Piearcey SINGLE CRYSTAL METALLIC PART US3494709A
US3494709A_19700210
Title: SINGLE
CRYSTAL METALLIC PART
3,494,709
Patented Feb. 10, 1970
I 3,494,709
Barry J. Piearcey, Cheshire, Conn., assigner
to United Aircraft corporation, East Hartford, Conn., a corporation of Delaware
Continuation-in-part of application Ser. No. 459,391, May 27, 1965. Ibis
application Feb. 17, 1966, Ser. No. 540,114 Int. Cl. F01d 5128; C22c 5100 U.S.
Cl. 416-232 7 Claims
ABSTRACT OF THE DISCLOSURE
A cast metallic alloy part for a gas turbine
power plant is formed as an elongated single crystal of a strong, heat
resistant and corrosion-resistant alloy having a face-centered cubic crystal
structure, said crystal being oriented with its <001> direction being
less than 20o from the elongated axis of the crystal, said single
crystal part having an air-foil portion and a laterally enlarged integral base
portion, the whole being a single integral crystal.
This application is a continuation-in-part of
my prior application Ser. No. 459,391 filed May 27, 1965 now abandoned.
The present invention relates to a novel and
improved process and mold for the formation of elongated shaped objects
comprising a single crystal oriented in a particularly desirable direction and
to an apparatus useful in carrying out the process, as well as to novel and
such improved single-crystal blades and vanes for a gas turbine engine,
especially those blades and vanes formed from certain nickel-based alloys.
The present invention has for its object the
provision of a novel and improved single-crystal blade or vane for use in a gas
turbine engine, which blades and vanes exhibit exceptional combination of physical
and mechanical properties including tensile strength, ductility, creep
resistance, low-cycle fatigue resistance, and thermal shock resistance. Still
another object is the provision of a novel and improved single crystal object
which is suitable for use as a blade or vane in a gas turbine engine, this
single crystal object being preferably formed from a nickel base alloy.
The development of nickel base superalloys
towards increases in temperature capability has resulted in a family of cast
alloys whose properties are very similar.
Increases in strength have only been obtained,
however, with a loss in ductility, particularly in the intermediate temperature
range 1400' F.). This loss in ductility has been attributed to the presence of
grain boundaries transverse to the major stress axis of a component.
Furthermore, most failure modes are observed to be associated with grain
boundaries, e.g. during creep-rupture, low cycle fatigue and thermal shock
testing.
Improvements in thermal shock resistance and ductility
were obtained by a process which resulted in single crystalline leading and
trailing edges of a turbine blade.
This improvement was made only at the expense
of creep strength, however. A significant improvement in thermal shock
resistance and ductility was obtained without loss in creep strength by a
casting process which resulted in columnar grained material with a [001]
preferred orientation. The creep strength of this material was further improved
by a heat treatment. Nevertheless, this material contains grain boundaries
which contribute to failure by several modes.
The present invention provides materials with
an exceptional combination of physical and mechanical properties including
tensile strength, ductility, creep resistance, 2 low-cycle fatigue resistance,
and thermal shock resistance, and, in particular, materials which are suitable
for use as blades or vanes in a gas turbine engine.
While, heretofore it has been recognized that
some properties of metal objects would be improved were the object in the form
of a single crystal, it has not been recognized that exceptional physical and
mechanical properties can be developed in certain structures formed from
certain nickel-base alloys where the orientation of the single crystal bears a
specific relation to the stress axis of the single-crystalline object. Also,
various techniques are known for growing single crystals, but those heretofore
proposed have had various inherent disadvantages and are not adapted to the
consistent, economical production of substantially uniform single crystal
shaped objects having a predetermined crystal axis.
It has been discovered that the tensile
strength, modulus of elasticity, minimum creep rate, rupture life and ductility
of single crystals of certain alloys are markedly dependent on orientation.
