CA1044921A - Nickel base alloys having a low coefficient of thermal expansion - Google Patents

Abstract

PATENT APPLICATION
OF
JAMES F. BALDWIN
AND
DOUGLAS H. MAXWELL
FOR

NICKEL-BASE ALLOYS HAVING
A LOW COEFFICIENT OF THERMAL EXPANSION
ABSTRACT OF THE DISCLOSURE
Alloy compositions consisting predominantly of nickel, chromium, molybdenum, carbon, and boron are disclosed. The alloys possess a duplex structure consisting of a nickel-chromium-molybdenum matrix and a semi-continuous network of refractory carbides and borides. A combina-tion of desirable properties is provided by these alloys, including elevated temperature strength, resistance to oxidation and hot corrosion, and a very low coefficient of thermal expansion. The nickel base alloys of this invention are particularly useful for making hard facing welding rods both in cast wire and powder form; components for use in the glass forming industry; and components for use in hot sections of gas turbine engines, such as integral wheels, turbine shrouds, cases, seals, and the like.

Description

! FI:ELD OF INVENTION:
I .
The present invention pertains to nickel base alloy compositions consisting predominantly of nickel, chromium, molybdenum, and carbon.
Ii Preferably, the alloys also contain boron. The alloys of the present inven-5 ~ tion provide a unique, and previously unavailable combination of propertiesIl including elevated temperature strength, resistance to oxidation, resistance 1, to corrosion at elevated temperatures, and a very low coefficient oF thermal ,1 expansion. The nickel base alloys of this invention are particularly useful il for making har d facing welding rods both in cast wir e and powder form;
10 ~I components for use in the glass forming industry; and components for use in hot sections of gas turbine engines, such as integral wheels, turbine shrouds, cases, seals, and the like.
; BACKGROUND OF INVENTION:
il In recent years, there has developed a need for alloys having low 15 1; thermal expansion characteristics coupled with elevated temperature capabilities. 'rhe need for such alloys, for the most part, has arisen in ' connection with gas turbine technology. With the growing demand for 1, improved engine efficiency, attention has been focused upon increasingly ,i sophisticated engine designs. Low thermal expansion characteristics of 20 1i alloys from which gas turbine engine components are fabricated9 is important '.

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if maximum engine efficiency is to be achieved under all modes and conditions of operation. Specifically, as adjacent engine components heat and cool, critical clearance dimensions change. In many cases, the ability to Ij' substantially maintain critical clearance dimensions throughout the full 5 I,, spectrum of engine operating conditions, will determine the success or failure of a particular engine design.
A typical situation is presented with respect to gas turbine engine shrouds. Gas turbine engine shrouds may be visualized as an open ended, Il thin-walled cylinder. Within the cylinder, a disk with radially attached 10 ,I blade air foils rotates about an axis which is common with the longitudinal ~, axis of the eylinder, 'l'he elearance between the tips of the rotating blades il and the inside surface of the cylinder will to a large extent, control the ,' efficiency of the engine. If the shroud expands more than the blade air Il foils during engine operation, the clearance increases and the engine l` efficiency falls off sharply. ~ !
il Gas turbine engine components fabricated from alloys having ¦¦ low coeffieients of thermal expansion are advantageous for reasons other Il than maintaining eritieal elearanee climensions. It has been determined that j~ a low eoefficient of thermal e~pansion is an essential physieal property 20 ¦I for improving thermal fatigue or thermal shoek cycling resistance in ¦¦ high temperature alloys.

