EP1819838B1 - Titanium aluminide based alloy - 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 invention relates to alloys based on titanium aluminides prepared using melt and powder metallurgy techniques with an alloy composition according to claim 1

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, making them attractive as a material for moving parts at elevated service temperatures. 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%, a number of phase transformations occur on solidification from the melt and subsequent cooling. The solidification can take place either completely via the β-mixed crystal with cubic body-centered structure (high-temperature phase) or in two peritectic reactions in which the α-mixed crystal with hexagonal structure and the γ-phase are involved.

Furthermore, it is known that the element niobium (Nb) leads to an increase in strength, creep resistance, oxidation resistance, but also ductility. With the element boron, which is virtually insoluble in the γ-phase, grain refining can be achieved both in the cast state and after the forming with subsequent heat treatment in the α-region. An increased proportion of β-phase in the microstructure due to low aluminum contents and high concentrations of β-stabilizing elements can lead to coarse dispersion of this phase and cause a deterioration of the mechanical properties.

The mechanical properties of γ-titanium aluminide alloys are the preferred because of their deformation and fracture behavior, but also because of the microstructural anisotropy lamellar structure or duplex structure strongly anisotropic. For a specific adjustment of structure and texture in the production of components made of titanium aluminides casting methods, different powder metallurgy and forming methods and combinations of these production methods are used.

From the publication of YW. Kim and Dimiduk in Structural Intermetallics 1997, Eds. MV Nathal, R. Darolia, CT Liu, PL Martin, DB Miracle, R. Wagner, M. Yamaguchi, TMS, Warrendale PA, 1996, p. 531 It is known that in various development programs the effect of a larger number of alloying elements with respect to constitution, microstructure adjustment in different production processes and individual properties has been investigated. The relationships found here are similarly complex, as is the case with other structural metals, eg steels, and can only be summarized in rules in a limited and very general form. Therefore, certain compositions may have outstanding combinations of properties.

Out EP 1 015 650 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.

A common problem of all titanium aluminide alloys is their low ductility. So far, the high brittleness and low damage tolerance of titanium aluminide alloys due to the nature of the intermetallic phases has not been achieved by alloying effects decisively improve (see " Structural Intermetallics 1997, p. 531 , see above). For the applications mentioned in the introduction, plastic elongations at break of ≥ 1% are often sufficient. Manufacturers of turbines and engines, however, demand that this minimum level of ductility be guaranteed in large-scale industrial production. Since the ductility is sensitive to the microstructure, it is extremely difficult in the industrial manufacturing process to ensure as homogeneous a microstructure as possible. For high-strength alloys, the maximum tolerable defect size, eg the maximum grain or lamellar colony size, is particularly small, so that a very high structural homogeneity is desirable for such alloys. However, this can only be achieved with difficulty because of the unavoidable fluctuations in the alloy composition of, for example, ± 0.5 atom% in the Al content.

Currently, of the many types of microstructures possible in γ-titanium aluminide alloys, only lamellar or so-called duplex microstructures are considered for high temperature applications. The former arise during cooling from the single-phase region of the α-mixed crystal by precipitating plates of the γ-phase crystallographically oriented from the α-mixed crystal.

In contrast, duplex microstructures consist of lamellar colonies and γ grains and are formed when the material is annealed in the two-phase region α + γ. The α grains which are present there are converted back into two-phase lamellar colonies on cooling. Coarse microstructure constituents are formed in γ-titanium aluminide alloys mainly by forming large α grains as they pass through the α region. This can already happen during the solidification when large columnar crystals of α-phase form from the melt. Thus, as far as possible, the single-phase region of the α-mixed crystal must be avoided during processing. Since in practice, however, fluctuations of the composition and process temperatures occur and therefore the constitution varies locally in the workpieces, the formation of coarse lamellar colonies can not be excluded.

In addition revealed EP-0 549 181 a titanium aluminide of the γ-type comprising titanium, aluminum, chromium and niobium in the composition TiAl _46-48 Cr _1-3 Nb _6-14 , wherein the alloy is made by casting and hot isostatic pressing (HIP).

In JP-A-06 116691 discloses a heat treatment process for TiAl alloys of composition Ti - (45 - 50) atomic% Al - (3 - 12) atomic% (Nb, Mo, Cr) wherein in one or two of said elements Nb, Mo and Cr in the alloys are included. According to the technical teaching, the alloys in the α-phase region are annealed and then rapidly cooled to suppress the equilibrium conversion α → α + γ → α ₂ + γ and instead cause a massive conversion of α → y. Following this, an age glow will be carried out.

