Note: Descriptions are shown in the official language in which they were submitted.
6~
- 1 - 13DV-8137
NICKEL-BASE SUPERALLOYS FOR PRODUCING
SINGLE CRYSTAL ARTICLES HAVING IMPROVED
TOLERANCE TO LOW ANGLE GRAIN BOUNDARIES
CROSS-REFERENCE TO RELATED APPLICATION
The invention disclosed and claimed herein is
related to the invention disclosed and claimed in
Canadian Application Serial No. 400,748, filed April 8,
1982.
BACKGROUND OF THE INVENTION
This invention pertains generally to nickel-base
superalloys castable as single crystal articles of
manufacture, which articles are especially useful as
hot-section components of aircraft gas turbine engines,
particularly rotating blades.
The efficiency of gas turbine engines depends
significantly on the operating temperature of the various
engine components with increased operating temperatures
resulting in increased efficiencies. The search for
increased efficiencies has led to the development of
heat-resistant nickel-base superalloys which can
withstand increasingly high temperatures yet maintain
their basic material properties. The requirement for
increased operating temperatures has also lead to the
development of highly complex cast hollow shapes, e.g.,
blades and vanes, which provide efficient cooling of the
material used to produce such shapes.
The casting processes used with early
generations of nickel-base superalloys, commonly
. .
~.,
^~
-- 1337624
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--2--
referred to as conventionally cast nickel-base
superalloys, generally produced parts whose
microstructures consisted of a multitude of equiaxed
single crystals (grains) of random ~nonoriented)
crystallographic orientation with grain boundaries
between the grains. Grain boundaries are regions of
highly nonoriented structure only a few atomic
diameters wide which serve to accommodate the
crystallographic orientation difference or mismatch
between adjacent grains.
A high angle grain boundary (HAB) is generally
regarded as a boundary between adjacent grains whose
crystallographic orientation differs by more than about
5-6 degrees. High angle grain boundaries are regions
Of high surface energy, i.e., on the order of several
hundreds of ergs/cm2, and of such high random misfit
that the structure cannot easily be described or
modelled. Due to their high energies and randomness,
high angle grain boundaries are highly mobile and are
preferential sites for such solid-state reactions as
diffusion, precipitation and phase transformations;
thus, high angle boundaries play an important role in
the deformation and fracture characteristics and
chemical characteristics (e.g., resistance to oxidation
and hot corrosion) of polycrystalline metals.
Also, due to the high energies and disorder of
HABs, impurity atoms are attracted preferentially
(segregated) to high angle grain boundaries to the
degree that the concentration of impurity atoms at the
grain boundary can be several orders of magnitude
greater than the concentration of the same impurity
atoms within the grains. The presence of such high
concentrations of impurity atoms at high angle grain
boundaries can further modify the mechanical and
chemical properties of metals. ~or example, in
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nickel-base superalloys, lead and bismuth are
deleterious impurities which segregate to the grain
boundaries. At high temperatures, even small amounts
(i.e., a few ppm) of such impurities in the grain
boundaries of nickel-base superalloys degrade the
mechanical properties (e.g., stress-rupture strength)
and failure generally occurs at the grain boundaries.
In contrast to high angle grain boundaries,
low angle grain boundaries, sometimes also called
subgrain boundaries, are generally regarded as
boundaries between adjacent grains whose
crystallographic orientation differs by less than about
5 degrees. It is to be understood, however, that the
classification of a boundary as high angle or low angle
may vary depending upon the person or organization
doing the classification. For the limiting case of a
low angle boundary (LAB) where the orientation
difference across the boundary may be less than 1
degree, the boundary may be described (modelled) in
terms of a regular array of edge dislocations, i.e., a
tilt boundary. While the mismatch is technically that
between any two adjacent grains, and not that of the
boundary per se, the extent of the mismatch is commonly
assigned to the boundary; hence the terminology of, for
example, a 5 degree low angle boundary, which usages
shall be used herein interchangeably.
