Note: Descriptions are shown in the official language in which they were submitted.
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Descrip'ion
Minor element Additions to Single Crystals
For Improved Oxidation Resistance
Technical Field
Nickel base superalloy single -rystal articles are
provided with enhanced oxidation resistance by the ad-
dition of .05-.2~ of a material selected from a group
consisting of hafnium and silicon and mixtures thereof.
Additions of these elements improved the oxidation re-
sistance of the articles in both coated and uncoated
form.
Background Art
Nickel base superalloys are widely used in gas tur-
bine engines. Originally such alloys were used in con-
ventionally cast form consisting of many randomly orient-
ed e~uiaxea grains. Substantial property improvements
were obtained by a casting technique known as directional
solidification which was initially used to produce
columnar grain articles consisting of a multiplicity of
elongated oriented grains whose axis of elongation is
constrained to be parallel to the axis of maximum stress
A subsequent refinement permits the production of single
crystal articles and these articles represent the state-
of-the-art in superalloy technology. The present in-
vention concerns tne improvement of the oxidation re-
sistance of singie crystal superalloy articles through
the addition of small amounts of hafnium and/or silicon.
Silicon is known as a constituent of superalloys
and is shown for example in U.S. patents 2,621,122,
EH-7044
2,~94~605, 3,005,704, 3,411,8~8 and 3,524,744. Such
minor additions have, to our }cnowledge, been made only
to alloys intended for use in equiaxed form. We are
unaware that silicon has even been intentionally added
to single crystal nickel base superalloy articles.
Hafnium has also been used in nickel base super-
alloy articles although to a lesser extent. U.S. patent
3,005,705 suggests the use of .1-1.0% hafnium in a
equiaxed alloy article. The benefit attributed to haf-
nium in this patent is improved high temperature mechan-
ical properties and it does not appear that any improve-
ment in oxidation resistance was recognized. Hafnium
has also been widely used in directional solidification
columnar grained alloys where it provides improved trans-
verse grain boundary ductility. This is described forexample in U.S. patent 3,677,747. Again, in this patent
there is no discussion of enhanced oxidation resistance.
We are unaware that small hafnium additions have eve-r-
been made to single crystals for any purpose and in fact
it was previously thought that hafnium should be avoided
in single crystal articles as discussed in U.S. patent
4,116,723.
Disclosure of Invention
The present invention concerns the additions of
from .05 to .2% of a material selected from a group con-
sisting of hafnium, silicon and mixtures thereof to
nickel base superalloy articles. The addition of haf-
nium and silicon in these levels can provide from 2-4X
improvement in oxidation resistance. Improvement in
oxidation resistance are observed in both coated an
--3--
uncoated form. Other features and advantages will be
apparent from the specification and claims and from the
accompanying drawings.
Brief Description of Drawings
Figure 1 shows the coated oxidation resistance of
the single crystal article as a funetion of hafnium and
silicon additions.
Figure 2a and 2b show the effect of hafnlum and
silieon additions on gamma prime solvus temperature and
incipient melting temperature.
Figure 3a and 3b show the effect of hafnium and
silicon additions on rupture life and l ereep life.
Best Mode For Carrying Out The Invention
This invention eoneerns a method for substantially
improving the oxidation resistanee of niekel base single
crystal superalloy articles in both uncoated and eoated
forms. The invention results from the discovery that
small additions of silieon and hafnium to the substrate
alloy can significantly inerease the oxidation resis-
tanee of the article. This result is somewhat surpris-
ing and not predietable from the prior art sinee it was
not formerly appreeiated that the degree of proteetion
provided by eoatings employed on superalloy articles was
so sensitive to the substrate alloy composition.
The essenee of the present invention is the addition
of from about .05 to about .2 weight percent of silicon
or hafnium or mixtures thereof to niekel superalloy
single erystal articles. Such single erystal artieles
OI
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will in general contain from 5 to 18% chromium, at least5% of the material selected from the group consisting of
aluminum and titanium with the provision that aluminum
be present from 2 minimum of 2% to a maximum of 8% and
the titanium be present in from a minimum of 1% to a
maximum of 5%. Further, the alloy must contain at least
5% of an element selected from the group consisting of
up to 10% molybdenum, up to 15% tungsten, up to 12%
tantalum, up to 3% columbium, up to 7% rhenium and mix-
tures thereof with the balance being essentially nickel.This composition is presented in U. S. Patent 4,116,723.