Moreover disclosed US-A-5,393,356 a multi-phase, high-temperature material made of an alloy based on an intermetallic compound of the type y -TiAl, wherein the TiAl alloy in addition to a titanium content of 35 to 45 atom% and an aluminum content of 45 to 60 atom% also alloyed amounts of silicon (0.1 to 20 at.%) and niobium (0.1 to 15 at.%), thereby improving the oxidation resistance of the alloy.

Starting from this prior art, the present invention has the object to provide a titanium aluminide alloy with a fine and homogeneous Gefömgemorphologie, with occurring in industrial practice, variations in the alloy composition and unavoidable temperature fluctuations in the manufacturing process hardly or not appreciably on the homogeneity of the alloy especially without major changes to the manufacturing processes. Furthermore, the object is to provide a component of a homogeneous alloy.

This object is achieved by means of an alloy based on titanium aluminides produced using fusion and powder metallurgy techniques and having an alloy composition of 44.5 atom% ≦ Al ≦ 47 atom%, in particular 44.5 atom% ≦ Al ≦ 45.5 At% atomic% and at least 5 atom% ≦ Nb ≦ 10 atom%, molybdenum (Mo) in between 0.1 atom% to 3.0 atom%, optional B and / or C with contents ≥ 0.05 atom% and ≦ 0, 8 atom%, and the remainder titanium and common impurities, wherein the γ-titanium aluminide alloy is formed with a fine dispersion of the β-phase and the β-phase is present up to a temperature of below 1320 ° C. The rest of the alloy is Ti (titanium).

It has been shown by experiments that by adding molybdenum to titanium aluminides having a niobium content in which usually the β-phase is not stable over the entire temperature range and therefore remains of the high-temperature β-phase in the conventional process steps such as Dissolve extruders, a better structural homogeneity of the alloy is achieved. Thus, over the entire, relevant for the manufacturing process temperature range, a volume fraction of the β-phase without

Grain coarsening realized. This type of alloy according to the invention then has a homogeneous structure with high strength values due to the fine and very uniform dispersion of the β phase.

Thus, an alloy is provided which can be used as a lightweight material for high temperature applications, e.g. Turbine blades or engine and turbine components, is suitable.

The alloy of the invention is produced using casting metallurgy, melt metallurgy or powder metallurgy processes or using these processes in combination with forming techniques.

Especially with Ti - (44.5 at% to 45.5 at%) Al - (5 at% to 10 at%) Nb, the addition of molybdenum at a content of 1.0 at% to 3.0 at% led to good microstructures with a high structural homogeneity.

In addition, an alloy according to the invention has 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 atom% ≤ x ≤ 0.8 atom% and remainder titanium and usual impurities, or a composition 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 atom% ≤ w ≤ 0.8 atom% and the balance titanium and conventional impurities, each containing molybdenum (Mo) in the range between 0.1 atom% to 3 atom% and wherein the β-phase is present up to a temperature of below 1320 ° C.

Alternatively, an alloy 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 at%, 0.05 at% ≦ x ≦ 0.8 at% and 0.05 at% ≦ w ≦ 0.8 at% and additionally from molybdenum in the range between 0.1 at% to 3 at% wherein the β-phase is present up to a temperature below 1320 ° C.

By means of the specified alloys and the corresponding alloying contents, high strength γ-titanium aluminide alloys having a fine dispersion of β-phase are produced for a wide range of process temperatures.

In the present invention, the desired microstructure stability and process reliability is achieved by avoiding the occurrence of single-phase regions over the entire temperature range during the manufacturing processes and during use 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.

It is known from the literature that this phase can be stabilized by various elements such as Mo, W, Nb, Cr, Mn and V at lower temperatures. The particular problem with alloying these elements, however, is that the β-stabilizing. Elements must be tuned very precisely to the Al content. In addition, occur in the addition of these elements undesirable interactions, which leads to high levels of the β-phase and lead to a coarse dispersion of this phase. Such a constitution is extremely disadvantageous for the mechanical properties.

Furthermore, the properties of the β phase also depend on the respective alloying elements and their composition. In particular, the constitution must be chosen so that an excretion of the brittle ω-phase from the β-phase is largely avoided. Due to these relationships, an alloy composition is provided which can provide a mechanical properties optimum composition and dispersion of the β-phase for a wide range of process temperatures. At the same time the best possible strength properties are achieved.

According to an advantageous embodiment of the invention, the alloy also contains boron, preferably with a boron content in the alloy in the range of 0.05 atom% to 0.8 atom%. The addition of boron advantageously leads to the formation of stable precipitates which contribute to the mechanical hardening of the alloy according to the invention and stabilization of the microstructure of the alloy.

Moreover, it is advantageous if the alloy contains carbon, preferably with a carbon content in the range of 0.05 at% to 0.8 at%. The addition of carbon, preferably in combination with the above-described additive boron, leads to the formation of stable precipitates, which also contribute to the mechanical hardening of the alloy and to the stabilization of the structure.

