U.S. patent application number 13/391454 was filed with the patent office on 2012-07-19 for nickel-based superalloy and parts made from said superalloy.
This patent application is currently assigned to AUBERT & DUVAL. Invention is credited to Alexandre Devaux, Philippe Heritier.
Application Number | 20120183432 13/391454 |
Document ID | / |
Family ID | 42370984 |
Filed Date | 2012-07-19 |
United States Patent
Application |
20120183432 |
Kind Code |
A1 |
Devaux; Alexandre ; et
al. |
July 19, 2012 |
NICKEL-BASED SUPERALLOY AND PARTS MADE FROM SAID SUPERALLOY
Abstract
A nickel superalloy has the following composition, the
concentrations of the different elements being expressed as wt-%:
Formula (I), the remainder consisting of nickel and impurities
resulting from the production of the superalloy. In addition, the
composition satisfies the following equation, wherein the
concentrations of the different elements are expressed as atomic
percent: Formula (II).
Inventors: |
Devaux; Alexandre;
(Combronde, FR) ; Heritier; Philippe;
(Clermont-Ferrand, FR) |
Assignee: |
AUBERT & DUVAL
Paris
FR
|
Family ID: |
42370984 |
Appl. No.: |
13/391454 |
Filed: |
August 20, 2010 |
PCT Filed: |
August 20, 2010 |
PCT NO: |
PCT/FR10/51748 |
371 Date: |
April 5, 2012 |
Current U.S.
Class: |
420/448 |
Current CPC
Class: |
F05C 2201/0466 20130101;
C22C 1/023 20130101; C22C 19/056 20130101; C22F 1/10 20130101 |
Class at
Publication: |
420/448 |
International
Class: |
C22C 19/05 20060101
C22C019/05 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 20, 2009 |
FR |
0955714 |
May 7, 2010 |
FR |
1053607 |
Claims
1. A nickel-based superalloy of the following composition, the
contents of the various elements being expressed as weight
percentages: 1.3%.ltoreq.Al.ltoreq.2.8%; trace
amounts.ltoreq.Co.ltoreq.11%; 14%.ltoreq.Cr.ltoreq.17%; trace
amounts.ltoreq.Fe.ltoreq.12%; 2%.ltoreq.Mo.ltoreq.5%;
0.5%.ltoreq.Nb+Ta.ltoreq.2.5%; 2.5%.ltoreq.Ti.ltoreq.4.5%;
1%.ltoreq.W.ltoreq.4%; 0.0030%.ltoreq.B.ltoreq.0.030%; trace
amounts.ltoreq.C.ltoreq.0.1%; 0.01%.ltoreq.Zr.ltoreq.0.06%; the
remainder consisting of nickel and impurities resulting from the
production, and such that the composition satisfies the following
equations wherein the contents are expressed as atomic percentages:
8.ltoreq.Al at %+Ti at %+Nb at %+Ta at %.ltoreq.11 0.7.ltoreq.(Ti
at %+Nb at %+Ta at %)/Al at %.ltoreq.1.3
2. The superalloy according to claim 1, characterized in that its
composition satisfies the following equation wherein the contents
are expressed as atomic percentages: 1.ltoreq.(Ti at %+Nb at %+Ta
at %)/Al at %.ltoreq.1.3
3. The superalloy according to claim 1, characterized in that it
contains between 3 and 12% of Fe, as weight percentages.
4. The superalloy according to claim 1, characterized in that its
composition is, expressed as weight percentages:
1.3%.ltoreq.Al.ltoreq.2.8%; 7%.ltoreq.Co.ltoreq.11%;
14%.ltoreq.Cr.ltoreq.17%; 3%.ltoreq.Fe.ltoreq.9%;
2%.ltoreq.Mo.ltoreq.5%; 0.5%.ltoreq.Nb+Ta.ltoreq.2.5%;
2.5%.ltoreq.Ti.ltoreq.4.5%; 1%.ltoreq.W.ltoreq.4%;
0.0030%.ltoreq.B.ltoreq.0.030%; trace amounts.ltoreq.C.ltoreq.0.1%;
0.01%.ltoreq.Zr.ltoreq.0.06%; and its composition satisfies the
following equations wherein the contents are expressed as atomic
percentages: 8.ltoreq.Al at %+Ti at %+Nb at %+Ta at %.ltoreq.11
0.7.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.3 theinder
consisting of nickel and of impurities resulting from the
production.
5. The superalloy according to claim 4, characterized in that
1.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.3.
6. The superalloy according to claim 4, characterized in that its
composition is, expressed as weight percentages:
1.8%.ltoreq.Al.ltoreq.2.8%; 7%.ltoreq.Co.ltoreq.10%;
14%.ltoreq.Cr.ltoreq.17%; 3.6%.ltoreq.Fe.ltoreq.7%;
2%.ltoreq.Mo.ltoreq.4%; 0.5%.ltoreq.Nb+Ta.ltoreq.2%;
2.8%.ltoreq.Ti.ltoreq.4.2%; 1.5%.ltoreq.W.ltoreq.3.5%;
0.0030%.ltoreq.B.ltoreq.0.030%; trace
amounts.ltoreq.C.ltoreq.0.07%; 0.01%.ltoreq.Zr.ltoreq.0.06%; and
its composition satisfies the following equations wherein the
contents are expressed as atomic percentages: 8.ltoreq.Al at %+Ti
at %+Nb at %+Ta at %.ltoreq.11 0.7.ltoreq.(Ti at %+Nb at %+Ta at
%)/Al at %.ltoreq.1.3 the remainder consisting of nickel and of
impurities resulting from the production.
