Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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is invention relates to structures for operation at
elevated temperatures, for example, temperatures in excess of
1000C and in particular composite structures which are made up
of superalloys with suitable reinforcement.
In the past, it has been known certain alloys generally
re~erred to as superalloys having nickel or cobalt base and con-
- taining alloying agents such as carbon, tungsten~ molybdenum,
cobalt, chromium, aluminum, titanium, zirconium, hafnium and boron
and having an iron base and containing alloying agents including
at least chromium, aluminum~ yttrium and carbon, may be operated
at temperatures in the neighbourhood of 750C to 1000C.
Apparatus such as gas turbines~ may desirably operate in this
temperature range and the blade structures in particular,
must continue to retain substantial strength at their maximum
operating temperatures. It would be desirable if the maximum
; operating temperatures could be raised to at least 1100C and
` even higher temperatures such as 1200C might find application.
; Unfortunately, superalloys of the type referred to
begin to lose their mechanical strength about 1000C and indeed
may be relatively useless at temperatures in the neighbourhood
of 1100C.
It has been proposed to reinforce such alloys by the
i` introduction o~ filaments in particular, tungsten and tungsten
; alloy filaments. Such filaments may contain in addition to their
tungsten base, potassium, silicon, rhenium, thoria or hafnium
- carbide and the filaments may vary in size depending upon the
particu~ar application. Alternatively~ the filaments may
consist of graphi-te, silicon carbide or molybdenumO All such
` 30 reinforcements contribute to the strength of the composite
structure because they themselves are stronger than the matrix
alloy at the elevated temperatures.
Unfortunately9 in operation, at elevated temperatures
over a long period of time, materials wi-thin the alloy migrate
in-to the rein~orcing material and in general, i~ the nickel or
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cobalt migrates into the reinforcing material, it will weaken the
filaments.
In the case of tungsten filaments immersed in a cobalt
base alloy, there is a detrimental interaction due to the migrat-
lon of cobalt into khe tungsten filaments producing intermetallic
compounds which weaken the filament. In the case o~ tungsten
fllaments immersed in a nickel base alloy there is a detrimental
interaction due to the migration of nickel into the tungsten
causing recrystallization of the tungsten alloy which reduced the
strength of the filaments rendering them unsuitable as reinforce-
ment.
It has been proposed to coat such filaments with a pro-
jfl~1 ;bi f
tective coating to ~4_}~ migration of the materials from the
alloy into the filament. One example o~ the prior art was the
~ coating of tungsten filaments with tantalum carbide for use as
reinforcements in cobalt base superalloys as disclosed in Septem-
` ber 1973 Technical Report AD756867, Ahmad, I., et al, National
` Technical Information Service, U.S.-3-Department of Commerce,
Springfield, Va. 22151. This coating, however, proved to be in-
effecti~e in nickel base alloys.
SUMMARY OF THE INVENTION
~: It is an ob~ect of this invention to overcome the prior
problems and weakening of the reinforcing material due to migration
of alloy constituents into the reinforcing material. To this end,
it has been found that hafnium carbide, hafnium nitride, titanium
nitride, zirconium carbide and zirconium nitride form barriers
^ which protect the reinforcing material and inhibit migration of
the undesirable components of the alloys into the reinforcing
~ibres.
It has been found, for example, that tungsten alloy
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filaments for use as reinforcing material in nickel base super-
alloys and cobalt base superalloys may be protected from migration
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o~ alloy constituents, particularly of nickel from the super-
alloy by coating the filament with a layer of hafnium carbide or
nitride. The composite material so formed, that is, tungsten alloy
filaments coated with hafnium carbide or ni-tride embedded in
superalloy materials, has been found to have a useful operating
temperature as high as 1176C. Titanium nitride has also been
found to provide a suitable barrier layer over tungsten alloy
~ilament immersed in cobalt superalloys up to about 1095C.
In accordance with this invention, the reinforcing
materials are coated with suitable protective barrier layers in
accordance with the desired operating temperature and the consti-
; tuents of the superalloy. A protective layer may be considered to
be suitable and inhibit migration of the matrix components into
the filament if, when exposed to the maximum operating temperature~
the composite article shows no evidence of detrimental migration
in a 200 hour test and may therefore be predicted to have a
reasonable operating life at its operating temperature. Various
methods can be used to deposit these barrier layers such as
'` sputtering, electron beam evaporation ion plating or chemical
vapour deposition.
EXAMPLE "A"
Wires of tungsten plus 2% thoria having a diameter of
02 inches were coated with a layer of hafnium carbide about 8microns thick by means of R.F. sputtering~ The coated wires were
incorporated into a nickel base superalloy Mar-M-200 by surround-
ing the wires with powdered superalloy within an evacuated metal
container and subjecting the filled container to hot isostatic
pressure. The isostatic press provided a pressure of 12 Ksi of
argon while heating -the con-tainer to 1120C. The composite struc-
ture was removed from the press and cut into samplesa Each samplewas enclosed in a silica glass evacuated tube and subjected
to prolonged heating~ Samples were subjected to 1065C, 1121C
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and 1176C for 10, 30, 100, 1,000 and 27000 hours. The samples
were then subjected to metallographic examination by optical
and stereoscan electron microscopyO At all temperatures, samples
exposed up to -100 hours showed no signs of recrystallization.
