DE102004056582A1 - Alloy based on titanium aluminides - Google Patents

Abstract

The invention concerns alloys made through the use of melting and powdered metallurgical techniques on the basis of titanium aluminides with an alloy composition of Ti-z Al-y Nb where 44.5 Atom % ≰z≰47 Atom %, 44.5 Atom % ≰z≰45.5 Atom %, and 5 Atom % ≰y≰10 Atom % with possibly the addition of B and/or C at a content between 0.05 Atom % and 0.8 Atom %. Said alloy is characterized in that it contains a molybdenum (Mo) content ranging between 0.1 Atom % to 3.0 Atom %.

Description

The The invention relates to alloys based on use titanium aluminides produced by melt and powder metallurgy techniques with an alloy composition of Ti - z Al - y Nb with 44.5 atom% ≦ z ≦ 47 atom%, in particular with 44.5 atom% ≤ z ≤ 45.5 atom %, and 5 atom% ≦ y ≦ 10 atom% and optionally additives of B and / or C with contents between 0.05 atom% and 0.8 atom %.

Titanium aluminide alloys have properties that are particularly favorable for use as a lightweight material, especially for high temperature applications. For industrial practice in particular alloys are interesting, which are based on an intermetallic phase γ- (TiAl) with tetragonal structure and in addition to the majority phase γ- (TiAl) and minority components of the intermetallic phase α ₂ (Ti ₃ Al) with hexagonal structure. These γ-titanium aluminide alloys are characterized by properties such as low density (3.85-4.2 g / cm ³ ), high elastic moduli, high strength and creep resistance up to 700 ° C, which makes them a material for moving parts make elevated operating temperatures attractive. Examples of this are turbine blades in aircraft engines and in stationary gas turbines, valves in engines and hot gas fans.

in the technically important range of alloys with aluminum contents between 45 atom% and 49 atom% occur in solidification from the melt and on subsequent cooling a series of phase transformations. The solidification can either completely over the β-mixed crystal with cubic body-centered structure (high temperature phase) or occur in two peritectic reactions in which the α-mixed crystal with hexagonal structure and the γ-phase involved.

Further It is known that the element niobium (Nb) increases the Strength, creep resistance, oxidation resistance, but also the ductility leads. With in the γ phase practically insoluble element Boron can grain refining both in the cast state and after Forming with subsequent heat treatment in the α-area be achieved. An elevated one Proportion of β-phase in the structure due to low aluminum contents and high concentrations of β-stabilizing Elements can lead to coarse dispersion of this phase and a Cause deterioration of the mechanical properties.

The mechanical properties of γ-titanium aluminide alloys are due to their deformation and fracture behavior, as well because of the microstructural anisotropy the preferred set lamellar structure or duplex structure strong anisotropic. For a specific adjustment of structure and texture in the manufacture of titanium aluminide components, casting processes, different powder metallurgy and forming processes as well Combinations of these production methods applied.

Out the publication from Y-W. Kim and 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, p. 531, it is known that in different development programs the effect of a larger number of alloying elements in terms of constitution, microstructure attitude in different manufacturing processes and individual properties was investigated. The found relationships are similarly complex, as with other structural metals, e.g. Steels, the case is, and let only in limited and very general form into rules. Therefore, certain Compositions have outstanding combinations of properties.

Out EP 1 015 605 B1 is a titanium aluminide alloy is known, which has a structurally and chemically homogeneous structure. Here, the majority phases γ (TiAl) and α ₂ (Ti ₃ Al) are finely dispersed. The disclosed titanium aluminide alloy with an aluminum content of 45 atom% is characterized by exceptionally good mechanical properties and high-temperature properties.

One general problem of all titanium aluminide alloys is their low Ductility. So far, it has not succeeded by the nature of the intermetallic Phases predetermined high brittleness and low damage tolerance of titanium aluminide alloys over alloying effects crucial improvement (see "Structural Inter metallics 1997, p. 531, see above). For The applications mentioned in the introduction are often plastic Elongation at break of ≥ 1 % sufficient. From the manufacturers of turbines and engines will be However, this minimum level of ductility is required in the industrial production over size Lottery is guaranteed. Since the ductility is sensitive to the structure is It is extremely difficult in the industrial manufacturing process, one possible homogeneous microstructure education sure. For high strength alloys is the maximum tolerable defect size, e.g. the maximum grain or fin colony size, especially small, so that for such Alloys a very high Gefügehomogenität desirable. But this can already be because of the inevitable fluctuations of Alloy composition of e.g. ± 0.5 atom% in the Al content difficult to reach.

Becoming present of the many, in γ-titanium aluminide alloys potential Microstructure types only lamellar or so-called duplex structure for high temperature applications taken into consideration. The former arise during cooling from the single-phase region of the α-mixed crystal by γ-phase plates precipitate crystallographically oriented from the α-mixed crystal.

