EP0733716A1 - Intermetallic alloy based on titanium aluminide and suitable for casting techniques - Google Patents

Abstract

An alloy which is readily shaped by casting comprises in atoms 48.5-52.5% Ti, 45.5-48.5% Al, 0.5-2.5, pref. 1-2% Re, 0-2.0% W, 0-3.5, pref. 3% Nb and 0-1.0, pref. 0.2-0.8% Si, with Re+W = 2.0-2.5% and Re+W+Nb = 2.0-5.5%. A casting of the alloy has a centred cubic crystal structure of mainly beta phase. Each beta grain consists a plurality of colonies of laminated structure, consisting of alternate layers of gamma and alpha-2 crystallography, oriented in 12 ways relative to the grain with neighbouring colonies being differently oriented.

Description

The invention relates to an intermetallic alloy based on titanium aluminide for the production of foundry parts.

The transformation by foundry of intermetallic alloys derived from titanium aluminide γ (TiAl) is considered with interest for the production of parts of aeronautical turbomachines. Foundry is in fact generally less expensive than other shaping processes. In addition, it has the advantage of preserving in principle the mechanical resistance to hot of the castings because the size of the metallurgical grains obtained is relatively large.

Although significant differences have been noted in the flowability of these alloys, that is to say their ability to form foundry parts of good quality, guaranteeing reliability and reproducibility of mechanical performance, no data is available to explain these differences, in particular in connection with the behavior of the alloys during their solidification and / or with their chemical composition.

In order to develop alloy compositions suitable for the foundry, the inventors undertook a study on the influence of various refractory addition elements on the flowability. They analyzed numerous TiAl-based alloys in which 2 to 10% of the atoms consisted of one or more of the addition elements Nb, Ta, Cr, Mo, W, Fe and Re, and in particular examined their microstructures both in the raw state of casting and after heat treatments. They thus came to the conclusion that the solidification process constitutes an important parameter for the quality of the foundry parts. The different alloys examined can indeed be classified in two categories, for which initially formed during solidification a phase of hexagonal crystal structure α and a phase of centered cubic structure β respectively.

0>_α//<1 $\overline{\text{1}}$

0>_γ inhérente au mécanisme de transformation de phase impliqué.In the case of solidification in the α phase, the initial crystals of this phase tend to form columnar grains according to the thermal gradient during solidification and the columnar character of the microstructure in the raw pouring state is often extremely pronounced due to the preferential growth of the crystals parallel to the axis c which is unique in the hexagonal α structure. In addition, all the lamellae of the γ phase, which precipitate in each of the columnar grains during subsequent cooling to form the so-called lamellar structure γ + α ₂ , are oriented perpendicular to the axis c of the hexagonal phase due to the relationship orientation (0001) _α // (111) _γ and <11 $\overline{\text{2}}$

0> _α // <1 $\overline{\text{1}}$

0> _γ inherent in the phase transformation mechanism involved.

This phase transformation mechanism makes it possible to explain certain serious difficulties encountered during the production of products cast from the alloys concerned, in particular various defects such as cracks of thermal origin and porosities introduced into the intercolumnar zone as well as a character strongly anisotropic of the products (texture), which may be harmful in terms of their mechanical performance. Most of the alloys developed so far, the best known of which is the grade Ti ₄₈ Al ₄₈ cr ₂ Nb ₂ described in US-A-4879092, belong to this category of alloys which solidify essentially in α and, when these alloys are used for foundry, it is necessary to resort to various technological means, although often hazardous, in order to reduce the columnar character of solidification and the texture which is associated with it. Consequently, these "first generation" alloys should rather be regarded as intended to be wrought, since the elimination of defects and the texture reduction can be achieved using appropriate thermomechanical treatments.

1>_β//<11 $\overline{\text{2}}$

0>_α, conduit théoriquement à la formation de douze variants α. Lorsque le refroidissement se poursuit, la phase γ précipite sous forme lamellaire dans chaque variant α. La microstructure résultante est caractérisée par la présence de nombreuses colonies (théoriquement jusqu'à douze variants d'orientation) à l'intérieur de chaque grain β initial. Chacune de ces colonies est constituée de nombreuses plaquettes (ou lattes) α, ces plaquettes (ou lattes) étant parfois délimitées par des liserés de phase β résiduelle. Chaque plaquette (ou latte) présente enfin la structure lamellaire γ+α₂. Une telle séquence de transformation se traduit par une minimisation des difficultés rencontrées dans les alliages se solidifiant en α avec la réduction de la fréquence des défauts de solidification et une texture moins prononcée.In the case of solidification in β, the columnar character is on the other hand less pronounced, although the axis <100> of the β phase remains the preferred direction of crystal growth during solidification. However, during cooling after solidification, the crystals of the β phase, called initial grains, transform into crystals of the α phase. This transformation, which occurs according to the so-called Burgers orientation relationship (110) _β // (0001) _α and <1 $\overline{\text{1}}$

1> _β // <11 $\overline{\text{2}}$

0> _α , theoretically leads to the formation of twelve α variants. When the cooling continues, the γ phase precipitates in lamellar form in each variant α. The resulting microstructure is characterized by the presence of numerous colonies (theoretically up to twelve orientation variants) inside each initial β grain. Each of these colonies is made up of numerous α platelets (or slats), these platelets (or slats) being sometimes delimited by residual β-phase borders. Each plate (or slat) finally has the lamellar structure γ + α ₂ . Such a transformation sequence results in a minimization of the difficulties encountered in the alloys solidifying in α with the reduction in the frequency of the solidification defects and a less pronounced texture.

