hours at 20,000 psi stress. At this point, no rupture had yet taken place. The results of High Temperature Stress-Rupture tests are listed above in Table vIV and plotted as Larson- Miller parameters in FIG. 3. It is thus evident that the compositions and TaC fibers have a marked effect on the stress-rupture properties, particularly at higher temperatures and lower stress levels.

Thus, the alloys of the present invention exhibit a combination of desirable properties. As shown in Tables II and III, both at room temperature and at elevated temperatures, the unidirectionally solidified TaC reinforced cobalt-base alloys have good tensile strength and elongation. They also have good oxidation and corrosion resistance at high temperatures (not shown); and, as set forth in Table IV, they have excellent stressrupture properties at elevated temperatures as high as 2,012F, with ductility in excess of 5 percent total elongation.

It will be obvious to those skilled in the art upon reading the foregoing disclosure that many modifications and alterations in the specific compositions and microstructures disclosed as non-limiting examples may be made within the general context of the invention, and

that numerous modifications, alterations and additions v may be made thereto within the true spirit and scope of the invention as set forth in the appended claims.

What we claim as new and desire to secure by Letters Patent of the United States is: y

1. An article of manufacture comprising a unidirectionally solidified casting of a nominal composition consisting essentially in weight percent of about 15.0 percent chromium, 8.5 percent nickel, 6.0 percent tungsten, 18.67 percent tantalum, 1.33 percent carbon,

and the balance cobalt, said casting having the cobaltbase alloy matrix reinforced with a fibrous phase of tantalum monocarbide.

2. A cast article according to claim 1, said fibrous phase forming at least 12 volume percent of the article.

Claims (1)

1. 2. A cast article according to claim 1, said fibrous phase forming at least 12 volume percent of the article.

Images