Patent Application
20250043393
This invention relates to nickel-based alloys. More particularly, this invention relates to nickel-based alloys specifically designed for an application in turbine casings for aeronautical engines and comprising tantalum.
The objectives of ACARE, in line with the European Union's Green Deal for Europe, as well as the requirements for reducing the ownership costs imposed by aircraft manufacturers, are forcing engine manufacturers to significantly increase the performance of new-generation turbojets, in particular with a large reduction in specific consumption. This translates into a need to improve engine efficiency by reducing the ventilation of hot engine parts. As a result, materials will need to withstand increasingly hotter operating temperatures.
For example, in the case of a low-pressure turbine casing, certain areas are subjected both to fatigue and to creep at very high temperatures, the target being 800° C. with peaks at 850° C. for new-generation engines. However, a longer fatigue life is favored by a fine grain size (around 10 according to the ASTM E112 standard, abbreviated below as ASTM), while the best creep resistance is obtained with coarse-grained microstructures (around ASTM 0). A compromise is therefore necessary between these two antagonistic properties.
Today, the main known alloys for aeronautical turbine casing applications are Inconel 718, 718 Plus, and Waspaloy. Their maximum operating temperatures are around 650° C., 704° C., and 750° C. respectively. Beyond that, their mechanical properties drop due to a softening of their microstructure. These alloys are therefore not designed to withstand temperatures of around 800° C. over long periods of time.
Other alloys stemming from powder metallurgy allow reaching these high operating temperatures: for example, the alloy described in EP 1 840 232 B1. However, this alloy contains more than 43 vol. % of γ′ precipitates and its ductility is insufficient to consider ring rolling shaping, a technique used for the manufacture of parts such as turbine casings for aeronautical engines. The current upper limit that is commonly accepted is indeed around 40% of γ′ precipitates.
This is why Waspaloy, an alloy containing 25 vol. % of γ′ precipitates (in which the nominal composition is, in weight percent: Cr 18.00-21.00, Co 12.00-15.00, Mo 3.50-5.00, Al 1.20-1.60, Ti 2.75-3.25, B 0.003-0.01, C 0.02-0.10, Zr 0.02-0.08, Fe 0-2.00, Mn 0-0.10, Si 0-0.15, P 0-0.015, S 0-0.015, and Cu 0-0.10), is currently the one which allows the best compromise between fatigue life and creep resistance at high temperatures. This compromise is achieved by obtaining an intermediate grain size (between ASTM 2 and 6) over the entire part. But once again, this alloy was not designed to withstand an operating temperature of 800° C. for very long periods of time.
Some alloys with 36 vol. % of γ′ precipitates, such as AD730TM or Rene65, could have better properties than Waspaloy, but currently they do not allow reaching an intermediate and homogeneous grain size over large parts. Their grain size, which is solely controlled by the populations of primary γ′ precipitates, grows very quickly indeed when the temperature exceeds the γ′ solvus. Avoiding this excessive growth in grain size would require precise control to the nearest degree of the heat treatment temperature over the entire part, which is not achievable in an industrial furnace.
Thus, at present, there is no alloy which combines better heat resistance than Waspaloy, the ability to be shaped by ring rolling, and the ability to reach a homogeneous and intermediate grain size over the entire part through heat treatment to ensure the compromise between creep resistance and fatigue life necessary for the intended application.
There is therefore a need to have a new alloy which can meet the need to increase the operating temperature of the part, while retaining a method of manufacturing by ring rolling and without degrading the fatigue life in comparison to Waspaloy.
Thus, one of the objectives of this invention is to overcome at least one of the disadvantages mentioned above.
For this purpose, the invention provides a nickel-based alloy comprising tantalum, comprising, in weight percent:
Other optional and non-limiting features are as follows.
The nickel-based alloy may comprise, in weight percent, 0.02 to 0.05% carbon.
The nickel-based alloy may comprise, in weight percent:
The nickel-based alloy may comprise 5.0 weight % or less of molybdenum.
The nickel-based alloy may comprise 5.0 weight % or less of iron, preferably 2.0 weight % or less.
