Note: Descriptions are shown in the official language in which they were submitted.
CA 02586974 2007-05-08
Nickel-base superalloy
Field of the invention
The invention deals with the field of materials
science. It relates to a nickel-base superalloy, in
particular for the production of single-crystal
components (SX alloy) or components with a
directionally solidified microstructure (DS alloy),
such as for example .blades or vanes for gas turbines.
However, the alloy according to the invention can also
be used for conventionally cast components.
Background of the invention
Nickel-base superalloys of this type are known. Single-
crystal components produced from these alloys have a
very good material strength at high temperatures. This
makes it possible, for example, to increase the inlet
temperature of gas turbines, which boosts the
efficiency of the gas turbine.
Nickel-base superalloys for single-crystal components,
as are known from US 4,643,782, EP 0 208 645 and
US 5,270,123, for this
purpose contain alloying
elements which strengthen the solid solution, for
example Re, W, Mo, Co, Cr, as well as elements which
form y' phases, for example Al, Ta, and Ti. The level
of high-melting alloying elements (W, Mo, Re) in the
base matrix (austenitic y phase) increases continuously
with the increase in the temperature to which the alloy
is exposed. For example, standard nickel-base
superalloys for single crystals contain 6-8% of W, up
to 6% of Re and up to 2% of Mo (details in % by
weight). The alloys disclosed in the abovementioned
documents have a high creep strength, good LCF (low
cycle fatigue) and HCF (high cycle fatigue) properties
and a high resistance to oxidation.
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These known alloys were developed for aircraft turbines
and were therefore optimized for short-term and medium-
term use, i.e. the load duration is designed for up to
20 000 hours. By contrast, industrial gas turbine
components have to be designed for a load time of up to
75 000 hours.
By way of example, the alloy CMSX-4 from US 4,643,782,
when tested for use in a gas turbine at a temperature
of over 10000C, has a considerably coarsened y' phase
after a load time of 300 hours, and this phenomenon is
disadvantageously associated with an increase in the
creep rate of the alloy.
It is therefore necessary to improve the resistance of
the known alloys to oxidation at very high
temperatures.
A further problem of the known nickel-base superalloys,
for example the alloys which are known from
US 5,435,861, is that in the case of large components,
e.g. gas turbine blades or vanes with a length of more
than 80 mm, the casting properties leave something to
be desired. The casting of a perfect, relatively large
directionally solidified single-crystal component from
a nickel-base superalloy is extremely difficult, since
most of these components have defects, e.g. small-angle
grain boundaries, freckles, i.e. defects caused by a
series of identically directed grains with a high
eutectic content, equiaxed limits of variation,
microporosity, etc. These defects weaken the components
at high temperatures, and consequently the desired
service life or operating temperature of the turbine
are not achieved. However, since a perfectly cast
single-crystal component is extremely expensive, the
industry tends to permit as many defects as possible
without the service life or operating temperature being
adversely affected.
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One of the most common defects is grain boundaries,
which are particularly harmful to the high-temperature
properties of the single-crystal items. Whereas in
small components small-angle grain boundaries in
relative terms have only a minor influence on the
properties, they are highly relevant to the casting
properties and oxidation properties of large SX or DS
components at high temperatures.
Grain boundaries are regions with a high local disorder
of the crystal lattice, since the neighboring grains
collide in these regions, and therefore there is a
certain misorientation between the crystal lattices.
The greater the misorientation, the greater the
disorder, i.e. the greater the number of dislocations
in the grain boundaries which are required for the two
grains to fit together. This disorder is directly
related to the properties of the material at high
temperatures. It weakens the material if the
temperature rises to above the equicohesive temperature
(= 0.5 x melting point in K).
This effect is known from GB 2 234 521 A. For example,
in a conventional nickel-base single-crystal alloy, at
a test temperature of 871 C, the fracture strength
drops greatly if the misorientation of the grains is
greater than 6 . This has also been confirmed in
single-crystal components with a directionally
solidified microstructure, and consequently the
viewpoint has generally been that misorientations of
greater than 6 are unacceptable.