Low angle grain boundaries are more highly
ordered and have lower surface energies than high angle
grain boundaries. Higher order and lower energy result
in boundaries with low mobility and low attraction for
impurity atoms which, in turn, results in a lesser
effect on properties, mechanical and chemical, compared
to high angle grain boundaries. Thus, while no grain
boundaries constitute a preferred condition, low angle
boundaries are to be preferred over high angle grain
boundaries.
1~37624
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Improvements in the ability of conventional
superalloys to withstand higher temperatures without
impairing other needed qualities, such as strength and
oxidation resistance, was achieved through alloy
development and the introduction of improved processing
techniques. These improvements followed fro~ findings
that the strength of such superalloys, and other
important characteristics, were dependent upon the
strengths of the grain boundaries. To enhance such
conventional superalloys, initial efforts were aimed at
strengthening the grain boundaries by the addition of
vario~s grain boundary strengthening elements such as
carbon (C), boron ~B), zirconium (Zr), and hafnium (Hf)
and by the removal of deleterious impurities such as
lead (Pb) or bismuth (Bi) which tended to segregate at
and weaken the grain boundaries.
- Efforts to further increase strength levels in
conventional nickel-base superalloys by preferentially
orienting the grain boundaries parallel to the growth
or solidification direction were subse~uently
initiated. Preferential orientation of the grains
generally results in a columnar grain structure of
long, slender (columnar) grains oriented in a single
crystallographic direction and minimizes or eliminates
grain boundaries transverse to the growth or
solidification direction. The process used, i.e.,
directional solidification (DS), had long been used for
other purposes such as the manufacture of magnets and
grain-oriented silicon steel for transformers. That
process has been described and impro~ed upon, for
instance, in U.S. Patent 3,897,815 - Smashey.
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Compared with conventionally cast superalloy
articles, directionally solidified (~S'd) articles
exhibited increased strength when the columnar grains
were aligned parallel to the principal stress axis due
to the elimination or minimization of grain boundaries
transverse to the direction of solidification. In
addition, DS provided an increase in other properties,
such as ductility and resistance to low cycle fatigue,
due to the preferred grain orientation. ~owever,
reduced strength and ductility properties still existed
in the transverse directions due to the presence of
longitudinal columnar grain boundaries in such DS'd
articles. Additions of Hf, C, B, and Zr were utilized
to improve the transverse grain boundary strength of
such alloys as was done previously in conventional
equiaxed nickel-base superalloys. However, large
additions of these elements acted as melting point
depressants and resulted in limitations in heat
treatment which did not allow the development of
maximum strengths within such directionally solidified
superalloys.
It has been recognized for some time that
articles could be cast in various shapes as a perfect
single crystal, thus eliminating grain boundaries
altogether. A logical step then was to modify the DS
process to enable solidification of superalloy articles
as single crystals to eliminate longitudinally
extending high angle grain boundaries previously found
in DS'd articles.
In the single crystal metallic alloy arts, it
has heretofore been conventional teaching that elements
such as boron, zirconium, and carbon are to be avoided,
i.e., kept to the lowest levels possible with
commercial melting and alloying practice and
35 technology. For example, U.S. Patent 3,494,709 recites
- 1337~2~
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the deleterious effect of B and ~r, proposing limits of
O.U01~ and 0.01~ for those elements, respectively.
U.S. Patent 3,567,526 teaches that the fatigue
properties of single crystal superalloy articles can be
improved by the complete removal of carbon.
In U.S. Patent 4,116,723, there is disclosed
homogeneous single crystal nickel-base superalloy
articles having no intentional additions of cobalt
(Co), B, Zr or C which are said to have superior
mechanical properties, e.g., creep and time to rupture,
compared to similar nickel-base superalloys containing
Co, C, B, and Zr. Therein it is taught that cobalt
should be restricted to less than about 0.5~, and more
preferably to less than about 0.2~, to preclude the
formation of deleterious topologically close packed
phases (TCP) (e.g., ~ and ~). Further, it is taught
therein that no single element of the group carbon,
boron, and zirconium should be present in an amount
greater than 50 ppm, that preferably the total of such
impurities be less than 100 ppm and, most preferably,
that carbon be kept below 30 ppm and that ~ and Zr each
be kept below 20 ppm. In any event, it is taught that
carbon must be kept below that amount of carbon which
will form MC type carbides. Subse~uently, in U.S.