The nickel-based alloy may comprise 9.0 weight % of less of tungsten, preferably 6.2 weight % or less.
The nickel-based alloy may comprise 1.0 weight % or less of niobium, preferably 0.5 weight % or less.
Furthermore, the present invention provides a method for processing such an alloy, comprising:
Other optional and non-limiting features are as follows.
Manufacturing the billet may comprise:
Shaping the part may comprise:
Heat treating the part may comprise at least one treatment among:
The heat treatment may further comprise:
The invention also proposes an aeronautical part made of the alloy described above, in particular a turbine casing.
The tantalum-comprising nickel-based alloys according to the invention are suitable for the manufacture of parts intended to withstand temperatures of around 800° C. in their hottest portions and temperature spikes of up to 850° C., while maintaining good fatigue resistance over the whole parts.
This compromise is made possible due to controlling the grain size by heat treatment and forging, which allows obtaining an intermediate grain size of ASTM 2 to 6. The alloy is also suitable for vacuum casting and for ring rolling shaping, techniques which allow reducing manufacturing costs compared to other pathways such as powder metallurgy or direct manufacturing.
Other objectives, features, and advantages will become apparent upon reading the description with reference to the drawings presented below.
A tantalum-comprising nickel-based alloy according to this invention is described below. Throughout the following, the composition of the alloy will always be given in weight percent.
The composition of such an alloy is presented in Table 1 below. Nickel is not specified. Generally speaking, the amount of nickel represents the balance in order to reach 100%. Furthermore, as in any alloy composition, it is not technically possible to avoid residual impurities. Thus, although not mentioned in the compositions presented in this description, certain trace elements may be present. A person skilled in the art will be able to recognize whether an element is present as trace element or whether it has deliberately been added. Indeed, it is recognized that elements in trace form do not impart any particular properties to the alloy or alter any of the properties of the alloy.
Thus, the present alloy comprises the elements cobalt, aluminum, and titanium, which are intended to form a hardening γ′ precipitation of ordered structure L12 and of composition (Ni,Co)3(Al,Ti,Ta).
Furthermore, cobalt contributes to reinforcing the mechanical resistance to heat by solid solution hardening of the γ matrix and allows controlling the stability domains of the carbides of interest, MC (M═Ti) and M23C6 (M═Cr).
The chromium content in particular allows promoting the oxidation resistance of the alloy while reducing the precipitation of TCP phases (Topologically Close Pack phases, also known as Frank-Kasper phases). In addition, chromium participates in the formation of M23C6 carbides.
Titanium and tantalum participate in the formation of MC type carbides. Further, tantalum is a refractory element which participates in the reinforcement of the γ matrix and γ′ precipitates at high temperature.
Carbon is present to control grain growth through the precipitation of MC carbides, and to reinforce the heat resistance of grain boundaries by forming M23C6 carbides.
The boron and zirconium elements also allow reinforcing the strength of grain boundaries over the entire operating temperature range, in particular up to 850° C.
The alloy may further comprise 5.0 weight % or less of molybdenum. Molybdenum contributes to mechanically strengthening the alloy against heat. Its content has been optimized to maximize this strengthening while limiting the precipitation of σ or μ TCP phases considered to be weakening. The σ-type TCP phase is an intermetallic compound having no defined stoichiometric composition and having an electron/atom ratio of 6.2 to 7. It is a primitive unit cell of 30 atoms. The μ-type TCP phase has an ideal A6B7 stoichiometry. In addition, this element is part of the composition of MC (in this case, M═Ti, Mo) and M23C6 (in this case, M═Cr, Mo) type carbides. MC type carbides are intended to control grain size by anchoring grain boundaries during γ′ supersolvus treatment. Furthermore, as a first approximation, the TCP phases all have the same effect, in particular a reduction in the ductility of the alloy by creating potential crack initiation sites. In addition, the formation of TCP phases also contributes to reducing the solid solution reinforcement of the matrix because it pumps some of the atoms of the alloying elements.
