Note: Descriptions are shown in the official language in which they were submitted.
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LOW-DENSITY DIRECTIONALLY SOLIDIFIED SINGLE-CRYSTAL SUPERALLOYS
Field of the Invention
The present invention relates to nickel-base superalloys used to manufacture
gas turbine blades or vanes by means of directional solidification or in the
form of
single crystals. The present invention particularly relates to low-density
alloys which
can work under high temperature and high load conditions.
State of the Art
Nickel-base superalloys are widely used in the manufacture of components for
gas turbines. In the particular field of gas turbines for aircraft, apart from
the high
requirements from the stress and temperature point of view, it is also
important to
develop low-density alloys. A precursor of low-density alloys is the InlOO
alloy (density
7.76 gr/cm3) developed at that beginning of the 60s by The International
Nickel
Company (INCO) and covered by patent US 3,061,426. This alloy is still used
today to
manufacture equiaxed turbine blades although it is admitted that it has low
castability
and low corrosion resistance.
In100 has been used as the basis for developing many alloys. Among others,
In6212 (density 8.02 gr/cm3) covered by patent US 4,358,318 was also developed
by
INCO as a low-density material with better corrosion resistance and
castability than
those of In100 at the expense of a slight increase of density.
These two equiaxed materials, InlOO and In 6212, have been used as the basis
for developing several single-crystal alloys. InlOO was used as a reference
for
developing the RR2000 alloy, covered by patent GB 2105369A in 1983 whereas
In6212 was used as the basis for developing the CMSX-6 alloy, covered by
patent US
4,721,540.
Both single-crystal alloys were developed according to a similar strategy. In
both cases, the amount of grain boundary hardening elements such as carbon,
boron
and zirconium was eliminated to increase the melting point of the alloy. It
was thus
possible to carry out a solution heat treatment of the hardening gamma prime
phase
dissolving the microstructure obtained directly after the casting and
achieving a fine
and homogeneous distribution of precipitates in the subsequent heat
treatments.
There is therefore a need to develop alternative alloys to those used
currently.
Description of the Invention
The present invention provides a low-density superalloy (7.867 g/cm3) useful
for
manufacturing components by means of directional solidification or single-
crystal
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components with a relaxed grain structure specification.
A first aspect of the invention relates to a nickel-base superalloy comprising
the
following elements (percent by weight):
7-13% Chromium,
0-16% Cobalt,
2-5% Titanium,
4.5-7% Aluminium,
0-5% Tantalum,
0-2% Hafnium,
0-3% Tungsten
0-2% Vanadium
0-5% Molybdenum
0.06-0.12% Carbon,
0.01-0.03% Boron,
0.005-0.02% Zirconium,
Nickel and residual impurities
In a particular embodiment the present invention relates to a nickel-base
superalloy comprising: 0.07% carbon, 10% chromium, 15% cobalt, 3% molybdenum,
5.5% aluminium, 4% titanium, 1% vanadium, 1.4% hafnium, 0.015% boron and 0.01%
zirconium.
In a particular embodiment the present invention relates to a nickel-base
superalloy comprising: 0.07% carbon, 10% chromium, 5% cobalt, 3% molybdenum,
2%
tantalum, 4.8% aluminium, 4.7% titanium, 1.4% hafnium, 0.015% boron and 0.01%
zirconium.
A second aspect of the present invention relates to the use of a nickel-base
superalloy described above for obtaining a directionally solidified casting or
a casting in
single-crystal form.
A third aspect of the present invention relates to a process for obtaining a
superalloy as described above, comprising the following steps:
a) Solution heat treatment at a temperature comprised between 1190-1250 C for
1 to 5 hours
b) Intermediate heat treatment at a temperature comprised between 1000-1100 C
for 1 to 5 hours
c) Precipitation heat treatment at a temperature comprised between 850-900 C
for 1 to 16 hours
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A fourth aspect of the present invention relates to a gas turbine comprising
components manufactured with a superalloy as described above, or from alloys
obtained by means of a process comprising the following steps:
a) Solution heat treatment at a temperature comprised between 1190- 1250 C
for
1 to 5 hours
b) intermediate heat treatment at a temperature comprised between 1000-1100 C
for 1 to 5 hours
c) precipitation heat treatment at a temperature comprised between 850-900 C
for 1 to 16 hours
Brief Description of Drawings
Figure 1: Low-cycle fatigue of composition E compared to commercial
composition A.