In contrast, exist Duplex structure off Lamellar colonies and γ-grains and arise when the material is annealed in the two-phase region α + γ. In this case, the α grains present there change again on cooling into biphasic lamellar colonies. Coarse microstructure constituents are formed in γ-titanium aluminide alloys above all, that when passing through the α-area large α-grains be formed. This can happen already during the solidification, if big ones Stem crystals of α-phase form from the melt. Accordingly, if possible, the single-phase region of the α-mixed crystal be avoided during processing. As in practice, however, fluctuations the composition and process temperatures occur and therefore the Constitution locally in the workpieces varies, the formation of coarse lamellar colonies can not be ruled out.

outgoing from this prior art, the present invention is the Task based, a titanium aluminide alloy with a fine and homogeneous morphology to provide, occurring in industrial practice Variations in the alloy composition and inevitable temperature fluctuations during the manufacturing process hardly or not appreciably on the homogeneity of the alloy, especially without any fundamental changes the manufacturing process. Furthermore exists the task is to provide a component with a homogeneous alloy.

Is solved this object by means of an alloy based on using titanium aluminides produced by melt and powder metallurgy techniques with an alloy composition of Ti - z Al - y Nb with 44.5 atom% ≦ z ≦ 47 atom%, in particular with 44.5 atom% ≤ z ≤ 45.5 atom %, and 5 at% ≦ y ≦ 10 at%, which is further developed by the fact that these molybdenum (Mo) in Range between 0.1 at% to 3.0 at%. The rest of the alloy consists of Ti (titanium).

It has been shown in experiments that by the alloying of molybdenum with titanium aluminides with a niobium content, where usually the β-phase does not exceed the entire temperature range is stable and therefore remains of the high-temperature β-phase at the usual Process steps such as extrusion dissolve, a better structural homogeneity of the alloy is reached. Thus, over the whole, for the production process relevant temperature range a volume fraction the β-phase without grain coarsening realized. This alloy type according to the invention then indicates the β phase due to the fine and very uniform dispersion homogeneous structure with high strength values.

In order to an alloy is provided which is used as a lightweight material for high-temperature applications, such as. Turbine blade or engine and turbine components, suitable is.

The alloy according to the invention is using casting metallurgical, melt metallurgical or powder metallurgical process or using these methods in combination with forming techniques produced.

In front especially in Ti - (44.5 Atom% to 45.5 atom%) Al - (5 Atom% to 10 atom%) Nb has the addition of molybdenum with a Content from 1.0 atom% to 3.0 atom% to good microstructures with a high structural homogeneity led.

Furthermore has an alloy according to the invention a composition of Ti - z Al - y Nb - x B with 44.5 atom% ≦ z ≦ 47 atom%, in particular with 44.5 atom% ≤ z ≤ 45.5 atom %, 5 atom% ≤ y ≤ 10 atom% and 0.05 at% ≦ x ≦ 0.8 at%, or a composition of Ti - z Al - y Nb - w C with 44.5 atomic% ≤ z ≤ 47 atomic%, in particular with 44.5 atom% ≤ z ≤ 45.5 atom %, 5 atom% ≤ y ≤ 10 atom% and 0.05 at% ≤ w ≤ 0.8 atom % on, each molybdenum (Mo) in the range between 0.1 atom% to 3 atom% included.

alternative If an alloy consists of Ti - z Al - y Nb - x B - w C 44.5 atom% ≦ z ≦ 47 atom%, in particular with 44.5 atom% ≤ z ≤ 45.5 atom %, 5 atom% ≦ y ≦ 10 atom%, 0.05 atom% ≤ x ≤ 0.8 atom % and 0.05 at% ≤ w ≤ 0.8 atom % and additionally made of molybdenum in the range between 0.1 atom% to 3 atom%.

through the specified alloys and the corresponding alloy contents become high-strength γ-titanium aluminide alloys with a fine dispersion of β-phase for one wide range of process temperatures generated.

In the present invention, the desired structural stability and process reliability is achieved by the occurrence of single-phase regions over the entire, in the manufacturing processes and during operation passed through temperature area is avoided by the targeted incorporation of the cubic-body-centered β-phase. In principle, the β-phase occurs in all technical titanium aluminide alloys as a high-temperature phase at temperatures ≥ 1350 ° C.

Out The literature is known to this phase by different elements how Mo, W, Nb, Cr, Mn and V are stabilized at lower temperatures can. The particular problem with alloying these elements is however, that the β-stabilizing Elements must be tuned very precisely to the Al content. In addition, kick when adding these elements undesirable interactions, too high levels of β-phase and lead to a coarse dispersion of this phase. Such a constitution is for the mechanical properties extremely disadvantageous.

