EP0924308A1 - Titanium-based intermetallic alloys of the Ti2AlNb type with high yield strength and good creep resistance - Google Patents

Abstract

A titanium-based intermetallic alloy having a high yield strength, high creep resistance and sufficient ductility at room temperature has a chemical composition, in atomic percentages, in the following range: Al 16 to 26; Num 18-28; Mo 0 to 2; If 0 to 0.8; Ta 0 to 2; Zr 0 to 2 and Ti complement to 100 with the condition Mo + Si + Zr + Ta <0.4%. Elaboration, shaping and thermal treatment ranges adapted to the use of the material are also determined.

Description

The present invention relates to a family of titanium-based intermetallic alloys which combine a set of specific mechanical properties including a high yield strength, high creep resistance and sufficient ductility at room temperature.
Intermetallic alloys of the Ti ₃ Al type have shown interesting specific mechanical characteristics. Ternary alloys with Nb additions have been tested in particular and their mechanical properties joined to a lower density than that of nickel-based alloys since between 4 and 5.5 depending on the Nb content, arouse great interest for aeronautical applications . These alloys also have a higher titanium fire resistance than the Ti-based alloys previously used in the construction of turbomachines. The targeted applications relate to massive structural parts such as casings, massive rotating parts such as centrifugal impellers or as a matrix of composite materials for one-piece bladed rings. The desired operating temperature ranges go up to 650 ° C or 700 ° C in the case of parts made of long fiber composite material.

Thus, US 4,292,077 and US 4,716,020 describe the results obtained by titanium-based intermetallic alloys containing 24 to 27 Al and 11 to 16 Nb in percentages atomic.

• une phase B2 riche en niobium, constituant la matrice du matériau et qui assure une ductilité à la température ambiante,
• une phase dite O, de composition définie Ti₂AlNb, orthorhombique et formant des lattes dans la matrice B2. La phase O est présente jusque vers 1000°C et confère au matériau ses propriétés de résistance à chaud en fluage et en traction.

Ces alliages antérieurs connus présentent cependant certains inconvénients, notamment une ductilité insuffisante à température ambiante et une déformation plastique importante durant le fluage primaire qui limitent actuellement leur utilisation.US 5,032,357 has shown improved results thanks to an increase in the Nb content. The intermetallic alloys obtained generally have in this case a microstructure composed of two phases:

• a phase B2 rich in niobium, constituting the matrix of the material and which ensures ductility at room temperature,
• a phase called O, of defined composition Ti ₂ AlNb, orthorhombic and forming slats in the matrix B2. Phase O is present up to around 1000 ° C. and gives the material its properties of resistance to heat in creep and in traction.

These known prior alloys, however, have certain drawbacks, including insufficient ductility at room temperature and significant plastic deformation during primary creep which currently limit their use.

Consequently, the present invention relates to a family of titanium-based intermetallic alloys avoiding the drawbacks of the aforementioned known solutions and which are characterized by a chemical composition, in atomic percentages, belonging to the following field:
A116-26; Num 18-28; Mo 0 to 2; If O to 0.8; Ta O at 2; Zr O at 2 and Ti complement at 100 with the condition Mo + Si + Zr + Ta> 0.4%.

Appropriate thermomechanical treatments and a method of are further defined for these alloys intermetallic according to the invention, allowing improve their mechanical properties, in particular increase ductility at room temperature and limit plastic deformation during primary creep.