Patent 4,209,348 it was shown that 3-7~ Co could be
included in the single crystal nickel-base superalloys
disclosed therein without forming TCP.
Another purpose in limiting C, B, and Zr is to
increase the incipient melting temperature in relation
to the gamma prime solvus temperature thus permitting
solutionizing heat treatments to be performed at
temperatures where complete solutionizing of the gamma
prime phase is possible in reasonable times without
causing localized melting of solute-rich regions.
~ecently, however, it has been recognized, U.S. Patent
4,402,772, that the addition of hafnium in small
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amounts to certain of nickel-base superalloys for the
casting of single crystal articles is effective, for
example, in providing enhanced properties and enhanced
heat treatability in that such articles have a greater
s temperature range between the gamma prime solvus and
incipient melting temperatures than do most prior art
single crystal articles.
SU~ARY OF THE I~VENTION
There is provided by the present invention
nickel-base superalloys for producing single crystal
articles having improved tolerance to low angle grain
boundaries. The improved tolerance to low angle grain
boundaries arises from the discovery that nickel-base
superalloys suitable for casting as single crystal
articles can, contrary to the teachings of the prior
art, be improved by the addition of small, but
controlled, amounts of boron and carbon, and optionally
hafnium, and is manifested principally by improved
grain boundary strength. Additionally, the superalloys
of this invention also possess an improved balance
between cyclic oxidation and hot corrosion resistance
tue primarily to the carbon and hafnium and an
increased Al to Ti ratio.
As one result of this increased grain boundary
strength, grain boundary mismatches far greater than
the 6 limit for prior art single crystal superalloys
can be tolerated in single crystal articles made from
the nickel-base superalloys of this invention. This
translates, for example, into lower inspection costs
and higher yields as grain boundaries over a broader
range can be accepted by the usual inspection
techniques without resorting to expensive X-ray
techniques. The superalloys of this invention are
especially useful when directionally solidified as
hot-section components of aircraft gas turbine engines,
particularly rotating blades.
133~624
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Broadly, the single-crystal superalloys of
this invention consist essentially of about, by weight,
7-12~ Cr, 5-1~ Co, 0.5-5~ Mo, 3-12~ W, 2-6$ Ta, 2-5
Ti, 3-5~ Al, 0-2~ Cb, 0-2.0~ ~f, 0.03-0.25% C and
0.002-0.050% B, the balance being nickel and incidental
impurities.
D~lAILED DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a perspective schematic view of a
blade member for use in a gas turbine engine;
FIGURE 2 is a perspective schematic view of a
directionally solidified slab-like single crystal ingot
marked for removal of blanks to be processed into
mechanical property test specimens;
FIGURE 3 is a graph of comparative
stress-rupture life versus alloy boron content;
FIGURE 4 is a graph of comparative
stress-rupture life versus grain boundary misfit; and
FIGURE 5 is a graph of external metal loss in
cyclic oxidation as a function of exposure time.
DETAILEV DESCRIPTION OF THE INVENTION
Nickel-base superalloys castable as single
crystals have typically been used to manufacture
airfoil members, e.g., rotating blades and stationary
vanes, for the hot section of aircraft gas turbine
engines. ~uch a blade member 10 is shown schematically
in FI~. 1 and includes base (or root) portion 12 (shown
machined to a "fir-tree" configuration for attachment
to a disk), platform portion 14, and aerodynamically
curved airfoil portion 16. Blade member 10 may also be
provided with an internal passage or passages through
which a fluid (generally air) is circulated during
operation of the turbine for purposes of cooling the
blade. Frequently, the fluid is forced out of holes
situated at the leading and trailing edges of the
airfoil to effect skin cooling by laminar flow of the
- 1337624
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g
fluid over the surface of the airfoil portion 16.
Details of such cooling provisions are known in the art
and are not shown here since they are unnecessary to an
understanding of this invention. The art of
directionally casting such blades is also known in the
art as shown, for example, by U.S. Patent 3,494,709
and, therefore, also shall not be described here in
detail.
