EP1584697B1 - Titanium-aluminium alloy having high-temperature ductility - Google Patents

Abstract

Alloy is made of titanium aluminate in which a minor fraction of the aluminium and titanium atoms is replaced by other atoms. It contains (by atomic %) 44 to 49 of aluminium, 0.5 to 3 of zirconium. 0.5 to 2 of iron, 0.5 to 2 of molybdenum, 0.2 to 0.5 of silicon and 0 to 3 of niobium. An independent claim is also included for: heat treatment of the alloy.

Description

An alloy made of titanium aluminide in which a minority fraction of the aluminum and titanium atoms is replaced by other atoms.

TiAl-type intermetallic alloys find advantageous use at high temperatures in aeronautical turbomachines. TiAl is characterized by its low density which gives it a higher heat resistance to the density than conventional titanium alloys and even some nickel superalloys. This is due to a yield strength which remains constant typically between 20 and 700 ° C. However, the low ductility of this alloy is likely to compromise its use for rotating parts. Research is currently being conducted around the world for the development of transformation ranges that use TiAl to allow the introduction of this material into aerospace turbomachines. The work deals in parallel with the choice of the most suitable shades for this or that transformation range.

The object of the present invention is to provide a TiAl-type alloy having a high ductility when hot, while retaining the usual mechanical properties for these alloys.

The invention aims in particular at an alloy of the kind defined in the introduction, and provides that it contains in atoms 44 to 49% of aluminum, 0.5 to 3% of zirconium, 0.5 to 2% of iron, 0.5 2% molybdenum, 0.2 to 0.5% silicon, 0 to 3% niobium, the balance to 100% titanium and unavoidable impurities.

It has been proposed to add iron ( US-A-6,165,414 and US-A-6,174,495 ) to improve the flowability of TiAl alloys, zirconium ( US-A-4,983,357 , US-A-5,207,982 and US-A-5 997,808 ) to improve the resistance-ductility, toughness, oxidation resistance and heat resistance resistance of molybdenum ( US-A-5,350,466 US-A-6,214,133 , US-2002145174 ). to improve the heat resistance. However, none of these documents suggest a combination of Fe, Zr and Mo elements.

• Il contient en atomes 45 à 48 % d'aluminium, environ 1 % de zirconium, environ 1 % de fer, environ 1 % de molybdène et environ 0,2 % de silicium.
• Il est composé exclusivement d'aluminium, de titane, de zirconium, de fer, de molybdène, de silicium et le cas échéant de niobium, sous réserve d'impuretés éventuelles.

Optional features of the alloy according to the invention, complementary or substitution, are set out below:

• It contains in atoms 45 to 48% of aluminum, about 1% of zirconium, about 1% of iron, about 1% of molybdenum and about 0.2% of silicon.
• It is composed exclusively of aluminum, titanium, zirconium, iron, molybdenum, silicon and optionally niobium, subject to possible impurities.

The subject of the invention is also a process for the heat treatment of an alloy as defined above, in which its constituent elements are dissolved by heating at a temperature of between 1200 ° C. and 1350 ° C. for approximately 4 hours. cooled to room temperature and annealed at a temperature between 800 ° C and 950 ° C for about 4 hours.

Advantageously, the dissolution is carried out at about 1250 ° C for about 4 hours and annealing at about 900 ° C for about 4 hours.

The characteristics and advantages of the invention are described in more detail in the description below, with reference to the appended drawing, which shows in graphical form the variation of certain properties of the alloy according to the invention as a function of its content. in aluminium.

In the present description, unless otherwise indicated, all proportions are given in atoms.

It has been found that alloy grades of TiAl type (or more succinctly TiAl alloys) with a high aluminum content such as 48% are more ductile and less resistant than low aluminum grades such as 44%. However, for contents greater than 48% the tendency is reversed rapidly with a reduced ductility while creep resistance and oxidation resistance are improved. Thus, the aluminum content must be enclosed in a window of very narrow composition (47-48%) to ensure a good compromise of properties. However, this high sensitivity of the aluminum content of TiAl alloys is a serious handicap for their development which requires a very high precision in the amounts of added elements. An ingot weighing several kilograms can thus have variations in aluminum contents of greater than 1% in different places, with the consequence of different properties, which can deviate from the specifications of the users. In addition, aluminum is volatile during melting, causing a loss of aluminum concentration that is dependent on the number of fusions. This is another reason why it is difficult to scrupulously respect the nominal aluminum contents.