7. The superalloy according to claim 6, characterized in that
0.7.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.15.
8. The superalloy according to claim 6, characterized in that
1.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.3.
9. The superalloy according to claim 1, characterized in that it
comprises a gamma' phase fraction comprised between 30 and 44%,
preferably between 32 and 42%, and in that the solvus of the gamma'
phase of the superalloy is less than 1,145.degree. C.
10. The superalloy according to claim 1, characterized in that the
composition of the alloy satisfies the following equation, wherein
the contents of the elements are calculated in the gamma matrix at
700.degree. C. and are expressed as an atomic percent: 0.717 Ni at
%+0.858 Fe at %+1.142 Cr at %+0.777 Co at %+1.55 Mo at %+1.655 W at
%+1.9 Al at %+2.271 Ti at %+2.117 Nb at %+2.224 Ta at
%.ltoreq.0.901.
11. The superalloy according to claim 1, characterized in that the
Cr content (expressed as an atomic percentage) is, in the gamma
matrix at 700.degree. C., greater than 24 at %.
12. The superalloy according to claim 1, characterized in that the
Mo+W content (expressed as an atomic percentage) is .ltoreq.2.8 at
% in the gamma matrix.
13. A part in a nickel superalloy, characterized in that its
composition is according to claim 1.
14. The part in a nickel superalloy according to claim 12,
characterized in that this is a component of an aeronautical or
land gas turbine.
15. The superalloy according to claim 2, characterized in that it
contains between 3 and 12% of Fe, as weight percentages.
Description
[0001] The invention relates to the field of nickel-based
superalloys, notably intended for making parts for land or
aeronautical turbines, for example discs of turbines.
[0002] Improvement in the performances of turbines requires more
and more performing alloys at high temperatures. They should
notably be capable of supporting operating temperatures of the
order of 700.degree. C.
[0003] For this purpose, superalloys were developed for
guaranteeing high mechanical properties at these temperatures
(tensile strength, creep resistance and oxidation resistance, crack
propagation strength) for the aforementioned applications, while
retaining good microstructural stability providing a long lifetime
to the thereby manufactured parts.
[0004] Known alloys which may meet these requirements are generally
highly loaded with elements promoting the presence of the gamma'
phase Ni.sub.3(Al,Ti), the proportion of which is often greater
than 45% of the structure. This makes these alloys impossible to
apply with satisfactory results via the conventional route (ingot
route) where the casting of an ingot from liquid metal is followed
by a series of shaping treatments and heat treatments. These alloys
can only be obtained with powder metallurgy, with the major
drawback of very high cost for obtaining them.
[0005] In order to reduce the costs for obtaining them, alloys were
developed allowing an application via a conventional route. This is
notably the nickel-based superalloy known under the name of UDIMET
720, as notably described in documents U.S. Pat. No. 3,667,938 and
U.S. Pat. No. 4,083,734. This superalloy typically has the
composition, described in weight percentages: [0006] trace
amounts.ltoreq.Fe.ltoreq.0.5%; [0007] 12%.ltoreq.Cr.ltoreq.20%;
[0008] 13%.ltoreq.Co.ltoreq.19%; [0009] 2%.ltoreq.Mo.ltoreq.3.5%;
[0010] 0.5%.ltoreq.W.ltoreq.2.5%; [0011] 1.3%.ltoreq.Al.ltoreq.3%;
[0012] 4.75%.ltoreq.Ti.ltoreq.7%; [0013]
0.005%.ltoreq.C.ltoreq.0.045% for low carbon versions, the carbon
content may rise up to 0.15% for high carbon versions; [0014]
0.005%.ltoreq.B.ltoreq.0.03%; [0015] trace
amounts.ltoreq.Mn.ltoreq.0.75%; [0016]
0.01%.ltoreq.Zr.ltoreq.0.08%;
[0017] the remainder being nickel and impurities resulting from the
production.
[0018] The alloy known under the name of TMW 4 was also developed,
a possible composition of which in weight percentages is typically:
[0019] Cr=15%; [0020] Co=26.2%; [0021] Mo=2.75%; [0022] W=1.25%;
[0023] Al=1.9%; [0024] Ti=6%; [0025] C=0.015%; [0026] B=0.015%;
[0027] the remainder being nickel and impurities resulting from the
production.
[0028] With the superalloys of the UDIMET 720 or TMW 4 type it is
possible to partly achieve the targeted goals. At high
temperatures, they actually retain good mechanical properties
because of their high Co contents, and these alloys may be obtained
via a conventional route from an ingot, therefore in a less
expensive way than with powder metallurgy.
[0029] However, they still have a high cost just because of their
large Co content which is generally comprised between 12 and 27%.