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Alloys suitable for fabricating objects such as components for gas turbine engines desirably possess a number of other properties in addition to low coefficients of thermal expansion. Such alloys must simul-lltaneously possess a number of high temperature properties including resistanceto oxidation, sulfidation, and other forms of environmental deterioration. In the past, exhaustive research has been conducted to develop alloys exhibiting resistance to oxidation and sulfidation. It is well recognized in the art that resistance to environmental deterioration in alloy compositions is Illcontrolled by the interaction oE various alloying constituents, Chr omium ~lis by far the most influential solute element eE~ecting resistance environmental deterioration. ~Iowever, large amounts of chromium adversely afEect high temperature creep rupture strength. For applications such as gas turbine components, high temperature creep rupture strength is also an important !! consideration.
~ ~he alloys currently use~ commercially for high temperature applica-jtions possess one, or in some instances two, of the three characteristics described heretofore (low coeEficient oE thermal expansion, high temperature llcorrosion resistance, and good creep rupture strength at elevated tempera-,'tures) that are desired in alloys useful for the fabrication oE gas turbine I,components. For example, commercial nickel base alloys are available I; , I' l ,, I

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which exhibit remarkably low thermal expansion properties in comparison with typical high temperature alloys. However, the very low chromium i content of such alloys renders them unacceptable for use in the uncoated I~ condition at temperatures over about 1400F. in sulfidizing environments.
5 jl Such alloys deteriorate catastrophically at temperatures higher than 1800~.
under sulfidizing conditions.
Other commercially available alloys exhibit excellent resistance to environmental deterioration, but typically such alloys are confined to low ~I stress applications at temperatures over 1600r~ /lore irrlportantly, such 10 ,l alloys generally exhibit high thermal expansion properties typical of nickel ~1, base alloys.
I~ rrhere are a number of precipitation strengthened nickel base super 1, alloys which, because of their strength for resisting creep deformation '~ll at elevated temperatures, are used as materials for fabricating components 15 ll for use in high temperature sections of gas turbines. rrhe conventional strengthening mechanism employed involved precipitation ol an ordered intermetallic phase, generally referred to as gamma prime, having the generic formula Ni3(Al rri) . As amounts of aluminum and titanium have Il been increased9 to increase the amount of precipitate formed and thereby 20 !l increase strength, it is necessary to decrease the chromium content.

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'rhe chromiurn content must be decreased in order to maintain an overall alloy composition that possesses microstructural stability and high tempera-ture strength. As chromium content is decreased, the resistance to ~¦ oxidation and sulfidization necessarily decreases.
5 ~tl Despite the apparent dilemma of being able to select either a strong alloy or one with good resistance to environmental deterioration, a few compositions have evolved with a relatively good balance of both properties.
However, even these compositions are suitable for use only in gas turbine I engines employing high grade aviation fuels ancl operating conditions whereby 10 '¦ hot corrosion and sulficlization are minimized, unless an oxidation and sulfidization resistant coating is applied to components formulated from such , alloys.
Furthermore, despite the good combination of strength and corrosion i resistance, such alloys are not well suited for applications in which low 15 l~ thermal expansion is a primary concern. Such alloys have high thermal expansion properties typical of nickel base superalloys.
Cobalt based superalloys rely on solid solution strengthening and a ~i dispersion of primary carbides for elevated temperature strength. For 1,l this reason, cobalt based alloys will accommodate a significantly greater 20 I percentage of chromium than nickel base alloys. As a general proposition, . I .

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Il cobalt base superalloys may be categorized as weaker, but more corrosion I resistant, than nickel base materials. The expansion properties of eobalt base alloys are generally higher than nickel base alloys, making cobalt Il base alloys even less attraetive for applications which require low thermal 5 ,~' expansion.
l,i The present invention pertains to nickel base alloy compositions ¦I possessing a very low coeffieient of linear thermal expansion and sulfidation ¦I resistanee adequate to enable use o~ ~meoatecl eomponents Eabrieated from the ' Il alloys in eorrosive environments. In addition, the alloys possess elevatecl 10 I temperature strength eharaeteristics adequate to permit the alloys to be employed for numerous high temperature applications.
The alloys of the present invention contain unusually high levels of ehromium and molybdenum. In the vast majority of eases~ chromium and ',3 molybdenum eontaining eommereial niekel base alloys eontain coneentrations;
15 Il of ehromium and molybdenum whieh are below the respective solubility limit li of each element in nickel. In the alloys of the present invention, the eoncen~ra-il tion of ehromium and molybdenum far exceeds normal solubility limits in ~i niekel.
' Excess ehromium and molybdenum in the alloys are prevented from 20 1l forming deleterious embrittling phaces through the addition of boron and ;, , ~l 7 1!.