The iron has the effect of enlarging the window of aluminum content for which the good compromise of properties is respected. In other words, the ductility and mechanical strength properties remain constant over a wider range of aluminum content, making it less difficult to manufacture the alloys to achieve the desired properties.

TiAl alloys containing iron are singularized in another way. While virtually all cast TiAl alloys deform plastically only at temperatures above 800 ° C, iron-containing grades can deform plastically at lower temperatures. The brittle-ductile transition temperature is indeed very brutal for all TiAl alloys which can therefore be classified according to this characteristic. A large number of elemental additives have proven ineffective in the past to improve ductility at temperatures below 800 ° C. However, it has been found that the addition of iron and zirconium makes the TiAl alloys even more ductile at 800 ° C. The advantage that is then removed is to be able to manufacture them using traditional shaping methods using temperatures compatible with common tools.

The only penalizing effect of iron that has been observed is a decrease in creep resistance, which can lead to limiting the addition of iron to small quantities (around 1%). The use of silicon in the present invention can compensate for this effect by providing an extremely fast gain on creep resistance, which makes it possible to limit its concentration to 0.5%. Indeed, higher silicon contents are disadvantageous because they cause the precipitation of silicides which are known to be detrimental to ductility.

With regard to zirconium, it has been found that a high content (5%) has the effect of repelling the brittle-ductile transition towards high temperatures and thus counteracting the beneficial effect of iron. Therefore, the zirconium content to be used should be well below 5%. It is also preferable to limit the zirconium content for density reasons. Finally, the macroscopic properties of TiAl can be affected in the presence of zirconium because of steric effects which predominate over the electronic effects. The work done by the inventors made it possible to verify that it was undesirable to incorporate more than 2 to 3% of zirconium in order to maintain an acceptable compromise between strength and ductility.

Reduced addition of molybdenum (1%) together with the iron and zirconium elements provides additional ductility gain at 800 ° C, which was the desired result. However, a higher content is undesirable because this element causes an increase in the mechanical strength when hot, which then requires the use of a higher force of the press to deform it. It should also be remembered that creep resistance is also improved compared to alloys not containing this addition of molybdenum, an effect that can be modulated by the addition of Mo and Si.

The alloys according to the invention are cast to the requirements of high ductility when hot for their shaping by anisothermal forging.

The alloys described in the examples below were manufactured using vacuum arc melting. The ingots were then subjected to isostatic compaction at 1250 ° C for the closure of porosities and shrinkage. The comparative characterization of the alloys took place after compaction and heat treatment. The heat treatment comprises a solution of 4 hours at 1250 ° C followed by a cooling of the oven and a stress relieving annealing of 4 hours at 900 ° C. This treatment aims to generate two types of structures according to the aluminum concentration. It allows to stabilize the γ + α ₂ two-phase lamellar structure (TiAl + Ti ₃ Al) for the richest titanium grades and to stabilize the duplex structure composed of lamellar grains and γ-phase monolithic grains (TiAl) for the grades the richest in aluminum. This two-phase γ / α _{2 structure} which is beneficial for the ductility can not however be obtained for aluminum concentrations greater than 49%, the alloy then remaining single-phase γ even after heat treatment. This treatment also allows a more uniform redistribution throughout the structure of certain elements such as zirconium which may tend to segregate in the last liquid areas during solidification.

The alloys according to the invention comprise on the one hand an addition of zirconium, and on the other hand additions of elements Mo and Fe which are known as β- gene elements, in that they stabilize the formation of the β phase. Other elements, W and Cr, also known as β-genes have been tested, but have not been retained in the alloy compositions of the invention. The hot ductility is determined by the tensile properties at 800 ° C. Since these alloys must retain sufficient cold ductility to allow the machining and handling of parts, tensile properties at 20 ° C have also been determined.

