TITANIUM ALUMINIDE ALLOYS

Abstract

Alloys based on titanium aluminides, such as γ (TiAl) which may be made through the use of casting or powder metallurgical processes and heat treatments. The alloys contain titanium, 38 to 46 atom % aluminum, and 5 to 10 atom % niobium, and they contain composite lamella structures with B19 phase and β phase there in a volume ratio of the B19 phase to β phase 0.05:1 and 20:1.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Patent application Ser. No. 12/331,909 filed Dec. 10, 2008, which claims priority German patent application DE 10 2007 060 587.2, filed Dec. 13, 2007, the subject matter of these patents is incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to alloys based on titanium aluminide, in particular made through the use of casting or powder metallurgical processes, preferably based on γ (TiAl).

BACKGROUND OF THE INVENTION

Titanium aluminide alloys are characterized by a low density, a high rigidity and good corrosion resistance. In the fixed state, they have domains with hexagonal (α), two-phase structures (α+β) and cubically body-centered β phase and/or γ phase.

For industrial practice, alloys based on an intermetallic phase γ (TiAl) with a tetragonal structure and containing minority shares of intermetallic phase α₂(Ti₃Al) with hexagonal structure in addition to the majority phase γ (TiAl) are particularly interesting. These γ titanium aluminide alloys are characterized by properties like low density (3.85-4.2 g/cm³), high elastic modulus, high rigidity and creep resistance up to 700° C., which make them attractive as lightweight construction materials for high-temperature applications. Examples of such applications include turbine buckets in aircraft engines and in stationary gas turbines, and valves for engines and hot gas ventilators.

In the technically important area of alloys with aluminum content between 45 atom percent and 49 atom percent, a series of phase conversions occur during the solidification from the cast and during the subsequent cooling. The solidification can either take place completely via the β mixed crystal with a cubically body-centered structure (high temperature phase) or in two peritectic reactions, in which the α mixed crystal with hexagonal structure and the γ phase participate. Atom percent (at %) of an elemental material in an alloy indicates the proportion of the identified material as 100×[the number of atoms of the identified elemental material)/(total atoms in the alloy]. This is equivalent to mole percent of the material, or 100×[mole fraction X_A], where X_A=n_A/N_tot, where n_a is the number of moles of elemental material A in the alloy and N_tot is the total number of moles of atoms in the alloy.

Furthermore, it is known that aluminum in γ titanium aluminide alloys causes an increase in the ductility and the oxidation resistance. Moreover, element niobium (Nb) leads to an increase in the rigidity, creep resistance, oxidation resistance, but also the ductility. With the element boron (B), which is practically insoluble in the γ phase, a grain refinement can be achieved in both the as-cast state and after the reshaping with subsequent heat treatment in the α area. An increased share of β phase in the structure as a result of low aluminum contents and high concentrations of β stabilizing elements can lead to rough dispersion of this phase and can cause deterioration of the mechanical properties.

The mechanical properties of titanium aluminide alloys are strongly anisotropic due to their deformation and breaking behavior but also due to the structural anisotropy of the preferably set lamellar structure or duplex structure. Casting processes, different powder-metallurgical and reshaping processes and combinations of these production processes are used for a targeted setting of structure and texture in the production of components made of titanium aliminides.

Moreover, a titanium aluminide alloy, which has a structurally and chemically homogeneous structure, is known from EP 1 015 650 B1. The majority phases γ (TiAl) and α₂ (Ti₃Al) are hereby distributed in a finely disperse manner. The disclosed titanium aluminide alloy with an aluminum content of 45 atom percent (at %) is characterized by extraordinarily good mechanical properties and high temperature properties.

Titanium aluminides based on γ (TiAl) are characterized in general by relatively high rigidities, high elastic modulus, good oxidation and creep resistance with simultaneously lower density. Based on these properties, TiAl alloys should be used as high temperature materials. These types of applications are heavily impaired through the very low plastic malleability and the low fracture toughness. Rigidity and malleability, as with many other materials, behave hereby inversely. The technically interesting high-strength alloys are thereby often particularly brittle. Comprehensive examinations for the optimization of the structure were performed in order to eliminate these disadvantageous properties.

