US3617261A - Wrought nickel base superalloys - Google Patents

Abstract

A wrought nickel base superalloy having an exacting combination of tungsten, tantalum, aluminum, nickel, boron, zirconium, carbon, manganese, silicon, chromium, cobalt, molybdenum, and titanium for minimization of deleterious phase formations and a resultant alloy capable of being hot worked and having good strength properties at high temperatures.

Description

llnited States Patent [72] inventors Louis W. Lherbier Cuddy; Frank J. Rizzo, Pittsburgh, both of Pa. [21] Appl. No. 703,978 [22] Filed Feb. 8, 1968 [45] Patented Nov. 2, 1971 [73] Assignee Cyclops Corporation Specialty Steel Division Bridgeville, Pa.

[54] WROUGHT NICKEL BASE SUPERALLOYS 5 Claims, 3 Drawing Figs.

[52] US. Cl 75/171, 148/11.5, 148/325 [51] Int. Cl C22c 19/00 [50] Field of Search 75/171,

[56] References Cited UNITED STATES PATENTS 3,164,465 l/1965 Thielemann 75/171 3,304,176 2/1967 Wlodek 75/171 3,322,534 5/1967 Shaw et a1 75/171 Primary Examiner Richard 0. Dean Att0rney-Webb, Burden, Robinson & Webb ABSTRACT: A wrought nickel base superalloy having an exacting combination of tungsten, tantalum, aluminum, nickel, boron, zirconium, carbon, manganese, silicon, chromium, cobalt, molybdenum, and titanium for minimization of deleterious phase formations and a resultant alloy capable of being hot worked and having good strength properties at high temperatures.

Stress (psi x IO' o Lherbier et ol. Alloy u Present-day Alloy Parameter T(20+ log t) x IO PATENTED Neva I971 SHEET 2 BF 2 INVENTORS. Louis W Lherbier Frank J Rizzo M Mi THE IR A TLQRNE Y5 WROUGHT NICKEL BASE SUPERALLOYS This invention relates to high-temperature alloys. More particularly, it relates to a nickel base alloy capable of being hot worked and having an exacting combination of elements for achievement of adequate strength, creep, ductility, and corrosion properties up to 1,900 F.

Many superalloys have been developed for the aerospace industry and the like, but the utility of such alloys has been limited by the failure of the alloys to possessthe desired hightemperature strength properties, while at the same-time having adequate ductility for hot working. With the trend-toward the developmentof engines with higher and higher thrusts, increased demands have been made on the component parts of such equipment.

The desire to increase the strength of nickel base alloys has resulted in the continual addition of solid strengtheners such as chromium, molybdenum, and tungsten as well as other elements such as aluminum and titanium. This has led to the transition from lower strength nickel base alloys with adequate ductility to thehigher strength castnickel base alloys with very limited ductility. Alloys which must be cast rather than fabricated do not have the' necessary high degree of uniformity even when the finest precision casting procedure .is employed. The increased contentsof many of the elements has also led tooverall instability of the alloy because of the precipitation of phases detrimental to both the strength and the ductility of the alloy. This instability can occur afterheat treatment and/or after exposure for extended periods of time at elevated temperatures.

Our invention provides a nickel base alloy with acceptable strength properties up to 1,900 F. which is a significant improvement over currently employed alloys. The alloy still maintains adequate hot workability so it can be'extruded,

forged and/or hot rollerinstead of cast. An improvement in,

oxidation resistance has also been effected. Ductility and corrosion resistance have not been adversely affected.

Even though the amount of the alloying additions in'this material has been significantly increased overthe present day wrought alloys, the stability thereof has not been adversely affected. The term wrought, as used herein to define our al-1' loy, is intended to define an alloy which can be hot worked; that is, extruded, forged and/or hot rolled.