Further, they remain difficult to apply via a conventional ingot
route, because of low forgeability notably due to a volume fraction
of gamma' phase which remains substantial (about 45%). Indeed,
because of the large volume fraction of gamma' phase, the
temperature intervals in which forging is possible without any risk
of forming cracks, are narrow and impose that they be put back into
the oven frequently in order to permanently maintain a suitable
temperature during forging. Moreover, for these alloys, forging in
gamma' supersolvus (i.e. above the gamma' solvus temperature and
therefore at a temperature at which the gamma' phase is put into
solution) is impossible, because there would be a risk of
occurrence of cracks. These alloys can only be forged in subsolvus
(therefore at a temperature below the gamma' solvus), which leads
to heterogeneous structures comprising gamma' phase spindles and
causing permeability defects during non-destructive tests with
ultrasonic waves. For these alloys, the forging process is
therefore delicate, difficult to control and costly.
[0030] In order to reduce the costs for obtaining them, novel
nickel superalloys were developed allowing the aforementioned
applications at temperatures of use close to 700.degree. C. An
alloy of this type known under the name of 718 PLUS, which is
described in document WO-A-03/097888, typically has the following
composition in weight percentages: [0031] trace
amounts.ltoreq.Fe.ltoreq.14%; [0032] 12%.ltoreq.Cr.ltoreq.20%;
[0033] 5%.ltoreq.Co.ltoreq.12%; [0034] trace
amounts.ltoreq.Mo.ltoreq.4%; [0035] trace
amounts.ltoreq.W.ltoreq.6%; [0036] 0.6%.ltoreq.Al.ltoreq.2.6%;
[0037] 0.4%.ltoreq.Ti.ltoreq.1.4%; [0038] 4%.ltoreq.Nb.ltoreq.8%;
[0039] trace amounts.ltoreq.C.ltoreq.0.1%; [0040]
0.003%.ltoreq.P.ltoreq.0.03%; [0041]
0.003%.ltoreq.B.ltoreq.0.015%;
[0042] the remainder being nickel and impurities resulting from the
production.
[0043] In order to reduce the costs for obtaining them due to the
raw materials (alloy elements) used, relatively to the
aforementioned alloys, 718 PLUS has a less substantial Co content.
Moreover in order to reduce the costs for obtaining them due to the
thermomechanical treatment, the forgeability of this alloy was
improved by considerably reducing the volume fraction of the gamma'
phase. The lowering of the volume fraction of gamma' phase is
however accomplished to the detriment of the hot mechanical
properties and of the performances of the parts generally, which,
de facto, are clearly lower than those of the alloys mentioned
earlier.
[0044] In the field of land or aeronautical turbines, the use of
the 718 PLUS alloy is therefore limited to certain applications for
which the requirements in terms of thermomechanical stresses are
less critical.
[0045] Moreover, the 718 PLUS alloy has a high Nb content
(comprised between 4 and 8%), which is detrimental to its chemical
homogeneity during production. Indeed, Nb is an element which leads
to substantial segregations at the end of the solidification. These
segregations may lead to the formation of production defects (white
spots). Only narrow and specific remelting rate windows during the
production of the ingot allow reduction of these defects. The
production of 718 PLUS therefore involves a method which is complex
and difficult to control. High Nb contents in superalloys are also
known to be rather detrimental to the propagation of cracks at high
temperatures.
[0046] The object of the invention is to propose an alloy having a
low cost for obtaining it, i.e. with a less substantial cost in
alloy elements than that of alloys of the UDIMET 720 type, and for
which the forgeability would be increased relatively to alloys of
the UDIMET 720 type, and this while having high mechanical
properties at high temperatures (700.degree. C.), i.e. higher than
those of 718 PLUS. In other words, the aim is to propose an alloy
for which the composition would allow a compromise to be obtained
between high hot mechanical properties and an acceptable cost for
obtaining it for the aforementioned applications. This alloy should
also be able to be obtained under not too restrictive production
and forging conditions in order to make their obtaining more
reliable.
[0047] For this purpose, the object of the invention is a
nickel-based superalloy of the following composition, the contents
of the various elements being expressed as weight percentages:
[0048] 1.3%.ltoreq.Al.ltoreq.2.8%; [0049] trace
amounts.ltoreq.Co.ltoreq.11%; [0050] 14%.ltoreq.Cr.ltoreq.17%;
[0051] trace amounts.ltoreq.Fe.ltoreq.12%; [0052]
2%.ltoreq.Mo.ltoreq.5%; [0053] 0.5%.ltoreq.Nb+Ta.ltoreq.2.5%;
[0054] 2.5%.ltoreq.Ti.ltoreq.4.5%; [0055] 1%.ltoreq.W.ltoreq.4%;
[0056] 0.0030%.ltoreq.B.ltoreq.0.030%; [0057] trace
amounts.ltoreq.C.ltoreq.0.1%; [0058]
0.01%.ltoreq.Zr.ltoreq.0.06%;
[0059] the remainder consisting of nickel and impurities resulting
from the production,
[0060] and such that the composition satisfies the following
equations wherein the contents are expressed as atomic
percentages:
8.ltoreq.Al at %+Ti at %+Nb at %+Ta at %.ltoreq.11
0.7.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.3
[0061] Preferably its composition satisfies the following equation
wherein the contents are expressed as atomic percentages:
1.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.3
[0062] Preferably, it contains in weight percentages between 3 and
12% of Fe.