The previously developed structure types can be roughly categorized into a) coaxial gamma structures, b) duplex structures and c) lamellar structures. The currently achieved development state is represented in detail for example in: Y.-W. Kim, D. M. Dimiduk, in: Structural Intermetallics 1997, Eds. M. V. Nathal, R. Darolia, C. T. Liu, P. L. Martin, D. B. Miracle, R. Wagner, M. Yamaguchi, TMS, Warrendale Pa., 1996, pg. 531, and M. Yamaguchi, H. Inui, K. Ito, Acta mater. 48 (2000), pg. 307.

The structures made of titanium aluminides were previously mainly refined by boron additives, which leads to the formation titanium borides (see T. T. Cheng in: Gamma Titanium Aluminides 1999, Eds. Y.-W. Kim, D. M. Dimiduk, M. H. Loretto, TMS, Warrendale Pa., 1999, pg. 389; and

Y.-W. Kim, D. M. Dimiduk, in: Structural Intermetallics 2001, Eds. K. J. Hemker, D. M. Dimiduk, H. Clemens, R. Darolia, H. Inui, J. M. Larsen, V. K. Sikka, M. Thomas, J. D. Whittenberger, TMS, Warrendale Pa., 2001, pg. 625.)

For further refining and consolidation of the structure, the alloys are usually subjected to several high temperature reshapings through extruding or forging. Also refer to the following publications:

• Gamma Titanium Aluminides, Eds. Y.-W. Kim, R. Wagner, M. Yamaguchi, TMS, Warrendale Pa., 1995;
• Structural Intermetallics 1997, Eds. M. V. Nathal, R. Darolia, C. T. Liu, P. L. Martin, D. B. Miracle, R. Wagner, M. Yamaguchi, TMS, Warrendale Pa., 1997;
• Gamma Titanium Aluminides 1999, Eds. Y-W. Kim, D. M. Dimiduk, M. H. Loretto, TMS, Warrendale Pa., 1999; and
• Structural Intermetallics 2001, Eds. K. J. Hemker, D. M. Dimiduk, H. Clemens, R. Darolia, H. Inui, J. M. Larsen, V. K. Sikka, M. Thomas, J. D. Whittenberger, TMS, Warrendale Pa., 2001.

SUMMARY

The present invention resides in one aspect in an alloy which contains titanium, 38 to 46 atom percent (at %) aluminum, and 5 to 10 atom percent niobium, and has composite lamella that contain a B19 phase and a β phase in a volume ratio of B19:β of 0.05:1 to 20:1.

The present invention resides in another aspect in a method for the production of an alloy. The method includes providing a composition that contains titanium, 38 to 46 at % aluminum, and 5 to 10 at % niobium and subjecting the composition to a casting or powder metallurgical technique to produce an intermediate product. The intermediate product is subjected to a heat treatment. The heat treatment includes heating the intermediate product at a temperature above 900° C. for more than sixty minutes, and cooling the intermediate product at a rate of more than 0.5° C. per minute.

The present invention resides in another aspect in an alloy made by the method described herein.

A component may be made from the alloys described herein.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is an electron photomicrograph of an alloy according to one embodiment of the present invention.

FIG. 1B is an electron photomicrograph providing a detailed view of selected lamella structures T of FIG. 1A.

FIG. 1C is an electron photomicrograph of an alloy according to another embodiment of the present invention.

FIG. 2A is an electron photomicrograph providing a more detailed view of a lamella structure T of FIG. 1A.

FIG. 2B is an electron photomicrograph providing a still more detailed view of a lamella structure T of FIG. 1A.

FIG. 2C is a diffractogram derived from FIG. 2B.

FIG. 3 is an electron photomicrograph of a crack in the alloy of FIG. 1A.

FIG. 4 is a graph of a plot of force on the vertical axis vs. deflection on the horizontal axis, for a toughness test of an alloy as described herein.

