AU630623B2 - An improved article and alloy therefor - Google Patents

Description

11 Our Ref: 293142 63OK 3

AUSTRALIA

Patents Act FORM COMPLETE SPECIFICATION

(ORIGINAL)

Application Number: Lodged: Complete Specification Lodged- Accepted: Published: Priority: Related Art: IIt Applicant(s): Address for Service GENERAL ELECTRIC COMPANY 1 River Road, Schenectady, New York, U.S.A.

ARTHUR S. CAVE CO.

Patent Trade Mark Attornerys Level 10, 10 Barrack Street SYDNEY NSW 2000 Complete specification for the invention entitled "An improved article and alloy therefor".

The following statement is a full description of this invention, including the best method of performing it known to me:- 1 3085T/gs 0523L -la- 13DV-8630 CAST COLUMNAR GRAIN HOLLOW NICKEL BASE ALLOY ARTICLE AND ALLOY AND HEAT TREATMENT FOR MAKING This invention relates to cast directionally solidified columnar grain nickel base alloy articles and, more particularly, to such an article of outstanding elevated temperature surface stability as represented by oxidation resistance, particularly in thin wallid hollow articles, and to the alloy and heat treatment for making sucn article.

BACKGROUND OF THE INVENTION A significant amount of the published and well known casting technology relating to nigh temperature S• operating articles, for example turbine blades for gas Sturbine engines, has centered about improvement of certain properties through elimination of some or all 4 of the grain boundaries in the final article's microstructure. In general, such structures nave been generated by the well known precision casting techniques of solidifying a molten metal directionally S" 20 (directional solidification) to cause the solidifying Scrystals or grains to be elongated. If only one grain is allowed to grow in the article during solidification, for example, through choking out 13DV-8630 -2others or using a seed crystal, an article of a single crystal and substantially no grain boundaries results. However, if multiple grains are allowed to solidify at an area of a casting mold and allowed to grow generally in a single direction in which heat is withdrawn from molten metal in a casting mold, multiple elongated or columnar grains exist in the solidified casting. Sucn a structure sometimes herein is called "DS multigrain" in connection with a cast article. The direction of elongation is called the longitudinal direction; the direction generally normal oao to the longitudinal direction is called the transverse direction.

once Because the grain boundaries in such an article o 15 are substantially all longitudinal grain boundaries, oit is important in an article casting that longitudinal mechanical properties, such as stress 0 0rupture life and ductility, be very good, along with good transverse mechanical properties and good alloy surface stability. With this property balance in the article, the article alloy must be capable of being cast and directionally solidified in complex shapes and generally with complex internal cavities and relatively'thin walls without cracking. So called "thin-wall" hollow castings nave presented difficult Quality problems to article casters using the well known "lost wax" type of precision casting methods with alloys designed for improved properties: though the alloy properties are good and within desired limits, thin wall castings, for example with a wall less than about 0.03S inch thick, generally cracked during multicolumnar grain directional solidification.

13DV-8630 -3- SUMMARY OF THE INVENTION Briefly, in one form, the present invention provides an improved cast columnar grain nickel base alloy article characterized by outstanding elevated temperature surface stability for a directionally solidified article, resulting from an alloy specification enhanced, in one form, by heat treatment and by an improved combination and balance between longitudinal and transverse stress rupture properties. In one form, the article has at least one internal cavity and includes an integral cast wall substantially free of a major crack, the wall having a thickness of less 'than about 0.035 inch.

S, In respect of the alloy associated with the present invention, a particular combinati.on of tne o elemental addition of C, Hf, Co and Ta, and the 9 intentional limitation of the elements V, Zr, and Ti, provides outstanding elevated temperature oxidation resistance, good castability, and resistance to grain boundary and fatigue cracking in a Ni base alloy which also includes Cr, Mo, W, Al, Re and B, and which o o allows optional amounts of Cb and Y.

o o In one form, the alloy includes essentially, in 0 0 S* percentages by weight, the combination of 0.1-0.15 C, 0.3-2 Hf, 11-14 Co, 5-9 Ta, less than 0.05 Zr and the o substantial absence of V and Ti at no more than about 1 each, to provide the alloy with the capability of being made into a DS multigrain article through good castability and resistance to grain boundary and fatigue cracking, along with outstanding oxidation resistance. The remainder of the alloy is 5-10 Cr, 0.5-3 Mo, 4-7 W, 5-7 Al, 1.5-4 Re, 0.005-0.03 B, up to Cb, up to 0.5 Y and the balance Ni and incidental impurities.