Detailed Description of an Embodiment
The present invention provides a low-density superalloy useful for
manufacturing components by means of directional solidification or single-
crystal
components with a relaxed grain structure specification. The alloy of the
present
invention was developed taking two single-crystal alloys, RR2000 and CMSX-6,
as a
reference.
The following table shows examples of alloys according to this invention,
alloys
E to G, inclusive. Alloys A and B are commercial alloys for directional
solidification
whereas C and D are commercial alloys for manufacturing low-density single-
crystal
components. The latter alloys are only set forth as a comparison and are not
included
within the scope of this invention.
Alloy Co Cr Mo W Al Ta V Ti Re Hf C B Zr
A 9,2 8,1 0,5 9,5 5,6 3,2 0,7 1,4 0,07 0,015 0,007
B 9,3 6 0,5 8,4 5,7 3,4 0,7 3 1,4 0,07 0,015 0,005
C 15 10 3 5,5 1 4
D 5 10 3 4,8 2 4,7 0,1
E 15 10 3 5,5 1 4 1,4 0,07 0,015 0,005
F 6 12 3 2 4,5 4,7 1,4 0,07 0,015 0,005
G 5 10 3 4,8 2 4,7 1,4 0,07 0,015 0,005
Carbon, boron and zirconium were added to the base composition of RR2000
and CMSX-6 but without reaching the high levels of these elements in the
compositions
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In100 or of In6212. The C, B and Zr of the alloy of this invention were
maintained at the
same levels as other commercial allows that are usually used for manufacturing
directionally solidified components such as alloy A and B of the previous
table. The
maximum carbon content was limited to 0.12%, the maximum boron content to
0.03%
and the maximum zirconium content to 0.02%, while these limits are 0.5%, 0.1%
and
0.25% respectively in In100. Hafnium was added to the composition to favor
carbide
formation in the grain boundary.
The introduction of these elements involved a reduction in the melting
temperature of the alloy. Such that the maximum temperature at which the
supersolution heat treatment can be carried out is limited, and therefore it
is not
possible to reach the high temperatures that are used in the supersolution
treatments
of single-crystal materials. The gamma prime dissolution that was achieved
with the
supersolution treatments was thus not as effective as that achieved with the
high
temperature treatments used in single-crystals. Nevertheless, there are
commercial
alloys which can be used to manufacture components by means of directional
solidification with and without supersolution heat treatment. The absence of
supersolution heat treatment gave rise to a drop in the alloy temperature
capacity of
about 30 C.
Even with this reduction, the benefit obtained with the low density of the
alloy of
this invention makes it a suitable option for manufacturing gas turbine blades
or vanes.
The absence of supersolution heat treatment can also give rise to a loss of
the
resistance to low-cycle fatigue of the alloy with respect to the commercial
RR2000 alloy
from which it has been developed. However, as can be seen in Figure 1,
composition E
of Table 1 has fatigue properties that are greater than those of commercial
alloy A.
The introduction or grain boundary hardening elements allowed the use of this
alloy for manufacturing directionally solidified components, which is not
possible with
most single-crystal alloys. The fact of using an alloy in directional
solidification form
instead of in single-crystal form gave rise to reduction in the creep rupture
of the alloy.
Nevertheless, this decrease was considered very small and therefore the alloy
of this
invention is sufficiently attractive for a wide range of applications.
Finally, it must be mentioned that the main purpose of this alloy is to offer
a low-
density alternative to alloys that are currently used in gas turbines. The
presence of
elements such as C, B, Zr and Hf improved the tolerance of the alloy to the
presence of
grain boundaries at the expense of a small reduction in properties such as
fatigue or
creep rupture. But having been designed from low-density single-crystal
alloys, even
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with this decrease of properties, the alloy of the present invention offers a
clear
improvement with respect to the alloys that are currently used for
manufacturing
directionally solidified materials. This benefit will be even greater in the
design of
advanced gas turbines in which the rotational speed is higher and therefore
the
5 centrifugal forces are greater, and the use of a low-density material is a
clear
advantage.
Likewise, it must also be mentioned that the use of this material in gas
turbines
for aircraft involves a clear improvement with respect to current alloys
because it can
give rise to lighter components and therefore to a lower specific turbine
consumption.