• la figure 1 représente les résultats d'essais de fluage à 550°C sous 500MPa en reportant en ordonnées le temps en heures à 1 % de déformation, suivant différentes compositions d'alliages ainsi que les résultats de traction en reportant en ordonnées la limite d'élasticité en MPa ;
• la figure 2 représente les résultats d'essais de fluage à 550°C sous 500MPa en reportant en ordonnées la limite d'élasticité en MPa et en abscisses le temps en heures à 0,5 % de déformation, suivant différentes compositions d'alliages ;
• la figure 3 montre un exemple de microstructure obtenue à l'issue d'une élaboration d'un alliage intermétallique conforme à l'invention ;
• la figure 4 représente schématiquement par zones les résultats d'essais mécaniques effectués sur quatre types différents d'alliages, en reportant en abscisses les allongements en pourcentages et en ordonnées la limite élastique spécifique, à température ambiante ;
• les figures 5 et 6 représentent en diagramme de Larson-Miller les résultats de tenue au fluage, respectivement à 1 % de déformation et à rupture, en reportant en abscisses le paramètre de Larson-Miller et en ordonnées la contrainte spécifique en MPa pour différents alliages ;
• les figures 7, 8 et 9 représentent les résultats d'essais mécaniques obtenus pour un alliage conforme à l'invention, respectivement les contraintes en MPa, à rupture et en limite d'élasticité, à 20°C et à 650°C puis la déformation homogène en pourcentages à 20°C et à 650°C et enfin le temps en heures à 1 % de déformation lors de la tenue au fluage à 550°C sous 500 MPa, suivant quatre gammes différentes de traitement thermique appliquées à l'alliage ;
• la figure 10 représente les résultats d'essais de fluage en compression pour un alliage antérieur connu et deux alliages conformes à l'invention.

We give below the justification of the choice of the ranges of composition retained as well as the description of the tests carried out leading to the definition of the method of preparation and shaping, indicating the results obtained in measurements of mechanical properties and compared with the properties. of prior known alloys, with reference to the appended drawings in which:

• FIG. 1 represents the results of creep tests at 550 ° C. under 500 MPa by plotting the time in hours at 1% deformation, according to different alloy compositions as well as the traction results by plotting the limit d on the ordinate elasticity in MPa;
• FIG. 2 represents the results of creep tests at 550 ° C. under 500 MPa by plotting on the ordinates the elastic limit in MPa and on the abscissa the time in hours at 0.5% deformation, according to different alloy compositions;
• FIG. 3 shows an example of a microstructure obtained at the end of an elaboration of an intermetallic alloy according to the invention;
• FIG. 4 schematically represents by zones the results of mechanical tests carried out on four different types of alloys, by plotting the elongations in percentages and in ordinates the specific elastic limit, at room temperature;
• FIGS. 5 and 6 show in Larson-Miller diagram the results of creep resistance, respectively at 1% deformation and at break, by plotting the Larson-Miller parameter on the abscissa and on the ordinate the specific stress in MPa for different alloys ;
• FIGS. 7, 8 and 9 represent the results of mechanical tests obtained for an alloy in accordance with the invention, the stresses in MPa, at rupture and in yield strength, respectively, at 20 ° C. and at 650 ° C. then the homogeneous deformation in percentages at 20 ° C and 650 ° C and finally the time in hours at 1% deformation during creep resistance at 550 ° C under 500 MPa, according to four different ranges of heat treatment applied to the alloy ;
• FIG. 10 represents the results of compression creep tests for a known prior alloy and two alloys according to the invention.

The experimental results have shown that the contents selected for the three major elements of the composition, titanium, aluminum and niobium are the most appropriate, namely:
Al 16-26; Nb 18 to 28 and Ti basic element.

The variation of the contents within the indicated limits allows a adjustment of properties according to the type of application temperature and range of use corresponding.

Specifications in Al, Si: α-gene elements.

These two elements are elements which favor phase O and therefore they increase the heat resistance of the alloys. However, they tend to decrease ductility in especially at room temperature. The deformation plastic during primary creep decreases from 0.5% to 0.25 % with the addition of these elements (0.5% of Si or a passage of 22% to 24% Al). On the other hand, the elastic limit is greatly reduced as well as the ductility (from 1.5% to 0.5%). Thus, increasing the aluminum content from 22% to 24%, for the same heat treatment, greatly reduces the limit elasticity which drops from 600 MPa to 500 MPa at 650 ° C. The beneficial influence of the addition of 0.5% Si on the resistance to creep is illustrated in Figure 2.

Specifications in Nb, Mo, Ta: β-gene elements

These elements favor the B2 phase which is ductile to the room temperature they participate in the stability of the phase B2 at operating temperatures. A reduction in niobium content (25% to 20%) mainly affects the creep resistance, the tensile properties being little modified, as shown by the results shown on the figure 1. We will show that the addition of molybdenum allows a significant increase in the yield strength of 100 MPa at room temperature and 200 MPa at 650 ° C and this without reduction of ductility at room temperature. Molybdenum also allows better resistance to creep, it reduces very clearly the plastic deformation during creep primary (from 0.5% to 0.25%) and decreases the speed of plastic deformation during the secondary stage. These gains are accentuated when the alloy previously contains silicon. These results obtained in creep at 550 ° C. under 500 MPa are illustrated in FIG. 2 for alloys comprising additions of MB, Si or both.

