US3567526A - Limitation of carbon in single crystal or columnar-grained nickel base superalloys - Google Patents

Abstract

THE NICKEL-BASE SUPERALLOYS WHICH ARE FABRICATED BY CONTROLLED UNIDIRECTIONAL SOLIDIFICATION INTO ANISOTROPIC ARTICLES, PARTICULARLY IN SINGLE CRYSTAL OR COLUMNAR-GRAINED FORM, ARE LIMITED TO A MAXIMUM CARBON CONTENT OF ABOUT 100 PARTS PER MILLION TO PROVIDE SUPERIOR FATIGUE RESISTANCE.

Description

United States Patent 3,567,526 LIMITATION OF CARBON IN SINGLE CRYSTAL OR COLUMNAR GRAINED NICKEL BASE SUPERALLOYS Maurice L. Gell, Rocky Hill, and Gerald R. Leverant,

Wapping, Conn., assignors to United Aircraft Corporation, East Hartford, Conn. No Drawing. Filed May 1, 1968, Ser. No. 725,889 Int. Cl. C22c 19/00 US. Cl. 148-32.5 3 Claims ABSTRACT OF THE DISCLOSURE The nickel-base superalloys which are fabricated by controlled unidirectional solidification into anisotropic articles, particularly in single crystal or columnar-grained form, are limited to a maximum carbon content of about 100 parts per million to provide superior fatigue resistance.

BACKGROUND OF THE INVENTION This invention relates primarily to the strong, hightemperature alloys, particularly the nickel-base superalloys, and to casting compositions for use in the controlled solidification of these superalloys.

A nickel-base superalloy is typically a nickel/ chromium solid solution hardened by the additions of aluminum, titanium and/ or niobium to precipitate the intermetallic compound Ni (M) where M is aluminum, titanium, niobium or a combination thereof. Commercial superalloys, in addition, usually contain cobalt to raise the solvus temperature of the precipitate, refractory metal additions for solution strengthening, and carbon, boron and zirconium to promote creep-rupture ductility.

Carbon has traditionally been employed as a necessary alloying addition in the polycrystalline nickel-base superalloys. Most of the carbon combines with other alloying ingredients such as titanium, chromium, zirconium, niobium and tungsten to form metal carbides in a variety of phases (MC, M C M C and M C the particular phase or phases present depending on the composition and heat treatment. Since the high temperature stress-rupture life and ductility of the superalloys containing carbides are superior to those without the carbides, the carbon alloying additions have, in the past, generally been considered essential in the polycrystalline superalloys. This is confirmed by a recent study by J. P. Stroup and L. A. Pugliese in Metal Progress, vol. 93, pages 96-100 (February 1968). And the carbon requirement remains a fact despite certain studies which have indicated that a limited series of nickel-base alloys have been found to have improved impact resistance at low carbon levels. See, for example, British Pat. No. 1,033,715.

Recently there have been developed certain casting techniques concerned with the controlled or unidirectional solidification of the superalloys into articles of columnargrained or single crystal form. The particular preferred directional solidification techniques are discussed in the patent to VerSnyder 3,260,505, and in the copending application of Piearcey, Ser, No. 540,114, filed Feb. 17, 1966, now Pat. No. 3,494,709 entitled, Cast Metal Part and Process and Apparatus Therefor. Both the patent and the copending application share a common assignee with the present invention.

Generally speaking, the alloys heretofore utilized in the various directional solidification processes have been of the same composition as those alloys utilized in the conventional casting techniques and, accordingly, have included the usual carbon alloying additions. Representative nickel-base superalloys and their conventional compositions are set forth below.

ice

Alloy designation Composition (percent by weight) MAR-M20O 9 Cr, 10 O0, 2 Ti, 5 Al, 12.5 W, 1 Nb, .015 B, .05 Zr,

.15 0, balance Ni.

Udimet 700 14.6 Cr, 15.3 Co, 3.4 Ti, 4.3 Al, 4. 1 M0, .016 B, .07 0

balance Ni.

IN 10 Cr, 15 Co, 4.5 Ti, 5.5 A1, 3 Mo, .75 V, .015 B, .075 Zr, .17 0, balance Ni.

13-1900 8 Cr, 10 Co, 1 Ti, 6 A1, 6 M0, 4.3 Ta, .15 B, .07 Zr,

.11 0, balance Ni.

1 Sigma tree.

SUMMARY OF THE INVENTION Briefly stated, the present invention relates to the discovery that the carbon content of the columnar-grained and single crystal castings of the nackel-base superalloys should be limited to a maximum of about 100 parts per million (.01 percent by weight).

DESCRIPTION OF THE PREFERRED EMBODIMENTS It has recently been found that the physical properties of the directionally-solidified, nickel-base superalloys, particularly the fatigue resistance thereof, are adversely affected to a significant extent by the presence in these alloys of a large MC-type carbide phase. An examination of the microstructure of the nickel-base superalloys of conventional composition, in both columnar-grained and single crystal form, has revealed that the MC-type carbides are much larger in these forms than in the conventional castings. Further, these carbides can exist as interconnected networks. These large MC-type carbides result from the slower solidification rate and shallow thermal gradient at the solidification front used in the casting of the anisotropic structures. It was also discovered that many of these carbides were precracked parallel to their longest dimension. Since the cracks were present in the materials immediately after casting, it is evident that they were formed during solidification of the alloys. The precracking of the carbides had not been observed previously, and an extremely careful mechanical polish or electropolish was required to reveal them.