13DV-8630 -4- Another form of the present invention associated with such alloy is a heat treatment involved in the method for making the article. Such heat treatment comprises a combination of at least three progressive heating steps including a solutioning step, a preliminary, first aging step and a second aging step, to improve stress rupture properties of the article.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a graphical comparison of oxidation resistance of the alloy associated with the present invention with other alloys.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The nickel base alloy associated with the present Sinvention is particularly characterized by the relatively high C content in combination with a relatively large amount of Hf and additions of Co and Ta. This, along with the intentional control and limitation of the elements V, Zr and Ti, enabled the total alloy to have, for a DS structure, outstanding oxidation resistance and the good DS castability and resistance to grain boundary and fatigue cracking to the point at which thin walls of less than 0.035 incn r can be DS cast with elongated grains substantially i crack free. Other elements in the alloy, contributing to its unique mechanical properties and surface stability, in a nickel base, are Cr, Mo, W, Al, Re, B "and optional, limited amounts of Cb and Y. The S* resultant article, with an unusual, uniaue combination of mechanical properties and surface stability, is particularly useful in making hollow, air cooled, high temperature operating components such as blading 13DV-8630 members (blades and vanes) of the type used in the strenuous environment of the turbine section of gas turbine engines. In rotating turbine blades which are subject to high stress as well as high temperature oxidation and hot corrosion, the crack free condition of thin walls associated with internal cooling passages, is essential to safe, efficient engine operation.

A measure of the castability and crack resistance of high temperature directionally solidified columnar o o grained nickel base superalloys is the castability test and rating scale reported in U.S. Patent 4,169,742 Wukusck et al, issued October 2, 1979, go beginning in column 2 at line 41 and continuing into column 3. The disclosure of such patent is hereby incorporated herein by reference. The rating is repeated here in Table I.

TABLE I CASTABILITY RATINGS 20 A No cracks B Minor crack at tip, less than 1/2" long or in starter zone C One major crack, greater than 1/2" long D Two or three cracks E Several cracks, more than 3 and less than 8 F Many cracks most grain boundaries t, A selection of nickel base superalloys sometimes f4 used or designed for use in gas turbine engine turbine components is presented in the following Table II along with a form of the particular alloy associated with the present invention. The alloy identified as Rene' N5, designed for use in making single crystal 13DV-8630 -6alloy articles, is described in currently pending U.S.

patent application Serial No. 790,439 Wukusick et al., filed October 15, 1985; the alloy identified as Rene' 150, designed for use as a DS columnar grain article, is described in the above incorporated U.S.

Patent 4,169,742 Wukusick, et al. The disclosure of such copending application assigned to the assignee of this invention, also is hereby incorporated herein by reference. Also included in Table II are the castability ratings o: such alloys.

*c o An evaluation of varying Hf, Co and B in the alloy o identified in Table II as Rene' NS was conducted to 6 0 improve castability. Results of such evaluation are 0o snown in Table III.

0 00d 0 0 4~ 0 0 0 0 4 4. 0 0 00 0 0 4. 04.0 0~ 0 0 00 0 0 2 o 0 2 0 4.00 0 0 0 0 0 0 0 0 002 0 0 TABLE I I NOMINAL ALLOY COMPOSITIONS (WL 1, balance Ni and incidental linpuriLleS)

ALLOY

Rene' N4+ Rene' NS Rene' 150 Rene' 80H No Coat MA 754(b)

INVENTION

Hf is1 .15 1. 5 .75 Is Co 7.S 7.5 12 9.S5 7.5 Ta 4.8 6.5 6 5 6.35 V Zr Cr Mo 9.8 1.5S 7 1.5 2. 2 5 I .01 14 4 9.5S 1.5S W Ti Al 6 3. 5 4. 2 5 6.2 5 5.5S 4 4.8 3 6 1. 5 5. 6 Re B .004 3 .004 3 .015 .015 004 Cb 0.5S Y 2 CASTABI LITY

RATING

N.A. (a) N.A. (a) .01 0.6 0. 4

A

C-F

N.A. (b) 12 1.5S 12 6.8 1. 5 4.9 6. 15 2.8 .015 N.A. Not applicable single crystal wrought TABLE II!I DS CASTABILITY TESTS OF RENE' NS VARIATIONS (Based on Rene' N5 Alloy Composition, Table 11) ELEMENTS ADDED TO BASE CASTABILITY (Nominal W ellht Percent) RATING