The alloy may also comprise 5.0 weight % or less of iron, preferably 2.0 weight % or less. Iron is an inexpensive element and allows reducing the density of the alloy as well as its cost. Furthermore, taking into account the iron content when seeking a composition suitable for the intended applications makes it possible to recycle iron-containing alloys for the production of the nickel-based alloy of the invention, and consequently to expand the range of usable recycled resources.
The alloy may also comprise 9.0 weight % or less of tungsten, preferably 6.2 weight % or less. Tungsten, in addition to or as a substitute for molybdenum, allows improving the mechanical behavior of the alloy when hot, in particular by solid solution hardening of the γ matrix. The added amount of molybdenum and tungsten in the alloy, in atomic percent, may also be between 2% and 5%. This avoids promoting the precipitation of TCP phases; in this case, in the formula for carbides of type M23C6, M═Cr, W (and if need be Mo).
The alloy may also comprise 1.0 weight % or less of niobium, preferably 0.5 weight % or less. When the alloy comprises niobium, the L12 ordered structure has the composition (Ni,Co)3(Al,Ti,Nb) instead of (Ni,Co)3(Al,Ti). Taking a niobium content into account when seeking a composition suitable for the intended applications allows recycling alloys containing niobium for the production of the nickel-based alloy of the invention and consequently, expanding the range of usable recycled resources. The maximum limit of the range, i.e. 1.0%, allows preferential stabilization of titanium carbides over niobium carbides.
Thus, the composition of the alloy according to the invention is preferably according to Table 2, with nickel making up the balance.
Such an alloy has greater heat resistance than Waspaloy, in particular due to a higher mole fraction of γ′ precipitates. Thus, due to such a composition, this mole fraction is greater than 28%, in particular between 28 and 40%. In addition, it limits the mole fraction of γ′ precipitates to 40%. Furthermore, the solvus temperature of the γ′ precipitates is limited to 1120° C. This facilitates shaping of the alloy by ring rolling.
This composition also ensures that the sum of the atomic percents of elements Al, Ti, Ta and Nb is 7 to 10%; preferably 8.0 to 9.5%; which makes it possible to obtain a mole fraction of γ′ phases of between 28% and 40%. It ensures that the temperature of the γ′ phase solvus is equal to or greater than 1000° C. In addition, it ensures that the atomic ratio between the element Al on the one hand and the elements Ti, Ta and Nb on the other hand (Al/(Ti+Ta+Nb)) is between 1.2 and 1.4, thus promoting the precipitation of the γ′ phase over the η− Ni3Ti phase, the latter being undesirable from the point of view of mechanical properties. In other words, the Ti and Ta contents are optimized in the γ′ phase, which maximizes the mechanical strengthening of the alloy against heat, while avoiding any promotion of the formation of the η phase to the detriment of the γ′ phase.
This composition induces the precipitation of carbides at the grain boundaries, in particular M23C6 type carbides, which reinforces the creep resistance of the alloy at high temperature. In particular, carbides have a discrete distribution at the grain boundaries. They generally have a nodular shape that is smaller than 5 μm, advantageously smaller than 1 μm. The discrete distribution at the grain boundaries is made possible by combining the composition with an appropriate heat treatment described below.
The amount of M23C6 carbides may be between 0.4 and 1 mol %, advantageously between 0.5 and 0.75 mol %. This makes it possible to obtain both a sufficient population of carbides to ensure the desired hardening and to avoid saturation of the grain boundaries. Indeed, saturation of the grain boundaries promotes unwanted intragranular precipitation.
Furthermore, the solvus of M23C6 type carbides satisfies the criterion:
Compliance with this criterion allows a γ′ subsolvus heat treatment to be carried out without risking the precipitation of M23C6 type carbides at a temperature above 870° C. Indeed, at this temperature, the precipitation of M23C6 type carbides could preferentially take place in the form of films or platelets on the grain boundaries, which adversely impacts the crack propagation resistance.
Furthermore, the composition ensures a solvus of M23C6 type carbides that is above 900° C. Thus, redissolution of the carbides can be avoided during temperature spikes above 850° C. during operation, ultimately making it possible to avoid degrading the mechanical aspect.