Following directional solidification, which
typically progresses downwardly toward base 12, in the
direction indicated by arrow 18, the solidified blade
member 10 is inspected for the presence of grain
boundaries and verification of the axial growth
direction 18. The axial growth direction is determined
by X-ray analysis (typically by the well-known Laue
method) and for nickel-base superalloys is preferably
plus or minus 15 degrees of the ~001] crystal
direction.
Heretofore, only low angle grain boundaries,
~o such as the one shown schematically at 20, up to a
maximum of about 6 mismatch across adjacent grains
have been permitted in single crystal blades 10.
Skilled observers can generally visually detect LABs on
the order of 0-3. Towards the maximum permissible
mismatch of 6, however, visual techniques become
unreliable and additional Laue patterns on either side
of the boundary in question must be made. The Laue
patterns are not inexpensive and due to current single
crystal practice 3 to 4 Laue patterns generally are
required per casting. Presently, due in part to
uncertainties in detecting low angle grain boundaries,
the yield of castings is only about 45-55%.
It has now been discovered that nickel-base
superalloys suitable for casting as single crystal
3~ articles can be improved by the addition of small, but
1337624
13DV-8137
-10 -
controlled, amounts of boron and carbon, and optionally
hafnium, yielding a new family of single crystal
nickel-base superalloys.
The principal benefit, in addition to an
improved balance between cyclic oxidation and hot
corrosion resistance, following from this discovery is
that low angle grain boundaries in single crystal
articles made from the superalloys of the invention
herein are stronger than their prior art single crystal
articles. Therefore, LABs having greater than 6 of
mismatch may be tolerated and accepted in such articles
compared to about 6 maximum previously considered
acceptable. ~educed inspection costs and increased
yield of acceptable articles follows from the aforesaid
improved tolerance to low angle grain boundaries. It
will be appreciated that neither LABs nor ~ABs will be
present in a true "single crystal." It will further be
appreciated, however, that although there may be one or
more low angle boundaries present in the single
crystals discussed herein reference shall still be made
to single crystals.
As noted above, single crystal articles such
as blade 10 are subjected to an X-ray test to determine
orientation and to a visual test to determine the
presence (or absence) of high angle grain boundaries.
While the X-ray test must still be used with the new
superalloys of this invention to determine orientation,
the number of X-ray tests required to distinguish
between ~ABs and LABs is expected to be greatly reduced
or eliminated.
Stated another way, the tolerance limits for
accepting LABs visually can be increased from about
0-3 to about 0-9 for the airfoil articles made from
the new superalloys o~ this invention and Laue
determinations are only expected to be required for
- 133762A
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-11 -
boundaries greater than about 9. It should be noted
that large boundary mismatches are acceptable in the
new superalloys when compared to the approximately 6
mismatches allowed in the prior art alloys. In the
root and platform areas, there will be no limitation on
the boundaries, i.e., ~ABs will be acceptable, due to
the increased strength of the boundaries in articles
made from the superalloys of this invention and in
recognition of the lower temperatures in the platform
and root portions compared to those in the airfoil
portion. Thus, reference to a "single crystal article"
herein shall be to an article at least a portion of
which shall be in the nature of a "single crystal."
Overall, the estimated casting yield of articles made
from the new superalloys is expected to increase to
75-85~.
It will be appreciated, therefore, that the
new superalloys of this invention possess exceptional
properties even when processing by DS techniques
results in articles having oriented high angle grain
boundaries throughout. Exceptional properties are
anticipated even when the superalloys of this invention
are conventionally cast (CC) to produce articles having
a plurality of randomly oriented grains with high angle
grain boundaries therebetween.
Accordingly, there is provided by this
invention a new family of nickel-base superalloys
castable as single crystal articles having improved
tolerance to low angle grain boundaries consisting
essentially of chromium, cobalt, molybdenum, tungsten,
tantalum, titanium, aluminum, columbium, hafnium,
carbon, boron and (optionally) hafnium in the
percentages (by weight) set forth in Table I, below,
the balance being nickel and incidental impurities.