[0063] Preferably, its composition is expressed in weight
percentages: [0064] 1.3%.ltoreq.Al.ltoreq.2.8%; [0065]
7%.ltoreq.Co.ltoreq.11%; [0066] 14%.ltoreq.Cr.ltoreq.17%; [0067]
3%.ltoreq.Fe.ltoreq.9%; [0068] 2%.ltoreq.Mo.ltoreq.5%; [0069]
0.5%.ltoreq.Nb+Ta.ltoreq.2.5%; [0070] 2.5%.ltoreq.Ti.ltoreq.4.5%;
[0071] 1%.ltoreq.W.ltoreq.4%; [0072]
0.0030%.ltoreq.B.ltoreq.0.030%; [0073] trace
amounts.ltoreq.C.ltoreq.0.1%; [0074]
0.01%.ltoreq.Zr.ltoreq.0.06%;
[0075] and its composition satisfies the following equations
wherein the contents are expressed as atomic percentages:
8.ltoreq.Al at %+Ti at %+Nb at %+Ta at %.ltoreq.11
0.7.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.3
[0076] the remainder consisting of nickel and impurities resulting
from the production.
[0077] Preferably, for this alloy 1.ltoreq.(Ti at %+Nb at %+Ta at
%)/Al at %.ltoreq.1.3.
[0078] Better, the composition of the alloy is expressed in weight
percentages: [0079] 1.8%.ltoreq.Al.ltoreq.2.8%; [0080]
7%.ltoreq.Co.ltoreq.10%; [0081] 14%.ltoreq.Cr.ltoreq.17%; [0082]
3.6%.ltoreq.Fe.ltoreq.7%; [0083] 2%.ltoreq.Mo.ltoreq.4%; [0084]
0.5%.ltoreq.Nb+Ta.ltoreq.2%; [0085] 2.8%.ltoreq.Ti.ltoreq.4.2%;
[0086] 1.5%.ltoreq.W.ltoreq.3.5%; [0087]
0.0030%.ltoreq.B.ltoreq.0.030%; [0088] trace
amounts.ltoreq.C.ltoreq.0.07%; [0089]
0.01%.ltoreq.Zr.ltoreq.0.06%;
[0090] and its composition satisfies the following equations
wherein the contents are expressed as atomic percentages:
8.ltoreq.Al at %+Ti at %+Nb at %+Ta at %.ltoreq.11
0.7.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.3
[0091] the remainder consisting of nickel and impurities resulting
from the production.
[0092] In certain cases for this alloy 0.7.ltoreq.(Ti at %+Nb at
%+Ta at %)/Al at %.ltoreq.1.15
[0093] In certain cases for this alloy 1.ltoreq.(Ti at %+Nb at %+Ta
at %)/Al at %.ltoreq.1.3.
[0094] Preferably, these superalloys comprise a gamma' phase
fraction comprised between 30 and 44%, preferably between 32 and
42% and the solvus of the gamma' phase of the superalloy is below
1,145.degree. C.
[0095] Preferably, the composition of the alloy satisfies the
following equation, wherein the contents of the elements are
calculated in the gamma matrix at 700.degree. C. and are expressed
as atomic percentages:
0.717 Ni at %+0.858 Fe at %+1.142 Cr at %+0.777 Co at %+1.55 Mo at
%+1.655 W at %+1.9 Al at %+2.271 Ti at %+2.117 Nb at %+2.224 Ta at
%.ltoreq.0.901.
[0096] Preferably, the Cr content (expressed as an atomic
percentage) is in the gamma matrix at 700.degree. C., greater than
24 at %.
[0097] Preferably, the Mo+W content (expressed as an atomic
percentage) is .ltoreq.2.8 at % in the gamma matrix.
[0098] The object of the invention is also a part in a nickel
superalloy, characterized in that its composition is of the
previous type. This may be a component of an aeronautical or land
turbine.
[0099] As this will have been understood, the invention is based on
an accurate equilibration of the composition of the alloy in order
to obtain both mechanical properties, ease in forging and
preferably a material cost of the alloy as moderate as possible,
making the alloy suitable for economical production via the
standard ingot route of parts which may operate under high
mechanical and thermal stresses, notably in land and aeronautical
turbines.
[0100] The invention will now be described with reference to the
appended FIG. 1 which shows the respective forgeabilities
(represented by striction) measured on remelted and homogenized
ingots at temperatures from 1,000 to 1,180.degree. C., of alloys
according to the invention and of a reference alloy of the UDIMET
720 type, the substitution of which is aimed by the invention.
[0101] While providing good mechanical properties, the alloy
according to the invention has good forgeabilities by limited
contents of elements generating the gamma' phase, and notably of
Nb, in order to also avoid segregation problems during the
production. An alloy according to the invention is for example
forgeable in the domain of the supersolvus of the alloy by which it
is possible to ensure better homogeneity of the metal and to
significantly reduce the costs related to the forging process.
[0102] As this may be seen, a superalloy according to the invention
in addition to reducing the costs associated with the raw
materials, allows reduction of the costs relating to the production
processes and to the thermo-mechanical treatment processes (forging
and closed die-forging) of a part made in this superalloy.
[0103] The alloys obtained according to this invention are globally
obtained at a relatively low cost, in any case at a lower cost than
those of the alloys of the UDIMET 720 type, and this while having a
high mechanical properties at high temperatures i.e. greater than
those of alloys of the 718 PLUS type.