10'~9Zi 'l carbon. Boron and carbon react with chromium and molybdenum to form borides and carbides. Unusual and unexpected strength improvements result from the boride and carbide dispersions so produced.
High concentrations of chromium in both the metallic matrix and 5 1I the strengthening dispersoid result in unusually high resistance to sulfidation and corrosion at elevated temperatures. The presence of all four major alloying constituents (chromium, molybdenum, boron, and carbon) serve to lower the thermal expan,sion properties of the alloys. ~he ~¦ expansivity of specific alloy compositions within the scope of the present 10 ~~ invention is lower than any known commercial nickel, cobalt, or iron-base ,' alloy.
" 'rhe present invention provides a nickel-base alloy having a low , coefficient of thermal expansion as well as elevated temperature strength I and resistance to high temperature corrosion. In addition, the present 15 j' invention provides a nickel base alloy composition having a high elevated l~ temperature hardness and corrosion resistance suitable for use in high ¦l temperature hard facing applications. Furthermore, the present invention provides high strength nickel base alloys of sufficient chromium content ~, to resist the fluxing action of molten oxides and thu~s is suitable for fabrica-20 I ting components useful in the manufacture of glass shapes.
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. ,~ ', lu~9zl , ! SUMMARY OF INVENTION: l ~, i In general terms, the present invention pertains to nickel base alloy compositions consisting essentially of nickel, chromium, molybdenum, ~, carbon and boron. These alloys have good elevated temperature strength, 5 ~~ resistance to oxidation, and resistance to hot corrosion, as well as a very low coefficient of thermal expansion. The invention also concerns components for use in gas turbine engines and hard facing welding rod made from such alloys.
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Table I sets forth a broad range, an intermediate range, and two 10 ¦I different and narrower ranges, in terms of percent by weight, of elements l~ employed in the alloys of the present invention. It should be ~mderstood ! that the tabulation in Iable I relates to each element individually, and is not i intended to solely define composites of broad and narrow ranges. Neverthe-l~ less, composites of the narrower ranges specified in Table I represent 15 ~ particularly preferred embodiments.

In addition to the alloying constituents specifically set forth in Table I, the alloys of the present invention may contain minor amounts of other elements ordinarily included in nickel base alloys by those skilled in ~ the art which will no-t substantially deleteriously affect the important 20 ', characteristics of the alloy or which are inadvertently included in such alloys, ~,1 : !i ! `
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'il Impurities and incidental elements which may be present include titanium, ., .manganese and silicon in amounts nor.mally employed to achieve castability ~, and melt deoxidation. Typically, these elements would be present in 5 il amounts less than 1% and preferably manganese and silicon would each be " present in amounts of not more than 0. 5% while titanium would be present Il in amounts of not more than 0. 2%. Other impurities ancl incidental elements i l! which may be present in the alloys of the present invention include copper Il in amounts of not more than 0. 5U/o, sul.phur ancl phosphorous in amounts of 10 ,I not more than 0. 20% and iron and cobalt in amounts of not more than 2. 0%.
, Impurities such as nitrogen, hydrogen, tin, lead, bizmuth, calcium and !'' I~ .magnesium should be held to as low a concentration as practical.

BP~IEF DESCRIPTION OF THE DRAWINGS:

FIG, 1 is a graphical plot of thermal expansion properties of 15 'I commercial iron, nickel and cobalt-base superalloys.

' FIG. 2 is a graphical plot depicting 100 hour creep rupture life Il for various commercial alloys.