It is also known from the above-referenced
GB 2 234 521 A that enriching nickel-base superalloys
with boron or carbon during a directional
solidification produces microstructures which have an
equiaxed or prismatic grain structure. Carbon and boron
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strengthen the grain boundaries, since C and B cause
the precipitation of carbides and borides at the grain
boundaries, and these compounds are stable at high
temperatures. Moreover, the presence of these elements
in and along the grain boundaries reduces the diffusion
process, which is a primary cause of the grain boundary
weakness. It is therefore possible to increase the
misorientations to 10 to 12 yet still achieve good
materials properties at high temperatures. However,
these small-angle grain boundaries have an adverse
effect on the properties in particular of large single-
crystal components formed from nickel-base superalloys.
Document EP 1 359 231 Al describes a nickel-base
superalloy which has improved casting properties and a
higher resistance to oxidation than known nickel-base
superalloys. Moreover, this alloy is, for example,
particularly suitable for large gas turbine single-
crystal components with a length of > 80 mm. It has the
following chemical composition (details in 95 by
weight):
7.7-8.3 Cr
5.0-5.25 Co
2.0-2.1 Mo
7.8-8.3 W
5.8-6.1 Ta
4.9-5.1 Al
1.3-1.4 Ti
0.11-0.15 Si
0.11-0.15 Hf
200-750 ppm C
50-400 ppm B
remainder nickel and production-related impurities.
However, its compatibility with TBC (thermal barrier
coating) layers, which are used in particular in the
gas turbine sector to protect components exposed to
particularly high thermal stresses, still needs
improvement.
CA 02586974 2012-07-23
Summary of the invention
The aim of the invention is to avoid the abovementioned
5 drawbacks of the prior art. The invention is based on
the object of further improving the nickel-base
superalloy which is known from EP 1 359 231 Al, in
particular with a view to achieving better
compatibility with TBC layers to be applied to this
superalloy combined with equally good casting
properties and a high resistance to oxidation compared
to the nickel-base superalloy which is known from
EP 1 359 231 Al.
According to the invention, this object is achieved by
the fact that the nickel-base superalloy
consists of the following chemical composition
in % by weight:
7.7-8.3 Cr,
5.0-5.25 Co,
2.0-2.1 Mo,
7.8-8.3 W,
5.8-6.1 Ta,
4.9-5.1 Al,
1.3-1.4
0.11-0.15 Si,
0.11-0.15 Hf,
200-750 ppm C,
50-400 ppm B,
< 5 ppm S,
5-100 ppm Y and 5-100 ppm La,
remainder nickel and production-related impurities.
The advantages of the invention are that the alloy has
very good casting properties, a high resistance to
oxidation at high temperatures and is very compatible
with TBC layers that are to be applied.
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It is expedient if the alloy consists of the following
composition in % by weight:
7.7-8.3 Cr,
5.0-5.25 Co,
2.0-2.1 Mo,
7.8-8.3 W,
5.8-6.1 Ta,
4.9-5.1 Al,
1.3-1.4 Ti,
0.11-0.15 Si,
0.11-0.15 Hf,
200-300 ppm C,
50-100 ppm B,
max 2 ppm S,
10-80 ppm Y and 10-80 ppm La,
remainder nickel and production-related impurities.
An advantageous alloy according to the invention consists of
the following chemical composition in % by weight:
7.7 Cr,
5.1 Co,
2.0 Mo,
7.8 W,
5.8 Ta,
5.0 Al,
1.4 Ti,
0.12 Si,
0.12 Hf,
200 ppm C,
= 50 ppm B,
1 ppm S,
50 ppm Y, and
ppm La,
remainder nickel and production-related impurities.
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This alloy is eminently suitable for the production of
large single-crystal components, for example blades or
vanes for gas turbines.
Ways of implementing the invention
The invention is explained in more detail below on the
basis of an exemplary embodiment.