B

TEST Added Total Added Total Added Total 87-i 1.5 1.6 0 7.5 0 .004 D-F 87-2 1.5 1.6 3 10.5 0 .004 A 87-3 1.5 1.6 0 7.5 0.01 .014 D-E 87-4 0.5 0.6 3 10.5 0 .004 A 87-5 1.0 1.1 3 10.5 0 .004 A 42-1 0.9 1.0 3 10.5 0 .004 A 42-2 0.4 0.5 3 10.5 0 .004 B 42-3 0.6 0.75 3 10.5 0 .004 A 42-4 0.4 0.5 4.5 12.0 0 .004 A 42-5 0.6 0.75 1.5 9.0 0 .004 A 42-6 0.2 0.3 4.S 12.0 0 .004 A 85-1 0.4 0.5 0 7.5S 0 .004 D 85-2 0.3 0.45 1.5 9.0 0 .004 C 85-3 0.2 0.3 3 10.5S 0 .004 D 85-4 0.3 0.4 3 10.5 0 .004 D 85-5 0. 4 0.5S 3 10.5S 0 .004 A 85-6 0.6 0.75 3 10. 5 0 .004 85-7 0 0.15 4.5 12. 0 0 .004 1) 85-8 0.2 0.3 4.5 12.0 0 .004 A 85-9 0.3 0.4 4.5 12.0 0 .004 A

OIL.

13DV-8630 -9- The data of Table III show primarily the benefit and criticality of including Co at a level greater than 7.5 wt% (for example about 10 wt up to about 12 wt%, ir. combination with Hf in the range of about 0.3 1.6 wt%. However, even with such improved castability, the alloy modification of Rene' N5 alloy had reduced longitudinal stress rupture strength due to dilution of the hardening elements from the addition of more Co to the Rene' NS alloy base chemistry of Table II above, at a C level of about 0.05 wt%. With the nominal 3% additional Co to the Rene' NS Alloy composition (to make it a total of 10.5% Co) and nominally 1% Hf, longitudinal stress rupture life was about 65% of Rene' NS alloy; with 15 nominally 4.5% additional Co (to make it a total of 12% Co) and at 0.5% Hf, longitudinal stress rupture life was 30% of Rene' NS Alloy. This is indicative of !one critical balance of elements used in the present Sinvention, with an alloy composition including C in the range of about 0.1 0.15 wt% along with Co in the range of 11-14 wt and 0.3-2 wt Hf.

In respect to the balance between castability, and grain boundary and fatigue cracking, it has been recognized that too little Co results in loss of castability and grain boundary strengthening, wnereas above about 14 wt% Co can dilute the effect of certain alloy strengthening elements. The element Hf, if too t low, such as below about 0.3 wtt, increases the tendency toward grain boundary cracking in DS casting and in use; and if too high, such as above 2 wt%, Hf can result in problems relating to casting reactivity and incipient melting during heat treatment. Too much Ta and Al can affect castability by being too strong and can cause grain boundary cracking. Also it can form Topologically Close Packed (TCP) pnases.

13DV-8630 Therefore, the Ta content is maintained preferably in the range of about 6-7 wt% and the Al preferably is 6.5 wt% in the practice of this invention. As is known in the art, small amounts of Cb may be substituted for Ta.

In the evaluation of some of the alloys of Table II, it was recognized that vanadium can detract from the surface stability, hot corrosion and oxidation resistance; Zr can increase crackability; and Ti can seriously reduce oxidation resistance.

Therefore, these elements have been controlled and limited to the ranges in weight percent of less than about 1 V, 0.05 Zr and 1.5 Ti, preferably less than a o 0.1V, 0.03 Zr and 0.02 Ti. While yttrium is helpful OcOe 15 in improving oxidation resistance, it can cause grain boundary weakening; thus, it is limited to amounts Sless than 0.1% in the alloys of the invention. Cr is a included primarily for its contribution to oxidation s 'l and hot corrosion resistance; Mo, W and Re primarily for matrix strengthening and B to enhance grain boundary strength.