.The plastic deformation and matrix cracking of the as-cast or heat-treated structures, in tensile and fatigue tests, were found to initiate at the tips of the cracks already present in the MC-type carbides, and it was shown that the fatigue life of these as-cast alloys is a very sensitive function of carbide size. It was demonstrated, for example, that the fatigue lives of specimens taken from the tops of cast bars were reduced by a factor of more than three hundred when compared to samples taken from the bottom or middle of the same bars where the carbides were smaller because of a faster solidification rate and a steeper thermal gradient at the solidification front. This is clearly demonstrated by the data of Table I.

It has also been found that the creep-rupture lives of specimens taken from the tops of cast bars is approximately one-half that of specimens taken from the lower portion of the same bars.

The fatigue properties of the nickel-base superalloys in columnar-grained and single crystal forms can be substantially improved by eliminating the MC-type carbides from the alloy microstructure. This is accomplished by limiting the carbon content of the alloy to a maximum of about 100 parts per million (0.0-1 percent by weight). Such carbon levels may be achieved by melting and casting the superalloys under vacuum without intentionally adding carbon to the alloy. Any carbon that is present is accordingly the result of carbon impurities present in the various elements used in the alloying process. With the carbides no longer required or desired in the alloy, small adjustments may be made to the nominal composition of the various alloys to take into account the fact that certain amounts of the elements are no longer utilized in formation of the carbides. Typical low carbon MAR- M200 and UDIMET 700 compositions are compared in Table -H with the corresponding alloys at their normal chemistry.

TABLE II nickel-base superalloys in columnar-grained and single crystal form by maintaining the carbon content below' 100 parts per million to thereby eliminate the precracked MC- type carbides which act as the sites for slip and crack initiation in fatigue. While the inventiorr has been described in connection with certain examples for the purposes er illustration, no limitation is intended thereby. Numerous modifications and additions will be evident to those skilled in the art from the teachings herein In accordance with the true spirit of the invention, the scope thereof will be measured not by the illustrative material but by the appended claims.

What is claimed is; 7

1. In those processes wherein a nickel-base superalloy comprising a first phase consisting principally of a nickel/ 1 OWTiNbZrCoCrAlBMo Ni MAR-M200:

Normal 15 12. 5 2. 1. 0 I 10. 0 9. O0 5. 00 015 Bal. Low carbon 01 12. l 1. 7 6 04 10. 0 9. 00 5. 00 015 Bal. UDIMET 700 (sigma free) a Normal 07 3 4 15. 3 14. 6 4-. 3 016 4. 4 Bal.

Low carbon .s 01 3 2 15. 3 14. l 4. 3 4. 2 Bal.

I Single crystal articles prepared from the MAR-M200 alloy of the low carbon formulation have displayed superior fatigue life as compared to the corresponding articles containing the MC-type carbides. Examination has shown no MC-type carbide and, hence, no carbide cracking in the low carbon alloy. This is supported by the comparative data of Table III. t

e TABLE 11f Low cycle fatigue of nickel-base superalloys at 1,40 F. at total strain range of 1.6 percent] It is evident that the severity of the problem with precracked carbides in controlled solidification casting is a function of both the carbon content of the alloy and the solidification parameters in the casting process, insofar as they affect carbide size, type and distribution in the cast article. Below about 600 parts per million at the normal solidification rates, an improvement in the properties of theldirectionally solidified article will be found as the carbon content of the alloy is reduced and the carbide size is lessened whereby the 'precraclc ng becomes less frequent and more scattered. Below 100 parts per millioncarbon, no MC-type carbides are formed.

By the present invention there has been provided means for substantially improving the fatigue'properties of the chromium solid solution and a second phase consisting of an intermetallic compound of the composition Ni (M), where M is aluminum, titanium, niobium or a combination thereof is cast into a columnar-grained or single'crystal article by controlledunidirectional solidification, the improvement which comprises limiting the carbon content of the superalloy to a maximum, by weight, of. about 100 parts per million. 7

2. A unidirectionally-solidified article which consists of a first phase consisting principally of a nickel/ chromium solid solution and a second phase consisting of an intermetallic compound of the composition Ni (M), where M is aluminum, titanium, niobium or a combination there: of and a maximum carbon content of about 0.01 percent, the article being further characterized by the substantial absence of large carbides in the microstructure.

3. A unidirectional cast article of pronounced anistropy which consists of afirst phase principally consisting ola nickel/chromium solid solution and a second phase consisting of an intermetallic compound of nickel with aluminum arid titanium and a maximum carbon content of about 0.01 percent, the article being characterized by the substantial absence of carbides in the microstructure References Cited UNITED STATES PATENTS 2,766,156 10/1956- Hetteridge et al. 148-162 3,260,505 7/1966 VerSnyder -l71 3,322,534 5/1967 Shaw et al. 75 171 3,376,132 4/1968 Shaw et 61 75-171 3,457,066 7/1969 .Pohlman; et al. 75-171 3,494,709 2/1970 Piearcey 75 171 RICHARD o. DEAN, Primary Examiner US. 01. X.R.