Nickel-base superalloys which are known from the prior
art (comparison alloys CA1 to CA5) and the alloy
according to the invention Al having the chemical
composition listed in Table 1 were tested (details in %
by weight):
CA1 CA2 CA3 CA4 CA5 Al
(CMSX- (CMSX-6) (CMSX-2) (Rene N5) (in
11B) accordance
with
EP 1359231A)
Ni Remainder Remainder Remainder Remainder Remainder Remainder
Cr 12.4 9.7 7.9 7.12 7.7 7.7
Co 5.7 5.0 4.6 7.4 5.1 5.1
Mo 0.5 3.0 0.6 1.4 2.0 2.0
W 5.1 - 8.0 4.9 7.8
7.8
Ta 5.18 2.0 6.0 6.5 5.84 5.8
Al 3.59 4.81 5.58 6.07 5.0 5.0
Ti 4.18 4.71 0.99 0.03 1.4 1.4
Hf 0.04 0.05 - 0.17 0.12 0.12
C - - - - 0.02 0.02
B - - - 0.005
0.005
Si - - - - 0.12 0.12
Nb 0.1 - - - -
Re - _ _ 2.84 - -
S - - - -
0.0001
,Y - - - 0.005
La - - - - _- 0.001
Table 1: chemical composition of the alloys tested
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The alloy Al is a nickel-base superalloy for single-
crystal components, the composition of which is covered
by the patent claim of the present invention. The
alloys CA1, CA2, CA3, CA4 are comparison alloys which
are prior art known under the designations CMSX-11B,
CMSX-6, CMSX-2 and Rene N5. Inter alia, they differ
from the alloy according to the invention primarily by
virtue of the fact that they are not alloyed with C, B,
Si and Y and/or La. The comparison alloy CA5 is known
from EP 1 359 231 Al and differs from the alloy
according to the invention in terms of the S, Y and/or
La content.
Carbon and boron strengthen the grain boundaries, in
particular also the small-angle grain boundaries which
occur in the <001> direction in SX or DS gas turbine
blades or vanes made from nickel-base superalloys,
since these elements cause the precipitation of
carbides and borides at the grain boundaries, and these
compounds are stable at high temperatures. Moreover,
the presence of these elements in and along the grain
boundaries reduces the diffusion process, which is a
primary cause of the grain boundary weakness. This
considerably improves the casting properties of long
single-crystal components, for example gas turbine
blades or vanes with a length of approx. 200 to 230 mm.
The addition of 0.11 to 0.1595 by weight of Si, in
particular in combination with Hf in approximately the
same order of magnitude, significantly improves the
resistance to oxidation at high temperatures compared
to the previously known nickel-base superalloys CA1 to
CA4.
Restricting the composition according to the invention
to a sulfur content of < 5 ppm produces very good
properties, in particular good bonding of the TBC layer
which has been applied to the surface of the
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superalloy, for example by thermal spraying. If the
sulfur content is > 5 ppm, this has an adverse effect on
the TBC bonding, and the layer quickly flakes off in the
event of fluctuating thermal stresses.
The addition of Y and/or La in the specified range (in
each case 5 to 100 ppm), i.e. in total, that is to say
Y + La, 10 to 200 ppm, if both elements are present
produces very good bonding of the ceramic thermal barrier
coating (TBC layer) which is to be applied.
The Y content of 50 ppm and the La content of 10 ppm
specified for the alloy Al is particularly advantageous,
since Al is particularly compatible with the TBC layers
to be applied. Moreover, these two elements also increase
the resistance to environmental influences. The addition
of these elements in these low ranges stabilizes the
aluminum/chromium oxide scale layer on the alloy surface
and produces a significant resistance to oxidation. Y and
La are oxygen-active elements which improve the bonding
strength of the scale layer on the base material. The
resistance to spalling during cyclic oxidation is the key
factor for the stability of the TBC layer.
Table 2 in each case lists the number of cycles which
it takes for the A1203 and other oxide layers formed to
flake off under cyclic oxidation at 10500C/1h/air
cooling to room temperature for the alloys listed in
Table 1:
Alloy Number of cycles until spelling occurs
CA1 < 30
CA2 200
CA3 80
CA4 230
CA5 1500
Al 2500
CA 02586974 2007-05-08
Table 2: number of cycles until spalling occurs
The alloy according to the invention Al, compared to
5 the alloys which are known from the prior art, has by
far the highest number of cycles before the oxide layer
flakes off. This implies a high stability of a TBC
layer which is to be applied to the surface of the
superalloy, for example by thermal spraying.
If, in other exemplary embodiments, by way of example,
nickel-base superalloys with higher C and B contents
(at most 750 ppm of C and at most 400 ppm of B) are
selected in accordance with claim 1 of the invention,
it is also possible for the components produced from
these alloys to be cast conventionally; i.e. without
them producing single crystals.