'~ FIG, 3 is a plot OI thermal expansion properties for commercial iron, nickel and cobalt base superalloys and for example alloys of the 20 ii presen-t invention.

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Il FIG. 4 is similar to FIG. 2 but represents example alloys of the ~l present invention rather than commercial alloys.
1~ DESCRIPTION O:F EXAMPLES AND PE~EI;'ERRED EMBODIMEN'rS:
~I As previously noted, commercially available high temperature alloys ,i may possess some of the characteristics desired in an alloy useful for ,~ fabricating components of gas turbine engines, but such alloys do not possess'I all of the desired characteristics. This may be illustrated with reference to!~ several commercial alloys whose compositions are presented in Table II.
,lAs shown in FIG. 1, commercial alloys A ancl B of Table 11 show remarkably ¦l low thermal expansion properties in comparison with typical high temperature alloys. In FIG. 1, the shaded area designated 1 represents a range of i~ mean coefficients of linear thermal expansion at various temperatures for il 89 commercial iron, nickel and cobalt-base superalloys. Curves 2 and 3 ii represent plots of mean coefficients of thermal expansion against tempera-,', ture for, respectively, commercial alloys A and B, In the case of both commercial alloy A and B, their low thermal li expansion is attributed to the presence of unusually high levels of molybdenum, j~ a refractory element with low expansivity. rrhe total absence or very low 1~ chromium content in these alloys renders them unacceptable ïor service, ~ in an uncoated condition, at temperatures over about 1400F. in a sulfidizing "
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environment. Both alloys cleteriorate catastrophically at temperatures of 1 8Q0 F. and h igher under sulfidizing conditions .
In addition to inadequate environmental corrosion resistance, the elevated temperature strength of commercial alloy A is so limited that it 5 i, cannot be employed for components subjected to high stress at temperatures of above 1600F. This is illustrated in FIG. 2 which plots 100 hour creep rupture life in terms of temperature versus stress for a number of commercial alloys. Curve 1 of :Ei'IG. 2 represents commercial alloy A.
Il As maybe further seen from FIG. 2, commercial alloy C (cu:rve 2) also 10 li lacks adequate elevated temperature strength. Commercial alloys D
and E (curves 3 and 4, respectively, of FIG. 2) possess better high ~I temperature strength characteristics, but not as high as desired at ',, temperatures above about 1600F. Although the strength of commercial ,l alloy B is excellent through about 2200 F., the total lack of environmental 15 l corrosion resistance severely restricts its use.
Commercial alloys C, D and E possess exceptional resistance to environmental deterioration. However, the therrnal expansion properties of all of these alloys are high, typical of nickel-base alloys falling within the ~ shaded area 1 of FIG. 1. ~he high thermal expansion properties of these 20 elements is a major drawback with respect to their use for fabrication of `''I

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certain gas turbine engine components.
As shown by the formulations of 'rable Il, the use of chromium and ,~ molybdenum as major alloying constituents in high temperature nickel-base ; alloys is relatively common. The advantages and effect of each element 5 ,¦ is known to those skilled in the art. However, in certain compositions, it has been observed that these elements, if present together in sufficient quantity will cause the precipitation of brittle phases in the form of needles or platelets. The resuLtant e~tect on high temperature strength and cluctility il can be severe.
10 i In the high chromium, high-molybdenum alloys of the present 1~ invention, the amount of chromium available for brittle, acicular phase 'f formation, is reduced through the addition of carbon and boron. Chromium ' forms stable carbides and both chromium and molybdenum form stable I borides.
15 1, Evaluation of cast alloys in accordance with the present invention shows a noticeable increase in alloy hardness in comparison to similar ! alloys which do not contain borides and carbides. Microstructural examination confirms that refractory carbides and borides are formed on ~l solidification of the alloy. In addition microstructural examination shows , 20 1i that the carbide and boride constituents are rejected by the solidifying .