While the process and apparatus of the present
invention are of wide usefulness in the formation of single crystal objects of
relatively complex shape, having the crystal axis in a predetermined relation
to the shape of the object, the single-crystal objects of the invention, and
especially the blades and vanes suitable for use in a gas turbine engine are
most usefully formed from nickel-base super-alloys, especially those alloys
which are commercially known as SM-200, and most preferably from SM-200 which
is substantially free of both boron and zirconium and with extremely low carbon
content.
According to the present invention, the
preferred and illustrative apparatus for forming the cast single-crystal
objects having a [001] crystalline orientation substantially parallel to the
length of the cast object comprises a ceramic shell mold, usually formed by the
"lost wax method" which rests upon a thermally conductive surface,
preferably adapted to be water cooled, which shell mold is adapted to e
inductively heated by a high-frequency electro-magnetic field, so that it may
be brought to substantially the temperature of the melting point of the alloy
to be cast, or slightly higher initial temperature at the upper part of the
mold, while the water-cooled support remains substantially below the melting
point, as to facilitate solidification of the alloy to be cast.
The mold comprises an enlarged base cavity,
which is connected to one or more shaped mold cavities by inclined passageways
which are greatly restricted with respect to both the enlarged base portion and
mold cavity and a pouring cup which facilitates pouring of the molten alloy to
be cast and provides sprues which may be machined from the cast objects in the
same manner after the lower cast portions below the mold cavity have been
removed from the rough casting.
When molten metal of an alloy which
crystallizes as a face-centered cubic crystal is poured into the heated shell
mold, as they are supported on a water-cooled supporting member, the metal in
the lower enlarged central portion of the shell mold, on cooling, crystallizes
and grows more rapidly along the [001] axis, the lower portion of the
constricted portion of the shell mold becomes filled with [001] oriented
columnar crystalline alloy which tends to grow sidewise and upwardly, and
gradually induces the molten alloy filling the shaped portion of the molds to
solidify as a single [001] oriented crystal having its [001] direction
substantially coinciding with the elongated or principal stress axis of the
object being cast.
The casting operation is preferably carried
out in vacuum, or in an inert atmosphere, preferably argon, although for less
demanding uses, the casting operation may be carried out in air.
3 After casting, the cast objects may be heat
treated to improve their mechanical and physical properties, before or after
which the lower portion of the casting and the sprue may be cut away, and any
necessary machining may be done of the cast objects.
While the process of the present invention
finds its greatest usefulness in connection with nickel-base alloys of the type
generally referred to as nickel-base super alloys, it is especially useful in
connection with these nickel base alloys which are commercially designated as
SM200 or PWA659, and it is from this class of alloys which blades and vanes for
use in gas turbine engines are preferably formed.
While the mold of the present invention and
the process are of wide usefulness with many different metals, alloys and other
substances which crystallize on cooling, the blades and vanes to be used in a
gas turbine, according to the present invention are alloys having compositions
falling within the following weight percent ranges:
Percent
Chromium---------------------------------2-25
Cobalt-----------------------------------4-30
Aluminum----------------------------------0-9
Titanium----------------------------------0-6.0
Molybdenum and
tungsten-------------------2-14
Carbon------------------------------------0-0.5
Boron-------------------------------------0-0.1
Zirconium---------------------------------0-0.2
the balance of the
alloy being essentially nickel in an amount of at least 35%.
Alloys which are especially adapted for use in
the present invention and are preferred have the following elements in the
weight percentage ranges set forth below, it being understood that copper,
manganese, sulfur, and silicon are generally considered the impurities. (See PDF)
Even better results are obtained where the
quantities of boron and zirconium are further reduced, and most preferably the
boron is present in a maximum quantity of less than 0.001% by weight, while the
zirconium has a maximum of 0.01% by weight, and PWA Alloy No. 659 is especially
benefited by reduction of the boron and zirconium content to these very low
limits.
While the usual minor amounts of boron and
zirconium are highly advantageous in objects made of PWA Alloy No. 659 (SM 200)
which have a conventional heterogeneous, equi-axed crystalline structure, the
presence of boron and zirconium in a single crystalline structure is distinctly
disadvantageous in several respects, such as:
The boron and zirconium lower the melting
point of the alloy, and thus lead to a lowering of the creep-resistance of the
part made from the alloy.