The invention is illustrated hereinafter by the description of tests relating to various alloys, for each of which are successively indicated an identification number, the composition in atoms and the composition in mass.

A first series of tests is intended to test the combination of hardening elements (W, Mo) and a ductilizing element (Zr). Alloys containing either 2% W or 1% W + 1% Mo are prepared with, for each of these combinations, two values of Al + Mo, the aluminum content being thus chosen slightly lower in the presence of molybdenum because of the more low power β-gene molybdenum compared to that of tungsten. 1029 Ti-45Al-2W-2Zr (Ti-28,9Al-8,7W-4,3Zr) 1017 Ti-48Al-2W-2Zr (Ti-30,4Al-8,8W-4,4Zr) 1027 Ti-44Al-1W-1Mo-2Zr (Ti-28,7Al-4,4W-4,4Zr-2.3Mb) 1028 Ti-47Al-1W-1Mo-2Zr (Ti-31,1Al-4.5W-4,5Zr-2.3Mb)

The tensile results at 20 and 800 ° C. reveal that the introduction of the hardening elements is highly disadvantageous for the ductility since the elongations (A in%) at 20 ° C. and at 800 ° C. do not exceed 1.2% and 2%. 9% respectively (Table 1). In any case, molybdenum appears to be more beneficial than tungsten for this property. As for the hardening evaluated from the yield strength, it is certainly appreciable with in particular the addition of molybdenum, but does not justify sacrificing ductility as much. In summary, the substitution of 1 Mo to 1 W appears favorable for the resistance-ductility compromise at 20 and 800 ° C. <u> Table 1 </ u>: Traction characteristics of the alloys Ti-45Al-2W-2Zr (1029), Ti-48Al-2W-2Zr (1017), Ti-44Al-1W-1Mo-2Zr (1027) and Ti-47Al-1W-1Mo-2Zr (1028) Alloy T (° C) σ _0.2 (MPa) σ _max (MPa) AT (%) 1029 20 545 594 0.40 800 430 516 1.20 1017 20 431 509 0.76 800 390 458 0.98 1027 20 603 640 0.36 800 472 569 1.96 1028 20 500 626 1.21 800 462 588 2.88

The second series of shades is characterized by the following compositions: 1031 Ti-45Al-2W-2Cr-1ZR (Ti-29,1Al-8,8W-2,5Cr-2,2Zr) 1032 Ti-47Al-2W-2Cr-1ZR (Ti-30,7Al-8,9W-2,5Cr-2,2Zr) 1033 Ti-46Al-1FE-1W-1ZR (Ti-30,5Al-1,4Fe-4,6W-2,3Zr) 1034 Ti-48Al-1FE-1W-1ZR (Ti-32,6Al-1,4Fe-4,6W-2,3Zr)

The aim of this series is to test the combination of a single hardening element (W) and several ductilizing elements (Cr, Fe, Zr), which led to a reduction of the zirconium content to 1% (Table 2). The ductility results reveal that the elongations do not exceed 1% at 20 ° C. Tungsten clearly appears to be responsible for this low temperature fragility, which confirms the results of the first series. On the other hand, a 1% reduction in the hardening element content (W) is reflected in a softening manner. It should be noted that the two grades containing iron are characterized by the same ductility at 20 ° C despite the difference in aluminum content, a behavior that contrasts with other pairs of grades. Another interesting point concerns the elongations at 800 ° C which reach nearly 28% for the alloy Ti-48Al-1W-1Fe-1Zr, clearly demonstrating the beneficial effect of iron on hot ductility. <u> Table 2 </ u>: Traction Characteristics of Ti-45Al-2W-2Cr-1Zr (1031), Ti-47Al-2W-2Cr-1Zr (1032), Ti-46Al-1W-1Fe-1Zr Alloys (1033) and Ti-48Al-1W-1Fe-1Zr (1034) Alloy T (° C) σ _0.2 (MPa) σ _max (MPa) AT (%) 1031 20 522 570 0.41 800 418 511 3.62 1032 20 498 532 0.35 800 422 520 1.72 1033 20 474 574 0.87 800 390 456 3.80 1034 20 357 431 0.84 800 338 440 27.52