Although the castability of such alloys as Rene' 150 were very good and within the acceptable range for "t thin wall castings, their surface stabilities were S 25 unacceptable for certain high temperature applications a t under strenuous environments. A comparison of the elevated temperature surface stability of Rene' 150 S: alloy and the alloy of the present invention has shown that during 100 hours exposure to Mach 1 air, Rene' 150 alloy at 2075°F lost 50-65 mils of metal per 4, specimen side, whereas the alloy of the present Sinvention, in the form shown in Table II, at a higher temperature of 2150 F and a longer exposure time of 150 nours lost only 1.5 mils per specimen side, i.e.

less than about 5 mils per side according to this 13DV-8630 -11invention. In another test, Zu: a.ditional comparison, Rene' 150 alloy at 2075°F in Mach 1 airflow lost 40 mils per specimen side after 82 hours.

One nickel base alloy considered to have outstanding elevated temperature oxidation resistance is MA754 alloy, identified in Table II. Such alloy is a wrought rather than cast alloy but is included here for further comparison with the oxidation resistance of the present invention. After exposure of a specimen of MA754 at Mach 1 airflow and 2150F, loss of 10 mils per specimen side occurred after 140 hours exposure. Confirming the outstanding elevated temperature oxidation resistance of the present invention were tests conducted on specimens from a 3000 pound heat of the alloy of the present invention.

After 170 hours exposure at 21SOF and Mach I airflow, a specimen showed a metal loss of only 1.6 mils per side; after 176 hours at those conditions, a loss of only 2 mils of metal per side was observed.

Another form of a comparison of this outstanding elevated temperature surface stability, as represented by oxidation resistance, of the present invention with other alloys is shown in the graphical presentation of the drawing. That comparison shows surface loss of a specimen in terms of hours of exposure in high si velocity air (HVO) moving at a speed of Mach 1 at S2150°F. The Mach 1 oxidation test specimens referred to herein were 0.23" diameter by 3.5" long.

Twenty-four specimens were mounted on a Irund ietal 30 plate and tested in a furnace which is heatd by aircraft jet fuel. The test specimens were a,,ined Sabout every 24 hours. As can be seen, the present invention provides a cast article witn remarkable surface stability.

As was stated above, an important characteristic 13DV-8630 -12of the present invention is its improved longitudinal stress rupture strength and improved balance between longitudinal and transverse stress rupture properties along with the outstanding surface stability discussed above. It exhibits, in a DS columnar grain article, the good stress rupture strength of Rene' 150 alloy and outstanding oxidation resistance of the single crystal article of the Rene' NS composition in Table II above. The following Table IV compares certain stress rupture properties: eooen TABLE IV e o e LONGITUDINAL STRESS RUPTURE DATA (uncoated, 0.160 diameter bars) 0 0 TEMP STRESS ALLOY/RUPTURE LIFE (hours) (ksi) INVENTIONiDS) RENE' I50(DS) RENE' N4(a) S1800 40 40 70 40 70 1600 80 45 100 SO 90 63 .lngle crystal, diffusion aluminide coated.

SFor the alloy of the present invention, the transverse stress rupture strength at 1800*F and a 32,000 psi (32 ksi) nominally was in the range of about 80 120 hours, as shown in Table V below.

a, During th' evaluation of the present invention, several heat treatments were studied. In one series of heat treatment tests, the alloy associated with the present invention and nominally described in Table II was DS cast into 1/1" thick x 2" wide x 4" long columnar grain slabs from which standard stress rupture specimens were machined after heat treatment 7 13DV-8630 -13- 4 i 00 0*4 of the slabs. In previous evaluations, for example with Rene' ISO alloy columnar grain articles, only partial solutioning was necessary to develop desired properties and full solutioning (90 95%) seriously reduced transverse stress rupture properties.

However, it was found that the present invention requires substantially full solution heat treatment (at least 90% solutiinlng of the gamma gamma prime eutectic and coarse scondary gamma prime with no more than about 4% incopient melting) in order to develop desired properties. In addition to the initial substantially full solutioning, a preferred form of tne neat treatment of the present invention includes an additional progressive combination of aging steps: a primary, first aging to improve ductility and transverse stress rupture properties, and two additional aging treatments at temperatures consecutively lower than that of the primary age to further optimize the gamma prime precipitate.

An outline of a series of heat treatments evaluated, along with resulting stress rupture strength, is shown in the following table V. The heat treatments, identified as A, B, C and D, summarize the heating steps, first with a solution temperature in the range of 2300 2335 F for 2 hours. This is followed by a progressive combination and series of aging steps identified in a manner widely used and understood in the metallurgical art. The solution and aging steps were conducted in a non-oxidizing atmosphere: vacuum, argon or helium. Cooling below 1200*F, conducted between aging steps, need not be conducted in such an atmospnere. Of the neat treatment; evaluated, heat treatment D, involving a uniaue relatively slow cooling step from the first aging to 13DV-8630 -14the temperature at which the second aging temperature was to be conducted, resulted in the best combination of properties.