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` 10~49Zi metallic dendrites. The continuity of the metallic phase on a microstructural 1 scale can be controlled by varying the alloy composition, but the network of '` particulate carbides and borides remains fairly continuous.
Ij It has further been found that in addition to an improvement in room 5 lll temperature hardness, the elevated temperature-creep rupture strength of ,l alloys in accordance with the present invention which contain only 0. 5% to ' 1. 0% carbon approaches the strength of several commercial cast cobalt base j~ superalloys. The simultaneous addition of carbon and boron results in Il creep-rupture strength comparable to several widely usecl commercial cohalt-10 ¦ base cast alloys. The maximum creep rupture strength observed in alloysin accordance with the present invention containing both carbon and boron is 42, 000 psi for rupture in 100 hours at 1500 F. This value is approximately ~ll 10% higher than the strongest known cast cobalt-base superalloy.
il A number of example alloy compositions in accordance with the 15 , present invention were studiedJ using material melted and ~ast in air in !~ standard shell test bar and weld rod ~molds. Thir ty to 50 lb. heats were !l produced for each composition studied. Response to heat treatment was determined by subjecting the test materials to a 24-hour aging exposure ,'l at 1600 F. Alloys that demonstrated an aging response were given the

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j', 1600F. aging treatment prior to testing or were subjected to a 2150F. stress relief/solution anneal prior to aging and testing.
Creep rupture tests were conducted at temperatures between 1400F.
~ and 2000F. under loads that would enable comparison of properties with those 5 ,, of commercial alloys. The measurements of thermal expansion properties were conducted on ground cylindrical specimens 2 inches in length and 0. 200 inches in diameter using standard dialatometric methods.
Hot corrosion and resistance to sulfidization were studied by ~! subjecting 1 inch long,0. 50 inch diameter, cylindrical specimens to a 300 ll hour partial exposure immersion in molten 90% Na2SO4 - 10% NaCI salt mixture at 1600F. Resistance was determined by the measurement o~
1~ weight loss per unit area and by determination of surface recession rate by ,I metallographic means.
l1 Analysis of the example alloys is presented in Table III, in terms of 15 `.ll percent by weight of alloying constituents. The results of thermal expansion i ll studies are presented in Table IV and graphically represented FIG. 3, in I comparison with commercial alloys. In E~IG. 3, shaded area 1 represents 1~ the range of mean coefficient of linear thermal expansion over the tempera-li ture range between about 400F. and 1600F. for 89 commercial high tempera-20 il ture alloys while shaded area 2 represents the samè range for ll example , ~ ~

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TABLE III

Ni Cr Mo B C Mn ~i 1 8 - 0. 5 2 (1) 30 18 - 1. 0 - _

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4 (1) 35 18 0. 2 1. 0 (t) 35 18 0. 5 1. 0 6 (1) 40 1'L 0. 5 1. 0 7 (1) 40 l4 0. 05 1. 0 8 (1) 40 14 0.5 0.5 9 (1) 35 18 0. 05 1.0 10 (l) 35 18 0. 2 0. 5 11 (t) 30 t8 0. 05 0. 5 12 (1) 30 18 0. 2 0. 5 13 ~t) 30 18 0.2 0.5 0.50.5 14 (1) 30 18 0. 5 0. 2 t5 (t) 30 18 0. 2 0. 2 (1) Balance , . . ~

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alloys. As shown in FIG. 3, alloys within the scope of the present invention tend to have substantially lower thermal expansion properties than ' conventional commercial superalloys.
Il Creep rupture test results for various example alloys are set forth