The single figure represents, for the alloys of the two previous series, the elastic limit and elongation at break at 20 ° C as a function of the aluminum content. The points corresponding to each property are located approximately on a straight line. Clearly, there is an inverse relationship between ductility and the yield strength. The increase in ductility and the decrease in the yield strength as observed on the richer aluminum compositions are related to two microstructural changes. The richer aluminum grade firstly has a higher monolithic γ volume fraction (and correspondingly a lower lamellar fraction); however, it is known that duplex structures are more ductile and less resistant than fully lamellar structures. In addition, this shade is less rich in β phase. The Ti-48Al-1Fe-1W-1Zr grade is predominantly composed of monolithic γ-phase and the low volume fraction of lamellae no longer makes it possible to fractionate the grain size, which explains the level of ductility slightly reduced at ambient temperature than for other additions. In return, the incorporation of iron produces a particular effect since it makes it possible to reach a certain level of ductility for the standard heat treatment, in this case 0.8% for the alloy Ti-46Al-1Fe-1W- 1Zr, which is not the case for others shades rich in titanium. In the standard state, the Ti-46Al-1Fe-1W-1Zr grade is characterized by the presence of a high amount of β- phase while the Ti-48Al-1Fe-1W-1Zr grade has a majority of monolithic γ-phase. and a phase minority β .

The third series of shades is characterized by additions of iron, zirconium and molybdenum. Two shades correspond to a substitution of Mo to W with respect to the preferred shades above. The other two grades are characterized by the absence of molybdenum: 1144 Ti-46Al-1FE-1ZR-0,2Si (Ti-32A1-1,4Fe-2,4Zr-0,2Si) 1145 Ti-47Al-1FE-1ZR-0,2Si (Ti-32,9Al-1,4Fe-2,4Zr-0,2Si) 1147 Ti-46Al-1FE-1Mo-1ZR-0,2Si (Ti-31,6Al-1,4Fe-2.5Mo-2,3Zr-0,2Si) 1146 Ti-47Al-1FE-1Mo-1ZR-0,2Si (Ti-32,5Al-1,4Fe-2.5Mo-2,3Zr-0,2Si)

The addition of silicon is done to counterbalance the possible weakness provided by the iron vis-à-vis the creep resistance. The comparison of the results with the previous series reveals that the individual substitution of iron or molybdenum for tungsten reduces the brittleness of the alloys. The level of cold ductility is quite satisfactory with elongations of between 1 and 2% (Table 3). Previous results are confirmed in the sense that in the presence of iron, the ductility is relatively insensitive to the Al content. The beneficial effect of molybdenum on hot ductility is also confirmed, whereas this effect was much less visible. on the alloys of the first series in the presence of tungsten. There is therefore a synergistic effect between the iron and molybdenum elements as to their effect on hot ductility. Thus, the comparison of the last four shades reveals that the addition of molybdenum makes it possible to increase the elongations to 800 ° C from 27% on average to 73% on average. Such a level of ductility at 800 ° C makes it possible to expect good deformability during the subsequent forging, especially since this is done by starting at a higher temperature, typically 1000 ° C. An examination of the fracture facies of specimens lengthening by 73% in mean reveals that the structure is composed of numerous small lamellar grains, without dynamic recrystallization in the form of monolithic grains, which suggests a good creep behavior. <u> Table 3 </ u>: Traction Characteristics of Ti-46Al-1Fe-1Zr-0.2Si (1144), Ti-47Al-1Fe-1Zr-0.2Si (1145), Ti-46Al-1Fe Alloys -1Mo-1Zr-0.2Si (1147) and Ti-47A1-1Fe-1Mo-1Zr-0.2Si (1146) Alloy T (° C) σ _0.2 (MPa) σ _max (MPa) AT (%) 1144 20 442 539 1.26 800 396 480 21.0 1145 20 391 486 1.14 800 357 445 33.0 1147 20 438 565 1.23 800 380 451 60.0 1146 20 371 506 1.40 800 354 438 76.0

Creep tests were also carried out at 750 ° C. under 200 MPa on most alloys of the three previous series in order to test them under the conditions closest to use in aeronautical turbomachines.