[0104] By lowering the Co content to below 11% it is possible to
considerably reduce the cost of the alloy, Co being the most
expensive among the alloy elements massively present in the
invention. In order to maintain good mechanical properties during
creep and traction, lowering the Co content is on the one hand
compensated by adjusting Ti, Nb and Al contents forming the gamma'
hardening phase and, on the other hand, compensated by an
adjustment of the W and Mo contents which will harden the gamma
matrix of the alloy.
[0105] The inventors were able to notice that by adding Fe as a
partial substitution for the Co content (relatively to alloys of
the UDIMET 720 or TMW-4 type) it was also possible to significantly
reduce the cost of the alloy.
[0106] The inventors were able to notice that an optimum Co content
was comprised between 7 and 11%, better 7 to 10%, in order to reach
a significant increase in the mechanical properties such as creep
resistance while maintaining a low cost in raw materials,
preferably by adding 3 to 9% of Fe, better 3.6 to 7%, into the
composition. Beyond 11% Co, the inventors were able to notice that
the performances of the alloy were not significantly improved.
[0107] An alloy according to this composition gives the possibility
of reaching mechanical properties close to those of the most
performing alloys such as the aforementioned ones (UDIMET 720 and
TMW-4) while keeping a low cost for obtaining them since, for
example, it is possible to easily reach a cost of raw materials of
less than 24 /kg (a cost close to that of 718 PLUS, see the
examples hereafter). In order to determine the costs of the raw
materials making up the liquid metal from which the ingot will be
cast and forged, for each element the following costs per kg are
considered: [0108] Ni: 20 /kg, [0109] Fe: 1 /kg [0110] Cr: 14 /kg,
[0111] Co: 70 /kg, [0112] Mo: 55 /kg, [0113] W: 30 /kg, [0114] Al:
4 /kg, [0115] Ti: 11 /kg, [0116] Nb: 50 /kg, [0117] Ta: 130 /kg
[0118] Of course, these figures may strongly vary over time and the
equation (1) which will be shown, by which it is determined what
would represent an optimization of the composition of the alloy in
terms of costs of raw materials, only has an indicative value and
does not form a parameter which should be strictly observed so that
the alloy is compliant with the invention.
[0119] The targeted ratio of the sum of the Ti, Nb and Ta contents
and of the Al content gives the possibility of ensuring hardening
via a solid solution of the gamma' phase while avoiding the risk of
occurrence of a needled phase in the alloy which may alter its
ductility.
[0120] A minimum gamma' phase fraction (preferably 30%, better 32%)
is desired in order to obtain a very good strength during creep and
traction at 700.degree. C. The fraction and the solvus of the
gamma' phase should however be preferably less than 44% (better
42%) and at 1,145.degree. C. respectively so that the alloy retains
good forgeability, and also so that the alloy may be partly forged
in the supersolvus domain, i.e. at a temperature comprised between
the gamma' solvus and the melting onset temperature.
[0121] The proportions of the phases present in the alloy, such as
the volume fractions of gamma' phases and the molar concentrations
of the TCP phases (the definition of which will be given later on),
were determined by the inventors and according to the composition,
by resorting to phase diagrams obtained by thermodynamic
calculations (by means of the THERMOCALC software package currently
used by metallurgists).
[0122] The parameter Md, which is usually used as an indicator of
the stability of superalloys, should be less than 0.901 in order to
impart optimum stability to the alloy according to the invention.
Within the scope of the invention, the composition may therefore be
adjusted so as to reach an Md.ltoreq.0.901 without being
detrimental to the other mechanical properties of the alloy. Beyond
0.901, the alloy risks being unstable, i.e. giving rise during
extended use to the precipitation of detrimental phases, such as
the sigma and mu phases which embrittle the alloy.
[0123] The aforementioned conditions on the Mo+W content in the
gamma matrix are justified in order to avoid precipitation of
brittle intermetallic compounds of the sigma or mu type. The sigma
and mu phases, when they develop in an excessive amount, cause a
significant reduction in the ductility and in the mechanical
strength of the alloys.
[0124] It was also observed that excessive Mo and W contents
strongly alter the forgeability of the alloy and considerably
reduce the forgeability domain, i.e. the temperature domain where
the alloy tolerates large deformations for hot shaping. These
elements further have high atomic masses and their presence is
expressed by a notable increase in the specific gravity of the
alloy which for aeronautical applications is a predominant
criterion.
[0125] The composition according to the invention gives the
possibility of maintaining a TCP (Topologically
Close-Packed=topologically compact phases such as the mu+sigma
phases, the content of which is expressed as a phase molar
percentage) content of less than 6% at 700.degree. C. in the alloy.
This value allows confirmation that the superalloy according to the
invention has very good microstructural stability at high
temperatures.
[0126] The mandatorily or optimally observed equations by the
composition of the alloy according to the invention are:
[0127] (1) (optimally) cost (/kg)<25 with cost=20 Ni %+Fe %+14
Cr %+70 Co %+55 Mo %+30 W %+4 Al %+11 Ti %+50 Nb %+130 Ta % in
weight percentages, with the reservations expressed above on the
strict validity of this criterion, due to inevitable variations in
the price of the alloy elements.
