Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF THE INVENTION
Field of the Invention - This inven~ion relates
to the Eield of homogeneous single crystal superalloy
articles.
Description of the Prior Art - The nickel base
superalloy art area has been extensively investigated
for many years, and as a result ~here are very many
issued patents in this area~ Some of these disclose
alloys in which no intentional additions of cobalt,
carbon, boron, or zirconium are made, or alloys in
which these elements are optional. These include,
for example, U.S. Patent Nos. 27621,122; 2,781,264;
2,912,323; 2,994,605; 3,046,108; 3,166,~12; 3,188,40~;
3,287,110; 3,304,176 and 3,322,534. These patents do
not discuss single crystal applications.
U.S. Patent No. 3,494,709, assigned to the assignee
of ~he present invention, discloses the use of single
crystal articles in gas turbine engines. This patent
discusses the desirability of limiting certain elements
such as boron and ~irconium to low levels.
The limitation of carbon to low levels in single
crystal superalloy articles is discussed in U.S. Patent
No. 3,567,526 which is also assigned to the present
assignee.
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3~
U.S. Patent No. 3~915,761, assigned to the present
assignee discloses a nickel base superalloy article pro-
duced by a me~hod which provides a hyperfine dendritic
structure. As a result of the fineness of the structure,
the article may be homogenized in relatively short times.
The conventional nickel base superalloys which are ~
used to fabricate. such parts have evolved over the last ~;
30 years. Typically these alloys contain chromium in
levels of about 10% primarily for oxidation resistance,
aluminum and titanium in combined levels of about 5% for
the formation of the strengthening gamma prime phase and
refractory metals such as tungsten, molybdenum, tantalum ;`
and columbium in levels of about 5% as solid solution `~
strengtheners. Virtually all nickel base superalloys
also contain carbon in levels of about .1% which acts
as a grain boundary strengthener and forms carbides
which strengthen the alloy. Boron and zirconium are `~ :
also often added in small amounts as grain ~oundary
strengtheners.
Most commonly, gas turbine blades are formed by :`~
casting and the casting pro~ess most often utilized
produces parts having equiaxed nonoriented grains. It
is well known that the high temperature properties of
metals are usually quite dependent upon grain boundary `~
properties, consequently efforts have been made to
strengthen such boundaries (for example by the additions
discussed previously), or to reduce or eliminate the
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grain boundaries transverse to the major stress axis of
the part. One method of eliminating such transverse
boundaries is termed directional solidification and is
described in U.S. Patent No. 3,260,505. The effect of
directional solidification is to produce an oriented
microstructure of columnar grains whose major axis is
parallel to the stress axis of the part and which has
minimal or no grain boundaries perpendicular to the
stress axis of the part. A further extension of this
concept is the utilization of single crystal parts in
gas turbine blades. This concept is described in U.S.
Patent NoO 3,494,709. The obvious advantage of the
single crystal blade is the complete absence of grain
boundaries. In single crystals, therefore, grain
boundaries are eliminated as potential weaknesses,
hence, the mechanical properties of the single crystal
are completely dependent upon the inherent mechanical
proper~ies of the material.
In the prior art alloy development such effort was
devoted to the solution of the problems resulting from
grain boundaries, through the addition of elements such
as carbon, boron, and zirconium. Another problem which
prio-r art alloy development sought to avoid was the
development of deleterious phases after long term expo-
sures at elevated temperatures ~i.e. alloy instability). i `~
These phases are of two general types. One, such as
sigma, is undesirable because of its brittle nature while
the oth~r, such as mu, is undesirable because the phase
ties up large amounts of the refractory solid solution
strengtheners ~hus weakening the remaining alloy phases.
These phases are termed TCP phases for topologlcally
closed packed phases, and one of their common properties
is that they all contain cobalt. There are TCP phases
which can form in the absence of cobalt but these cobalt
free TCP phases contain other elements such as silicon
which are no~ commonly found in nickel base superalloys.
While an obvious remedy to control these deleterious
phases is the removal or minimization of cobalt, this ~-
has not proved practical in prior art alloys for poly-
crystalline applications. The problem is that if the
cobalt is removed or significantly reduced, the carbon
combines preferentially with the refractory metals to
form M6C carbides which are deleterious to the proper-
ties of the material as their formation depletes the
alloy of the strengthening refractory elements.
U.S. Patent No. 3,567,526 teaches that carbon can
be completely removed from single crystal superalloy `~
articles and that such removal improves fatigue properties.