DETAILED DESCRIPTION OF THE INVENTION

In one embodiment, the present invention provides a titanium aluminide alloy with a fine structure morphology, for example, a morphology in the nanometer range. In another embodiment, the present invention provides a component made from a homogeneous alloy.

In one aspect, the present invention provides an alloy based on titanium aluminides which may optionally be made through the use of casting or powder metallurgical processes, preferably based on γ (TiAl), using a composition that contains titanium (Ti), 38 to 42 atom percent (at %) aluminum (Al), and 5 to 10 at % niobium (Nb), and wherein the composition comprises composite lamella structures with B19 phase and β phase in each lamella, with a volume ratio of the B19 phase to the β phase in each lamella between 0.05:1 to 20:1. In an optional embodiment, the volume ratio is between 0.1:1 and 10:1.

It has been shown that in some alloys or intermetallic connections described herein, composite lamella structures, including composite lamella structures in the nanometer size, are created. The lamella structures include modulated lamellas made of the crystallographically different, and alternatingly formed, B19 phase and β phase. The created composite lamella structures are largely surrounded by γ-TiAI.

These types of composite lamella structures can be established in alloys using known production technologies, i.e. through casting, reshaping and powder technologies. The alloys are characterized by an extremely high rigidity and creep resistance with simultaneously high ductility and fracture toughness.

Example alloys as described herein can be provided with any of the following titanium-based compositions (wherein titanium makes up the balance of the at % of each composition):

• Titanium, 38.5 to 42.5 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Cr;
• Titanium, 39 to 43 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Zr;
• Titanium, 41 to 44.5 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Mo,
• Titanium, 41 to 44.5 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Fe,
• Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.1 to 1 at % La;
• Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.1 to 1 at % Sc;,
• Titanium, 41 to 45 at % Al, 5 to 10 at % Nb; and 0.1 to 1 at % Y;
• Titanium, 42 to 46 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Mn;
• Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % Ta;
• Titanium, 41 to 45 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % V; and
• Titanium, 41 to 46 at % Al, 5 to 10 at % Nb, and 0.5 to 5 at % W.

Each of the titanium aluminide alloys disclosed above can optionally include boron (B) and/or carbon (C). For example, any of the described titanium aluminide alloys may include 0.1 to 1 at % boron and/or 0.1 to 1 at % carbon. The already fine structure of the alloy is hereby further refined.

Within the framework of the invention, the remainders of the specified alloy compositions are made of titanium and unavoidable impurities.

In accordance with the invention, alloys are thus made available, which are suitable as a lightweight construction material for high temperature applications, such as turbine buckets or engine and turbine components.

According to one aspect, alloys as described herein can be produced using casting metallurgical or powder metallurgical processes or techniques, or using these processes in combination with reshaping techniques.

In some embodiments, the alloys with composite lamella structures have a very fine microstructure and a high rigidity and creep resistance with simultaneously good ductility and fracture toughness with respect to alloys without the composite lamella structures.

It is known that titanium aluminide alloys with aluminum contents of 38-45 at % and other additives, for example, refractory elements, contain relatively large volume shares of the β phase, which can also be present in a controlled form as B2 phase. The crystallographic lattices of these two phases are mechanically instable with respect to homogenous shearing processes, which can lead to lattice conversions. This property is mainly attributed to the anistropic bond ratio and the symmetry of the cubically body-centered lattice. The tendency of the β or B2 phase towards lattice transformation is thus very distinct. Different orthorhombic phases can be formed through a shear transformation of the cubically body-centered lattice of the β or B2 phase, to which phases B19 and B33 belong in particular.

Without wishing to be bound by any one particular theory, the invention is based on the idea of using lattice transformations through shear conversion for an additional refining of the microstructure of the titanium aluminide alloys. This type of process is not previously known for titanium aluminide alloys in scientific literature. In the case of the alloys as described herein, the formation of brittle phases like ω, ω′ and ω″, which are extremely disadvantageous for the mechanical material properties, are also avoided, due to shear conversions.

In some embodiments, the structural refining of the alloys described herein is achieved without the addition of grain-refining or structure-refining elements or additives such as boron (B) and the alloys thus contain no borides. Since the borides occurring in TiAl alloys are brittle, they lead to the brittleness of TiAl alloys as of a certain content and generally represent potential crack nuclei in boron-containing alloys.