The above results have been achieved by using an exacting combination of elements to achieve the high strengths without sacrificing any of the other properties normally-adversely affected by increased strength levels. This proper balance of elements employed achieves these results by minimizing the formation of deleterious phases which lead-to the instability of the alloy. The technique of avoiding undesirable phases by calculating electron vacancy number was used to design our alloy. 7

The structure of nickel base superalloys consists primarily of gamma prime, carbide precipitates, and a gamma matrix. If

the composition of the material is not adequately controlled, unwanted phases form by nucleating on carbides and feeding on the gamma matrix for their constituents. These unwanted phases deleteriously affect the stability and the strength of the material. They are the intermetallic phases sigma, mu, and Laves. Consequently, the composition of the matrix is of major concern in developing new nickel base superalloys. The matrix of our alloy consists of nickel, cobalt, chromium, molybdenum, tungsten, andtantalum. The relative amounts of these alloying elements in the matrix are determined by the other elements in the alloy such as aluminum, titanium, carbon, and boron, all of which have reacted to form precipitating phases.

The alloys of this invention have been vacuum melted although it is believed that with the use of other proper melting techniques the improvements mentioned herein would be attainable.

The carbon content should be between 0.25 percent and 0.45 percent with a preferred range of 0.30 percent to 0.40 percent. At least 0.25 percent which is high by present day standards is necessary in our alloy to make it workable. Howsolution ever, extremely high carbon contents (i.e., above 0.45 percent) are not desirable because the alloy will become brittle. The elements responsible for solid solution strengthening will form carbides, and the carbides will form in morphologies which are harmful to the desired properties.

sion resistance, solid solution strengthening and grain boundary strengthening. Chromium content below 11.0 percent will not give the desired resistance to oxidation and corrosion, and chromium in excess of 17.0 percent makes the alloy difficult to hot work and the stability will be s'everelyimpaired.

Cobalt is employed in the broad range from 8.0-12.0 percent (9.0-1 1.0 percent preferred) for its strength properties at elevated temperatures. It also improves ductility, workability, and creep properties. Cobalt in excess of 12.0 percent impairs oxidation andcorrosion properties.

Tantalum-is a critical element because of its solid solution hardening and carbide formation. Amounts of 1.0-3.0 percent are desirable with a preferred range of 1.25 to 1.75 percent. Levels above 3.0 percent detrimentally affect ductility and hot workability and can also lead to the formation of harmful second phases.

Molybdenum andtungsten also take part in carbide formation and solid solution-hardening. The broad range of molybdenum is.2.06.5 percent and the preferred range is 2.5-3.5 percent. The low end of the range is desirable because the presence of highmolybdenum can lead to the precipitation of deleterious phases. The tungsten content ranges from 4.0 to 8.0 percent with a preferred range of 5.5 to 6.5 percent.

Aluminum and titanium are also critical elements because of their contribution to the strengthening of nickel base superalloys. The broad composition range for aluminum is 4.0 to

5.0 percent. An improved composition range is 4.20 to 4.80 percent and the preferred range is 4.45 to 4.70 percent. This is a critical range because below 4.0 percent aluminum the alloy will not achieve the strength level and above 5.0 percent the alloy shows severe loss of ductility, workability, and stability. The broad composition range for titanium is 2.2 to 3.2 percent with a preferred range of 2.8 to 3.2 percent. Titanium which is also a carbide former has essentially the same effect as the aluminumf Boron and zirconium both enhance the'creep resistance at elevated temperatures. The broad range for boron is 0.0005 to 0.030 percent with a preferred range of 0.008 to 0.018 percent. Zirconium has a broad range of 0.001 to 0.25 percent and a preferred range of 0.005 to 0.150 percent. An excess of either of these elements will have deleterious effects on the ductility and stability of the alloy.

The base metal for the alloy is nickel. lts ability to harden by precipitation .of secondary phases and carbides in addition to solid solution strengthening makes it ideal for this application.

The nominal chemical analysis and the preferred ranges of the elements ofour alloy are summarized in table 1:

