RU2370561C2 - Alloy on base of titanium aluminides - Google Patents

Abstract

FIELD: metallurgy. ^ SUBSTANCE: invention refers to alloys on base of titanium aluminides produced by melting or by power metallurgy with composition Ti-zAl-yNb where 44.5ëñzëñ45.5 atom % and 5ëñyëñ10 atom %; also composition contains molybdenum 0.1ëñMoëñ3 atom % and has fine dispersed -phase in -titanium-aluminide alloy in temperature range up to 1320C. Alloy can additionally contain B and/or C at amount of 0.05ëñBëñ0.8 atom % and 0.05ëñCëñ0.8 atom %. ^ EFFECT: alloys possess high mechanical properties. ^ 5 cl, 4 dwg

Description

The invention relates to alloys based on titanium aluminides obtained using pyrometallurgy and powder metallurgy methods with an alloy composition of Ti-zAl-yNb with 44.5 at.% ≤z≤47 at.%, In particular 44.5 at.% ≤ z≤45.5 at.%, and 5 at.% ≤y≤10 at.%, and also, if necessary, additives B and / or C with a content between 0.05 at.% and 0.8 at. %

Titanium-aluminum alloys have properties that are particularly suitable for use as lightweight materials, in particular for high temperature applications. For industrial practice, alloys based on the γ- (TiAl) intermetallic phase with a tetragonal structure and, in addition to the γ- (TiAl) intermetallic phase, also contain a small fraction of the α ₂ (Ti ₃ Al) intermetallic phase with a hexagonal structure, are especially interesting for industrial practice. These γ-titanium aluminum alloys are characterized by properties such as low density (3.85-4.2 g / cm ³ ), high modulus of elasticity, high strength and creep resistance up to 700 ° C, which make them attractive as materials for mobile structural elements at elevated temperatures of use. Examples of these structural elements are turbine blades in aircraft engines and stationary gas turbines, valves for motors, and fans for hot gases.

In the technically important range of alloys with an aluminum content between 45 at.% And 49 at.%, A number of phase transformations are observed upon solidification from the melt and upon further cooling. Solidification can occur either completely through a β-solid solution with a cubic body-centered structure (high-temperature phase), or by two peritectic reactions in which an α-solid solution with a hexagonal structure and a γ-phase are involved.

The niobium (Nb) element is known to increase strength, creep resistance, oxidation stability, and ductility. By using boron which is practically insoluble in the γ phase, grinding of grains can be achieved both in the molten state and after pressure treatment followed by heat treatment in the α region. An increased proportion of the β-phase in the structure due to the reduced aluminum content and high concentrations of β-stabilizing elements can lead to a rough dispersion of this phase and cause a deterioration in mechanical properties.

The mechanical properties of γ-titanaluminide alloys are highly anisotropic due to their behavior during molding and fracture, as well as due to structural anisotropy of a preferably adjusted lamellar structure or duplex structure. For targeted regulation of the structure and texture in the manufacture of structural elements from titanium aluminides, casting methods, various powder metallurgy methods and pressure processing methods, as well as combinations of these manufacturing methods are used.

From the publications of Y-W.Kim, D.M. Dimiduk in “Structural Intermetallics 1997”, Eds. MVNathal, R. Darolia, CTLiu, PLMartin, DBMiracle, R. Wagner, M. Yamaguchi, TMS, Warrendale PA, 1996, p. 531, it is known that in various development programs the effect of a very large number of alloying elements on structure, regulation of the structure with various production methods and individual properties. The relationships found are in this case as complex as is the case with other structured metals, for example, steels, and allows conclusions to be drawn only in a limited and very general form. Therefore, certain formulations may be characterized by varying combinations of properties.

A titanium aluminum alloy is known from EP 1015605 B1, which has a structurally and chemically homogeneous structure. In this case, the main phases γ (TiAl) and α ₂ (Ti ₃ Al) are distributed in the form of a fine dispersion. The described titanium-aluminum alloy with an aluminum content of 45 at.% Is characterized by extremely good mechanical properties and high temperature properties.

A common problem of all titanium-aluminum alloys is their low ductility. So far, it has not been possible to significantly improve the high brittleness and low resistance to damage to titanium-aluminum alloys given by the nature of the intermetallic phases by the alloying effect (cf. “Structural Intermetallics 1997”, p. 531, see above), namely, for the applications mentioned in the introduction, multiple plastic elongations ≥1%. However, turbine and engine manufacturers require this minimum ductility to be guaranteed in industrial production through a large number of series. Since plasticity significantly depends on the structure, it is extremely difficult to ensure the formation of the most homogeneous structure in industrial production processes. For high-strength alloys, the maximum permissible size of defects, for example, the maximum grain size or colony of lamellas, is especially small, so a very high structural homogeneity is desirable for such alloys. However, this can only be achieved with great difficulty due to inevitable fluctuations in the composition of the alloy, for example ± 0.5 at.% In Al content.