Solidification in β phase can be obtained for binary alloys sufficiently rich in Ti, as for example in the case of the composition Ti ₆₀ Al ₄₀ , whose Ti / Al atomic ratio of 1.5 is very far from that of the composition equiatomic Ti ₅₀ Al ₅₀ equal to 1. However, alloys as rich in titanium are significantly heavier and less resistant to oxidation than the equiatomic alloy. Finally, after preparation, they have a two-phase structure γ + α ₂ in which the volume fraction of the slightly deformable α ₂ phase is excessively large, which makes them extremely fragile. It should be noted that the two-phase alloy of composition T1 ₅₂ Al ₄₈ with an atomic ratio equal to 1.08, which has optimal ductility thanks to a fraction volume of phase α ₂ of the order of 10%, can only solidify in α.

We therefore sought elements of addition suitable for promoting solidification in the β phase while maintaining the atomic ratio Ti / Al close to the optimal value 52/48, without this exceeding the value 1.16, and minimizing the addition of refractory elements so as not to weigh down the alloys. It has thus been found, surprisingly, that rhenium is the most effective element in this regard, followed closely by tungsten. Indeed, an addition of the order of 2% by atom of these elements in the basic binary alloy Ti ₅₂ Al ₄₈ is sufficient for solidification to occur almost entirely in the β phase, while the addition of approximately 5% in atoms is necessary for other elements. It also turned out that the addition effect was cumulative. For example, if 1% of Re and 1% of W are added simultaneously, the alloy solidifies in β, while the separate addition of each of these elements to the content indicated is not sufficient.

The invention relates in particular to an alloy of the kind defined in the introduction, and provides that its composition in atoms is included in the field defined below: Ti: 48.5 to 52.5% Al: 45.5 to 48.5% Re: 0.5 to 2.5% W: 0 to 2.0% Re + W: 2.0 to 2.5% Nb: 0 to 3.5% Re + W + Nb: 2.0 to 5.5% Yes : 0 to 1.0% The use of tungsten, as an element favoring solidification in β, rather than rhenium alone, presents a economic interest due to the high cost of rhenium. The addition of niobium provides good oxidation resistance, as well as a good level of heat resistance. Finally, the addition of silicon aims to obtain a beneficial effect on the mechanical properties of use such as creep.

• Il contient environ 2 % en atomes de Re + W.
• Il contient environ 1 à 2 % en atomes de Re.
• Il contient environ 3 % en atomes de Nb.
• Il contient environ 0,2 à 0,8 % en atomes de Si.
• Sa formule atomique est choisie parmi les suivantes:
  
          Ti_50,6Al_46,6Re₂Si_0,8     (1)
  
  
  
          Ti₅₂Al₄₆Re₁W₁     (2)
  
  
  
          Ti_51,8Al₄₆Re₁W₁Si_0,2     (3)
  
  
  
          Ti₄₉Al₄₆Nb₃Re₁W₁     (4)
  
  
  
          Ti_48,8Al₄₆Nb₃Re₁W₁Si_0,2     (5).
  
  
• Il est propre à former lors de sa solidification une phase de structure cubique centrée β.

Optional characteristics of the alloy according to the invention, complementary or alternative, are set out below:

• It contains approximately 2% of Re + W atoms.
• It contains around 1 to 2% Re atoms.
• It contains about 3 atomic% of Nb.
• It contains about 0.2 to 0.8 atomic% of Si.
• Its atomic formula is chosen from the following:
  
  Ti _50.6 Al _46.6 Re ₂ Si _0.8 (1)
  
  
  
  Ti ₅₂ Al ₄₆ Re ₁ W ₁ (2)
  
  
  
  Ti _51.8 Al ₄₆ Re ₁ W ₁ Si _0.2 (3)
  
  
  
  Ti ₄₉ Al ₄₆ Nb ₃ Re ₁ W ₁ (4)
  
  
  
  Ti _48.8 Al ₄₆ Nb ₃ Re ₁ W ₁ Si _0.2 (5).
  
  
• It is able to form during its solidification a cubic structure phase centered β.

The invention also relates to a foundry piece made of an alloy as defined above, comprising the juxtaposition of a multiplicity of colonies within each initial β grain, colonies themselves comprising the juxtaposition of a multiplicity platelets each formed by an alternating stack of lamellas of crystallographic structure γ and layers of crystallographic structure α ₂ . The platelets of the same colony are oriented according to one of the 12 α variants defined by the Burgers relationship from said β grain, the platelets of two neighboring colonies being oriented according to different variants.