6 0 400t t «4 1* L. TABLE V INVENTION ALLOY RUPTURE TESTS FROM 1/4" THICK SLABS vs HEAT TREATMENT A TO D (Fast Cools Unless Otherdlse Noted) Direct ion* TempI 0 F Stress/KSI Li fe/flours EL/% RA/% A 2300F/2 Hours 197SF14 Hours 1650/4 Hours 1600 1600 1icoo 1800 1800 2000 1600 1800 2000 1600 1800 2000 1600 1800 1800 35 38.5S 40 is 319.1 47.5 73.4 49. 9 38.3 97.0 49.9 87.0 85. 21. 5 11.6 16.6 18. 8 22. 3 11.5s 37.0 27.7 39.4 41. 6 38. 8 45. 9 B 2335F/2 Hours 1975F/4 Hours 1650F/4 flours 113.5S 45.9 161.0 50.4 1 4. 6, 72. 2 13.0 22.9 13. 3 3.5S 4. 3 1. 6, 1 .5 27. 9 51.3 45.9 3.8 3. 7 1. 3, 1.3

EIL

TABLE V (Con~tinued) C 2335F/2 Hours 2050F/4 Hlours 197SF/4 flours 16SOF/4 flours L 1600 75 121.4 L 1800 40 56.9 L )0 0 Is 293. 4 T 1600 6S 2.0 T 1800 32 107.2 T 2000 10 72.3 14.5S 21.0 21.4 0.8 2. 7 2.0 28. 4 46. 2 63.0 0.0 0.6 D 2335F/2 Hlours 2OSOF/4 flours, One Hour Cool to 197SF 197SF/4 Hours 16S0F/4 Hours 1600 1800 1800 2000 80 40 30 17.5S 99. 2, 81.9 81.7 376.2 61.5, 67.6 27. 2, 129.4, 117.3 189. 1 75. 1, 100.4, 159. 1 22. 6 13.1, 12.0 16. 5 21.4 15. 8, 12. 8 0.6, 2.3, 2.4 3.4 2. 4, 2. 8, 4.0 2. 5 25. 3, 22. 8 42.5S 52. 2 50.4, 32.0 1. 2, 2. 6.7 5. 6 1.2, 3.1 1.9 1600 1800 1800 2000 Longitudinal L, Transverse T 13DV-8630 -17- In che neat treatment of the present invention, a substantially full solutioning step is included.

This is in contrast with tne partial solutioning commonly used with such DS articles made from alloys from Table II such as Rene' 150, certain properties of which are affected detrimentally by a full solution heat treatment. In this invention, solutioning of at least about 90% of the gamma gamma prime eutectic and coarse secondary gamma oro 10 prime and with less than about 4tincipient melting o~ o is important because the stress rupture life is ol"a increased with increased solutioning of the gamma no prime eutectic and coarse secondary gamma prime.

Fsee The following Table VI compares amount of o.rF 15 solutioning and stress rupture life for the alloy o o associated with the present invention.

o TABLE VI Effect of Solutioning on Stress Rupture Life 4' Unsolutioned 1800 0 F Stress Rupture Life A I 20 x 10 15 2x 0 S 3x After solutioning, it is preferred that cooling, for example to a temperature in tie range of about 2025 2075°F, be at a rate of at least 100F per minute. As was described in the above identified copending, incorporated patent application Ser. No.

790,439, more rapid cooling rates have a beneficial effect on properties such as stress rupture strength.

The heat treatment of the present invention is 1 13DV-8630 -18- 9 9 9999I 9.1 o 99 0 0 o 0 9 9( 99 further characterized by a progressive combination of aging steps after solutioning. The first or primary age is conducted in a temperature range of about 2025 2075SF in a non-oxidizing atmosphere, for example for about 1 10 nours, to improve ductility and stress rupture strength of the article. After the first solutioning, it is preferred tnat cooling, for example to the range of about 1950 2000*F, be at a rate of about 75°F per hour prior to further cooling.