Cobalt 9-0010 The stress rupture properties of our alloy are given in table Molybdenum 2.00 to 6.50 2.50 to 3.50 I. Tungsten 4.0010 8.00 5.50 10 5.50 Tam, LOO, 300 L75 TABLE III.-S'IRESS RUPIURE PROPERTIES $12 :28 288 2'2: 2'28 5 tIest Ruptture Rupture Elongation Redurctlon em s ress pcrcen 0 area Boron 0.0005 to 0.030 0.008 to 0.018 F?) (p.s.1.) (hours) 1") (percent) Zirconium 0.001 to 0.250 0.005 to 0.150 H t N (58 0.: N H. f ki iff? 1 1, 500 70,000 100.1 4.1 3.0 1, 500 70,000 63. 8 3. 4 7. 5 1,500 70,000 177. 7 7. 7 14.11 1, 600 55, 000 52. 0 0. 0 0. 0 1,000 55,000 01.0 7.5 8.5 All percents prelented herein unless otherwise specified refer to weight percent. i g 1, 000 55,000 78. 0 2. 7 7. 0 1,000 55,000 81. 0 5. 0 10.11 1,0 55,000 70. 2 s. 5 11.11 1, 800 20, 088 02. 0 11. 1 1 15 1,800 20,0 11 .0 0.7 2.: Trace elements such as sulfur, phosphorous, lead, etc. and 1.388 .88 residual iron are permitted as residuals resulting from normal 800 20:000 2 melting practices. iron may be present in amounts up to 2.00 1,388 fgdfi 3 2 percent, but it is preferred that the iron content be kept below 1:900 000 5 1 7 1 0 percent 20 1,000 15,888 28.11 20 1 2 1 1 000 14. J .5 0 .1' Some of the mcchamcal properties of this new alloy comared to those currentl available are shown in table 11: 1 in order to com are the stress ru ture ro erties of our P y P P P P A 1 alloy with the present day alloy, the Larson-Miller parameter method, which is well known to those skilled in the art, was 25, employed on our stress rupture data to provide the rupture stress at 100 hours at various test temperatures. The results TABLE II.MECHANICAL PROPERTIES g are shown 1n table IV:

T t Ultimate 0i21, Elonga- R d n es ensie y 0 tion 0 uc 0n temp strength strength (percent of area 1 TABLE Iv F.) (p.s.i.) (p.s.i.) in 1") (percent) Heat No.: 4 i 1 201 000 153 200 12.2 1 .1 2. 1071000 170: 200 0. 2 9. a one Hundred Hour 3. 218, 08g 172, 300 12. 3 :3; 1 206 1 155 15. tr 3 Pro I 6m gggggg 3% g 12% S ess Ruptur pe ms 7... 150 1 .1 1. 10 I700 1471000 0. 0 12.1 2. 175,000 100, 300 4.0 8. 1 1. 105, 200 142,000 12. 0 111. 0 Ruvwrs Sims ti. 173, 500 12g, 1% 2. 7 8.1 Alloy Test Temp. (5) (P151) 7. 105,700 1 7 8.7 13.2 1. 125,400 1001 800 s. 7 1s. 0 3. 155,288 187,988 18.3 40 Lherbieret 01. 1,500 71,000 1 8 11:11:17.. 122: 2.2:: 0. 124, 300 104, 700 5. 0 7. 8 7 118 500 08 000 7. s 13. 4 Day 0: 1, 113000 02: 100 10.0 40.1 Lherbier 0111. 1.700 35,000 Present day alloy. 204, 000 140, 000 Present Day 1,700 27,000 400 000 000 1.551111" =1 11. 1,1100 22.000 600 921000 Present Day 1,1100 15,000 1.11mi" =1 .11. 1,000 11.000 s- 2 Present Day 1,900 7,000

notch sensitivity. The present day alloy because of its exces- This data is also presented in graph form in the accompanying FIG. 1 where log stress is plotted against a rupture life term which includes both time and temperature. The rupture life term is represented by the LarsomMiller parameter T(20+log t) where T is temperature in F. and t is time in hours.

The results of table 1V show a significant improvement in rupture stress at 100 hours for all test temperatures when comparing our alloy to the present day alloy. The consistency of the improvement can best be seen by referring to FIG. 1 where the relationship of stress and rupture life of our alloy is compared to the modern day alloy.

The chemical analysis of the experimental heats and the present day alloy used in the compilation of mechanical properties are set forth in table V:

TABLE V.--AC'1UAL COMPOSITION (WEIGHT PERCENT) ANI) ELECTRON VACANCY NUMBER.