This patent also suggests that up to 3.5% hafnium might
be preset. The present invention calls for the addition
of from about .05 to .2% hafnium and the range suggested
in the present invention is critical as will subsequently
be demonstrated. The present invention was evaluated
using a particular alloy denoted as Alloy 454 having a
nominal composition of 10 weight percent chromium, 5%
cobalt, 5% aluminum, 1.5% titanium, 12% tantalum, 4%
tungsten, balance nickel. This single crystal alloy
formulation is described in and claimed in U. S. Patent
4,209,348. This single crystal composition has an
outstanding combination of properties but is generally
representative of a wide range of such compositions.
Additions of .1%, .4, and .6 weight percent hafnium as
well as additions of .1 and .3 weight percent silicon
were made to this nominal alloy composition.
The resultant samples were tested under a variety
of test conditions with the following resultsO Figure
1 shows the effect of minor additions of silicon and
hafnium on coated oxidation resistance of Alloy 454. In
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this instance the coating was a coating known as an
overlay coating with a nominal composltion of 22% cobalt,
20% chromium, 12% aluminum, and .8% yttrium This is
a NiCoCrAlY overlay coating and is described in some
S detail in U.S. patent 3,928,026. Overlay coatings
typically contain 10-35% Cr, 8-25% Al, .1-1% Y balance
selected from the group consisting of nickel and cobalt.
Small additions of other elements including silicon,
hafnium and tantalum may also be present Overlay
coatings are generally applied by vacuum vapor deposition
process to produce a thin uniform adherent layer of the
overlay coating alloy on the substrate surface. The
data presented in Figure 1 is the test time to penetra-
tion of the coating divided by the coating thickness.
This type of measurement gives a relatively accurate
indication of coating performance and eliminates coating
thickness as a variable. From the figure it can be
seen that the additions of small amounts of hafnium and
silicon to the substrate composition have a marked ef-
fect on coated article life. The data in Figure 1 are
the result of testing at 2150F using a burner rig test.
In this type of test, hot gases are generated by the
combustion of jet fuel and these gases are impinged on
the sample in test. The particular cycle employed in-
25 cluded 29 minutes of exposure at 2150F followed by a
7 minute forced air cooling period. The purpose of the
forced air cooling is to simulate the thermal fluctuations
which occur during the operation of a gas turbine engine.
From Figure 1 it can be seen that the addition of about
.1% silicon improves the coated oxidation life by about
90 or 100% while the addition of an equivalent amount of
hafnium increases the coated oxidation life by about
150%. Hafnium is seen to be somewhat more effective
~21;~
than silicon in improving coated oxidation life and it
is significant tha-t with additions of both hafnium and
silicon that no significant increase in oxidation re-
sistance is provided by additions of more than about
.2 weight percent of the element. From figure l the
broad range of element additiGns can be set at from
about .05 to about .2%.
The effects of small additions of hafnium and
silicon on the uncoated oxidation re~-istance of single
crystal articles are described in Table 1. Alloy 454
is the previous single crystal material. The table
shows the effect of small additions of hafnium and sili-
con on the various parameters which are evaluated in
oxidation testing. soth weight loss and maximum depth
of penetration are substantially reduced by minor ele-
ment additions. The column labeled oxidation resistance
is determined by dividing the hours of testing into the
maximum 2epth of penetration and the column labeled
relative oxidation resistance compares the oxidation
resistance of the various alloys to unmodified Alloy 454
material. From this latter column, it can be seen that
additions of both hafnium and silicon significantly in-
crease the uncoated oxidation resistance of the material.
Hafnium appears to be more effective than silicon at
the same level and additions of hafnium in the amount
of .~ weight percent are substantially more effective
than .1% hafnium additions. As has been seen, this
latter effect is not observed in testing of coated
oxidation resistance,
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From the preceding discussion it appears that
hafnium is more effective than silicon in improving the
oxidation resistance of single crystal articles in both
coated and uncoated form. It is also obvious that
hafnium has less deleterious effects on mechanical
properties (as shown in Figure 3) and on incipient
melting temperature (as shown in Figure 2). For this
reason ha~nium is preferred over silicon.