Farther hang also the properties of the β-phase of the respective alloying elements and their composition from. In particular, the constitution must be chosen so that an excretion of the brittle ω-phase the β-phase is largely avoided. Because of these relationships will be an alloy composition provided with one for the mechanical properties optimal composition and dispersion of the β-phase for a wide range of process temperatures can be realized. At the same time the best possible strength properties are achieved.

According to one advantageous embodiment of the invention, the alloy also contains Boron, preferably with a boron content in the alloy in the range from 0.05 at% to 0.8 at%. The addition of boron advantageously leads for the formation of stable precipitates, for the mechanical hardening of the alloy according to the invention and stabilization of the structure contribute to the alloy.

Furthermore it is advantageous if the alloy contains carbon, and although preferably with a carbon content in the range of 0.05 atom % to 0.8 at%. Also, the addition of carbon, preferably in combination with the above-mentioned additive boron leads to Formation of stable precipitates, which also contribute to the mechanical hardening alloy and stabilize the structure.

The The object is further achieved by a component which is produced from an alloy according to the invention is. To avoid repetition is to the preceding versions expressly directed.

The Invention will be described below without limiting the general inventive concept of exemplary embodiments with reference to the attached schematic drawings described by way of example, to the rest with respect to the Revelation of all unspecified in the text details of the invention is referenced. Show it:

1 a scanning electron micrograph of a cast billet with a Ti - 45 Al - 8 Nb - 0.2 C alloy (atom%);

2a to 2c in each case a photograph of a structure in an alloy Ti - 45 Al - 8 Nb - 0.2 C (atom%) by means of a scanning electron microscope according to various process steps;

3a and 3b in each case a picture of a microstructure in an alloy according to the invention Ti - 45 Al - 5 Nb - 2 Mo (atom%) according to various method steps and

4 a diagram with stress-strain curves of samples of the alloy Ti - 45 Al - 5 Nb - 2 Mo (atom%).

In 1 Two photographs of a structure in a cast block of the alloy Ti - 45 Al - 8 Nb - 0.2 C (atom%) are shown. The images as well as all further images in the following figures were recorded by means of backscattered electrons in a scanning electron microscope.

The structure ( 1 ) shows lamellar colonies of the α ₂ and γ phases, which originated from former γ lamellae. The former γ-lamellae are separated by strips of light-emitting grains of the β or B2 phase. The α-lamellae initially formed in the β-α transformation decompose on further cooling in α ₂ - and γ-lamellae.

In the 2a to 2c Further images of the microstructure of the alloy T-45 Al-8 Nb-0.2 C are shown after various process steps in the scanning electron micrographs. 2a shows the structure after extrusion at 1230 ° C. The extrusion direction is horizontal. The microstructure shows grains of the α ₂ and γ phases, with the cubic body-centered β phase disappearing.

2 B shows the structure of the alloy after extrusion at 1230 ° C and another forging step at 1100 ° C. The microstructure shows grains of the α ₂ and γ phases and a few α ₂ / γ lamellar colonies.

In 2c the structure of the alloy after extrusion at 1230 ° C and a subsequent heat treatment at 1330 ° C is shown. The microstructure also shows grains of the α ₂ and γ phases. The picture shows a fully lamellar microstructure with lamellae of α ₂ and γ phase. The lamellar colony size is about 200 microns, which also colonies occur that are significantly larger than 200 microns.

As with the in 2a shown structure also occurs in the 2 B and 2c represented structures the cubic body-centered phase no longer on. Thus, the β-phase is thermodynamically unstable in this temperature range with heat treatment after extrusion.

In the 3a and 3b Microstructures of an alloy according to the invention are shown in two scanning electron micrographs. Starting from an alloy Ti - 45 Al - 5 Nb, the alloy molybdenum was alloyed with 2 atom%. This resulting alloy Ti - 45 Al - 5 Nb - 2 Mo is based on a composition as described in the European patent specification EP 1 015 650 B1 is described.

The 3a and 3b show the microstructures of this alloy according to the invention, which after extrusion at 1250 ° C and a subsequent heat treatment at 1030 ° C ( 3a ) and at 1270 ° C were observed ( 3b ).

The structure in 3a shows grains of the α ₂ , γ and light-imaging β phases, the latter being arranged in stripes. The structure in 3b shows lamellar colonies of the α ₂ - and γ-phase and grains of the light-imaging β-phase, from which in turn the γ-phase has been eliminated.

The microstructure in 3a and 3b are fine, very homogeneous and show a uniform distribution of the β-phase. After heat treatment at 1030 ° C., a globular structure is present, with β-phase grains arranged in strips parallel to the extrusion direction ( 3a ), while the material heat-treated at 1270 ° C has a very homogeneous, fully lamellar microstructure with evenly distributed β-grains ( 3b ).