In single crystal articles which are free from carbon
there are two important strengthening mechanisms. The
most important strengthening mechanism is the inter-
metallic gamma prime phase, Ni3(Al, Ti). In modern
nickel base superalloys the gamma prime phase may occur
in quantities as great as 60 volume percent. The second
strengthening mechanism is the solid solution strengthen-
ing which is produced by the presence of the re~ractory
metals such as tungsten and molybdenum in the nickel
solid solution matrix. For a constant volume fraction
of gamma prime, considerable variation.s in the strength-
ening effect of this volume fraction of gamma prime may
be obtained by varying the size and morphology of the
gamma prime precipitate particles. The gamma prime
phase is characterized by having a solvus temperature
above which the phase dissolves into the matrix. In
many cast alloys, however, the gamma prime solvus
temperature is in fact above the incipient melting
temperature so that it is not possible to effectively
solutionize the gamma prime pha~e without incipient
melting. Solutionizing of the gamma prime is the only
way in which the morphology of t:he as cast gamma prime
phase can be modified, hence for many modern commercial
nickel base superalloys the gamma prime morphology is
limited to the morphology which resulted from the
original casting process. The other strengthening
mechanism, solid solution strengthening, is most effect-
ive when the solid solution strengthening elements are
uniformly distributed throughout the nickel solid solu-
tion matrix. Again this strengthening is reduced in
effectiveness because of the nature of the casting and
solidification process. Practical nickel base super-
alloys freeze over a wide temperature range. The
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~.
freezing or solidification process involves the forma-
tion of high melting point dendrites followed by the
subsequent freezing of the lower temperature melting
interdendritic liquid. This solidification process
leads to significant compositional inhomogeneities
throughout the microstructure. It is theoretically
possible to homogenize such a microstructure by heating
at elevated temperatures to permit diffusion to occur~
however, in practical nickel base superalloys the
maximum homogenization temperature, which is limited ;`
by the incipient melting temperature, is too low to
permit significant homogenization in practical time in-
tervals.
SUMMARY OF_THE INVENTION
This invention includes three interrelated aspects.
The first aspect is the particular alloy employed. The
alloy is a nickel base alloy containing from about 8
to about 12% chromium3 from about 4.5 to about 5.5V/o
aluminum, from about 1 to 2% ti~anium9 from 3 to 5%
tungsten, and from 10 to 14% tantalum. The cobalt con-
tent is controlled to fall within the range of 3-7%, and
the balance is essentially nickel. The alloy employed
in the present invention is ~ree from intentional addi-
tions of carbon, boron and zirconium, although obviously
these elements may be present as unintentional impuri-
ties. The alloy is characterized by having an incipient
melting temperature in excess of about 2300F. Thus,
~ w
this alloy may be heat treated under conditions which
permit solutionizing of the gamma prime phase without
incipient melting. At the same time the high incipient
melting temperature permi~s essentially complete homo-
geni~ation of the alloy in commercially practicable times.
The high incipient melting temperature of the alloy is a
result of the absence of carbon, boron and zirconium.
The low cobalt content inhibits the formation of dele- -~
terious TCP phases.
The second important aspect of the invention is the
formation of the previously described alloy into single
crystal articles.
The third aspect of the invention is the heat treat-
ment sequence by which the gamma prime morphology may be
modified and refined at the same time that significant
homogenization of the as cast microstructure is performed.
The resultant single cryskal article will have a micro-
structure whose typical gamma prime particle size is
about one third of the gamma prime particle size found
in the as cast material. At the same time the heat
treated single crystal microstructure will be essentially
free from compositional inhomogeneities and this uniform
microstructure combined with the increased gamma prime
solvus temperature will permit the article of the present
invention to exhibit temperature capabilities, for equal
mechanical propertiesg which are at least 30F greater
than the temperature capabilities of comparable prior
3~
art single crystal articles which are formed from conventional
alloys containing carbon, boron and zirconium and conventional
levels of cobalt. me alloys have advantages over conventional
alloys even if not heat treated, but the heat treatment is the
preferred embodiment.
In accordance with a broad aspect of the invention,
there is provided a heat treated nickel base superalloy article
suited for use at elevated temperatures having a composition of
a. from about 8 to about 12% chromium,
b. from about 4.5 to about 5.5% aluminum,
c. from about 1 to about 2% titanium,
d. from about 3 to about 5% tungsten,
e. from about 10 to about 14% tantalum,
f. from about 3 to about 7% cobalt,
g. balance essentially nickel, and
said article being free from internal grain boundaries
and having an average gamma prime particle size of less than
about .5 micron.