At elevated temperatures, there is a need for high strength properties and optimum ductility. Excessive ductility at elevated temperatures results in poor creep rates and insufficient ductility results in notch sensitivity for brittle fracture. Our strength levels, as measured by ultimate tensile strength and yield strength, are significantly superior to the present day alloy while the ductility, as measured by elongation and reduction of area, is optimized to prevent excessive creep rates or sive ductility at elevated temperatures is prone to both excessive creep rates and resultant dimensional instability.

C Cr Co Mo W Ta Ti Al Zr B Ni N Nominal composition (weight percent) 0.06 15.00 15. 00 5. 25 3. 00 4. 40 0. 03 1301111100.. 0.26 11.81 11.07 5. 6.10 1.00 3.06 5.111 0.078 0.017 ...do.... U7

1 Present day alloy. 2 Ch. g

Oxidation resistance as determined by weight gain at measured time intervals and elevated temperatures and corrosion properties as determined by sulfidation resistance are comparable to the present day alloy.

The hot workability of the alloy is evidenced by the fact that the ingots produced have been extruded, forged, or hot rolled. The aforementioned three methods of hot workinghave also been successfully accomplished in various combinations thereof. Ingots have also been hot rolled directly into billets, bars and sheets.

The method of calculating an electron vacancy number (N for an alloy is known to those skilled in the art.=rBasically, the average number of electron vacancies (N..) in an alloy is calculated by forming the atom'fractions of the elements involved. The alloy matrix composition then becomes a critical part in N calculations and all precipitated phases are subtracted'from the total alloy composition. The type, amount, and compositions of the precipitated phases are first determined -by various empirical means also well known to those skilled in the art.

More specifically, as will be shown in the example, the alloying elements in the material are converted to atomic percent. Following calculation of boride, carbides, and gamma prime phases precipitated, the residual alloying elementsare assumed to constitute the matrix. The matrix elementamounts are scaled to 100 percent and the new matrix composition is then used to calculate the mean electron vacancy number by summation.

A sample calculation of the electron vacancy number ofone of our heats designated as heat 5 is illustrated next.

The Actual Procedure Employed in Calculating the Electron Vacancy Number (N,) for Heat No.5

Initially the matrix contains atoms of each element l presented by the atomic percent. The following reactions take place:

a. Loss of elements to formation of borides 0.5 o.15 o.25 o.1o)a z Residual element:

Ni 57.90 (0.30 =='57.89 Cr 13.36- (0.75 X 13.33

Left after boride.

b. Loss of elements to formation of carbides where M represents one or more metallic elements.

MC+M C[Ni CoM0 )C] Residual Element L66 Percent Carbon available MC0.83 percent Carbon Zr=0.070.07=0 Left after MC reaction .C. Loss of elements to formation of gamma prime.

Ni (Al+Ti+0.l Cr) Residual Element Cr=l 3.33l.33=l2.00 Left after gamma prime formation The matrix now contains the atoms ofeach element which has not taken part in the above reactions. That leaves:

Residual Percentage of .Elcment Atoms (5b) Residuals in Matrix C 0.0 Cr 12.00 33.71 Co 8.87 24.92 M0 0.0 0.0 W l.07 3.01 Ta 0.0 0.0 A1 0.0 0.0 Ti 0.0 00 Zr 0.0 B 0.0 0.0 Ni 13.66 38.37

The electron vacancy number (N,.) is determined fromthe following equation:

'FIG. ,2, and an unacceptable microstructureFlG. 3. FIG. 2 is a photomicrograph of a-sample from heat ,5 having anelectron vacancy number of 2.39. FIG. 3 is a photomicrograph of a sample from heat "9 which has a chemical-composition outside of our broad range (see table V) and has an electron vacancy number of 2.97. Both photomicrographs are taken from sam- ,ples in theidentical as heat-treated state and are taken at 4,000 magnifications.

vIn FIG. 2 the .large almost spherical particles are MC carbide. The smaller somewhat spherical particles precipitated at grain boundaries are M C carbides. Within the grains, numerous angular particles of gamma prime have precipitated.

From grain to grain, the orientation of the gamma -.pr,ime is changed.

In FIG. 3, several large spherical MC carbides surrounded by smaller almost spherical particles of primary gamma prime are evident. The grains contain many small angular gamma prime precipitates. The undesirable needlelike" phase present throughout the structure is of the sigma type. The presence of this undesirable phase results in instability of the alloy and poor mechanical properties. The poor mechanical properties are evident by comparing heat 9 in table II with the properties of our alloy.