Furthermore, the alloy has an intermediate grain size between ASTM 2 and 6, which represents a good compromise between the creep resistance at high temperature fostered by a coarse grain size, and the fatigue resistance fostered by a fine grain size.
This intermediate grain size is obtained in particular thanks to the presence of a controlled population of MC type carbides, which allows limiting grain enlargement during forging and during heat treatment at a temperature above the γ′ solvus. These carbides generally have a nodular shape, sometimes angular, in the presence of trace nitrogen, and a size smaller than 5 μm. Preferably, the molar quantity of MC type carbides is between 0.1 and 0.3% at a temperature higher than the solvus of the γ′ phases, for example at a γ′ solvus temperature of +40° C.
The presence of MC type carbides allows anchoring the grain boundaries 2 on the MC type carbides 3 during heat treatment (
The mole fraction of MC type carbides, limited to 0.3%, makes it possible to avoid degrading the fatigue life via the formation of coarser carbides and carbonitrides (i.e. of a size greater than 5 μm) which is inherent to production by casting and forging.
Another way to limit the formation of coarse carbides is to have a solvus temperature for MC type carbides that is lower than the solidus of the alloy; which is made possible by the composition.
Finally, the composition ensures a tantalum content in MC type carbides equal to or greater than 10 at %, which reinforces their stability at high temperature during grain size enlargement processings.
A preferred composition is according to the following Table 3, or to Table 4 when taking into account the amounts of Mo, Fe, W and Nb.
A method for manufacturing a part made of a nickel-based alloy as described above is described below with reference to
This method comprises manufacturing 100 a billet which has the same composition as that of the nickel-based alloy, shaping 200 the part, and heat treating 300 the part (
Manufacturing 100 the billet may in particular comprise producing 110 an ingot and converting 120 the ingot into billets (
Converting 120 the ingot into billets may be carried out by forging after cutting 121 the ingot, in particular by successive operations of upset forging 122 and drawing 123 the nickel-based alloy in order to refine the solidification structure of the nickel-based alloy (
Shaping 200 the part may comprise forging 210 the billet, in particular by upset forging the nickel-based alloy that forms the billet. Shaping the part may also comprise rolling 220 after forging, in particular ring rolling (
Heat treating 300 the part in particular comprises at least one treatment among γ′ supersolvus solution heat treatment 310 and γ′ subsolvus solution heat treatment 320 (
Heat treating 300 the part may further comprise a tempering 330 to precipitate M23C6 type carbides, in particular after γ′ supersolvus solution heat treatment 310 and/or γ′ subsolvus solution heat treatment 320. For example, this tempering 330 to precipitate M23C6 type carbides is carried out by heating to a temperature of between 825° C. and 870° C., preferably between 840° C. and 860° C., for example approximately 850° C., in particular for 4 to 8 hours.
Heat treating 300 the part may further comprise a tempering 340 to stabilize the populations of γ′ precipitates, in particular at a temperature close to the targeted operating temperature, for example between 760° C. and 825° C. (
Thus, heat treating the part may in particular include the following combinations (referring to the differentiating number in references 310, 320, 330 and 340): 1+3, 1+4, 2+3, 2+4, 1+2+3, 1+2+4, 1+3+4, 2+3+4, 1+2+3+4.
Table 9 gives the mass composition of twelve examples according to this invention (Ex. 1 to Ex. 12) and a comparative example (Ex. C1). Table 10 gives the properties for these examples.
where P1 is the atomic ratio Al/(Ti+Ta+Nb); P2 is the sum of the atomic percents of elements Al, Ti, Ta and Nb; P3 is the mole percent of M23C6 type carbides determined at 850° C.; P4 is the γ′ solvus (° C.); P5 is the solvus of M23C6 type carbides (° C.); P6 is the difference between the γ′ solvus and the solvus of M23C6 type carbides (° C.); P7 is the mole percent of MC type carbides, 40° C. above the γ′ solvus temperature; P8 is the solvus of MC type carbides (° C.); P9 is the solidus (° C.); and P10 is the atomic percent of Ta in MC type carbides.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2113492 | Dec 2021 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2022/052355 | 12/14/2022 | WO |