- 1337624
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TABLE I
ALLOY CO~lPOSITIO~S
(weight ~)
Elements Base Preferred Most Preferred
Cr 7-12 7-10 9.5-10.0
~o S-15 5-10 7.0-8.0
Mo 0.5-5 1-3 1.3-1.7
W 3-12 4-8 5.75-6.25
Ta 2-6 3-5 4.6-5.0
Ti 2-5 3-4 3.4-3.6
Al 3-5 4-4.5 4.1-4.3
Cb 0-2 0-1 0.4-0.6
Hf 0-2.0 0.05-0.5 0.1-0.2
C 0.03-0.25 0.03-0.1 0.05-0.07
B 0.002-0.050 0.002-0.020 0.003-0.005
In Table II there is set forth the
co~positions of the various alloys, including those of
the present invention, referred to herein.
- 13~7624
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-13-
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1337624
13DV-8137
-14-
Shown schematically in FIG. 2 is the top
portion of a slab-like ingot 30 directionally
solidified in the direction of arrow 18' to produce
material for testing. The material produced was either
S a single crystal which had no LA~s or, as depicted in
FIG. 2, had at least one LAB 20' parallel to
solidification direction 18', or was conventionally
DS'd to produce ingots having a plurality of HABs
oriented parallel to solidification direction 18' (not
illustrated). The ingots produced so as to have a
plurality of oriented HABs were likewise produced by
the same DS process but without the use of the
techniques required to produce single crystals and will
be referred to herein simply as DS or DS'd material.
For comparative purposes, some of the alloys of Table I
were also cast conventionally to produce ingots having
a plurality of randomly oriented grains with high angle
grain boundaries in between.
The heat treatment method used with the
superalloys of the present invention to substantially
fully develop a duplex gamma prime structure was to
slowly heat the as DS'd ingot (or article) to about
2310 F and hold thereat for about 2 hours to place the
gamma prime phase into solid solution; cool at a rate
of 100 F to 150 F per minute to below about 1975 F
then at a rate of about 75 F to 150 F per minute to
about 1200 P; reheat to about 1975 F for about four
hours; cool at a rate of about 75 F to lS0 F per
minute to about 1200 F; heat to about 1650 F for
about 16 hours; and, lastly, cool to ambient
temperature.
The aforementioned specimens for physical
property measurements were fabricated in conventional
fashion from bar-like sections 32 taken transverse to
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-15-
solidification direction 18' of the heat treated
ingots. Each single crystal specimen from section 32
contained either no JABs or an LAB of ~known orientation
established by X-ray analysis. Similarly, specimens
from DS'd slabs contained a plurality of oriented
grains and oriented high angle grain boundaries and
specimens from CC slabs contained a plurality of
randomly oriented grains and randomly oriented high
angle grain boundaries.
By reference to FIG. 3 and Table III, it may
be seen that boron has been discovered, contrary to the
teachings of the prior art, to be beneficial to the
stress-rupture strength of single crystals and, with
carbon, strengthens any LABs present in single crystals
made from the alloys of this invention. In FIGS. 3 and
4 and Tables III and IV, reference is made to "~ of
Perfect Crystal Life" which is the stress-rupture life
of an alloy of the Base composition (Table II) DS'd to
form no LABs and tested with its ~110] direction
perpendicular to the ~S direction (and parallel to the
specimen stress axis) at the same conditions of stress
and temperature as the superalloy for which it serves
as the comparative standard. Also in some Tables,
there is set forth for comparative purposes the
2~ stress-rupture lives of specimens of the Base
composition having a LAB with the degree of mismatch
shown and for specimens of the Base composition in the
DS'd condition.