The boron and zirconium content of the alloy
collects in small areas in the dendritic structure of the crystal, thus produce
weakening discontinuities in the structure.
While the cast single crystal turbine blades
and vanes of the present invention are preferably produced with the apparatus
and process of the present invention, they may be produced by thermal gradient
solidification from a properly oriented seeding crystal, and are sometimes
produced in an elongated mold which normally produces uncontrolled
polycrystalline growth or alternatively improperly oriented single crystals.
According to the present invention, the resulting cast objects are almost
always in the form of single crystals properly oriented with respect to the
stress axis of the cast object.
Of the drawings:
FIGURE I is a schematic vertical sectional
view through a mold of the present invention.
FIGURE 2 is a cross sectional view taken on
the line 2-2 of FIGURE 1.
FIGURE 3 is a similar schematic vertical
section showing a modified embodiment of a mold in accordance. with the present
invention.
FIGURE 4 is a cross-sectional view taken on
the line 4-4 of FIGURE 3.
FIGURE 5 is a schematic vertical section
showing a further modification of the mold of the present invention, especially
adapted for the casting of a plurality of elongated objects each formed of a
single crystal of metal or other material.
FIGURE 6 is a vertical section showing a
casting apparatus for use in accordance with the present invention, together
with legends showing the several materials and the states of the molded object.
FIGURE 7 is a perspective view of an
illustrative form of rotor blade for a gas turbine in accordance with the
present invention.
FIGURE 8 is a cross-sectional view showing a
modified I embodiment of a turbine blade in accordance with the present
invention.
FIGURE 9 is a perspective view of an
illustrative form of a vane member for a-as turbine in accordance with the
present invention.
FIGURE 10 is a stereo-triangle diagram
depicting the crystal orientation of various specimens on which tensile
strength test data was obtained, and is reported in the specification.
FIGURE 11 is a similar stereo-triangle diagram
depicting the crystal orientation of various specimens on which creep data was
obtained, and is reported in the specification, and FIGURES 12, 13 and 14 are
graphical representations of test data obtained on notched test specimens which
3 were subjected to stress-strain tests.
Referring now in detail to the present
preferred and illustrative embodiments of the present invention, a simple form
of mold for carrying out the process of the present invention is shown in
FIGURES I and 2, in which there is provided a shell mold 20 having an interior
shape appropriate to the object to be molded. This mold comprises a relatively
thin-walled shell which has preferably been formed by shell molding technique for
use in the lost-wax method of casting, and is to be used in a relatively
vacuum, less preferable in an inert atmosphere of argon or helium, or sometimes
in an atmosphere of air.
The mold 20 is formed to rest on a relatively
cool, heat conductive, and preferably water-cooled block 22, which is
conveniently made of a relatively thick piece of copper or copper alloy. The
block during the casting process is maintained at a temperature considerably
below the solidification temperature of the alloy or other material to be cast.
The lower portion of the mold 20 comprises a
relatively wide cavity 24-which communicates with a restricted passageway 26
connecting the base cavity 24 with the mold cavity proper 28. The passageway 26
may be of circular cross-section, as shown in FIGURE 2, or may be otherwise
shaped, but is non-linear and has a relatively small cross section compared
with the cross section of the lower portion, and is preferably upwardly
inclined to communicate with the mold cavity 28.
The mold is preferably formed of ceramic
material from a conventional slurry of alumina or other high melting point
refractory material, in accordance with standard shell-molding techniques.
FIGURES 3 and 4 illustrate an improved and
preferable form of molding apparatus in accordance with the present invention.