The creep results of the first series reflect more strongly the strong influence of aluminum content and indirectly the microstructure on creep resistance than the effect of substitution of Mo to W (Table 4). Indeed, the comparison of the TiAl + 2W + 2Zr grades with different aluminum contents (1017 and 1029) reveals the high sensitivity of the creep resistance to aluminum. There is almost an order of magnitude between the secondary creep rates to the advantage of the aluminum rich shade. For low aluminum grades, the residual β phase is quite coarse. On the other hand, for the grades with a high aluminum content, the β phase is smaller and is characterized by precipitation. dense in the lamellae. These precipitates act as obstacles to deformation and explain the unexpected improvement in creep. For the difference in properties observed between 1017 and 1028, it is difficult then to incriminate the non-beneficial presence of molybdenum because the aluminum content is slightly lower for the molybdenum-containing grade. In general, the creep properties obtained on these four grades are excellent compared to known alloys based on TiAl. <u> Table 4: </ u> Creep characteristics of the alloys Ti-45Al-2W-2Zr (1029), Ti-48Al-2W-2Zr (1017), Ti-44Al-1W-1Mo-2Zr (1027) and Ti-47Al-1W-1Mo-2Zr (1028) Alloy Duration (h) Primary Secondary ε̇ (s ^-1 ) A = 0.2% A = 0.5% AT (%) Duration (h) 1029 33 153 0.13 12 5,0.10 ^-9 1017 392 1160 - - 0.9. 10 ^-9 1027 18 129 0.32 46 3.7.10 ^-9 1028 85 574 0.15 52 0.67.10 ^-9

The creep results obtained on three of the grades of the second and third series reveal the detrimental effect on the creep resistance of the chromium and iron ductilizing elements (Table 5). However, the addition of 0.2% silicon for a grade of the third series makes it possible to significantly reduce the secondary creep rate to reach a level comparable to that of the first series. <u> Table 5 </ u>: Creep Characteristics of Ti-45Al-2W-2Cr-1Zr (1031), Ti-46Al-1W-1Fe-1Zr (1033) and Ti-46Al-1Fe-1Zr-0 Alloys , 2Si (1144) Alloy Duration (h) Primary Secondary ε̇ (s ^-1 ) A = 0.2% A = 0.5% AT (%) Duration (h) 1031 17 77 0.22 22 11.10 ^-9 1033 2.5 32 0.38 20 26.10 ^-9 1144 30 250 0.50 250 2.5.10 ^-9

Independently of the present invention, the alloys used in aeronautical turbomachines must also have good resistance to oxidation. In this context, the oxidation resistance of the preferred alloys may, if necessary, be improved by the introduction of a certain amount of niobium, an element known for its favorable action on this property.

Claims (7)

1. Alloy consisting of titanium aluminide in which a minor proportion of the aluminium and titanium atoms are replaced by other atoms, characterised in that it contains, in atoms, 44 to 49% aluminium, 0.5 to 3% zirconium, 0.5 to 2% iron, 0.5 to 2% molybdenum, 0.2 to 0.5% silicon, 0 to 3% niobium, plus titanium and unavoidable impurities to make up 100%.
2. Alloy according to claim 1, which contains, in atoms, 45 to 48% aluminium, about 1% zirconium, about 1% iron, about 1% molybdenum and about 0.2% silicon.
3. Alloy according to claim 2, which contains, in atoms, about 46% aluminium.
4. Alloy according to claim 2, which contains, in atoms, about 47% aluminium.
5. Alloy according to one of the preceding claims which is made up exclusively of aluminium, titanium, zirconium, iron, molybdenum, silicon and, optionally, niobium, subject to any impurities.
6. Method of heat-treating an alloy according to one of the preceding claims, wherein the constituent elements of the alloy are put into solution by heating to a temperature of between 1200°C and 1350°C for about 4 hours, cooling to ambient temperature and annealing at a temperature of between 800°C and 950°C for about 4 hours.
7. Method according to claim 6, wherein the putting into solution is carried out at 1250°C for about 4 hours and annealing is carried out at about 900°C for about 4 hours.

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