In another aspect, some alloys as described herein comprise composite lamella structures with the B19 phase and β phase in each lamella, wherein the lamellas are surrounded by the TiAl-γ phase.

In various embodiments, the volume ratio of the B19 phase and β phase each in a lamella is between 0.05:1 and 20:1, for example, 0.1:1 and 10:1.In some embodiments, the volume ratio of the B19 phase and β phase in a lamella is between 0.2:1 and 5:1, and the volume ratio may be between 0.25:1 and 4:1. In certain embodiments, the volume ratio of the B19 phase and β phase in a lamella is between 1:3 and 3:1. For example, the volume ratio may be between 0.5:1 and 2:1. Embodiments having a particularly fine structure in the alloy composition have a ratio, in particular the volume ratio, of the B19 phase and β phase in a lamella, between 0.75:1 and 1.25:1, for example, particular between 0.8:1 and 1.2:1. In one instance, the volume ratio may be between 0.9:1 and 1.1:1.

Moreover, it is possible in a further embodiment of the alloys according to the invention that lamellas of the composite lamella structures are surrounded by lamellas of type γ (TiAl), preferably on both sides of the lamella.

The alloys are further characterized in that the lamellas of the composite lamella structures have a volume share of more than 10%, optionally more than 20%, of the total alloy.

Moreover, the fine lamella-like structure in the composite structures are retained if the lamellas of the composite lamella structures TiAl have the phase α₂-Ti₃Al with a volume share of up to 20%, wherein in particular the (volume) ratio of the B19 phase and β phase in the lamellas remains unchanged and constant.

The alloys according to the invention are suitable as high temperature lightweight construction material for components that are exposed to temperatures of up to 800° C.

An alloy as described herein can be produced using casting or powder metallurgical techniques. The casting or powder metallurgical techniques are used to produce an intermediate alloy product containing the titanium, aluminum, niobium and optional other components, if any, in the appropriate proportions. The intermediate alloy product is then subjected to heat treatment including heating at temperatures above 900° C., preferably above 1000° C., in particular at temperatures between 1000° C. and 1200° C., for a predetermined period of time of more than 60 minutes, preferably more than 90 minutes, yielding a heat-treated intermediate alloy product. The heat-treated intermediate alloy product is then cooled with a predetermined cooling rate of more than 0.5° C. per minute.

In one embodiment, the heat-treated intermediate alloy product is cooled with a predetermined cooling rate between 1° C. per minute and 20° C. per minute, preferably up to 10° C. per minute.

Light (high temperature) materials or components for use in thermal engines like combustion engines, gas turbines, and aircraft engines may be made of an alloy as described herein, e.g., from an alloy based on an intermetallic bond of type γ-TiAl made through casting or powder metallurgical processes or techniques and heat treatment.

Accordingly, an alloy as described herein can be used for the production of a component. To avoid repetitions, reference is made expressly to the above expositions.

As indicated above, the alloys described herein may be created through the use of conventional metallurgical casting methods or through known powder metallurgical techniques, and can for example be processed through hot forging, hot pressing or hot extrusion and hot rolling.

Examples of composite lamella structures of the type described herein are shown in the figures. The example composite lamella structures are based on an alloy comprised of titanium (Ti), 42 atom % aluminum (Al) and 8.5 atom % niobium (Nb).

FIG. 1A shows a picture of a structure alloy, which was taken with the help of a transmission electron microscope. The overview picture in FIG. 1 shows that the composite lamella structures, which are labeled with T in FIG. 1, have a striped contrast to the structure of the γ phase surrounding the structures.

FIG. 1B shows a picture of the alloy structure with a higher magnification, whereby it can be seen that the modulated composite lamella structures (reference letter T) are surrounded by the γ phase respectively are embedded in the γ phase.

The structures shown in FIG. 1A and 1b were obtained or set through extrusion.