In accordance with another broad aspect of the inven-
tion, there is provided a method for producing a single crystal
nickel base superalloy article suited for use at elevated
temperatures including the steps of:
a. providing an alloy containing
from about 8 to about 12% chromium,
from about 4.5 to about 5.5% aluminum,
from about 1 to about 2% titanium,
from about 3 to about 5% tungsten,
from about 10 to about 14% tantalum,
from about 3 to about 7% cobalt,
balance essentially nickel,
b. forming -the alloy into a single crystal article, and
3~
c. solution heat treating the article at a temper-
ature of from about 2350 to about 2400F.
In accordance with yet another broad aspect of the
invention, there is provided an intermediate single crystal
article useful in the production of articles for use at elevated
temperatures, said intermediate article having a composition of:
a. from about 8 to about 12% chromium,
b. from about 4.5 to about 5.5% aluminum,
c. from about 1 to about 2% titanium,
do from about 3 to about 5% tungsten,
e. from about 10 to about 14% tantalum,
f. from about 3 to about 7% cobalt,
g. balance essentially nickel, and
said article being free from internal grain bound-
aries and having an as cast microstructure.
The foregoing, and other objects, features and ad-
vantages of the present invention will become more apparent in
the light of the following detailed description of the preferred
embodiment thereof.
DESCRIPTION OF THE PREFERRED EMBODIMENT -;
In the description which follows, all percent figures
are in weight percent unless otherwise specified,
This invention relates to an article made of a speci-
fic alloy by a critical series of process steps. Although
other articles may be produced according to this invention,
this invention has particular utility in the fabrication of air-
foils (blades and vanes) for use in gas turbine enginesO In
particular, the strength of articles made according to this
invention make them especially suited for use as blades in `;-
gas turbine engines.
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,,' ~ .: '
~.~3;~
A primary feature in the alloys employed in the pre-
sent invention is the substantial elimination of the grain
boundary strengthening agents, carbon, boron and zirconium and
the reduction in cobalt content relative to conventional
superalloys. The alloys of the invention
-9b-
are intended for use as gas turbine components in a
single crystal form~ No intentional additions of the
elements, carbon, boron and zirconium are made, how- ;
ever, some will invariably be present as an impurity.
In order to ensure that ~CP phases will not form in
the alloy over a wide range of compositions and operating
conditions, the level of cobalt is controlled to fall
within the range of 3 to 7%.
Likewise, with regard to the grain boundary strength-
ening agents carbon, boron and zirconium, no intentional
additions are made. I the maximum benefit is to be
obtained from this invention, no single element of the
group carbon~ boron and æirconium should be present in
an amount greater than 50 ppm and it is preerred that
the total of such impurities be less than 100 ppm. Most
preferably carbon is present in an amount less than 30
ppm and the remaining elements are each present in
quantities less than 20 ppm. In any event, the carbon
level must be restricted to be below that amount of
carbon which will form MC type carbides. It must be
emphasized that no intentional addition of these elements
is contemplated and that their presence in the alloy or
single crystal article of the inventiorl is unintentional
and undesirable.
Alloys which can be produced using the concept of
the present lnvention will contain: :~
1) from 8 to 12% chromium,
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3~ :
2) rom 4.5 to 5.5% aluminum, and from 1-2%
titanium,
;` 3) from 3-5% tungsten and from L0~14% tantalum,
4~ from 3-7% cobalt~
5) balance essentially nickel.
Within the preceding ranges~ certain relationships
are preferred. The sum o tungsten an~ tantalum levels
is preerably at leastl5.5% to insure adequate solid
~- solution strengtheni~g and improved e~evated temperature
creep strength. ~ eantalum level of at least l:l% :Ls
preferred for oxidation resistance. T'he elements
aluminum~ titanium and tantalum participate i~ ~he
formation of the gamma prime phase ~N~3Al~ T:L~ Ta~ and
for ma~imum strengthening by the gamma pxime phase the
total content of aluminum plus titanium plus tantalum
is preferabl~ at lea~t 17.5%. ~luminum and titanium
are the principal elements which form t:~e gamma prîme
phase ~nd the xatio of aluminum to titanium must ~e
con~rolled to be greater than 2.5 an~ prefera~ly gxeater
than 3 O to insure adequate oxidation resistanceO ~t
i least ~% chromî~m should be present if the article îs s/~a/~7
to be used in environments whexe sulfidation is a problem.
The minor addition of cobalt also aids in improving
sulfidation resistance.