The object is further achieved by a component which is produced from an alloy according to the invention. To avoid repetition, reference is expressly made to the above statements.

Fig. 1

eine Rasterelektronenmikroskopieaufnahme ei- nes Gussblocks mit einer Legierung Ti - 45 Al - 8 Nb - 0,2 C (Atom %);

Fig. 2a bis 2c

jeweils eine Aufnahme eines Gefüges in einer Legierung Ti - 45 Al - 8 Nb - 0,2 C (Atom %) mittels eines Rasterelektronenmikroskops nach verschiedenen Verfahrensschritten;

Fig. 3a und 3b

jeweils eine Aufnahme eines Gefüges in einer erfindungsgemäßen Legierung Ti - 45 Al - 5 Nb - 2 Mo (Atom %) nach verschiedenen Ver- fahrensschritten und

Fig. 4

ein Diagramm mit Spannungs-Dehnungskurven von Proben der Legierung Ti - 45 Al - 5 Nb - 2 Mo (Atom %).

The invention will now be described by way of example without limitation of the general inventive idea by means of embodiments with reference to the accompanying schematic drawings, to which reference is otherwise made with respect to the disclosure of all unspecified in the text details according to the invention. Show it:

Fig. 1

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

Fig. 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;

Fig. 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 (atomic%) according to different method steps and

Fig. 4

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

In Fig. 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 recordings as well as all further recordings in the following figures were recorded by means of backscattered electrons in a scanning electron microscope.

The structure ( Fig. 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 FIGS. 2a to 2c Further images of the microstructure of the alloy Ti - 45 Al - 8 Nb - 0.2 C are shown after different process steps in the scanning electron micrographs. Fig. 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.

Fig. 2b 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 Fig. 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, which are significantly larger than 200 microns.

As with the in Fig. 2a shown structure also occurs in the Fig. 2b 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 FIGS. 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 FIGS. 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 ( Fig. 3a ) and at 1270 ° C were observed ( Fig. 3b ).

The structure in Fig. 3a shows grains of the α ₂ , γ and light-imaging β phases, the latter being arranged in stripes. The structure in Fig. 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 Fig. 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 ( Fig. 3a ), while the material heat-treated at 1270 ° C has a very homogeneous, fully lamellar microstructure with evenly distributed β-grains ( Fig. 3b ).

The colony size of the microstructures of the Ti - 45 Al - 5 Nb - 2 Mo alloy is between 20 and 30 μm, which is at least a factor of 5 smaller than otherwise in fully lamellar microstructures of γ-titanium aluminide alloys. Moreover, within the β-phase, the γ-phase is precipitated, so that the β-grains are very finely subdivided. As a result, a very fine and homogeneous structure is achieved overall.

In experiments it has been found that this fine and homogeneous Gefömgemorphologie is present after heat treatments in the entire high temperature range up to 1320 ° C. The microstructures thus clearly show that over the entire temperature range relevant for the production processes a sufficient volume fraction of the β-phase is present and grain coarsening is effectively suppressed.

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

Fig. 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 alloying low molybdenum contents, a very uniform microstructure in the alloy is achieved, so that these alloys can be used well as high-temperature materials.

In addition, in Fig. 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 of the alloys according to the invention does not depend on technically unavoidable variations in temperature or composition in the range of relevant process temperatures.

The titanium aluminide alloys of this invention were prepared using casting or powder metallurgy techniques. For example, the alloys of the invention can be processed by hot forging, hot pressing or hot extrusion and hot rolling.

The invention offers the advantage that, despite the variations in alloy composition and process conditions occurring in industrial production, a titanium aluminide alloy having a very uniform microstructure and high strength is provided more reliably than heretofore.

The titanium aluminide alloy according to the invention achieves high strength up to a temperature in the range of 700 ° C. to 800 ° C. and good room temperature ductility. Thus, the alloys suitable for numerous applications and can be used eg for particularly highly stressed components or for titanium aluminide alloys exceptionally high temperatures.

Claims (3)

1. A titanium aluminide based alloy, produced by using melt metallurgical and powder metallurgical techniques with an alloy composition of 44.5 atom % ≤ Al ≤ 47 atom %, 5 atom % ≤ Nb ≤ 10 atom %, molybdenum between 0.1 atom % to 3 atom %, optionally B and/or C with contents ≥ 0.05 atom % and ≤ 0.8 atom %, and the remainder being titanium and conventional impurities, wherein the γ-titanium aluminide alloy is formed with a fine dispersion of the β phase and the β phase is present up to a temperature of less than 1320°C.
2. An alloy as claimed in claim 1, characterised in that the proportion of aluminium in the alloy composition is between 44.5 atom % to 45.5 atom %.
3. A component produced from an alloy as claimed in claim 1 or 2.

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