EP0237378B1 - Superalloy having a nickel base matrix, manufactured by powder-metallurgical processing, and gas turbine discs made from this alloy - Google Patents

Abstract

1. A superalloy comprising a nickel-based matrix and having good mechanical properties when hot, as regards tensile strength, creep strength, oligocyclic fatigue and resistance to cracking, characterised in that the percentage chemical composition by weight is within the following range : Cr 11 to 13 Co 8 to 17 Mo 6 to 8 Nb not more than 1.5 Ti 4 to 5 Al 4 to 5 Hf not more than 1 C, B, Zr none more than 500 ppm Ni remainder of 100.

Description

• - tenue en traction jusqu'à 750_°C : limite d'élasticité et allongement,
• - tenue en fluage jusqu'à 750_°C : résistance élevée et absence de sensibilité à l'entaille,
• -tenue en fatigue oligocyclique,
• - vitesses de propagation de fissure aussi faible que possible même compte tenu des effets de l'environnement et des temps de maintien prolongés sous charge dont on sait qu'ils deviennent critiques dans la plage de température visée ; l'importance de cette propriété est attestée par l'introduction d'exigences de tolérance au dommage comme, par exemple, dans l'édition de Novembre 1984 de la norme MIL-STO-17-83 de l'USAF.

The present invention relates to nickel-based superalloy compositions, more particularly intended for the manufacture of turbomachine disks, the use of which can be extended up to 750 _° C. so as to respond to the evolution of the thermodynamic cycles of very high efficiency and specific power turbomachines. These discs require the use of moderate density materials having a particular set of hot mechanical properties such as:

• - tensile strength up to 750 _° C: elastic limit and elongation,
• - creep resistance up to 750 _° C: high resistance and lack of sensitivity to notching,
• - maintained in oligocyclic fatigue,
• - crack propagation rates as low as possible even taking into account the effects of the environment and the prolonged holding times under load which are known to become critical in the target temperature range; the importance of this property is evidenced by the introduction of damage tolerance requirements such as, for example, in the November 1984 edition of USAF standard MIL-STO-17-83.

• - soit des matériaux qui présentent une bonne résistance à la fissuration avec une faible sensibilité à l'environnement mais dont la limite d'élasticité et la résistance au fluage sont insuffisantes à haute température ; US 3.147.155 fournit des exemples de compositions de superalliages de ce type (voir alliage A au tableau 1 ci-après).
• - soit des matériaux qui présentent une limite d'élasticité élevée mais dont la sensibilité à l'entaille en fluage, la tenue à la fissuration et la sensibilité à l'environnement ne sont pas satisfaisantes; US 3.061.426 et FR-A 2.244.827 fournissent également des exemples de compositions de superalliages de ce type (voir alliages R et au tableau 1 qui figure plus loin.)

The materials developed by powder metallurgy are today the most suitable for meeting these technical requirements and in the current state of the art we use:

• - Either materials which have good resistance to cracking with low sensitivity to the environment but whose yield strength and creep resistance are insufficient at high temperature; US 3,147,155 provides examples of compositions of superalloys of this type (see alloy A in Table 1 below).
• - Either materials which have a high elastic limit but whose sensitivity to creep notch, resistance to cracking and sensitivity to the environment are not satisfactory; US 3,061,426 and FR-A 2,244,827 also provide examples of compositions of superalloys of this type (see alloys R and in Table 1 which appears below.)

Some examples of these known alloy compositions are given in Table 1.

The improvement of certain mechanical properties (resistance to cracking for example) can be obtained by producing specific microstructures (large grains, collar structure). These improvements are however made to the detriment of other characteristics (elastic limit for example) and the object of the present invention is to achieve an optimal set of the properties mentioned above by new alloy compositions.

le reste étant essentiellement du nickel.The present invention relates to a new family of nickel-based superalloys having the properties noted above, characterized in that the chemical composition in weight percentages, belongs to the following field:

the remainder being essentially nickel.

• Co:14 à 17%
• C : O à 200 ppm
• B : O à 200 ppm.

Advantageously, the following preferential contents are used:

• Co: 14 to 17%
• C: O at 200 ppm
• B: O at 200 ppm.

Two examples of alloys belonging to the present invention are given below (N 14 and N 16):

Advantageously, these superalloys according to the invention are capable of being implemented by powder metallurgy techniques and the turbomachine disks constitute an interesting application.

The nickel-based superalloys generally have an essentially two-phase structure with: - a phase y of Ni, Co hardened mainly by elements in solid solution (W, Cr, Mo) - a hardening phase y of type A _s B in which A is mainly formed of Ni, Co, Cr and B of AI, Ti Nb, Ta, Hf, V, Ta.