In this form, the shell mold 30 is formed to provide a base cavity 32 which
communicates with a laterally extending and preferably upwardly inclined,
non-linear passageway 34 which leads to a second restricted laterally extending
passage 36, which preferably extends in a different direction, and communicates
with the mold portion proper 38. Mold 30 is open at the top to receive the
molten metal from which the object is to be molded, and Tests upon a relatively
cool and preferably water-cooled copper block 22 which establishes a
temperature gradient within the molten metal filling the mold, so that
solidification of the alloy within the mold begins at the bottom of the mold.
As shown in FIGURE 4, the restricted
passageway 34 is preferably a relatively narrow slot, and the portion 36 is
similarly shaped, to assist in insuring that the solidified metal within the
mold portion proper 38 is in the form of a single crystal, the crystal axis
extending lengthwise of the mold portion 38, that is, in a substantially io
vertical direction.
FIGURE 5 illustrates a form of molding
apparatus in which a plurality of mold cavities 50 are connected with a single
base cavity 52 resting on a copper cooling block 22. Each of the cavities 50,
is connected with the base cavity by means of a restricted, laterally and
upwardly extending passageway 56. At their upper ends, mold cavities 50 are
connected with a central mouth 58 through which the molten metal is introduced
to fill the several parts of the molding apparatus.
FIGURE 6 of the drawings illustrates
schematically and in a more complete manner a molding apparatus according to
the present invention for carrying out the process of the present invention.
The entire apparatus shown in FIGURE 6 is preferably enclosed within a vacuum
chamber (not shown) or within a chamber which may be filled with argon or other
inert gas. The mold portion 60 provides an enlarged base cavity 61, above which
is the portion 62 providing an upwardly and laterally extending restricted
passageway 63 communicating with the mold cavity 65 formed by the shell 64.
Above the mold cavity 65 is the pouring mouth 66 formed by the uppermost
portion of the shell, and within which the sprue 67 forms.
Surrounding the shell mold are the means for
heating the mold to the desired temperature for casting. Preferably, the shell
mold is surrounded by a graphite susceptor 70, and this in turn is surrounded
by an induction coil 72 supplied with high frequency electric current as is
usual in a high frequency induction furnace. Prior to casting, the shell mold
is seated on the cooling block 22, the chamber is evacuated or filled with
inert gas and the coil 72 is supplied with current to heat the shell mold to the
desired temperature for casting. When the desired temperature has been
attained, the molten metal, heated to the proper temperature for casting, is
poured into the mold mouth 66 to fill the mold, the copper chill block being
maintained relatively cool so as to establish a temperature gradient within the
molten metal filling the mold as the metal solidifies. Power is shut off from
the coil 72, and the assembly is allowed to cool.
After completion of the process of the present
invention, the shell mold and cast metal are removed from the furnace, and the
shell mold is, broken away from the cast object, after which the surplus-metal
is machined away to provide the cast blade or vane member formed by the mold
cavity 65.
The metal within the base cavity 61, when the
metal is a face-centered cubic crystalline alloy, has a controlled columnar
crystalline structure, with the crystals extending upwardly within the base
portion and into the restricted passageway 63. With the restricted passageway
63, the solidified metal becomes a single crystal which fills the mold cavity
65, the [001] crystal axis extending substantially vertically along the length
of the blade or vane member. This single crystal structure extends into the
mouth 66 of the shell mold, and the sprue portion 67 generally exhibits an
uncontrolled polycrystalline growth. FIGURE 7 of the drawings illustrates a
rotor blade for use in a gas turbine, which blade is of conventional shape, but
is differentiated from the rotor blades of the prior art by being formed of a
single crystal of a face centered cubic crystalline alloy, the single crystal
having a [001] orientation with respect to the elongated axis of the blade
member. Exact coincidence between the <001> direction of crystallization
of the single crystal forming the blade member and the longitudinal axis of the
blade member is not essential and as much as a 20' deviation between the
<001> crystal direction and the longitudinal axis is acceptable, it being
understood that the closer the <001> crystal direction and the
longitudinal axis coincide, the more fully the principal objects of the present
invention are achieved.