[0128] (2) (optimally) Md=0.717 Ni at %+0.858 Fe at %+1.142 Cr at
%+0.777 Co at %+1.55 Mo at %+1.655 W at %+1.9 Al at %+2.271 Ti at
%+2.117 Nb at %+2.224 Ta at %.ltoreq.0.901, the contents (at %) of
the various elements being calculated in the gamma matrix at
700.degree. C. (an equation resulting from thermodynamic
calculations made with models customarily known to metallurgists
working in the field of nickel-based superalloys).
[0129] (3) (optimally) Cr.gtoreq.24 at % in the gamma matrix at
700.degree. C. for optimizing the oxidation resistance
(optimization resulting from thermodynamic calculations).
[0130] (4) (mandatorily) 0.7.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at
%.ltoreq.1.3 for ensuring hardening of the .gamma.' and limiting
the risk of occurrence of a needled phase, and optimally
1.ltoreq.(%Ti+%Nb+%Ta)/%Al.ltoreq.1.3 for better hardening, and
optimally 0.7.ltoreq.(Ti at %+Nb at %+Ta at %)/Al at %.ltoreq.1.15
in order to avoid the risk of occurrence of a needled phase.
[0131] (5) (mandatorily) 8<Al at %+Ti at %+Nb at %+Ta at %<11
for ensuring an adequate fraction of gamma' phase.
[0132] (6) (optimally) 30%<.gamma.' fraction<45% and .gamma.'
solvus<1,145.degree. C. (optimization resulting from
thermodynamic calculations): better: 32%<.gamma.'
fraction<42%; it is in this interval where the best compromise
is obtained between creep strength and tensile strength on the one
hand and forgeability on the other hand; the optimum value is about
37%.
[0133] (7) (optimally) molar percent of TCP phases.ltoreq.6% at
700.degree. C. in order to ensure good microstructural stability at
high temperatures (optimization resulting from thermodynamic
calculations).
[0134] (8) (optimally) Mo at %+W at % in the gamma phase at
700.degree. C..gtoreq.2.8 in order to ensure proper hardening of
the gamma matrix (optimization resulting from thermodynamic
calculations), but without exceeding Mo weight contents of 5% and W
weight contents of 4% in order to avoid precipitation of brittle
intermetallic compounds of the sigma or mu type.
[0135] The selections of the contents according to the invention
will now be motivated in detail, element by element.
[0136] Cobalt
[0137] The cobalt content was limited to contents of less than 11%,
better less than 10%, for economical reasons, insofar that this
element is one of the most expensive of those entering the
composition of the alloy (see equation (1) where this element has
the second strongest weighting after Ta). Advantageously, a minimum
content of 7% is desired in order to retain very good creep
strength.
[0138] Iron
[0139] Substitution of the nickel or cobalt with iron has the
advantage of significantly reducing the cost of the alloy. Addition
of iron however promotes precipitation of the sigma phase harmful
for ductility and notch sensitivity. The iron content of the alloy
should therefore be adjusted so as to obtain a significant cost
reduction while guaranteeing a highly stable alloy at a high
temperature (equations (2), (7)). The iron content in the general
case is comprised between trace amounts and 12%, but is preferably
comprised between 3 and 12%, better between 3 and 9%, better
between 3.6 and 7%.
[0140] Aluminum, Titanium, Niobium, Tantalum
[0141] The weight contents of these elements are from 1.3 to 2.8%,
better 1.8 to 2.8% for Al, 2.5 to 4.5%, better 2.8 to 4.2% for Ti,
0.5 to 2.5%, better 0.5 to 2% for the sum Ta+Nb.
[0142] Although the precipitation of the gamma' phase in the
nickel-based alloys is essentially a matter of the presence of
aluminum in a sufficient concentration, the elements, Ti, Nb and
Ta, may promote the occurrence of this phase if they are present in
the alloy with a sufficient concentration: the elements aluminum,
titanium, niobium and tantalum are elements said to be
gamma'-genes. The stability domain of the gamma' phase (the gamma'
solvus of which the alloy is representative) and the gamma' phase
fraction therefore depend on the sum of the atomic concentrations
(at %) of aluminum, titanium, niobium and tantalum. These elements
have thus been adjusted so as to obtain optimally, a .gamma.' phase
fraction comprised between 30% and 44%, better between 32% and 42%,
and a gamma' phase solvus of less than 1,145.degree. C. An adequate
gamma' phase fraction in the alloys of the invention is obtained
with a sum of the Al, Ti, Nb and Ta contents greater than or equal
to 8 at % and less than or equal to 11 at %. A minimum gamma' phase
fraction is desired in order to obtain very good creep and tensile
strength at 700.degree. C. The fraction and the solvus of the
gamma' phase should however preferably be less than 40% and
1,145.degree. C. respectively so that the alloy retains good
forgeability, and may also be partly forged in the supersolvus
domain, i.e. at a temperature comprised between the gamma' solvus
and the melting onset temperature. A .gamma.' phase fraction and a
solvus temperature exceeding the upper limits mentioned earlier
would make the application of the alloy more difficult via the
conventional ingot route, which would risk attenuating one of the
advantages of the invention.