Tantalum is a β-gene element very similar to niobium to which it is often mixed in ores. In the titanium alloys it increases their mechanical strength and gives them better resistance to corrosion and oxidation.

Zr specifications: β-neutral element

Zirconium is a neutral element and the methods of alloys and the origin of the elements brought, by recycling or not, can bring the presence of Zr, which may in some cases be desired.

The atomic percentage used for the alloys of the invention for Zr, as for Ta, is is between 0 and 2%.

These specifications and the experimental tests carried out have led to the addition, for the composition of intermetallic alloys in addition to the three major elements noted above, of elements in the following atomic percentages:
Mo 0 to 2; If 0 to 0.8; Ta 0 to 2 and Zr 0 to 2
with the additional condition of the presence of at least one of the addition elements: Mo + Si + Zr + Ta> 0.4%.

Elaboration and shaping processes

A process for developing the material has also been developed. point in accordance with the invention and makes it possible to obtain the mechanical properties sought and previously described.

In this development, the first step consists in homogenizing the composition of the material, using for example the VAR process (Vacuum Arc Remelting), this step is important because it determines the homogeneity of the material. The material is then deformed at high speed to reduce the grain size either by forging with a pestle in the β domain, or by high speed extrusion still in the β domain. These bars are then cut into pieces to undergo the last stage of the thermomechanical treatment: isothermal forging. This isothermal forging takes place in a temperature range from T _β -125 ° C to T _β -25 ° C and with deformation rates from _5.10 ^-4 s ^-1 to _5.10 ^-2 s. ^-1 . T _β is the transition temperature between the high-temperature single-phase β domain and the two-phase domain α ₂ + B ₂ , α ₂ is a phase of defined composition Ti3Al transforming into phase 0 below about 900 ° C. T _β is around 1065 ° C for example, for a Ti 22 Al 25 Nb alloy.
Depending on the particular applications, the bars obtained by forging or extrusion can, as a variant, be subjected to a rolling operation where the deformation rates are of the order of 10 ⁻¹ s ⁻¹ . It is also possible to carry out precision forging in a byphase domain α ₂ + B ₂ which leads to a structure of equiaxed grains with a globular shape of the phase α2 / 0. In this case, forging takes place in a temperature range from T _β - 180 ° C to T _β - 30 ° C.
The preparation of the material ends with a heat treatment which consists of three stages.

The first step is a step of re-solution at a temperature between T _β -35 ° C and Tβ + 15 ° C for less than 2 hours.

The second stage allows the growth of the hardening phase O and this aging is carried out between 750 ° C and 950 ° C for at least 16 hours.

The third treatment is carried out within a range of temperature of 100 ° C around the operating temperature of the material.

• effet de la température de forgeage : le forgeage à une température élevée assure une meilleure résistance au fluage à 550°C, le temps à rupture est multiplié par 10 et la déformation à rupture passe de 0,8 % à 1,3%, ceci avec une augmentation de 50°C de la température de forgeage ;
• effet de la vitesse de forgeage : Pour une vitesse 20 fois plus grande, on constate une réduction d'un facteur 10 du temps à rupture lors de fluage à 550°C sous 500MPa.

The choice of the cooling rate between the different stages is important because it determines the size of the slats of the hardening phase O. The determination of a particular program is done according to the characteristics of use that are sought.
Figure 3 shows an example of microstructure obtained at the end of this development of an intermetallic alloy according to the invention.
In the case where a structure of grains equiaxed by precision forging in the domain α2 + B2 is sought, during the first stage of the heat treatment, the temperature of re-solution is close to the forging temperature. The choice of this temperature is critical because it influences both the size of the equiaxed grains that we are targeting and the relative proportion of the populations of remaining globularized primary hardening phase and of secondary needle hardening phase which will form in the following stages.
In the adjustments carried out, it has been shown that thermomechanical treatments have a great influence on the mechanical properties:

• effect of the forging temperature: forging at a high temperature ensures better creep resistance at 550 ° C, the breaking time is multiplied by 10 and the breaking strain changes from 0.8% to 1.3%, this with a 50 ° C increase in the forging temperature;
• effect of the forging speed: For a speed 20 times greater, there is a reduction by a factor of 10 in the breaking time during creep at 550 ° C under 500MPa.