As shown in FIGURE 7, the rotor blade member
comprises a root member 80, a shroud portion 82 and an intermediate airfoil
portion 84, all portions of which are formed as a single crystal of the
face-centered cubic crystalline alloy, having the composition of the broad
range of nickel base alloys set forth above, and most preferably having a
composition set forth with respect to the allow designated as PWA 659 (SM 200).
Airfoil gas turbine members which are to be
subjected to internal cooling during operation may be provided with an internal
passage or passages through which a cooling fluid is circulated during
operation of the turbine.
Such a blade is shown in section in FIGURE 8,
the blade otherwise being shaped in accordance with that shown in FIGURE 7. In
FIGURE 8, the airfoil section, root and shroud portions are formed with a
smooth internal passage or passages, and as shown the passage 86 is formed in
the blade. Like the blade of FIGURE 7, the blade of FIGURE 8 having a
longitudinally extending interior passage 86 is formed as a single crystal of a
face-centered cubic crystalline alloy having its < 001> orientation substantially
coinciding with the longitudinal axis of the blade member.
FIGURE 9 of the drawings illustrates a
conventional form of vane member 88 for use as an airfoil member in a gas
turbine, and which is formed as a single crystal of a face-centered cubic
crystalline alloy in which the <001> direction substantially coincides
with the principal longitudinal axis of the vane member.
The process of the present invention is
illustratively described especially with respect to the apparatus shown in
FIGURE 6 of the drawings:
A shell mold having a mold cavity 65 of the
desired shape, an enlarged base cavity 61, a laterally and upwardly directed
restricted passageway communicating between the base cavity 61 and the mold
cavity 65 and provided at its top with an enlarged, upwardly extending mouth 66
is firmly seated on a copper chill block 22 within a vacuum induction furnace.
The shell mold is preheated by current supplied to the induction coil 72,
thereby heating the susceptor element 70 and the shell mold itself.
The shell mold at its lower end is maintained
at a lower temperature by means of the copper chill block 22 which is cooled by
means of water circulating in a lower portion of the block 22.
The shell mold is preferably heated to a
temperature of about 2600 F. for the casting of PWA 659, and the temperature of
the upper face of the chill block 22 is preferably maintained at a temperature
of not more than 200' F.
The interior-of the furnace is either
evacuated to a pressure of 10-2 mm. (Hg) or less, or is purged and filled 6o
with an inert gas, preferably argon.
A suitable quantity of the alloy to be cast,
such as PWA 659, is then melted within the furnace by high frequency inductive
heating, and when the molten alloy has been heated to a temperature above its
melting point, preferably to a temperature of about 2600 F., the alloy is
poured into the mold so as to completely fill the mold.
The molten alloy immediately 'begins to
solidify at its lower portion within the base cavity 61 where the molten alloy
is in contact with the cool, chill plate 22. Initially there is formed a very
thin layer of uncontrolled poly-crystalline solidified alloy on the surface of
the chill, block. These uncontrolled crystals having a haphazard orientation
give away to the more rapid upward growth of the. [100] crystals so that in the
upper portion of the base cavity 61 the crystals are substantially all of
<001> orientation. As the crystal growth proceeds upwardly through the
cooling mass of metal in the mold, a few of the upwardly growing crystals
having an [001] orientation enter the restricted laterally and upwardly
directed passageway 63 and one crystal continues to grow laterally and then
upwardly into the mold cavity 65, and the growth in the major portion of the
restricted passageway 63 and completely in the mold cavity 64 is a single
crystal of the face-centered cubic crystalline alloy.
During the solidification of the alloy heat is
continually drawn away by the water-cooled copper chill block 22 so that a
temperature gradient is always maintained between the bottom portion and the
upper portion of the metal within the shell mold.
After the casting and solidification of the
elongated object within the shell mold has been completed and has cooled to a
moderate temperature at which the single crystal cast part is no longer subject
to deleterious action by exposure to air, the chamber may be opened to, break
the vacuum or to allow air to enter the furnace chamber, and the shell mold and
its enclosed cast part may be removed from the furnace. When the shell mold and
part have cooled, the shell mold may be broken away from the cast part, and the
cast part-is then ready for machining to accurately finish its root and shroud
portions, and for any finishing which may be required on the airfoil section,
although such machining of the airfoil section is generally not required.
