CZ342696A3 - Intermetallic base alloy of nickel and aluminium - Google Patents

Abstract

A nickel-aluminium intermetallic basis alloy has a structure mainly made of the binary phase NiAl and further contains the elements chromium and tantalum. The total proportion of the elements chromium and tantalum amounts to maximum 12 % by atoms. The preferable content ranges lie from 0.3 to 3.8 % by atoms tantalum and from 1.0 to 9.0 % by atoms chromium. The nickel-aluminium intermetallic basis alloy is characterised in particular by a high oxidation resistance at high temperatures, such as 1350 DEG C. It is therefore suitable for producing pieces exposed to a high and continuous thermal stress, such as gas turbine blades. This high oxidation resistance allows additional anti-oxidation layers to be dispensed with.

Description

Intermetallic base alloy of nickel and aluminum

Technical field

BACKGROUND OF THE INVENTION The present invention relates to an intermetallic base alloy of nickel and aluminum comprising a double phase NiAl. The invention also relates to the use of an intermetallic base alloy of nickel and aluminum.

BACKGROUND OF THE INVENTION

This intermetallic nickel-aluminum base alloy is known from NiAl Alloys for High Temperature Structural Applications, published in the Journal of Metals of March 1991, p. 44 et seq.

DE-AS 18 12 144 describes a process for producing high strength nickel-aluminum materials with good oxidation resistance. In this process, the nickel powder is mixed with the aluminum powder and then compressed and cold formed to form a self-supporting and cohesive molding with a fibrous or laminar structure. The proportion of nickel in this material is at least 80% and the proportion of aluminum at most 20%. The cohesive molding is then subjected to hot forming in the next phase at gradually increasing temperatures. In addition to the solid solution of aluminum in nickel, the Ni ₃ Al compound is additionally produced. This solid solution as well as the Ni ₃ Al compound can be detected by X-ray analysis. Whether other nickel and aluminum compounds are formed in the material according to the invention is not apparent from this specification.

SUMMARY OF THE INVENTION The object of the present invention is primarily to improve the thermomechanical properties of nickel-aluminum alloys. These properties include, in particular, sufficient heat strength, oxidation resistance and thermal shock resistance. Another object of the invention is to find a suitable application for this new nickel-aluminum alloy.

SUMMARY OF THE INVENTION

This object is achieved by an intermetallic nickel-aluminum alloy which contains predominantly the binary phase NiAl and additionally chromium and tantalum, both chromium and tantalum being present in the alloy in a total proportion of up to 12 at.% And optionally additionally containing at least one an element from the group containing iron, molybdenum, tungsten, niobium and hafnium, the individual proportions of which may be up to 1 at.% and the total sum of the proportions of these elements shall not exceed 3 at.%. The proportion of the NiAl binary phase in the total alloy composition is in an atomic amount of 70 to 95 at%, in particular 85 to 90 at%. Preferably, the alloy contains 0.3 to at

9.0 at.% Amazement. Within this range, a range of 0.3 at.% To 0.9 at.% Tantalum and 1.0 at.% To 3.0 at.% Or 0.7 to 3.0 at. , 0 to 9.0 at.

The ratio of tantalum to amazement is 1: 3 or less. At such a ratio, the concentration of the substitution elements in NiAl reaches its maximum. By the addition of tantalum and astonishment, the intermetallic base nickel-aluminum alloy at the grain boundaries of the NiAl binary phase forms precipitates in the coarser raultary Laves phase, in which the elements Ni, Al, Cr and Ta can participate. In addition, fines of the fine-grained Laves phase and α-chromium are contained within the NiAl grains. In this case, it is preferred that the alloy structure consists of 5% to 11% Laves phase,% to 10% precipitates in NiAl and the remainder being NiAl. Particularly preferred is an alloy structure which contains about 11% of the Laves phase at the grain boundaries and about 10% of the precipitate in NiAl in bulk, the remainder being NiAl.

A further improvement of certain properties can be achieved if the alloy contains at least one element from the group comprising iron, molybdenum, tungsten, niobium and hafnium in atochrome

2a to a maximum of 1%, not more than 3% total. In addition, the alloy may contain trace elements such as oxygen, nitrogen and sulfur, as well as impurities whose presence cannot be avoided for manufacturing reasons.

By adding tantalum and chromium in the amounts mentioned in the previous section, said coarser or fine-grained Laves phases and α-chromium are formed. These precipitates generally occur at the wedge points of the gaps between the different NiAl grains. Greater amounts of tantalum and chromium than the above limits may result

that the amount of precipitates reaches an undesirable amount. If too much Laves phase is added too much, a cell structure arises in which the Laves phase assumes the function of the matrix. Too much of the Laves phase in the alloy structure makes this interraetal alloy brittle and more difficult to process.