A second aging step, at a temperature lower than the first aging, for example in the range of about 1950 2000°F for about 4 12 hours, generally about 4 8 nours, enables growth of the gamma prime to improve ductility. As can be seen from the data of Table V, tnis unioue progressive combination of heating steps results in a structure of improved mechanical properties and enables heat treatment of castings having thin walls without detrimental affect on such walls.

20 After the above aging steps, a final aging step generally is beneficial, for example, in the range of about 1625 -1675SF for about 2 10 hours, typically about 4 8 hours.

The heat treatment of the present invention, in 25 connection with the DS cast article utilizing the alloy associated with tnis invention maximizes longitudinal stress rupture strength while retaining acceptable transverse strength and ductility. Tnis i! due, at least in part, to the increased solutioning ol tne gamma prime at a relatively higher temperature, Introduction of a primary or first aging in the range of about 2025 2075SF followed by a relatively slow cool (for example, about 1 hour) to a temperature in the range of about 1950 2000°F before further cooling resulted in a further improvement in 09; 9 9 r iP'" 13DV-8630 -19longitudinal stress rupture life coupled with improved transverse stress rupture properties.

The combination of alloy selection, casting practice, and heat treatment, according to the present invention, enables provision of an improved DS columnar grain article including a thin wall of less than about 0.035 inch substantially free of cracks. In the form of a gas turbine engine turbine blade, which has a radial centerline, the grain boundaries and primary dendritic orientation is approximately straight and parallel. In addition, it is preferred in such an article, and is capable through this invention, that any emergent grain from the airfoil of such a blade intersect the airfoil leading edge or trailing edge at S' 15 an angle no greater than 15° with the edge and that all other grain boundaries and primary dendrites are witnin of the radial centerline.

As a result of evaluations of the type described above, it was recognized tnat tne article and neat treatment of the present invention can be used with a particular alloy range. A specific allay range is particularly unique in the combination with the heat treatment. The following Table VII identifies such useful and the novel specific alloy range.

'1 13DV- 8630 Table VII ALLOY COMPOSITION FORMS Wt%, balance Ni and incidental impurities RAkNG ES

ELEMENTS

C

Hf Co Ta

V

Zr Cr Mo w Ti tit# Al 'It Re

B

Cb

Y

B ROAD 0. 1-0. is 0. 3-2 11-14 5-9 no more than 1 less than .05 S-10 0.5-3 4-7 no more than 1 S-7 l.S-4 .OOS-.03 0-1.5 0-0.5

PREFERRED

0.1-0.15 1-2 11-13 6-7 less than 1 0-.03 6-7 1-2 4.5-5. 5 less than 1 5.5S-6. 5 5 .01-.02 0-0.5S 0-0.5

SPECIFIC

0.1-0.14 1. 2-1. 7 11. 7-12. 3 6. 2-6.5S 0-0.1i 0-0.03 6.6-7 1. 3-1. 7 4. 7-S. 1 0-0. 02 6-6. 3 2. 6-3 .01-.02 0-0.1 0-0. 2 Tnis invention has been described in connection witn specific examples and embodiments. However, it will be understood by those skilled in the metallurgical arts involved that the invention is capable of a variety of other forms and embodiments within tne scope of tne appended claims.

Claims (5)

1. A nickel base superalloy having improved elevated temperature oxidation resistance and creep-rupture strength, capable of being thin-walled cast and directionally solidified, comprising, in weight percent, 0. 1-0. 15 C, 1. 2-1. 7 Hf, 11. 7-12. 3 Co, 6. 2-6. 5 Ta, up to 0. 1 V, up to 0. 03 Zr, 6. 6-7 Cr, 1. 3-1. 7 Mo. 4. 7-5. 1 W, less than 0. 02 Ti,