- 133762~
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TABLE III-A
TRANSV~RSEl STRESS-RUPTURE P~OPERTIES
SIRESS-~UPTU~E PROPERTIES
~ OF
NO. HEAT B Hf LAB TEMP ST~ESS LIFE ELONG A
_ (ppm) (~) (de~) () (ksi) (hrs) (~
1 47 - 0.1512.6 1600 58 24.6 0.4 0.0
2 47 - 0.1511.9 1600 58 10.3 0.6 1.2
3 48 20 0.159.2 1600 58 146.0 0.6 0
4 48 2~ 0.1512.2 1600 58 77.7 1.3 0
50 30 0.2012.03 1600 ~55 175.1 2.4 1.8
6 49 43 0.15 14.0 1500 75 185.02 2.1 2.5
7 49 43 0.15 14.0 1600 58 304.04 3.8 2.5
8 49 43 0.15 ~31 1600 5848.8 1.3 0.6
9 49 43 0.15 ~31 1600 5846.3 1.8 0.6
49 43 0.15 15 1600 58109.8 0.9 1.2
11 59 75 0.20 13.6 1600 58347.9 1.9 1.8
12 90 46 0.15 11 1600 58380.1 3.9 24.9
13 90 46 0.15 14 1600 58171.4 1.8 2.5
14 90 46 0.15 16 1600 58168.0 2.5 3.7
49 40 0.15 14.0 1700 4592.2 2.5 0.7
16 49 43 0.15 14 1800 30108.7 1.9 1.3
17 49 43 0.15 15 1800 24124.7 2.5 0.6
18 49 43 0.15 15 1800 3033.3 0.9 0.0
19 50 30 0.20 123 1800 28234.05 NA NA
90 46 0.15 11 1800 30118.8 2.6 0.6
21 90 46 0.15 14 1800 24296.1 1.8 0
22 90 46 0.15 14 1800 3051.0 1.6 2.5
23 9~ 46 0.15 16 1800 3073.1 3.3 0.8
1 Transverse across LABs (or HABs~ and transverse
- to solidification direction.
2 No failure in time shown - was step loaded to
104.8 ksi/3 hrs then step loaded to 134.7 ksi/
failure in 1 min.
3 In radius section of specimen
4 No failure in time shown - s~ep loaded to 78 ksi/
failure in 4.7 addn'l hrs.
5 No failure in time shown - step loaded to
50 ksi/failure.
1337624
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133762~
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13DV-8137
-18-
That the superalloys of the invention have
superior stress-rupture strengths compared to
conventional single crystal superalloys at any given
angle of mismatch from O to about 18 is shown in FIG.
4. Similarly, at any given level of ~ of no LAB
rupture life the superalloys of the invention can
tolerate larger degrees of misfit, on the order of
about 2 times, than can single crystal superalloys of
the prior art. As may be noted from Table IV, even
when ~S'd to form HABs, the superalloys of the
inrention have superior str r" " "
/
133762~
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-19 -
~ABLE IV
ST~ESS-RUPTURE STRENGTHS1
(DS'd High Angle Boundary Specimens)
COMPARATIVE
STKESS-
RUPTURE
LIV~S (H~S)
ST~ESS-RUPTUXE P~UPERTIES ~A~E
R UF NO DS CC
H~AT B TEMP STRESS LIFE ELONG A LAB ~ASE R80
(ppm) () (ksi) (hrs) (%) (~) 110
47 0 1400 90 4.0 0.9 0.0 220 NA 100
1600 55 1.9 1.0 ~.0 230 ~3 45
18U0 26 2.3 2.1 2.7 250 cl 65
2000 12 3.1 1.0 0.0 250 c4 10
48 20 1400 90 3.3 0.8 0.0 220 NA 100
1600 5515.6 ~.6 0.8 230 c3 4S
1800 26 9.2 1.1 0.0 250 ~1 65
2000 12 4.5 0.0 0.0 250 c4 10
1400 90 184.42 1.9 3.8 220 NA 100
1600 55 69.2 1.5 0.0 230 c3 45
1800 26 65.6 1.0 0.0 250 ~1 65
2000 12 9.1 1.6 1.3 25~ c4 10
49 43 1400 90 92.53 3.7 6.2 220 NA 100
1600 55 133.8 1.3 2.5 230 c3 45
1800 2650.0 1.2 0.0 250 cl 65
2000 12 2.9 1.9 2.0 250 c4 10
2000 12 1.8 NA 0.0 250 c4 10
59 75 1400 90 92.44 10.8 32.0 220 N~ 100
1600 55 54.1 0.9 0.0 230 c3 45
1800 26 98.1 1.7 0.6 250 ~1 6S
2000 12 4.1 NA 0.6 250 c4 10
AA - 1600 50 0.3
1 All transverse to DS direction except for CC
35 2Step Loaded 3Step Loaded 4Step Loaded
to 100 KSI to 110 KSI + 110 KSI + 21.8 hrs
+ 2.2 hrs 21.9 hrs 120 KSI +2.2 hrs
to 110 KSI 120 KSI + 2.1 hrs 130 KSI +.1 hrs
+ .8 hrs 130 KSI + .2 hrs 140 KSI +.2 hrs
to 120 KSI 140 KSI + 1.3
+ .2 hrs 150 KSI + .3 hrs
133~ 624
-
13DV-8137
-20-
Table V presents the results of cyclic