Obtaining the targeted mechanical properties is achieved by intervening respectively on the two hardening modes, which leads to specifying on the one hand the contents of AI, Ti, Nb, Hf, V, Ta and on the other hand in W, Mo , and Cr.

The invention will be better understood and the advantages specified with the aid of the description which follows of the justification of the main choices and of the exemplary embodiments, with reference to the single figure which shows the influence of the Mo / W ratio on the creep breaking service lives.

Specification in Nb. HAVE. Ti. Hf and V:

It is known that the introduction of Nb and Ta contributes greatly to the increase in the elastic limit and in the smooth creep behavior, but Table 2 below shows that this beneficial effect is acquired at the expense of notch sensitivity and cracking resistance in fatigue creep from 650 _° C (see in particular the examples of alloys R and N 13 for the influence of Nb and the examples of alloys NA10 and NA9 for the influence of Ta).

Tantalum also has, with respect to niobium, the disadvantage of increasing the density more strongly. For these reasons, the alloys of the present invention do not contain Ta and are limited to 1.5% of Nb.

Due to this limitation, it is necessary, in order to obtain properties in the target temperature range, to have a volume fraction of y of at least 50%, acquired by adding AI and Ti which does not cause the above drawbacks. The invention provides contents of AI and Ti such that their ratio is close to 1 because, if it is known that Ti is a more favorable element than AI for the curing of the y phase beyond 650 ° C, it very quickly increases the solid solution re-temperature of this phase, making the implementation of the alloy difficult. For the same reason the sum of the elements AI + Ti is limited to 10% by weight.

Additional hardening may be obtained by the addition of Hf, up to a limit of 1% for processing reasons (reduction of the solidus and increase of the solvus Y).

Likewise, if it is known that an increment of hardening can be obtained by the addition of vanadium, it is found that the cracking fatigue fatigue creep rates at 650 ° C. are then excessive. For this reason, the alloys of the present invention do not contain vanadium.

Specifications in MB. W and Cr

• - le rapport de la concentration du Mo dans la phase y à sa concentration dans la phase y est 2 à 3 fois plus élevé que le rapport correspondant pour le W,
• - la figure 1 montre que la substitution du Mo au W diminue la sensibilité à l'entaille au fluage à 650 °C pour une tenue en fluage sur éprouvette lisse légèrement inférieure. Sur la figure 1, t_R en ordonnées et en échelle logarithmique représente la durée de vie en heures, en fluage-rupture, à 650_°C sous charge de 1000MPa et en abscisses sont reportées les teneurs en Mo et W en pourcentages atomiques; la courbe en trait plein représente les résultats de fluage, effectués sur éprouvette avec entaille et la courbe en trait tireté, des résultats en fluage sur éprouvette lisse.
• - la pénalisation en densité est moindre avec l'élément Mo qu'avec W.

Given the limitations set out above, it is necessary to strongly harden the phase in solid solution y. To do this, the elements W and Mo which are known as effective hardeners of the matrix are used. The hardening by Mo is, in the present invention, preferred to that by W because:

• - the ratio of the concentration of Mo in phase y to its concentration in phase y is 2 to 3 times higher than the corresponding ratio for W,
• - Figure 1 shows that the substitution of Mo for W reduces the sensitivity to notch creep at 650 ° C for creep resistance on a slightly lower smooth specimen. In FIG. 1, t _R on the ordinate and on a logarithmic scale represents the lifetime in hours, in creep-rupture, at 650 _° C. under load of 1000 MPa and on the abscissa are shown the contents of Mo and W in atomic percentages; the curve in solid line represents the results of creep, carried out on test-piece with notch and the curve in dashed line, results in creep on smooth test-piece.
• - the density penalty is less with the Mo element than with W.

The present invention recommends a Mo content of between 6 and 8% by weight which, as shown in Table 2 (see in particular the examples of alloys according to the invention N 14 and N 16), results in high resistance values in tension and creep, the alloy nevertheless retains particularly low cracking fatigue-creep rates.

It is known that the addition of chromium is necessary for resistance to oxidation and participates in the hardening of the alloy. However, for contents of AI, Ti and Mo reaching the levels recommended by the invention, the work which led to the invention has shown that a chromium concentration greater than 13% by weight causes an abundant precipitation of intergranular carbides. which leads to a deterioration of the properties of ductility, sensitivity to notching and cracking, as shown by the results of alloy N 17 reported in Table 2 shown below.