Alloys made according to the precedin~ limitation
will comprise a nickel chromium solid solutîon containing
at least 30/O by volume o~ the ordered phase of the
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compositlon Ni3M where M is aluminum, titanium, tantalum~
and tungsten to a lesser degree.
The alloys within the ranges set forth above are
, ~
~ thermally stable and deleterious microstructural insta-
, ~
:~ bilities such as the cobalt containing TCP phases will
~i
',)J~ not form, even after extended exposure at elevated
,,:.
~j temperature as for example 500 hours at either 1600,
1800 or 2000F. Further, the alloys have good fatigue
,;.
~; properties since the formation of deleterious carbide
, 10 particles is prevented. The refractory metals which
would normally combine with carbon or precipitate in
TCP phase formation remain m solid solution and result
` in an alloy having exceptional mechanical properties
An important benefit whiclh arises from the elimina-
tion of boron, carbon and zirconium is an increase in
the incipient melting temperature. Typically the inci-
~; pient melting ~emperature of the present alloys, that
temperature at which the alloy first begins localized
melting, will be increased by at leas~ 50F over the
20 incipien~ melting temperature of a similar (prior art)
alloy which contains normal amounts of carbon, boron
:
and zirconium. The incipient melting temperature of
the alloy of this invention will typically exceed 2300F
while conventional high strength, high volume fraction
gamma-gamma prime alloys have incipient melting tempera-
tures below 2300F. This increased temperature permits
solutionizing heat treatments to be performed at
.~
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.
,:
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.. . . .
temperatures where complete solutionlzing of the precipitated
gamma prime is possible while simultaneously permitting a
significant amount of homogenization within reasonable times.
e alloys of the present invention will not form the
carbides which have been found necessary for grain boundary
strengthening in polycrystalline nickel base superalloys. For
thi~ reason the alloys of the present invention must be used
' as single crystal articles. The formation of the alloy into
single crystal form is a critical aspect of the present in-
vention, but the method of single crystal formation is un-
important. Typical articles and solidification techniques are
described in U.S. Patent No. 3,494,709 to Piearcey, which is
assigned to the assignee of the present applicationO
The final aspect of the invention involves the
specific heat treatment applied to the single crystal article.
The as cast single crystal article will contain the gamma prime
phase in dispersed form wi~h a typical particle size on the
order of 1.5 microns. The gamma prime solvus of the alloy
will typically fall in the range of 23S0-2400F and the inci-
pient melting temperature will be in excess of about 2360F.Thus, heat treatment in the range of 2350-2400F (but below the
incipient melting temperature) will place the precipitated
gamma prime
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3Z~' ,
phase into solution without deleterious localized
melting. Times on the order of 1/2 to 8 hours will
normally be satisfactory although longer times may be
.,, j -,.
employed. Such heat treatment temperatures are about
100F higher than those which can be employed with poly-
crystalline articles of conventional superalloys. This
(~ elevated temperature permits a substantial amount of
homogenization to occur during the solutionizing steps.
, Following the solutionizing treatment, an aging ;
10treatment at 1600-2000~F may be utilized to reprecipitate
the gamma prime in refined form. Typical gamma prime
particle sizes ater reprecipitation will be less than
about .5 micron~
The preceding discussion oE the preferred embodiment
will be clarified through reference to the following
illustrative examples
Example 1
; Alloys having compositions set forth in Table I
were prepared.
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lli73ZO
.~ ~,~
s~ l ~ o
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i I I ~D
. o
. , .
U)~
o o
, r~t ~
.
~ 1 , , ~, -
, .
.~ .
ol Ino o
V, , ~t ~ .:
: . ,~
~ oU~ oo
P~ .,,
E~ E~ ~, ,t ~ ~t , ..
r~
;-
~, .
~ .
t . ,_
~, ~
c,t
'~:
o C~
~t ~_
," C~l U~ ..
~*~
~.--,
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~* U~ ~3
o o oo
r-t
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~ ¢ ¢ ¢ ..
- 15 -
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Alloy 444 is disclosed in U. S. Patent No. 4,116,723
issued September 26, 1978 to United Technologies Corporation,
which is the parent case to the present application. Alloy
454 is the alloy of the present invention. Both o~ these
alloys were solidified in single crystal form, Alloy PWA 1422
is a co~mercial alloy which is extensively used as a blade mat-
erial in gas turbine engines, It is noted for its high tempera-
ture mechanical properties. Alloy PWA 1422 was produced in a
directionally solidified form having elongated columnar grains.