5 1' in Tables V and VI and in FIG. 4. The data tabulated in ~able V for each of ' the example alloys includes time to rupture in hours under various conditions ! of temperature and stress, the tolerated final total elongation or linear creep strain, the reduction in area of the specimen diameter in the area of ¦~ fracture, and a calculated equivalent stress to produce rupture in tO0 hours ¦
10 ¦ at 1500 F. The temperature of 1500F. was selected because it would enable comparison with other alloys which are candidates for use in 1 applications which require low expansion.
ii Table VI tabulates creep rupture test results for notched specimens. i ¦ Time to rupture in hours at 1600F. under stress of 22, 000 psi is given for !1 a number of example alloys.
'~ FIG. 4 represents a plot of 100 hour creep rupture life as !~ temperature versus ~:=i~for a number of example alloys. In FIG. 4, /~;
curves 1, 2 and 3, respectively, represent example alloys 4, 6 and 14.
~ Example alloys 1 and 2 represent additions of relatively large 20 1I percentages of carbon to ternary nickel-chromiom-molybdenum alloys I~
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TABLE V 1, ! Creep-Rupture Properties Test Conditions Example Temp. Stress, Life, 1500F.-lO0 Hr.
Alloy F. psi Hrs. % El.~oRA ~re Stress ' 1 1600 20, 000 8. 6 19. 0 33. 1 --i 1600 20, 000 9. 9 26. 7 40. 2 --2000 5, 000 8. 5 12. 6 24. 3 -- ' 2000 5, 000 8. 8 19. 0 26. ~
~ ~ ~ ~ ~ ~ ~ ~ ---- 20, 000 ij 2 1600 20, 000 9. 3 50. 6 46. 0 --I! 1600 20, 000 8. 4 55.5 47. 1 --il 2000 5, 000 6. 1 16.6 26. 5 --2000 5, 000 6. 0 20. 0 30. 1 ---- -------- ---- ---- 20, 000 ll l 3 1600 20, 000 I0. 020. 1 24. 6 --1600 20, 000 81. 0t9. 1 19. ~
lj 2000 5, 000 t5. 27. 9 I0. 3 --i~ 2000 5, 000 12. 36. 0 8. 5 --!l __ __ -- -- -- 26, 500 ,1 ' I 4 1600 20, 000 48. 817. 5 16. 9 --il 1600 20, 000 58. 328. 3 32. 5 --I 2000 5, 000 7. 6 35.0 47.0 --ti 2000 5, 000 10. 224. 6 41.5 --j~ __ __ __ ---- ---- 25~ 500 ~' 5 1600 20, 000 69. 719. 0 27. 1 --1600 20, 000 57. 613. 9 17. 3 --2000 5, 000 lB. 227. 5 29.4 --2000 5,00010.4 20. 617.8 --I ~ _ _ _ _ _ _ --- --- 26, 000 'I 6 1400 50, 000 132. 43. 0 -- --1400 50, 000 228. 43. 1 3. 9 --1600 35, 000 65. 62. 4 2. 8 --1600 35, 000 35. 82. 0 3.2 --1800 15, 000 12. 22. 0 ~ 4. 0 --i?j 1800 15, 000 153. 21. 2 1. 5 --2000 5, 000 5~. 21. 3 1. 8 --'; 2000 5, 000 14. 75. 5 18. 6 --2000 5, 000 95. 11. 7 2. 2 -~
,' 2000 5, 000 24.0 7. 6 9 0 --!l -- -- -- -- -- ~2, 000 ; 7 1600 20, 000 60. 93. 8 4. 5 --i) 1600 20, 000 64. 93. 9 4. 3 --2000 5, 000 3. 7 1. 8 3. 0 --2000 5, 000 3.6 2. 3 3. 5 --i __ __ __ ---- ---- 25, 500 11 , ~. ,, ,;i: ~- , . ..