FIG. 1C shows a cast structure of the same alloy, i.e., an alloy containing titanium, 42 at % aluminum, and 8.5 at % niobium, in which a composite lamella structure (indicated in the Figure by the reference letter T) is also formed, which is surrounding by the γ phase.

FIG. 2A shows a high resolution illustration of the atomic structure of the composite lamella structures above the γ phase. The composite lamella structures are made up of the controlled B19 phase and the uncontrolled β phase, which border the γ phase (in the lower area). It can be seen from the picture in FIG. 2A that the composite lamella structures contain the two crystallographically different phases B19 and β/B2, which are arranged at separation distances of a few nanometers. The composite lamella structures contain the phases B19 and β, which are both considered ductile. The volume ratio of the B19 phases to the β phases in a composite lamella structure is 0.8:1 to 1.2:1. Due to the ductile phases B19 and β, the structure is mainly made of easily malleable lamellas, which are embedded in the previously relatively brittle γ phase.

FIG. 2B shows an illustration of a B19 structure with a magnified representation. The corresponding diffractogram, which was calculated from the section shown in FIG. 2B and is characteristic for the B19 structure, is shown in FIG. 2C.

FIG. 3 shows an electron-photomicrograph of a crack C in the aforementioned alloy. It can be seen from the figure that the crack C is diffracted at the modulated composite lamella structures (T) and that the composite lamella structures form ligaments that can bridge the edge of the crack. This type of behavior is considerably different from the crack propagation in the previously known Ti—Al alloys, in which a cleavage fracture occurs in the microscopic dimension observed here. In the alloy according to the invention, crack propagation is prevented due to the formed composite lamella structures.

The fracture toughness of structure important for the technical application was determined with the help of notched Chevron samples in the bending test at different temperatures. The recorded register curve of such a test is shown in FIG. 4. The indentations marked by the arrows can be seen in the curve, which indicate that crack propagation intermittently occurs during the loading of the sample, but is stopped again and again. Such a behavior is typical for alloys that are made up of a brittle phase (γ phase), in which the relative ductile phases B19 and β are embedded.

As mentioned above, the alloys according to the invention can be made through the technologies known for TiAl alloys, i.e. via casting metallurgy, reshaping technologies and powder metallurgy. For example, alloys are melted in an electric arc furnace and are re-melted multiple times and are then subjected to a heat treatment. Moreover, the production methods of vacuum arc casting, induction casting or plasma casting, which are known for primary cast blocks made of TiAl alloys, can be used for production. After the solidification of casting primary cast material, hot-isostatic presses can also be used as the compression method at temperatures of 900° C. to 1,300° C. or heat treatments in the temperature range of 700° C. to 1,400° C. or a combination of these treatments, in order to close pores and to establish the microstructure in the material as described herein.

Although the invention has been described with reference to particular embodiments thereof, it will be understood by one of ordinary skill in the art, upon a reading and understanding of the foregoing disclosure, that numerous variations and alterations to the disclosed embodiments will fall within the scope of this invention and of the appended claims.

Claims

1. A method for the production of an alloy, comprising:

providing a composition that comprises titanium, 38 to 46 at % aluminum, and 5 to 10 at % niobium;

subjecting the composition to a casting or powder metallurgical technique to produce an intermediate product; and

subjecting the intermediate product to a heat treatment, the heat treatment comprising heating the intermediate product at a temperature above 900° C. for more than sixty minutes, and cooling the intermediate product at a rate of more than 0.5° C. per minute.

2. The method of claim 1 wherein the heat treatment comprises heating the intermediate product at a temperature above 1000° C.

3. The method of claim 1 wherein the heat treatment comprises heating the intermediate product at a temperature between 1000° C. and 1200° C.

4. The method of claim 1 wherein the heat treatment comprises heating the intermediate product at said temperature above 900° C. for more than 90 minutes.

5. The method of claim 1 wherein the heat treatment comprises heating the intermediate product at a temperature above 1000° C. for more than 90 minutes.

6. The method of claim 1, comprising cooling the intermediate product at a rate of 1° C. per minute to 20° C. per minute.

7. The method of claim 1, comprising cooling the intermediate product at a rate of 1° C. per minute to 10° C. per minute.