Figures 2a and 2b illustrate the effect of hafnium
and silicon additions on the gamma prime solvus tempera-
ture and the incipient melting temperature of the sub-
strate alloy. The gamma prime solvus temperature of the
alloy is that temperature which must be exceeded if the
gamma prime strengthening phase is to be dissolved into
solid solution. The gamma prime solvus temperature must
be approached and preferably exceeded for effective heat
treatment of nickel base superalloys. The incipient
melting temperature is that temperature above which
localized melting of the superalloy will occur. For
2~ optimum heat treating results the gamma prime solvus
temperature must be exceeded while the incipient melt-
ing temperature should not be exceeded. Further, as a
consequnce of the limitations of practical heat treating
equipment, it is desirable that a temperature span of at
least 10F, and preferably a greater span, exist between
the gamma prime solvus and the incipient melting point.
In the case of single crystals it is not catastrophic if
minor incipient melting occurs, but it is preferable to
avoid incipient melting if possible. Turning now to
Figure 2a the effect of hafnium additions on the gamma
prime solvus and incipient melting temperatures of Alloy
454 are shown and it can be seen that for hafnium ad-
ditions in excess of about .25 weight percent the
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incipient temperature lies below the gamma prime solvus
leading to an undesirable heat treating situation. A
similar situation can be seen to exist in Figure 2b with
respect to silicon additionsr The information presentea
in Figures 2a and 2~ leads to the conclusion that ad-
ditions of silicon and/or hafnium in excess of about
.25~ are deleterious from the standpoint of heat treat-
ment capability.
II1 metallurgy as in most highly developed art areas,
it is unusual that a modification which improves one
property will not have deleterious effect on other prop-
erties. This is the case in the present invention.
Figures 3a and 3b show the effect of hafnium and silicon
additions on the creep properties of A11O~5~. Figure
3a shows that additions of small amounts of hafnium lead
to the steady decrease in both the rupture life and the
time to l creep for specimens tested at 1600F with an
applied load of 70 ksi. Figure 3b shows a similar ef-
fect on the creep properties of Alloy 454 as a consequence
of additions of small amount of silicon. However, it
appears that small silicon additions have more of a
deleterious effect on the creep properties than similar
amounts of hafnium. The data presented in Figures 3a
and 3b demonstrates another reason for limiting the
silicor'hafnium additions to the lowest possible level
consistent with the achievement of improved oxidation
resistance.
While it has be., suggested in the prior art that
additions of various elements including hafnium and
silicon to the overlay coating composition itself pro-
duces improvements in oxidation resistance, such prior
art suggestions have to our knowledge been limited to
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the suggestion that the elements be added to the coating
itself and further have generally sugsested that ele-
mental additions substantially in excess of those con-
templated in the present invention be made. It appears
that by making the addition o:E hafnium or silicon to
the substrate alloy that the substrate 'herebv acts as
a large reservoir of silicon and/or hafnium and that
these elements diffuse into and through the coatinS
and aEfect the oxidation process at the free surface of
the coating. Because of the relatively large amounts
of hafnium and silicon which are present in the total
substrate, the diffusion through the coating can occur
for long periods of time without significantly de-
creasing the effective amount of silicon and hafnium
which is available. It is somewhat surprising, and
unexpected, that these protective overlay coatings are
so sensitive to the presence of silicon and hafnium in
the substrate in such small amounts.
The overlay coatings derive their effectiveness
from the development of a thin adherent scale of
aluminum oxide on the free surface. It appears that the
improvement in coated oxidation life results from some
modification of this aluminum oxide layer by the sili-
con and hafnium in the substrate. The other significant
type of protective coating employed on superalloys is
that referred to as aluminide coatings. Such coatings
are produced by the diffusion of aluminum into the
superalloy surface to produce a mcdified surface layer
relatively rich in aluminum This aluminum rich surface
layer develops an oxide on its free surface which pro-
tects the coated part in a manner similar to that Of the
protection derived by the overlay coatings. In view of
this it is fully expected that additions of silicon
%~
and hafniu~l to the substrate will produce a similar
improvement in coated oxidation resistance of parts
of superalloy single crystals which have been given
a protective aluminide coating.
Finally, although the invention has been extensive-
ly investigated with regard to an alloy known as Alloy
454, this alloy is representative of many other single
crystal alloys and it is anticipated that similar
results will be obtained on other al]oys. Another
alloy containing esser amounts of tantalum and greater
amounts of tungsten was also tested with and without
the addition of l hafnium and an improvement of
oxidation life of about 70~ was obtained. This tends
to confirm the belief that the effect of silicon and
hafnium will be generally observed in nickel base
superalloys of the type previously described.
It should be understood that the invention is not
limited to the particular embodiments shown and de-
scribed herein, but that various changes and modifica-
tions may be made without departing from the spirit and
scope of this novel concept as defined by the following
claims.