When the sigma-type needles form in the microstructure, the properties aredetrimentally effected because alloying elements employed for solid solution strengthening are used to form the needle1ike" phase instead. Thus, the strength decreases as the needles form and grow. Failure occurs because of excessive slip and the needles" act as excellent planes upon which slip can occur. Therefore, the more sigmatype phase present, the greater the resultant instability of the alloy.

The difference in microstructure becomes more acute after the alloy has received exposure, i.e., exposure for extended periods of time at elevated temperatures. The presence of these unwanted second phases greatly reduce the stability of the alloy and therefore limit the type of use for which the alloy can be employed. Therefore, to maintain the stability of the alloy, it must meet a satisfactory N, range of values. At the same time, the composition must have the necessary high strength mechanical and stress rupture properties.

Alloys having electron vacancy numbers below 1.9 do not possess the requisite high-temperature properties. As the electron vacancy number is increased above 1.9, the requisite strength properties increase and the stability of the alloy remains satisfactory. The first notable transition from stability to the presence of deleterious second phases occurs above an N of 2.5 in the exposed state. In the heat-treated state before exposure, the formation of the deleterious phases usually occurs above an electron vacancy number of 2.7. In addition, above an N of 2.5 the strength and ductility properties start to diminish. However, an alloy having an N number from 2.5 to 2.7 is still quite satisfactory for many applications both from the standpoint of mechanical properties and stability. However, the optimum high-temperature properties are found in compositions having an N, range from 2.3 to 2.5.

We claim:

1. A wrought nickel base alloy for use up to l,900 F. composing by weight percent 0.25 to 0.45 carbon, 0 to 2.00 manganese, 0 to 1.50 silicon, 11.00 to 17.00 chromium 8.00 to 12.00 cobalt, 2.00 to 6.50 molybdenum, 4.00 to 8.00 tungsten, 1.00 to 3.00 tantalum, 4.00 to 5.00 aluminum, 2.20 to 3.20 titanium, 0.0005 to 0.030 boron, 0.001 to 0.250 zirconium, 2.0 max. iron, and the balance nickel.

2. An alloy of the composition set forth in claim 1 characterized by an electron vacancy number of 1.9 to 2.7.

3. A wrought nickel base alloy for use up to 1,900 F. composing by weight percent 0.30 to 0.40 carbon, 1.00 max. manganese, 1.00 max. silicon, 11.00 to 13.00 chromium, 9.00 to 11.00 cobalt, 2.50 to 3.50 molybdenum, 5.50 to 6.50 tungsten, 1.25 to 1.75 tantalum, 4.45 to 4.70 aluminum, 2.80 to 3.20 titanium, 0.008 to 0.018 boron, 0.005 to 0.150 zirconium, 1.0 max. iron, and balance nickel.

4. An alloy of the composition set forth in claim 3 characterized also by an electron vacancy number in the range of 2.3 to 2.5.

5. The alloy of claim 3 containing up to 0.50 percent by weight misch metal, the misch metal composing a mixture of rare earth elements in metallic form.

* I l i i

Claims (4)

1. 2. An alloy of the composition set forth in claim 1 characterized by an electron vacancy number of 1.9 to 2.7.
2. 3. A wrought nickel base alloy for use up to 1,900* F. composing by weight percent 0.30 to 0.40 carbon, 1.00 max. manganese, 1.00 max. silicon, 11.00 to 13.00 chromium, 9.00 to 11.00 cobalt, 2.50 to 3.50 molybdenum, 5.50 to 6.50 tungsten, 1.25 to 1.75 tantalum, 4.45 to 4.70 aluminum, 2.80 to 3.20 titanium, 0.008 to 0.018 boron, 0.005 to 0.150 zirconium, 1.0 max. iron, and balance nickel.
3. 4. An alloy of the composition set forth in claim 3 characterized also by an electron vacancy number in the range of 2.3 to 2.5.
4. 5. The alloy of claim 3 containing up to 0.50 percent by weight misch metal, the misch metal composing a mixture of rare earth elements in metallic form.

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