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~',8 160020,000 48.6 5.5 6.4 --160020,000 60.1 6.5 7.2 --20005,000 9.6 2.6 3.8 20005,000 11.8 5.0 6.8 ---- 25,000 9 160020,000 30.9 13.522.6 --160020,000 42.0 10.917.1 --i 20005,000 5.2 17.818.1 --20005,000 5.0 11.814.3 --I ~ _ __ _ - - - - - - 24,000 il lO 160020,000 42.7 20.127.6 --1600'~0,000 43.8 1~.120.9 --20005,000 4.5 23.652.5 --20005,000 9. 1 20.746. ~ --, _ __ __ __ - - 24,000 :
ll 160020,000 21.2 24.230.9 --160020,000 18.6 20.135.0 --20005,000 3.5 18.629.9 --20005,000 4.0 21.220.0 --__ __ __ 22,500 12 160020,000 24.8 17.828.7 --' 160020,000 35.4 11.614.9 --20005,000 9.0 6.611.3 --20005,000 11.7 4.611.2 --1l __ __ __ __ __ 23,000 .j 13 160020,000 20.8 13.532.4 --I 160020,000 24.2 18.033.1 --20005,000 4.9 12.618.1 --20005,000 4.8 7.425.0 --__ __ __ - - - - 23,000 14 160020,000 96.2 8.417.3 --160020,000 107.0 7.5 6.9 --20005,000 40.5 5.6 7.5 --20005,000 37.5 5.0 6.7 --__ __ - - - - - - 27,500 160020,000 69.9 19.838.3 --160020~ 000 44.1 17.127.0 --20005,000 9.4 11.324.0 --1 20005,000 11.9 16.227.1 ---- 24,500 ,- , , ~;

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1~449Zl ¦ that would show, absent the relatively large amount of carbon, micro-¦ structural instability. Structurally, these alloys consist of primary ¦ metallic dendrites and primary "herring bone" eutectic chromium-~ ¦ molybdenum carbides. Example alloys 1 and 2 exhibited Rockwell hardness ¦ numbers C-scale, (Rc) of 33 and 42 respectively. Example alloy 2 showed ¦ a slight softening to Rc 38 upon aging. Rupture strength of both alloys is ¦ relatively low, but approaches that of cast cobalt-base superalloys.
¦ Increasing chromium content of nickel base alloys generally results ¦ in a lowering of elevated temperature strength. However, as shown by ¦ the data in ~able V with respect to example alloys 3, 4, 5, and 6, increasi~g ¦, chromium content while simultaneously adding relatively large percentages ¦ of carbon and boron results in sharp increases in strength. In the case of ¦ exarnple alloy 6, the stress to produce rupture in lO0 hours at 1500F is ¦ more than doubled in comparison to example alloys l and 2. Of course, ¦ this is an unusually large and unexpected increase in strength. By ¦ comparing FIGS. 2 and 4 it may be seen that the level of strength of ¦ example alloy 6 is approximately 10% above that of commercial alloy E.
Alloy E is one of the strongest cobalt base alloys which has been developed.
Example alloy 4 not only has good strength, it possesses a lower mean coefficient of thermal expansion from 80F. to 1600 F. than any other " ~)443Z~L I
¦ known nickel-base alloy. The surprisingly low mean coefficient of thermal ¦ expansion of example alloy 4 from 801~ to 160lD F. is shown in ~able IV.
¦ A comparison of this data with the curves of FIG. 1 illustrates the low ¦ degree of thermal expansion of example alloy 4 compared to various 5 ¦ commercially available superalloys~
¦ Exa~nple alloys 4 and 69 respectively, show a weight loss of 50. 4 ¦ and 48.1 mg/cm and surface recession rates of 0. 0035 and 0. 002 inches ¦ in 300 hours in the sulfidization test. This represents excellent resistance ¦ to the severe test condition6 employed and demonstrates that these alloys 10 ¦ may be categorized as hot corrosion resistant.
¦ Despite the fact that example alloy 6 showed a remarkable increase ¦ in stren~th, example alloy 4 may be the more attractive material for certain ¦ types of use. The very low expansivity combined with excellent hot corrosion ¦ resistance and moderate strength makes example alloy 4 very attractive for 15 ¦ fabricating components which require a very low degree of thermal expansion ¦ at elevated temperatures. Compositional modifications around example alloy ¦ 4 and 6 resulted in some strength improvement over alloy 4, in example alloy ¦ 14, but at some sacrifice in expansion properties.
¦ In producing the alloys of the present invention, and objects prepared 20 ¦ from the alloys of the present invention, no special skilis or techniques are ~.