[0143] According to a remarkably advantageous aspect of the
invention, the aluminum, titanium, niobium and tantalum contents
are such that the ratio between the sum of the titanium, niobium
and tantalum contents and the aluminum content is greater than or
equal to 0.7 and less than or equal to 1.3. Indeed, hardening in a
solid solution in the gamma' phase provided by Ti, Nb and Ta is all
the higher since the ratio (Ti at %+Nb at %+Ta at %)/Al at % is
high. A ratio greater than or equal to 1 will be preferred for
guaranteeing better hardening. However for a same aluminum content,
too high Ti, Nb or Ta contents promote precipitation of needled
phases of the eta type (Ni.sub.3Ti) or delta type (Ni.sub.3(Nb,Ta))
but which are not desired within the scope of the invention: these
phases if they are present in too large amounts may alter the hot
ductility of the alloy by precipitating as needles at the grain
boundaries. The ratio (Ti at %+Nb at %+Ta at %)/Al at % should
therefore not exceed 1.3 and preferably 1.15 in order to prevent
precipitation of these detrimental phases. The Nb and Ta contents
on the other hand are less than the titanium content so that the
density of the alloy remains acceptable (less than 8.35), in
particular for aeronautical applications. It is also known to one
skilled in the art that too high niobium contents are detrimental
to resistance to hot crack propagation (650-700.degree. C.). The
niobium is preferably present in a larger proportion than tantalum
insofar that tantalum has a higher cost and a higher atomic mass
than niobium. Equations (1), (4) and (5) take these conditions into
account.
[0144] Molybdenum and Tungsten
[0145] The Mo content should be comprised between 2 and 5% and the
W content between 1 and 4%. Optimally, the MO content is comprised
between 2 and 4% and the W content comprised between 1.5 and
3.5%.
[0146] Molybdenum and tungsten provide strong hardening of the
gamma matrix by a solid solution effect. The Mo and W contents
should be adjusted with care in order to obtain optimum hardening
without causing precipitation of brittle intermetallic compounds of
the sigma or mu type. These phases, when they develop in an
excessive amount, cause a substantial reduction in the ductility
and the mechanical strength of the alloys. It was also observed
that excessive Mo and W contents strongly alter the forgeability of
the alloy and considerably reduce the forgeability domain, i.e. the
temperature domain where the alloy tolerates substantial
deformations for hot shaping. These elements further have high
atomic masses, and their presence is expressed by a notable
increase in the specific gravity of the alloy, which is not
desirable for aeronautical applications notably. Equations (2), (7)
and (8) take these conditions into account.
[0147] Chromium
[0148] Chromium is indispensable for resistance to oxidation and
corrosion of the alloy and thus plays an essential role for the
resistance of the alloy to environmental effects at high
temperature. The chromium content (14 to 17% by weight) of the
alloys of the invention was determined so as to introduce a minimum
concentration of 24 at % of Cr in the gamma phase at 700.degree.
C., by taking into account the fact that a too high chromium
content promotes precipitation of detrimental phases such as the
sigma phase and therefore deteriorates hot stability. Equations
(2), (3) and (7) take these conditions into account.
[0149] Boron, Zirconium, Carbon
[0150] The B content is comprised between 0.0030 and 0.030%. The Zr
content is comprised between 0.01 and 0.06%. The C content is
comprised between trace amounts and 0.1%, optimally between trace
amounts and 0.07%.
[0151] So-called minor elements such as carbon, boron and zirconium
form segregations at the grain boundaries, for example as borides
or carbides. They contribute to increasing the strength and the
ductility of the alloys by trapping detrimental elements such as
sulfur and by modifying the chemical composition at the grain
boundaries. Their absence would be detrimental. However, excessive
contents cause reduction in the melting temperature and strongly
alter forgeability. They therefore have to be maintained within the
limits which have been stated.
[0152] Examples, tested in the laboratory, for applying the
invention will now be described and compared with reference
examples. The contents of Table 1 are indicated in weight
percentages. None of these examples contains tantalum in notable
proportions, but this element has a comparable behavior with that
of niobium, as this was stated.
TABLE-US-00001 TABLE 1 compositions of the samples tested in the
laboratory example Al Co Cr Fe Mo Nb Ni Ti W B C Zr P Ref 1 1.4 9.0
18.0 10.2 2.8 5.6 remainder 0.7 1.0 0.0052 0.002 -- 0.009 Ref 2 1.7
9.0 15.5 5.0 3.0 1.4 remainder 3.9 2.5 0.0110 0.002 0.03 -- Inv 3
2.2 9.0 15.5 5.1 3.0 1.3 remainder 3.9 2.5 0.0110 0.003 0.03 -- Ref
4 2.1 9.0 15.5 5.1 3.0 3.4 remainder 2.4 2.5 0.0100 0.004 0.03 --
Inv 5 2.1 11.0 15.0 11.0 2.5 1.0 remainder 3.6 1.5 0.0100 0.040
0.03 -- Inv 6 2.1 9.0 15.5 5.1 3.0 1.0 remainder 3.6 2.5 0.0110
0.005 0.03 -- Inv 7 2.1 6.1 15.5 3.1 3.4 1.0 remainder 3.6 3.0
0.0120 0.011 0.03 -- Inv 8 1.8 2.1 16.0 9.2 2.8 1.0 remainder 3.3
2.5 0.0110 0.006 0.03 -- Inv 9 2.3 9.1 15.0 3.1 3.1 1.2 remainder
4.0 2.2 0.0110 0.007 0.03 -- Inv 10 2.4 8 15.3 4 3 0.7 remainder
3.3 3 0.0120 0.01 0.04 --
[0153] Examples 1 to 4 were elaborated by VIM (vacuum induction
melting) in order to produce 10 kg ingots.