The heat treatment in the vicinity of the temperature of the transition T _β causes the recrystallization of the grains B2 and makes it possible to significantly increase the creep resistance at 650 ° C. However, this treatment reduces the elastic limit, but increases the ductility around 350 ° C. A heat treatment at a temperature further (-25 ° C) from that of the T _β transition increases the elastic limit and increases the creep resistance at 550 ° C. In addition, this treatment achieves a ductility plateau around 10% from 200 ° C to 600 ° C.

These findings result in particular from the following tests:

EXAMPLE 1 - Role of the forging temperature;

We looked at the influence of two forging temperatures on creep resistance. Forging is followed by the same heat treatment at high temperature. We therefore show that the forging temperature is important on creep resistance because it determines the morphology of the phases present in the material, as shown below the creep resistance results of a Ti22Al 25Nb alloy at 550 ° C under 450 MPa: FORGING TEMPERATURE TIME AT 0.5% (%) BREAKING TIME (H) PRIMARY DEFORMATION (%) DEFORMATION SPEED T _β -100 ° C 30.3 H 168 H 0.44% 5 10 ^-9 S. ^-1 T _β -50 ° C 123.3 H 1037.5 H 0.35% 2.10 ^-9 S. ^-1

Finally, the creep resistance of the Ti 22 Al 25 Nb alloy at 650 ° C. under 300 MPa, as a function of the temperature of isothermal forging gives the following results: FORGING TEMPERATURE TIME AT 0.5% (%) BREAKING TIME (H) PRIMARY DEFORMATION (%) SECONDARY DEFORMATION SPEED T _β -100 ° c 7 a.m. 980 H 1% 1.10 ^-8 S. ^-1 T _β -50 ° C 12.7H 1526 H 0.8% 6.9.10 ^-9 S. ^-1

Example 2 - Effect of heat treatment;

Here we show the influence of the return temperature solution on mechanical properties and creep resistance, for the high temperature forged roller. We can find that a re-solution at a high temperature leads to recrystallization and a fall in properties in traction. However, these two treatments allow choose the temperature at which the material is resistant in creep, either at 550 ° C, or at 650 ° C. A temperature of low solution allows good creep resistance at 550 ° C, while a higher temperature allows better resistance at 650 ° C, this for all characteristics: breaking time, plastic deformation primary, strain rate.

The following results were obtained in yield strength measured in MPa, as a function of the test temperature for two re-solution temperatures: PROCESSING TEMPERATURE 20 ° C 350 ° C 450 ° C 550 ° C 650 ° C T _β -5 ° C (MPa) 792.4 637.6 659 668 505 T _β -25 ° C (MPa) 846.7 711.01 734.3 695 645.4

Likewise, the following results were obtained in creep resistance at 550 ° C. under 500 MPa, as a function of the temperature of the solution treatment: PROCESSING TEMPERATURE TIME AT 0.5% (%) BREAKING TIME (H) PRIMARY DEFORMATION (%) DEFORMATION SPEED T _β -5 ° c 123 H > 1000 H 0.37% 2 10 ^-9 S. ^-1 T _β -25 ° C 211 H 1220 H 0.47% 1.3.10 ^-9 S. ^-1

Example 3 - Ductility adjustment at temperature ambient;

We will present the ductility obtained at room temperature according to the temperature of the last heat treatment, the duration of this treatment is between 16 and 48 H. We can see that the higher the temperature of the last treatment, the more the ductility increases. These results were obtained on a quaternary alloy containing molybdenum. It is therefore possible with an appropriate treatment to obtain a ductility adapted to a particular use, as indicated below: T ° last treatment 900 ° C 750 ° C 600 ° C 550 ° C Ductility 10% 6.4% 2.5% 1.25%

Intermetallic alloy samples including the composition belongs to the field of the invention have been tested and showed improvements in results compared to the prior known alloy of standard composition Ti 22Al 25Nb.