For this reason, the invention recommends a chromium content of between 11 and 13% by weight.

Other specifications

It is recognized that a content of at least 8% by weight of cobalt is necessary for the creep resistance. This element also lowers the solvent temperature of phase y and, due to the high values of AI and Ti of the present invention, a cobalt content of at least 14% by weight is retained by the invention so as to facilitate the implementation of the material. This content must be limited in the use of the material. This content must be limited above 17% to maintain a volume fraction of y sufficient at the considered temperatures of use.

Boron and carbon are known elements which can improve the creep resistance, but taking into account the chromium and molybdenum contents of the invention, and in order to avoid the excessive formation of carbides and borides, the invention limits their concentration by weight at 500 ppm.

Zirconium may be useful for fixing possible embrittling traces of sulfur, but the invention however limits its content to 500 ppm by weight to avoid the formation of phases with a low melting point. Other elements such as Mg, Ca, Si, Y etc ... often used for the development of the alloy, can remain present at the level of traces without harming the properties of the alloys according to the invention.

By way of example, two alloys (examples N14 and N16) of the family have been studied more particularly according to the invention. Their composition is given in Table 1 below where the contents of each element are expressed in weight concentration.

For each alloy grade, mechanical tests were carried out on the one hand on test pieces, the development of which led to a coarse-grained structure (greater than or equal to 50 μm) or to "collar" and on the other hand test tubes, the development of which leads to a structure with small grains (less than or equal to 10 μm). Each specimen produced is subjected to a sequence of heat treatments before testing in order to optimize the properties of the alloys.

• - des essais en traction pour lesquels sont notés les limites élastiques RO,2 en MPa à 650_°C et à 750_°C et les allongements A% à 750_°C,
• - des essais de fluage à 750_°C à l'air sous charge de 600 MPa pour lesquels sont notés les temps à rupture sur éprouvette lisse tRL en heures et le rapport τ entre le temps à rupture sur éprouvette entaillée / temps à rupture sur éprouvette lisse.
• - des essais de fissuration cycliques à 650_°C à l'air pour lesquels sont notées les valeurs de vitesse de fissuration da/dN :
  

These characterization tests include:

• - tensile tests for which the elastic limits RO, 2 in MPa at 650 _° C and 750 _° C and the elongations A% at 750 _° C are noted,
• - creep tests at 750 _° C. in air under a load of 600 MPa for which the break times on a smooth test piece tRL in hours are noted and the ratio τ between the break time on the notched test piece / break time on the test piece smooth.
• - cyclic cracking tests at 650 _° C in air for which the values of cracking speed da / dN are noted:
  

The results obtained are collated in Table 2 below which also groups together the comparative results obtained with alloys known from the prior art, the corresponding compositions of which are also given in Table 1 above. These results are obtained by applying a cooling rate of 100 ° C. per minute to the test pieces after re-dissolving the γ phase. This speed corresponds to a cooling speed at the heart of parts capable of being made of an alloy according to the invention.

These results show that the superalloys of the invention make it possible to achieve the desired optimal set of hot mechanical properties combining good results in cracking resistance with good results also in tension and creep up to 750 ° C.

The use of superalloys in accordance with the invention can use any method such as to avoid the appearance of major segregations of the kind which appear when such alloys are used according to conventional foundry methods. Thus the production of superalloys according to the invention can in particular be obtained by the known techniques of powder metallurgy and parts made of these alloys such as turbomachine rotor discs can for example be manufactured by known isostatic compaction processes with hot.

Claims (8)

1. A superalloy comprising a nickel-based matrix and having good mechanical properties when hot, as regards tensile strength, creep strength, oligocyclic fatigue and resistance to cracking, characterised in that the percentage chemical composition by weight is within the following range:

2. A superalloy comprising a nickel-based matrix according to claim 1, characterised by the following contents in percentage by weight:

3. A superalloy comprising a nickel-based matrix according to claim 2, characterised by the following chemical composition, in percentage by weight:

4. A superalloy comprising a nickel-based matrix according to claim 2, characterised by the following chemical composition, in percentage by weight:

5. A superalloy comprising a nickel-based matrix according to any of the preceding claims, characterised in that it is manufactured by methods of operation starting from powders.

6. A turboshaft engine rotor disc, characterised in that it is made of a superalloy comprising a nickel-based matrix according to any of claims 1 to 4.

7. A turboshaft engine rotor disc according to claim 6, characterised in that it is made of a superalloy comprising a nickel-based matrix and manufactured by methods using powders.

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