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required other than normal conventional foundry practice. ~he alloys may be readily cast in sand, shell, or investment molds and melted and cast in air or under vacuum. Although the alloys were developed for use in the cast condition, several specific compositions within the ambit of the present invention may be employed in wrought for~n if produced by powder metallurgy techniques .
~ he alloys of the present invention may generally be described as a class of nickel-base alloys possessing a duplex structure consisting of a nickel-chromium-molybdenum matrix and a semi-continuous network of refractory carbides and borides. The alloy compositions possess a combina-tion of physical and mechanical characteristics which have generally been considered mutually exclusive.
Although the present invention has been described in conjunctivn with preferred embodiments, it is to be understood that modifications and varia-tions may be resorted to without departing from the spirit and scope of the invention. Such modifications are considered to be within the purview and scope of the invention and appended claims.

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Claims (16)

WHAT IS CLAIMED IS:

1. A nickel base alloy having elevated temperature strength, resistance to oxidation and hot corrosion, and a low coefficient of thermal expansion, consisting essentially of the following elements in the weight percent ranges set forth:

the balance of the alloy being essentially nickel and minor amounts of impurities and residual elements which do not detrimentally affect the basic characteristics of the alloy.

2. The nickel base alloy of claim 1 wherein the carbon content is about 0. 5% to about 1.2% by weight.

3. A component for use in a gas turbine engine formed of the alloy of claim 1.

4. A hard facing welding rod formed of the alloy of claim 1.

5. The alloy of claim 1 which contains, on a weight basis, about 28% to about 42% chromium, about 12% to about 20% molybdenum, about 0.15% to about 1. 2% carbon, and about 0.04% to about 0.7% boron.

6. The alloy of claim 5 which contains, on a weight basis, about 16% to about 20% molybdenum and about 0.2% to about 0.7% boron.

7. A component for use in a gas turbine engine formed of the alloy of claim 5.

8. A component for use in a gas turbine engine formed of the alloy of claim 6.

9. A hard facing welding rod formed of the alloy of claim 5.

10. A nickel base alloy having elevated temperature strength, resistance to oxidation and hot corrosion, and a low coefficient of thermal expansion consisting essentially of the following elements and the weight percentage ranges set forth the balance of the alloy being essentially nickel and minor amounts of impurities and residual elements which do not detrimentally affect the basic characteristics of the alloy.

11. A component for use in a gas turbine engine formed of the alloy of claim 10.

12. A nickel base alloy having elevated temperature strength, resistance to oxidation and hot corrosion, and a low coefficient of thermal expansion consisting essentially of the following elements in the weight percentage ranges set forth:

the balance of the alloy being essentially nickel and minor amounts of impurities and residual elements which do not detrimentally affect the basic characteristics of the alloy.

13. A component for use in a gas turbine engine formed of the alloy of claim 12.

14. The alloy of claim 1 which contains not more than 0. 2%
titanium.

15. The nickel base alloy as claimed in claim i, 10 or 12 wherein the carbon and boron are effective to prevent the formation of deleterious embrittling phases through formation chromium and molybdenum borides and carbides.

16. A nickel base alloy having elevated temperature strength, resistance to oxidation and hot corrosion, and a low coefficient of thermal expansion, consisting essentially of the following elements in the weight percent ranges set forth:

the balance of the alloy being essentially nickel and minor amounts of impurities and residual elements which do not detrimentally affect the basic characteristics of the alloy, the carbon and boron being effective to prevent the formation of deleterious embrittling phases through formation of chromium and molybdenum borides and carbides.