[0154] Examples 5 to 10 were elaborated by VIM and then by VAR
(vacuum arc remelting) in order to produce 200 kg ingots.
[0155] Reference Example 1 corresponds to a conventional 718 PLUS
alloy.
[0156] Reference Example 2 is then outside the invention because of
a ratio (Ti at %+Nb at %)/Al at %=1.5, therefore greater than
1.3.
[0157] Reference Example 4 is outside the invention because of a
too high Nb content which theoretically corresponds to the Nb
content beyond which the delta phase may occur.
[0158] Examples 5, 7, 8 and 9 correspond to the invention, although
to non-optimized alternatives thereof.
[0159] Examples 3, 6 and 10 correspond to the preferred version of
the invention.
[0160] The optimum composition was obtained for Example 6. By
comparison with this Example 6: [0161] Example 5 contains more Fe,
Co and C and less Mo and W; [0162] Example 7 contains less Fe and
Co and more Mo and W; [0163] Example 8 is less loaded with alloy
elements such as Al, Co, Mo, Ti and more loaded with Fe; [0164]
Example 9 is more loaded with alloy elements such as Al, Ti, Nb and
less loaded with Fe and W; [0165] Example 10 has a lower ratio (Ti
at %+Nb at %)/Al at % and includes more W, less Co and less Fe;
[0166] Reference Example 2 contains more Ti and Nb and less Al, for
an equal fraction of gamma' phase; the ratio (Ti at %+Nb at %)/Al
at % is higher. [0167] Example 3 contains more Al and Nb and Ti,
therefore a higher fraction of gamma' phase; [0168] Example 4, for
an equal fraction of gamma' phase, contains more Nb and less
Ti.
[0169] Table 2 shows additional characteristics of the tested
alloys, with their main mechanical properties: tensile strength Rm,
yield strength Rp.sub.0.2, elongation at break A, creep lifetime at
700.degree. C. under a stress of 600 MPa. The mechanical properties
are given in values relative to those of Reference Example 1 which
is of the usual 718 PLUS type.
TABLE-US-00002 TABLE 2 complementary characteristics and mechanical
properties of the samples (Rationalized with respect to 718 PLUS)
Creep Gamma' Gamma' lifetime fraction solvus (Ti + Nb + Cost Rm
Rp.sub.0.2 A % 700.degree. C. Example (%) (.degree. C.) Ta)/Al Md (
/kg) 700.degree. C. 700.degree. C. 700.degree. C. 600 MPa Ref 1 26
950 1.35 0.904 23.9 1.0 1.0 1.0 1.0 Ref 2 36 1100 1.5 0.892 23.6
1.3 1.3 0.8 1.8 Inv 3 40 1115 1.17 0.895 23.7 1.3 1.3 1.2 8 Ref 4
37 1070 1.13 0.899 24.4 1.1 1.2 0.6 0.1 Inv 5 37 1095 1.1 0.896
23.7 1.2 1.15 1.3 3.5 Inv 6 37 1095 1.1 0.894 23.6 1.3 1.2 1.4 5.3
Inv 7 37 1105 1.1 0.895 22.6 1.2 1.2 1.5 3 Inv 8 32 1070 1.2 0.891
19.2 1.2 1.1 1.5 1.1 Inv 9 42 1125 1.15 0.895 23.9 1.2 1.3 1.1 8.3
Inv 10 40 1095 0.85 0.895 23.2 1.15 1.1 1.5 6.2
[0170] The tensile strength and the creep lifetime of the alloys of
the invention are all clearly greater than that of the 718 PLUS
alloy (Example 1), while the cost of the alloy is comparable or
lower. The gain in tensile strength, in yield strength and in
resistance to creep is less than for Example 8, but the cost of
this alloy is much less than that of 718 PLUS. Examples 2 and 4,
which are not part of the invention, show a reduction in the hot
ductility relatively to the one obtained with 718 PLUS, which is
expressed by a lesser elongation at break.
[0171] The mechanical properties of the alloys of the invention are
thus much superior to those of 718 PLUS and close to those of
UDIMET 720.
[0172] The alloys of the invention have a cost of raw materials
which is less than or equal to 718 PLUS, and therefore they are
much less expensive than UDIMET 720, for which the cost of raw
materials, calculated according to the same criteria, would amount
to 26.6 /kg.
[0173] Another advantage of the alloys of the invention with
respect to UDIMET 720 is unquestionably better forgeability which
facilitates application of the alloys and reduces the manufacturing
costs. Indeed, FIG. 1 shows that the alloys of the invention have a
better striction coefficient and therefore excellent forgeability
in the stage of an ingot homogenized between 1,100 and
1,180.degree. C., and that these alloys unlike UDIMET 720 tolerate
forging at a temperature above the solvus of the gamma' phase. With
this, it is possible to obtain less complex transformation ranges
and more homogeneous microstructures: the refining of the grain may
be carried out during the first transformation stages in the
absence of gamma' phase.
* * * * *