In the accompanying drawings and views, FIGS. 1 and 2 schematically represent two successive stages in the solidification of an intermetallic alloy based on titanium aluminide.

FIG. 3 is a sectional view of an alloy conforming to that of FIG. 2.

Figures 4 and 5 illustrate the structure of an alloy according to the invention.

Figures 1 and 2 illustrate the α phase cooling process described above. FIG. 1 shows by way of example a cylindrical sample 1 of an alloy in the process of cooling in which columnar grains 2 of crystallographic structure α are formed. These grains are elongated in the crystallographic direction c, which coincides with the direction of the temperature gradient indicated by the arrow F, that is to say the radial direction of the cylinder 1. FIG. 2 shows, on a larger scale, these same columnar grains 2 further cooled. Each of them contains lamellas 3 of crystallographic structure γ oriented perpendicular to the longitudinal direction of the grain, separated from each other by layers 4 of crystallographic structure α ₂ .

Figure 3 highlights the structure of such an alloy of the "first generation".

In the center of Figure 4, section of an alloy according to the present invention, clearly appears the border 5 of an initial β grain. In this grain, each colony 6 is highlighted by the orientation of the platelets which compose it. Each orientation follows the Burgers relationship.

FIG. 5 is a section of the same alloy showing, on the one hand, the orientation of the plates 7 in each colony 6 and, on the other hand, the alternating stack of lamellae of crystallographic structure γ and layers of structure crystallographic α ₂ .

The alloys according to the invention can be produced and used in the same way as the known intermetallic alloys based on titanium aluminide, so that it is not necessary to provide particular information in this regard.

Tests have confirmed the superiority of the alloys according to the invention compared to the alloys of the prior art, as regards the resistance to creep at high temperature which is a key factor for the industrial use of these materials.

The alloy of formula (1) above and the aforementioned alloy of formula Ti ₄₈ Al ₄₈ Cr ₂ Nb ₂ underwent the same heat treatments, four hours at 1250 ° C., then four hours at 900 ° C. After these treatments, the two alloys exhibited comparable tensile properties at 25 ° C., respectively 484 and 459 MPa for the elastic limit, 1.4% and 0.9% for the elastic elongation or ductility. On the other hand, a deformation of 0.5% in creep at 800 ° C. under 180 MPa was obtained in 145 hours for the alloy according to the invention against 5 hours for the known alloy. For the latter alloy, the resistance to hot creep could be improved by eliminating the aforementioned heat treatments, but this would result in a collapse of the ductility at room temperature due to the poor flowability associated with solidification in the α phase.

The alloys of formulas (1), (2) and (3) above, and an alloy of formula Ti ₄₈ Al ₄₆ Nb ₃ W ₁ developed by Allison and considered to be very resistant to creep, were subjected to a test of creep at 750 ° C at 200 MPa. A deformation of 0.5% was obtained after 625 hours, 212 hours, 740 hours and 56 hours respectively for the four alloys, ie durations four to thirteen times higher for the alloys according to the invention than for the alloy of the prior art.

Claims (8)

Intermetallic alloy based on titanium aluminide for the production of foundry parts, characterized in that its composition in atoms is included in the field defined below: Ti: 48.5 to 52.5% Al: 45.5 to 48.5% Re: 0.5 to 2.5% W: 0 to 2.0% Re + W: 2.0 to 2.5% Nb: 0 to 3.5% Re + W + Nb: 2.0 to 5.5% Yes : 0 to 1.0% Alloy according to claim 1, characterized in that it contains approximately 2% of Re + W atoms. Alloy according to claim 2, characterized in that it contains approximately 1 to 2% in Re atoms. Alloy according to one of the preceding claims, characterized in that it contains approximately 3 atomic% of Nb. Alloy according to one of the preceding claims, characterized in that it contains approximately 0.2 to 0.8% Si atoms. Alloy according to either of the preceding claims, characterized in that its atomic composition is chosen from the following:

Ti _50.6 Al _46.6 Re ₂ Si _0.8



Ti ₅₂ Al ₄₆ Re ₁ W ₁



Ti _51.8 Al ₄₆ Re ₁ W ₁ Si _0.2



Ti ₄₉ Al ₄₆ Nb ₃ Re ₁ W ₁



Ti _48.8 Al ₄₆ Nb ₃ Re ₁ W ₁ Si _0.2 .

Alloy according to one of the preceding claims, characterized in that it is capable of forming, during its solidification, a phase of cubic structure centered β. Foundry piece made of an alloy according to claim 7, comprising the juxtaposition of a multiplicity of colonies (6) within each initial β grain, colonies themselves comprising the juxtaposition of a multiplicity of platelets (7) each formed by an alternating stack of lamellas of crystallographic structure γ and layers of crystallographic structure α ₂ , the platelets of the same colony being oriented according to one of the 12 variants α defined by the Burgers relationship from said grain β, and the platelets from two neighboring colonies being oriented according to different variants.

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