Currently, of the many possible types of structures in γ-titanium-aluminum alloys for high-temperature applications, only lamellar or so-called double (duplex) structures are taken into account. The former arise upon cooling from a single-phase region of an α-solid solution, in which γ-phase plates are crystallographically oriented from the α-solid solution.

In contrast, duplex structures consist of colonies of lamellas and γ grains and are formed when the material is annealed in the two-phase region α + γ. At the same time, the α-grains existing there, upon cooling, again turn into two-phase lamella colonies. The coarse components of the structure appear in γ-titanium-aluminum alloys primarily due to the fact that large α-grains are formed during the passage of the α region. This can occur already during solidification, when large columnar crystals of the α phase are formed from the melt. As a result of this, the single-phase region of the α-solid solution should be avoided whenever possible. However, since in practice there are fluctuations in the composition and temperature of the process, and therefore the structure in the processed product varies locally, the formation of large colonies of lamellas cannot be excluded.

Based on this prior art, the present invention is based on the task of making available a titanium-aluminum alloy with a fine and homogeneous structural morphology, the variations in the alloy composition encountered in industrial practice, as well as the inevitable temperature fluctuations during the production process, should not have a worthy effect on the alloy homogeneity , in particular without fundamental changes in the method of obtaining. Further, the task is to make available a structural element of a homogeneous alloy.

This problem is solved by means of an alloy based on titanium aluminides obtained using pyrometallurgy and powder metallurgy methods with an alloy composition of Ti-zAl-yNb with 44.5 at.% ≤z≤47 at.%, In particular 44.5 at. % ≤z≤45.5 at.%, And 5 at.% ≤y≤10 at.%, Which is improved due to the fact that it contains molybdenum (Mo) in the range between 0.1 at.% And 3.0 at.%. The remainder of the alloy consists of Ti (titanium).

It was shown in experiments that by doping with molybdenum in the case of titanium aluminides with a certain fraction of niobium, for which the β phase is mostly unstable in the entire temperature range, and therefore the remaining high-temperature β phase decomposes during ordinary process steps, such as hot pressing The best homogeneity of the alloy structure is achieved. Thus, over the entire temperature range essential for the process of obtaining, the volume fraction of the β phase is realized without grain enlargement. This type of alloy according to the invention, due to the fine and very uniform dispersion of the β-phase, has a homogeneous structure with high strength values.

Thus, an alloy is proposed that is suitable as a lightweight material for high temperature applications such as, for example, turbine blades or components of turbines and motors.

The alloy according to the invention is obtained using metallurgical casting methods, pyrometallurgical methods or powder metallurgy methods, or using these methods in combination with pressure treatment.

First of all, for the Ti - alloy (from 44.5 at.% To 45.5 at.%), Al - (from 5 at.% To 10 at.%) Nb is a molybdenum additive with a content of from 1.0 at.% To 3.0 at.% Leads to a good microstructure with high structural homogeneity.

In addition, the alloy according to the invention is characterized by a composition of Ti-zAl-yNb-xB with 44.5 at.% ≤z≤47 at.%, In particular 44.5 at.% ≤z≤45.5 at.%, 5 at.% ≤y≤10 at.%, and 0.05 at.% ≤x≤0.8 at.%, or a composition of Ti-zAl-yNb-wC with 44.5 at.% ≤z≤47 at %, in particular 44.5 at.% ≤z≤45.5 at.%, 5 at.% ≤y≤10 at.% and 0.05 at.% ≤w≤0.8 at.%, which accordingly contains molybdenum (Mo) in the range between 0.1 at.% and 3 at.%.

Alternatively, the alloy consists of Ti-zAl-yNb-xB-wC with 44.5 at.% ≤z≤47 at.%, In particular 44.5 at.% ≤z≤45.5 at.%, 5 at.% ≤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.% and 3 at.%.

Using these alloys and the corresponding alloy content, high-strength γ-titanium aluminum alloys are produced with a fine dispersion of the β-phase for a wide range of process temperatures.

In the present invention, the desired stability of the structure and reliability of the process is achieved due to the fact that the occurrence of single-phase regions is avoided throughout the entire process of preparation and when applying the temperature range by targeted introduction of a cubic volume-centered β-phase. Fundamentally, the β-phase appears in all technical titanium-aluminum alloys as a high-temperature phase at temperatures ≥1350 ° С.

It is known from the literature that this phase can be stabilized with various elements, such as Mo, W, Nb, Cr, Mn, and V, to low temperatures. However, a particular problem with doping with these elements is that the β-stabilizing elements must be very precisely matched to the Al content. In addition, when these elements are added, undesirable interactions arise that lead to a high proportion of the β phase and to a rough dispersion of this phase. A structure of this kind is extremely harmful to mechanical properties.