EXAMPLE 4 - effect of molybdenum;

The table below shows the elastic limit for different temperatures and we clearly see the effect of adding 1% Mo on the elastic limit. In the second table, we show the advantage of the presence of molybdenum on creep resistance. The materials were treated according to the same thermomechanical treatment. This thermomechanical treatment is characterized by low temperature forging T _β -100 ° C and heat treatment at T _β -25 ° C before a 24 hour plateau at 900 ° C and aging at 550 ° C for at least 2 days.

Example 5 - Effect of silicon;

We present the contribution of silicon on the creep resistance always from materials developed by applying the thermomechanical treatment described above in Example 4. We thus show the reduction in the plastic deformation of the primary creep and the significant decrease in the secondary creep speed.

Example 6 Effect of Tantalum

Castings of a reference alloy Ti-24 Al-20 Nb and of a modified alloy of composition Ti-24 Al-20 Nb-1 Ta, the values being given in atomic percentages, were produced, then cylindrical samples have been machined and the heat treatments applied have been: 1160 ° / 30 minutes, cooling in an oven to 750 ° C. then holding for 24 hours. The mechanical compression tests carried out gave the following results:

Example 7 Effect of Zirconium

The same operations as in Example 6 for a Ti-24Al-20Nb-1Zr alloy gave the following results:

The compression creep tests in these two examples also show the interest of the elements Ta and Zr for increase the creep resistance by decreasing the amplitude of the primary creep and reduction of the speed of secondary creep. The results are shown in the figure 10 for compression creep tests at 650 ° C under 310MPa, on curve 5 for the Ti-24 Al-20Nb alloy, on the curve 6 for the Ti-24Al-20Nb-1Ta alloy and curve 7 for Ti-24Al-20Nb-1Zr alloy.

The experimental results obtained show the advantages previously noted alloys according to the invention. In addition, Figure 4 shows a comparison of the properties specific mechanical traction at room temperature of these alloys with those of alloys commonly used in aeronautics, of the type based on nickel or titanium or development courses such as intermetallic γ Ti Al and these results confirm the interest of alloys according to the invention. Likewise, the compared results of creep of known nickel-based alloys such as Inco 718 and a nickel-based superalloy A in accordance with EP-A-0 237 378, based on titanium, such as IMI 834 or intermetallic γ Ti Al and an alloy according to the invention are reported in Figures 5 and 6 following Larson-Miller diagrams.

• mise en solution à 1030°C/ 1 heure
• vieillissement à 900°C/24 heures
• revenu à 550°C/48 heures ;

les niveaux 2a....g correspondent au traitement thermique :

• mise en solution à 1030°C/1 heure
• vieillissement à 900°C/24 heures ;

les niveaux 3a....g correspondent au traitement thermique :

• mise en solution à 1060°C/1 heure
• vieillissement à 900°C/24 heures
• revenu à 550°C/48 heures ;

et les niveaux 4a....g, au traitement thermique :

• mise en solution à 1030°C/1 heure
• vieillissement à 800°C/24 heures
• revenu à 600°C/48 heures

Finally the results obtained in mechanical tests on an alloy in accordance with the invention of composition in atomic percentages 22Al, 25Nb, 1Mo and Ti complement to 100 were reported on the diagrams of Figures 7, 8 and 9 where the levels 1a ... .g correspond to a heat treatment comprising:

• dissolved at 1030 ° C / 1 hour
• aging at 900 ° C / 24 hours
• returned to 550 ° C / 48 hours;

levels 2a .... g correspond to the heat treatment:

• dissolved at 1030 ° C / 1 hour
• aging at 900 ° C / 24 hours;

levels 3a .... g correspond to the heat treatment:

• dissolved at 1060 ° C / 1 hour
• aging at 900 ° C / 24 hours
• returned to 550 ° C / 48 hours;

and levels 4a .... g, heat treatment:

• dissolved at 1030 ° C / 1 hour
• aging at 800 ° C / 24 hours
• returned to 600 ° C / 48 hours

Claims (10)