Alloy 1455 is a commercial alloy which has been used as a gas
turbine blade material. It is noted for its high temperature
oxidation resistance. This alloy was produced by conventional
casting methods with equiaxed nonoriented grains. The experi-
mental alloys were heat treated according to the invention,
the treatment used was a 4 hour solution heat treatment at
2350F with subsequent aging treatments at 1975F for 4 hours
and 1600F for 32 hours. Alloy PWA 1422 was treated at 2200F ; ~
for 2 hours followed by aging treatments at 1975F for 4 hours `
and 1600F for 32 hours and the alloy P~A 1455 was tested as
; 20 cast. Both of these conventional alloys were tested in the
condition in which they are commonly used.
Example 2
Some of the alloy samples produced in Example 1 were
tested to evaluate their creep rupture properties. The test
conditions and results are set forth below in Table II.
,',.
.,
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~1 16
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TABLE II
A-lloy Cree~-R_pture
'
Test Time to Rupture
y Condition_ 1% Creep Life ~`
454 1700F/50 ksi 46.2 165.6
44~ " 2~.5 82.6
PWA 1422 " 17 76
454 1800F/29 ksi 143.9 350
444 " 110~0 310
;~ 10 PWA 1422 " 60 160
454 2000~F/12 ksi 409.9 776.4
44~ " 303.9 3~5.7
~ PWA 1422 " 31; 61 ;
- Referring to Table II, it is apparent that under~` the test conditions employed, the invention alloy (454)
was superior to the alloy in the parent case (~44) and
the commercial alloy (PWA 1422). The proportionate
;~ degree of superiority of the invention alloy, in creep,
to alloy 444 can be seen to diminish somewhat with
increasing temperature~ However, in creep, the superi-
ority of the inven~ion alloy ~o ~he commercial alloy,
1422, can be seen to increase significantly with in-
creasing test temperature.
In terms of rupture life superiority of the inven-
tion alloy to the 1422 alloy is seen to increase with
increasing temperature. The invention alloy displays
- superior properties under all conditions tested. Since
~ 17 -
the trend in gas turbine engines is toward increased
efficiency through higher temperature, the improved -
elevated temperature properties of the present invention
are significant.
Example 3 -
Samples of the materials described in Example I
were tested for resistance to sulfidation and oxidation
at elevated temperatures. The sulfidation test involved
the application of Na2S0~ at the rate of lmg/cm2 every
twenty hours. The failure criteria was a weight loss
of 250 mg/cm2. The oxidation tests were perormed both -
on the unprotected alloys at 2100F under cy~clic condi- -
tions and on the alloys protected with a NiCoCrAlY type
of coating under cyclic conditions at 2150F. NiCoCrAlY
is a commercial coating material having a nominal com-
position o~ 18% Cr, 23% Co, 12~5~/o Al, .3% Y, balance
nickel. The tests on coated samples were normalized to
minimize the efect o different coatin~ thicknesses
This coating is described in U.S. Patent 3,928,026
~ie~ ~e~ ~he~er~-b~-ref~re~ee. The test
results are shown below in Table III.
- 18 -
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b~ :
o l ~
d V O U~ . ,
o ~ ~ o o c`~ o
h c~ O O ~ o n
t~ o 4 ~ ,~
~ z ~ 'Ei :~ ~
'd
C~
o~ ~ .~ , :
.~ ~ U~ ~ :.
a~ u
C~ ~ t~ t~ ::
H ~:1 t~ ~ ~
Hr-l C~ ~ ~ . ~ :
~:.1 ~ o 4 ~ 5~
rl O ~ Z
O
i~ ~ .1o : ` '
o ~ O `:
4~
~ ~g
~ri O ~ ~ ~ ~
~1
o 4~ ~ ~ ~ `
u) ~1 0 r~ S~ .
,J ~ `--h 4~ ~:
.~ :
U~ r,~
o t~
¢ ~ ~ p~ ~c
:~ ~
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The stlL~idation resistanee of the invention alloy
is clearly superior to that of the other alloys tested.
Likewiseg i~ cyclic oxidation evaluation of uncoated
samples, the invention alloy outperforms even alloy 1455,
an alloy noted for inherent oxidation res-lstance. Even
when a protective coating is employed, the invention
allo~ displays superior resistance to el ~ated tempera~
tllre cyclic oxidation.
Although he invention has been-shown and described
1~ with respect to a preferred embodiment ~hereof, it should
be understoocl by those skilled in the art that various
changes and omissions in the form ai~ detail thereo may
be made therein without departing from the spirit and
scope of tlle invention.
.
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