Test specimens of face-centered cubic
crystalline alloy parts, blades and vanes produced in accordance with the
present invention exhibit surprisingly superior properties compared with
uncontrolled polycrystalline and directionally solidified parts of the same
alloys.
FIGURE 10 shows the orientations of four
single crystalline bars of PWA 659 and the tensile properties of the four
crystals at 70 and 1400 F. are shown in Table 1.
FIGURE 11 shows the orientation of three
additional crystals of PWA 659, which were tested after being heat treated in
accordance with the heat treatment process of U.S. patent application Ser. No.
405,410 the data being shown for 1400 F. and a constant load of 100,000 p.s.i.
Table H shows the rupture lives, percentage
elongation at 1400' F., and modulus of elasticity of 70 and 1400 F.
The specimens subjected to test and shown in
FIGURES 10 and 11 and in Tables I and 2 below had the following relation of
their crystal axes to the longitudinal axis of the test specimens: (See PDF)
The tensile data in Table 1 shows that the
crystals with orientations of [001] and [111] have superior strength to
crystals closer to the [011] orientation. Similarly, the data in Table 2 shows
that the one closest to the [001] orientation has a remarkably superior rupture
life.
Consideration of the value of ductility and
modulus of crystals with the [001] and [111] orientations indicates that the
crystal with the [111] orientation is more limited in ductility. Furthermore,
since thermal stress is dependent upon modulus, a crystal with the [001]
orientation would be expected to be superior under conditions of thermal-shock.
Although the qualitative variation in
properties quoted may be predicted by a consideration of the deformation
behavior of face-centered cubic crystals, a class to which nickel-base
superalloys belong, the variation in rupture life of single crystalline PWA 659
with orientation is outstanding and unpredictable.
Confirmation of the unique properties of [001]
oriented single crystalline PWA 659 may be obtained by further comparisons.
Table 3 shows the creep-rupture properties of random polycrystalline and [100]
columnar crystalline PWA 659 indicating that grain boundaries in PWA 659
contribute towards failure to a greater or lesser extent depending on whether
they are transverse and/or longitudinal to the stress axis.
In addition, a comparison of the creep
properties of other preferentially oriented columnar-grained nickel-base
superalloys shows that the improvement obtained by the absence of transverse
grain boundaries and the preferred orientation is not the same for each alloy.
The intrinsic strength of the alloys is more dependent on composition than
would appear by a determination of the properties of random polycrystalline
castings.
Table 4 shows the rupture lives for random
polycrystalline cast test bars of the three alloys PWA 659, PWA 658 and PWA
663. All three alloys have short lives at 1400' F. and 85,000 p.s.i. The alloy
PWA 663 appears to show slightly superior life and percent elongation to PWA
658 and PWA 659.
Table 5 shows that in the [0011 preferentially
oriented columnar crystalline condition the three alloys behave quite
differently, PWA 659 showing obvious superiority.
Since each of the latter group of materials
might be considered as an assembly of [001] oriented single crystals, it
follows therefore that a [0011 oriented single crystal of PWA 659 would
demonstrate superior properties to similar crystals made in PWA 658 and PWA
663. The highly desirable properties of the single crystal cast objects of the
present invention fall off gradually as the crystal axis of the < 001>
direction departs from the longitudinal axis of the part, but unexpectedly
superior properties are achieved where there is no more than a 20 deviation
between the <001> crystal direction and the longitudinal axis of the
part.
In addition to the improved properties of the
single crystal parts of the present invention as shown above, the parts show greatly
improved ballistic impact properties and greater low cycle fatigue resistance.
FIGURES 12, 13 and 14 of the drawings show
test data plotted as stress-strain curves of notched and unnotched test
specimens. Extension of the test specimens is plotted against stress in
thousands of pounds per square inch of the original specimen.