Samples exposed to 1176C for 2,000 hours showed a recrystallization
zone of` only about 15 microns thickness.
The single figure of drawings illustrates an apparatus
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for chemical vapour deposition of such barrier materials. The
tungsten filament 3 passes through a chamber 4, preferably formed
of glass, entering and exiting through mercury seals 5 and 6 which
are arranged to permit the tungsten filament to pass through the
seal but prevent gases from entering or leaving the chamber 4.
The wire 3 pas~es centrally down the chamber through a coaxial
tube 7 which is supported and sealed to the outer wall of the
chamber 4 by means of an annular member 8~ A metallic chloride
generator 9 may consist simply of a chamber for retaining metall~c
particles of a suitable material such as hafnium and having an open
end into which hydrogen chloride gas or chlorine gas may be intro-
duced. The reaction products consist of chlorides of hafnium
which flow out through a tube at the bottom o~ the chamber desig-
nated 11. A further inlet in the tube 12 at 13 permits the intro-
duction of a suitable reactive gas and hydrogen, for example
; methane and hydrogen. The two outputs mixed at the end of tube 11
will flow into the chamber 4, down the chamber 4 to its right hand
end and into the coaxial tube 7, back through the chamber 4 to
its ~ar end and out through outlet 1~ which is connected to a suit-
able trap, an arrangement for venting the gases. In traversing
the chamber 4~ the gas flows over the filament 3.
Heating and insulating members cover the reactive areas
to maintain suitable temperatures. The wire is maintained at a
higher temperature by means of current passing through the wire
between electrodes 15 and 16 which are immersed in the mercury
seals. This current is ad~usted to maintain the wire at the des-
ired temperature usually between 900 and 1300C.
To produce a nitride coating, nitrogen is substituted
for the methane at inlet 13. Titanium can be added to the process
by the direct introduction o~ titanium chloride which is commer-
cially available.
` In order that the coatJng produced may be su~iciently
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adherent, it will be necessary that wire be carefully cleaned,
for example by treatment with triclorethylene followed by exposure
to hydrogen at about 900C.
The result of -this whole process is an adherent coating
two/ten microns thick of titanium nitride, hafnium nitride~ haf-
nium carbide, zirconium nitride or zirconium carbide as desired.
me finished wire coated with the selected material may
now be assembled together with the superalloy to produce the des-
ired composite memberO me fabrication of the composite member
can be accomplished by several techniques including hot isostatic
; pressing, diffusion bonding, or hot die bonding~ By proper selec-
tion of the superalloy and the coating for the fibres, the com-
posite structure may be designed to withstand much higher temper-
atures than the matrix material. By proper selection of the
coatings, the filaments may be protected from the deleterious
effect of the matrix and may maintain strength even in the pres-
ence of prolonged exposure to high temperatures.
- EXAMPLE "B''
Tungsten 2% thoria alloy wires .020" in diameter were
coated with hafnium nitride by chemical vapour deposition to pro-
duce a layer about 5 microns thicko The coated wires were sur
rounded with powdered cobalt base superalloy Y~S~w~ within an
evacuated metal containerO The container was subjected to hot
isostatic pressing at a temperature of 1200C a~d an argon pres-
sure of 10 Ksi. The composite structure was removed from the
press and cut into samples each of which was placed in an evac-
- uated silica glass container and subjected to prolonged heating.
Samples were heatsd to temperatures of 1186C and 1095~C for
~ periods o~ 100 and 170 hours.
- 30 The samples were then subjected to metallographic exam-
ination by optical and stereoscan electron microscopyO No inter~
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action layer was observed in any of the samples. The protective
layer was therefore considered to inhibit migration of the
cobalt into the tungsten fibres~
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E~MPLE ''C''
: The experiment of Example B was repeated substituting
titanium nitride for hafnium nitride. When subjected to metal-
lographic examination, the sample subjected to 1186C for 170
hours showed only a very thin inter action layer, less than 1
micron thick~ Here again the titanium nitride layer was
considered to effectively inhibit the migration of cobalt
into the tungsten at this temperature.
While the invention has been described primarily in
association with tungsten alloys, it is noted that other fila-
: ments as indicated earlier in the specification such as graphite
and silicon carbide and molybedenum may be used which may require
coating with suitable barriers as selected for the particular
, combinations of filament material and alloy constituents and
while the superalloys described were those having nickel or
cobalt base, other alloys may be encountered, including those
having an iron base as previously identified which require
filament reinforcing.
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