Titanium-based intermetallic alloy with a high yield strength, high creep resistance and sufficient ductility at room temperature characterized in that its chemical composition, in atomic percentages, belongs to the following field:
Al 16-26; Num 18-28; Mo 0 to 2; If 0 to 0.8; Ta 0 to 2; Zr 0 to 2 and Ti complement to 100 with the condition Mo + Si + Zr + Ta> 0.4%. Intermetallic alloy according to claim 1 characterized in that it is produced by carrying out at least the following steps in the order indicated: a) fusion allowing an ingot of homogeneous composition to be obtained; b) high speed deformation resulting in a reduction in grain size; c) isothermal forging at a temperature between the temperature of β transus T _β minus 125 ° C and the temperature of β transus T _β minus 25 ° C, with deformation rates between 5.10 ^-4 s ^-1 and 5.10 ^-2 s ^-1 . d) heat treatment comprising the following sub-steps: d1) dissolving at a temperature between the temperature of β transus minus 35 ° C and the temperature of β transus plus 15 ° C, for a period of less than two hours, d2) aging at a temperature between 750 ° C and 950 ° C for a period greater than 16 hours, allowing growth of an orthorhombic hardening phase 0; d3) treatment carried out in a temperature range of 100 ° C around the use temperature determined for the material, the cooling rates between the stages of the heat treatment being determined as a function of the characteristics of use sought for the material, taking into account their influence on the size of the slats of the orthorhombic hardening phase 0. Intermetallic alloy according to claim 1 characterized in that it is produced by carrying out at least the following steps in the order indicated: a) fusion allowing an ingot of homogeneous composition to be obtained; b) high speed deformation resulting in a reduction in grain size; c) rolling at a deformation speed of the order of 10-1 s-1. d) heat treatment comprising the following sub-steps: d1) dissolving at a temperature between the temperature of β transus minus 35 ° C and the temperature of β transus plus 15 ° C, for a period of less than two hours, d2) aging at a temperature between 750 ° C and 950 ° C for a period greater than 16 hours, allowing growth of an orthorhombic hardening phase 0; d3) treatment carried out in a temperature range of 100 ° C around the use temperature determined for the material, the cooling rates between the stages of the heat treatment being determined according to the characteristics of use determined for the material, taking into account their influence on the size of the slats of the orthorhombic hardening phase 0. Intermetallic alloy according to claim 1 characterized in that it is produced by carrying out at least the following steps in the order indicated: a) fusion allowing an ingot of homogeneous composition to be obtained; b) high speed deformation resulting in a reduction in grain size; c) precision forging, at a temperature between the temperature of β transus T _β minus 180 ° C and the temperature of β transus T _β minus 30 ° C, obtaining an equiaxed grain structure; d) heat treatment comprising the following sub-steps: d1) dissolving at a temperature close to the forging temperature, for a period of less than two hours, d2) aging at a temperature between 750 ° C and 950 ° C for a period greater than 16 hours, allowing growth of the orthorhombic hardening phase O; d3) treatment carried out in a temperature range of 100 ° C. around the temperature of use determined for the material, the cooling rates between the stages of the heat treatment being determined as a function of the characteristics of use sought for the material, taking into account their influence on the size of the slats of the orthorombic hardening phase 0. Intermetallic alloy according to one of claims 2 or 3 characterized in that in step a), the fusion is produced by double vacuum arc fusion. Intermetallic alloy according to any one of claims 1 to 4 characterized in that a heat treatment giving it an optimized creep resistance is applied to it, comprising the following steps: a) dissolving at a temperature of β transus minus 25 ° C for one hour; b) aging at a temperature between 875 ° C and 925 ° C for 24 hours, followed by rapid cooling; c) tempering treatment carried out at the temperature of use determined for the material. Intermetallic alloy according to claim 5 characterized in that the income processing is carried out at 550 ° C for 48 hours for a temperature operating temperature of 550 ° C. Intermetallic alloy according to claim 5 characterized that the income processing is carried out at 650 ° C for 24 hours at an operating temperature 650 ° C. Intermetallic alloy according to claim 1 characterized in that a heat treatment giving it a deformability of at least 10% at ambient temperature is applied to it, comprising the following steps: a) dissolving at a temperature between the temperature of β transus minus 35 ° C and the temperature of β transus minus 15 ° C for at least two hours; b) aging at a temperature of 900 ° C ± 50 ° C for a period greater than 16 hours. Intermetallic alloy according to claim 8 characterized in that an income is made in a range temperature of 100 ° C around the temperature of determined use for the material and gives it additional hardening after its implementation.

Images