The results shown in FIGURE 12 were obtained
on single crystal specimens of PWA 659 in which the [001] crystal orientation
was substantially coincident with the longitudinal axis of the test specimen.
The results shown in FIGURE 13 were obtained
on single crystal specimens of PWA 659 in which the crystal orientation was
substantially in accordance with the angular measurements given for crystal 2A
in the first table.
The results shown in FIGURE 14 were obtained
on single crystal specimens of PWA 659 in which the crystal orientation was
substantially as given in the first table for crystal 3A.
It is thus seen that the single crystal
objects in accordance with the present invention, to a surprising degree, are
not notch sensitive, thereby rendering such single crystalline objects
especially suited for use as rotor blades in gas turbines.
The [001] single crystal parts of PWA 659 are
preferably heat treated to develop their optimum properties, and gas turbine
blade or vane members may also be provided with a protective coating in
accordance with 12 normalizing by cooling in a gas, preferably an inert
atmosphere, being carried out between each of the three heat treating operations.
The heat treatment may be varied, while achieving the results of the present
invention by initially heating the coated blade or vane at about 2000 F. for
about 4 hours, normalizing by cooling in air or an inert atmosphere, heating at
a temperature of from 2200 to 2300 F. for from I to 4 hours, followed by
cooling in air or an inert gas, then heating at 1550 to 1650 F. for a period of
from one to three days, again followed by air cooling or in an inert
atmosphere.
Alternatively, the heat treatment may comprise
initially heating the coated vane or blade at an increasing temperature in the
range of 1800 to 2200 or 2300 F., the temperature being increased at the rate
of about 100 F. per hour, and thereafter holding the blade or vane at a
temperature of 2200' to 2300 F., preferably 2250' F. for from I to 4 hours, all
preferably in an inert atmosphere.
Alternatively the coating may be applied after
the treatment at 2250 F. in which case the prior treatment of 2000 F. is
unnecessary.
The cooling in gas is preferably in an inert
atmosphere, such as argon, or less preferably in air.
Improvement in the properties of the single
crystal cast pieces, such as blades and vanes, when the boron and zirconium
content of the nickel-base alloys is shown by the following comparative tests.
The comparative specimens had the following
analyses: (See PDF)
Any of these specified coating treatments
provides the blade or vane member with a surface layer of a composition
selected from the group of metals comprising aluminum, magnesium, chromium,
columbium, cobalt, titanium, tantalum, tungsten, silicon, alloys thereof,
oxides thereof and mixtures of the foregoing, which has been sintered on the
surface of the blade or vane, and preferably comprises about 64% titanium, and
36% aluminum, the weight of the coating being about 30% of the weight of the
coated vane or blade; or alternatively the coating consists of a mixture of
finely divided particles of aluminum and silicon comprising about 90% aluminum
and about 10% silicon.
According to the process of the present
invention, the coated blades or vanes are subjected after coating to heat
treatment and the preferred procedure for this heat treatment comprises heating
the coated blade or vane members in vacuum or an inert gas, such as argon, or
less preferably in air at 1600 F. to 2000 F. for a period of about four hours,
followed by about one to four hours heating at 2250 F., followed by heat
treatment at 1600 F. for a period of from 32 to 64 hours; the step of In Table
7, the values given for SM 200 are based upon directionally cast specimens
according to specifications PWA 664 as fully disclosed in the application of
Francis L VerSnyder Ser. No. 361,323 filed Apr. 17, 1964 now Patent No.
3,260,505 granted July 12, 1966. All of the specimens were subjected to heat
treatment of 1 hour at 2250 F. followed by 32 hours at 1600 F. all values being
averaged from all specimens tested.
The invention in its broader aspects is not
limited to the specific articles, apparatus, steps, processes and combinations
shown and described but departures may be made there from within the scope of
the accompanying claims without departing from the principles of the invention
and without sacrificing its chief advantages.
Claims: (See PDF)