The Colony size of the structure of the alloy Ti - 45 Al - 5 Nb - 2 Mo is between 20 to 30 microns and is thus smaller by at least a factor of 5 than otherwise in fully lamellar structures of γ-titanium aluminide alloys. Within the β phase will also the γ phase excreted, so that the β grains very much be finely divided. This will be a very fine overall and homogeneous structure reached.

In Try it has been found that this fine and homogeneous microstructure morphology after heat treatments throughout the high temperature range up to 1320 ° C is present. The microstructures show that clearly that over the whole, for the manufacturing processes relevant temperature range sufficient Volume fraction of the β-phase is present and grain coarsening effectively suppressed becomes.

In Tensile tests performed on material heat treated at 1030 ° C was at room temperature, a yield strength of 867 MPa, a tensile strength of 816 MPa and a plastic elongation at break measured at 1.8%.

4 shows measured stress-strain curves of samples of the alloy Ti-45 Al-5 Nb-2 Mo in a tensile test. The sample material was extruded at 1250 ° C and then subjected to a heat treatment of 2 hours at 1030 ° C and oven cooling. The tensile curves recorded at 700 ° C and 900 ° C show that the alloy is suitable for many high temperature applications.

By the alloying of low molybdenum contents becomes a very uniform microstructure achieved in the alloy, making these alloys as high-temperature materials can be used well.

In addition, in 4 the result of a tensile test at room temperature (25 ° C) shown on the material according to the invention, wherein the tensile stress σ in MPa against the strain ε in% is plotted. In this case, a yield strength increase was found, which was otherwise not observed on γ-Titanaluminid-alloys so far. This is an indication of a particularly fine and homogeneous structure. The yield strength increase indicates that the material can react to local stresses by plastic flow, which is very favorable for the ductility and damage tolerance.

The homogeneity the alloys of the invention hangs in the Range of relevant process temperatures not technically unavoidable fluctuations the temperature or the composition.

The titanium aluminide alloys according to the invention were prepared using pouring or powder metallurgy techniques. For example can by hot forging, hot pressing or hot extrusion and hot rolling the alloys according to the invention to be edited.

The invention offers the advantage that despite occurring in industrial production Variations in alloy composition and process conditions more reliable than previously provided a titanium aluminide alloy with a very uniform microstructure and high strength.

The Titanium aluminide alloy according to the invention achieves high strength up to a temperature in the range from 700 ° C up to 800 ° C as well as a good room temperature ductility. Thus, the alloys are Legie for many Applications suitable and can e.g. For particularly heavily loaded components or for titanium aluminide alloys exceptionally high Temperatures are used.

Claims (5)

An alloy based on titanium aluminides prepared using fusion and powder metallurgy techniques and having an alloy composition of Ti - z Al - y Nb of 44.5 at% ≤ z ≤ 47 at%, in particular at 44.5 at% ≤ z ≤ 45 , 5 at%, and 5 at% ≦ y ≦ 10 at%, characterized in that it contains molybdenum (Mo) in the range between 0.1 at% to 3 at%. Alloy based on using melting and powder metallurgical Techniques produced titanium aluminides with an alloy composition made of Ti - z Al - y Nb - x B with 44.5 atom% ≦ z ≦ 47 atom%, in particular with 44.5 atom% ≤ z ≤ 45.5 atom %, 5 atom% ≤ y ≤ 10 atom% and 0.05 at% ≤ x ≤ 0.8 atom %, characterized in that these molybdenum (Mo) in the range between 0.1 atom% to 3 atom%. Alloy based on using melting and powder metallurgical Techniques produced titanium aluminides with an alloy composition made of Ti - z Al - y Nb - w C with 44.5 atom% ≦ z ≦ 47 atom%, in particular with 44.5 atom% ≤ z ≤ 45.5 atom %, 5 atom% ≤ y ≤ 10 atom% and 0.05 at% ≤ w ≤ 0.8 atom %, characterized in that these molybdenum (Mo) in the range between Contains 0.5 atom% to 3 atom%. Alloy based on using melting and powder metallurgical Techniques produced titanium aluminides with an alloy composition made of Ti - z Al - y Nb - x B - w C with 44.5 atom% ≦ z ≦ 47 atom%, in particular with 44.5 atom% ≤ z ≤ 45.5 atom %, 5 atom% ≦ y ≦ 10 atom%, 0.05 atom% ≤ x ≤ 0.8 atom % and 0.05 at% ≤ w ≤ 0.8 atom %, characterized in that these molybdenum (Mo) in the range between 0.1 atom% to 3 atom%. Component made of an alloy according to one the claims 1 to 4.