Further, the properties of the β phase also depend on the corresponding alloying elements of the alloy and its composition. In particular, the structure should be chosen such that the isolation of the brittle ω phase from the β phase is substantially avoided. For this reason, the composition of the alloy is proposed, with which the composition and dispersion of the β-phase, optimal for mechanical properties, can be realized for a wide range of process temperatures. At the same time, as good as possible strength properties are achieved.

According to a preferred embodiment of the invention, the alloy also contains boron, preferably the boron content in the alloy is in the range from 0.05 at.% To 0.8 at.%. The addition of boron preferably leads to the formation of stable precipitates, which contribute to the mechanical hardening of the alloy according to the invention and the stabilization of the alloy structure.

In addition, it is preferable when the alloy contains carbon, and particularly preferably the carbon content is in the range from 0.05 at.% To 0.8 at.%. The carbon addition also, preferably in combination with the boron additive described above, leads to the formation of stable precipitates, which also contribute to the mechanical hardening of the alloy according to the invention and the stabilization of the structure.

Next, the problem is solved using a structural element, which is made of an alloy according to the invention. In order to avoid repetitions, reference is specifically made to previous implementations.

Further, without limiting general ideas, the invention is approximately described using examples of implementation with reference to the attached schematic diagrams, which indicate in the rest with respect to the disclosure all the details of the invention not explained in detail in the text. They show the following:

figure 1 is a photograph in a scanning electron microscope of an ingot from an alloy of Ti-45Al-8Nb-0.2C (at.%);

figa-2c, respectively, a snapshot of the structure of the alloy Ti-45Al-8Nb-0.2C (at.%) using a scanning electron microscope after various stages of the method;

figa and 3b, respectively, a snapshot of the structure of the alloy according to the invention Ti-45Al-5Nb-2Mo (at.%) after various stages of the method;

4 is a diagram with stress-elongation curves of a sample of a Ti-45Al-5Nb-2Mo alloy (at.%).

Figure 1 shows two snapshots of the structure in the ingot of the alloy Ti-45Al-8Nb-0.2C (at.%). These images, as well as all other images in the subsequent drawings, were made using backscattering electrons on a scanning electron microscope.

The structure (Fig. 1) shows the colonies of α ₂ and γ phase lamellae that arose from the previous γ lamellas. The old γ-lamellas are separated by bands of light grains of the β or B2 phase. The α-lamellas formed initially during the α-β transformation decompose upon further cooling into α ₂ and γ-lamellas.

On figa-2c shows another pictures in a scanning electron microscope of the structure of the alloy Ti-45Al-8Nb-0.2C (at.%) After various stages of the method. Figure 2a shows the structure after hot pressing at 1230 ° C. The direction of hot pressing is horizontal. The structure shows grains of the α ₂ and γ phases, and the cubic body-centered β phase has disappeared.

Fig.2b depicts the structure of the alloy after hot pressing at 1230 ° C and the subsequent stage of forging at 1100 ° C. The structure shows grains of the α ₂ and γ phases and several small colonies of α ₂ / γ lamellas.

Figure 2c shows the alloy structure after hot pressing at 1230 ° C and subsequent heat treatment at 1330 ° C. The structure also exhibits grains of the α ₂ and γ phases. The picture shows a completely lamellar structure with lamellae of the α ₂ and γ phases. The size of the lamella colonies is approximately 200 μm, and colonies that are clearly larger than 200 μm also occur.

As in the structure shown in FIG. 2a, and in the structures shown in FIGS. 2b and 2c, there is no longer a cubic body-centered phase. Thus, the β phase in this temperature range with heat treatment after hot pressing is not thermodynamically stable.

Figures 3a and 3b show the alloy structures according to the invention in two photographs using a scanning electron microscope. Based on the Ti-45Al-5Nb alloy, this alloy was alloyed with 2 at.% Molybdenum. This obtained alloy Ti-45Al-5Nb-2Mo is based on a composition similar to that described in European patent EP 1015650 B1.

On figa and 3b shows the structure of this alloy according to the invention, which was observed after hot pressing at 1250 ° C and subsequent heat treatment at 1030 ° C (Fig.3a), as well as at 1270 ° C (Fig.3b).

The structure of FIG. 3a shows grains of α ₂ -, γ - and the light β-phase, the latter being arranged in strips. The structure of FIG. 3b shows the lamellar colonies of the α ₂ and γ phases, as well as the grains of the light β-phase, from which the γ-phase is again distinguished.

The structures in FIGS. 3a and 3b are thin, very homogeneous, and exhibit a uniform distribution of the β phase. After heat treatment at 1030 ° C, a globular structure takes place, with β-phase grains arranged in strips parallel to the hot pressing direction (Fig. 3a), while the material subjected to heat treatment at 1270 ° C has a very homogeneous, completely lamellar structure with evenly distributed β-grains (fig.3b).