6-6.3 Al, 2.6-3 Re, 0.01-0.02 B, up to 0. 1 Cb, up to 0. 2 Y, and the balance Ni and incidental impurities. 2. A nickel base superalloy in accordance with claim 1, comprising, in weight percent, about 0. 12% carbon, about 1. 5% hafnium, about 12% cobalt, about 6. 35% tantalum, about 6. 8% chromium, about 1. 5% molybdenum, about 4. 9% tungsten, about 6. 15% aluminum, about 2. 8% rhenium, about 0. 015% boron, and the balance nickel and incidental impurities. 3. A cast columnar grain, directionally solidified, nickel base superalloy article having improved elevated temperature oxidation resistance and creep-rupture strength, capable of being thin-walled cast, the superalloy of the article S4.. comprising, in percentages by weight, 0. 1-0. 15 C, 1. 2-1. 7 Hf, 11. 7-12. 3 Co, 6. 2-6. 5 Ta, up to 0. 1 V, up to 0.03 Zr, .o 6. 6-7 Cr, 1. 3-1. 7 Mo, 4. 7-5. 1 W, up to 0. 02 Ti, 6-6. 3 Al, 2. 6-3 Re, 0. 01-0. 02 B, up to 0. 1 Cb, up to 0. 2 Y, and the balance Ni and incidental impurities. 4. A nickel base superalloy article in accordance with claim 3, comprising, in weight percent, about 0. 12% carbon, about 1.5% hafnium, about 12% cobalt, about 6.35% tantalum, Sabout 6. 8% chromium, about 1.5% molybdenum, about 4.9% tungsten, about 6. 15% aluminum, about 2. 8% rhenium, aboul 0. 015% boron, and the balance substantially nickel and incidental impurities. 480e/gg 22 The cast article in accordance with claim 3 or claim 4, having an internal cavity within an outside article surface, the cavity including an integral cast wall, substantially free of cracks, and a wall thickness of less than 0. 035 inch. 6. The cast article in accordance with claim 5 in which the internal cavity is separated from the outside surface by an article wall across a thickness of less than 0. 035 inch.

7. The cast article in accordance with claim 5 in the form of a turbine blading member having a radial centerline and including an airfoil having a leading edge and a trailing edge in which: grain boundaries and primary dendritic orientation is approximately straight and parallel; and, any emergent grain which intersects the airfoil leading or trailing edge forms an angle no greater than 15' with the edge, and all other grain boundaries and primary dendrites are within 15" of the radial centerline.

8. A method of heat treating a cast nickel base alloy oo. article made of the alloy defined in claim 1 or claim 2, characterised by 0:4 heating at a solutioning temperature in a non-oxidizing atmosphere for a time sufficient to solution at least 90% of the gamma gamma prime eutectic and coarse secondary gamma prime and so that there is no more than 4% incipient 0 melting, and then cooling in the atmosphere to a temperature in the range of 2025-2075"F; heating at a first aging temperature in the range of

2025-2075' in a non-oxidizing atmosphere for 1-10 hours and then cooling in the atmosphere to a temperature in the range of 1950-2000"F; and heating at a second aging temperature lower than the first aging temperature in the range of 1950-2000'F for 4-12 hours. 480e/gg 4: 23 9. The method of claim 8 including a third aging step of: heating at a temperature range of 1625-1675' for 2-10 hours. The method of claim 8 or claim 9 in which the solutioning temperature is in the range of 2275-2360'F and the heating time is at least 30 minutes. 11. A method of making the cast columnar grain nickel base superalloy article of claim 5, of improved elevated temperature oxidation resistance, the article having an internal cavity including an integral cast wall of a wall thickness of less than 0. 035 inch, characterised by precision casting the article from the alloy of claim 1 or claim 2, with the cast wall integral with the casting by columnar multigrain directional solidification casting; and heat treating the cast article in accordance with the method of any one of claims 8 to 12. A method for making the cast columnar grain nickel base superalloy gas turbine engine turbine blading member of claim 7, of improved elevated temperature oxidation resistance, the article having at least one internal cavity including an integral cast wall of a wall thickness less than 0.035 inch, characterised by providing the superalloy of claim 1 or claim 2, precision casting said superalloy to provide an article having at least one internal cavity including an integral cast wall of a wall thickness of less than 0. 035 inch; and heat treating said cast article in accordance with the method of any one of claims 8 to 13. A nickel base superalloy having improved elevated temperature oxidation resistance and creep-rupture strength, substantially as herein described with reference to the examples. r r 1 r air or oe~~i r oia~ (Is O 000 U~F(i*O 1) D owrr or r o r a orco I, P r D O D1 OuOO O (I D ;D -r L; I [<1 -71 24 14. An article cast of nickel base superalloy, substantially as herein described with reference to the examples. A method of heat treating a cast nickel base alloy article, substantially as herein described with reference to the examples. 16. A method of making a cast columnar grain nickel base superalloy article, substantially as herein described with reference to the examples. DATED this 31th day of August, 1992. GENERAL ELECTRIC COMPANY By Its Patent Attorneys DAVIES COLLISON CAVE 0400 0ir 00000C 000 480e/gg