oxidation tests on uncoated 1/4" x 3" long round pin
specimens conducted under the conditions shown in the
table using a natural gas flame at Mach 1 gas
velocity. The specimens were rotated for uniform
exposure and cycled out of the flame once per hour to
cool the specimens to room temperature. External metal
loss was measured on a section cut transverse to the
length dimension of the specimen. Metal loss per side
was found by dividing the difference between the pin
diameter before and after test by two. The data in the
table are the average of two such measurements at 90
to each other across the diameter of the specimen.
The data of Table V are presented in graphical
form in FIG. 5. It may be noted that while the
resistance of the superalloys of the invention to
cyclic oxidation is not as good as exemplary alloy BB,
the superalloys of the invention possess highly
acceptable resistance to cyclic oxidation which is an
improvement over the cyclic oxidation resistance of the
Base alloy and R125. The improved cyclic oxidation
resistance of the superalloys of this invention
compared to that of the Base superalloy is believed to
be due primarily to the increased Al to Ti ratio.
Comparison of the data for heats 44 and 49/50 shows the
further increased cyclic oxidation resistance provided
by the addition of hafnium.
133~624
13DV-8137
-21-
O U~ ~ O N ¢~
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I ~ ~ 1~ 1`~ ~ 0 u~ 0 Ir~ 0
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Z~_o
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- ~ ~ O ~
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O ~ O O C`~
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N
- 1337624 13DV-8137
-22-
Table VI presents the results of hot corrosion
tests on uncoated 1/8" x 2" long round pin specimens
conducted under the conditions shown in the table using
a JP-5 fuel-fired flame with salt in parts per million
(ppm) shown added to the combustion products. The
specimens were rotated for uniform exposure and were
cycled out of the flame to room temperature once every
day. The data of Table VI show that the presence of
carbon in the superalloys of the invention is required
for hot corrosion resistance and that the hot corrosion
resistance of the superalloys of the invention is
superior to that of alloys AA and BB - prior art single
crystal alloys.
The superalloys of the invention thus have an
improved balance between cyclic oxidation and hot
corrosion resistance due primarily to the carbon and
hafnium and an increased Al to Ti ratio in comparison
to the Base alloy.
TABLE ~I
HOT COXROSION TESTS
TEMP SALT TIME METAL L~SS
- HEAT ~) (ppm) (Hrs) (mils/side)
44 1600 1 613 1.7
Base 1600 1 613 1.0
25 18 1600 2 402 36.0
44 1600 2 620 1.0
Base 1600 2 620 1.5
AA 1600 2 470 11.8
BB 1600 2 620 28.0
30 44 1700 5 478 6.6
Base 1700 5 478 11.3
AA 1700 5 478 30.1
- 1337624
13DV-8137
-23-
There being extant evidence that the inventive
concepts herein of adding small, but controlled,
amounts of boron and carbon, and optionally hafnium, to
improve the low angle grain boundary tolerance of
nickel-base superalloys suitable for casting as single
crystal articles are applicable to other nickel-base
single crystal superalloys, it will be understood that
various changes and modifications not specifically
referred to herein may be made in the invention herein
described, and to its uses herein described, without
departing from the spirit of the invention particularly
as defined in the following claims.