The colony size in the structure of the Ti-45Al-5Nb-2Mo alloy is between 20 and 30 μm and is thus at least 5 times smaller than that typical in the fully lamellar structures of γ-titanium aluminum alloys. In addition, a γ-phase is released inside the β-phase, so that β-grains are distributed very finely. Thus, in general, a very fine and homogeneous structure is achieved.

Studies have shown that this subtle and homogeneous structural morphology takes place after heat treatment in the entire high temperature range up to 1320 ° C. The structure is definitely characterized by the fact that throughout the entire range of temperatures important for the process of obtaining, there is a sufficient volume fraction of the β phase and grain coarsening is effectively suppressed.

In tensile tests, which were carried out on a material subjected to heat treatment at 1030 ° C, a yield strength of 867 MPa, a tensile strength of 816 MPa, and a plastic ultimate elongation of 1.8% were measured at room temperature.

Figure 4 shows the stress-elongation curves measured in tensile tests of a Ti-45Al-5Nb-2Mo alloy specimen. A sample of the material was hot pressed at 1250 ° C and then heat treated for 2 hours at 1030 ° C and cooled in an oven. Tensile curves obtained at 700 ° C and 900 ° C show that the alloy is suitable for many high temperature applications.

Using additional alloying with a small amount of molybdenum, a very uniform microstructure is achieved in the alloy, so that these alloys can be easily used as high-temperature materials.

In addition, figure 4 presents the results of a tensile test at room temperature (25 ° C) for the material according to the invention, the tensile stress σ is given in MPa, and the elongation ε is in%. In this case, a yield strength peak was found, which up to now has not usually been observed in γ-titanium-aluminum alloys. This is a sign of a particularly fine and homogeneous structure. The peak yield strength indicates that the material can respond to local stresses by plastic flow, which is very favorable for ductility and resistance to damage.

The homogeneity of the alloys according to the invention in the range of important process temperatures does not depend on the technically unavoidable fluctuations in temperature or composition.

The titanium-aluminum alloys according to the invention were obtained using casting methods or powder metallurgy. For example, the alloys of the invention can be machined by hot forging, hot pressing or hot extrusion and hot rolling.

The invention has the advantage that, despite fluctuations in the composition of the alloy and the process conditions existing in industrial production, a more reliable titanium aluminum alloy with a very uniform microstructure and high strength is provided.

The titanium aluminum alloys according to the invention achieve high strength up to a temperature in the range of 700 ° C to 800 ° C, as well as good ductility at room temperature. Thus, these alloys are suitable for a large number of applications and can, for example, be used for particularly high loaded structural elements or at extremely high temperatures for titanium-aluminum alloys.

Claims (5)

1. Alloy based on titanium aluminide obtained by melting or powder metallurgy, with the composition Ti-zAl-yNb, where 44.5≤z≤47 at.%, In particular 44.5≤z≤45.5 at.% And 5 ≤y≤10 at.%, Characterized in that it additionally contains molybdenum in an amount of 0.1≤Mo≤3 at.% And has a finely divided β-phase in the γ-titanium-aluminum alloy in the temperature range up to 1320 ° C. 2. Alloy based on titanium aluminide obtained by melting or powder metallurgy, with the composition Ti-zAl-yNb-xB, where 44.5≤z≤47 at.%, In particular 44.5≤z≤45.5 at.% , 5≤y≤10 at.% And 0.05≤x≤0.8 at.%, Characterized in that it additionally contains molybdenum in an amount of 0.1≤Mo≤3 at.% And has a finely divided β-phase in γ-titanium aluminum alloy in the temperature range up to 1320 ° C. 3. Alloy based on titanium aluminide obtained by melting or powder metallurgy, with the composition Ti-zAl-yNb-wC, where 44.5≤z≤47 at.%, In particular 44.5≤z≤45.5 at.% , 5≤y≤10 at.% And 0.05≤w≤0.8 at.%, Characterized in that it additionally contains molybdenum in an amount of 0.1≤Mo≤3 at.% And has a finely divided β-phase in γ-titanium aluminum alloy in the temperature range up to 1320 ° C. 4. Alloy based on titanium aluminide obtained by melting or powder metallurgy, with the composition Ti-zAl-yNb-xB-wC, where 44.5≤z≤47 at.%, In particular 44.5≤z≤45.5 at .%, 5≤y≤10 at.%, 0.05≤x≤0.8 at.% And 0.05≤w≤0.8 at.%, Characterized in that it additionally contains molybdenum in an amount of 0, 1≤Mo≤3 at.% And has a finely divided β-phase in the γ-titanium-aluminum alloy in the temperature range up to 1320 ° C. 5. A structural element made of an alloy according to any one of claims 1 to 4.

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