By adding one or more of iron, molybdenum, tungsten, niobium and hafnium in an atomic amount of up to 1%, in total not more than 3%, an increase in short-term strength can be achieved, but the creep resistance of the material is reduced. The addition of hafnium results in a better cohesiveness of the oxide layer after the first occurrence of corrosion.

According to the invention, the task of using the alloy is solved by the fact that components of a gas turbine, in particular those exposed to high temperatures such as gas turbine blades, can be manufactured from a NiAl base alloy. The components of a gas turbine made of a base alloy, in particular a turbine blade, are particularly suitable for permanent use in high temperature environments, for example above 1100 ° C, in particular 1350 ° C, due to the high oxidation resistance of the material. Depending on the specific requirements, the application of an additional protective layer can be omitted in this component, unlike the materials of high-alloy alloys. The turbine blades thus produced consist only of a single alloy without additional coatings, so that their manufacture is considerably simpler and does not have the problems of securing the bond between the individual layers.

The intermetallic nickel-aluminum alloy is a suitable material especially for the production of such articles which must have high strength, high refractoriness, good toughness, good oxidation resistance and good thermal shock resistance. The strength lies at an elongation limit of 0.2% at room temperature above 600 MPa, the hot strength at an elongation limit of 0.2% is greater than 200 MPa at 800 ° C and greater than 90 MPa at 1000 ° C, the toughness is at least 7 MPa / m and oxidation resistance is of the order of 5. . 10 “ ¹⁴ g ² c” ⁴ p.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be further elucidated by means of the following examples of an intermetallic nickel-aluminum alloy.

The compositions of controlled alloys (in at%) are given in Table 1 below.

Table 1

Ni AI The Cr Other SSM 364 45,00 45,00 2.50 7.5 VA 2823 44.50 44.50 2.50 8.99 0.39 Fe, 0.105 C USM 2823 44.50 43.90 2.90 8.50 0.14 Fe, 0.02 C USM 2922 45,00 45,00 2.00 8.00 PM 75/76 44.10 44.10 2.40 7.70 0.09 Fe, 0.06 C, 0.09 0 33 ppm N, 14 ppm S VA 892 44.50 45.20 2.53 7.60 90 ppm Hf, 0.04 C 20 ppm S, 61 ppm 0

The structure formed, i.e. the grain size, the distribution of the precipitated particles, the size of the precipitated particles, varies greatly depending on the production process. The thermodynamic treatment, extrusion (SP) or use of the powder metallurgy (PM) production line homogenises the phase particle distribution.

Also, the mechanical properties of the alloys are strongly dependent on the selected production process. The following manufacturing processes were used to manufacture these alloys:

- directed solidification as a possibility of creating a flawless structure by casting technology. Production parameters correspond to the parameters of superalloys (see U. Paul, VDI-Fortschrittbericht No. 264, VDI Publishing),

- powder metallurgy: injection of inert gas through a nozzle and subsequent hot isostatic pressing at 125 ° C,

- extrusion to homogenize the structure and adjust the defective grain size at 1250 ° C,

- hot pressing in a multiaxial state of stress and 1100 ° C.

The rectified solidified samples have a clearly higher strength while, for example, the extruded material has a reduced or very low strength. The following table 2 shows the yield strength values of 0.2% achieved in the upsetting test for various alloys and also for NiAl.

Table 2

Temperature in ° C Alloy No .: 23 200 400 600 800 900 1000 1100 1200 SSm 364 650 520 520 451 201 146 94 60 46 VA 2823 640 524 414 264 137 83 USM 2823 1501 1494 584 404 186 125 88 PM 75 814 593 456 284 126 65 PM 76 869 562 466 275 133 ⁵¹ VA 892 133 SP 75 730 581 344 294 113 69

The creep resistance (in MPa) of the alloys under pressure test (secondary stationary creep strength as a function of elongation rate [v / s] at 1000 ° C and 1100 ° C) is shown in Table 3.

Table 3

Elongation speed in 1 / sec Sv 1.00E-08 at 1000 ° C 1.00E-07 at 1000 ° C 1.00E-06 ' 1.00E-08 at 1100 ° c 1.00E-07 at 1100 ° c 1, OOE-06 at 1100 ° c at 1000 ° C SSm 364 19.90 36.10 55.50 14.60 20,00 34.60 VA / SM 16.5 23.20 33.40 USM 2823 casting 79,00 98.00 28.50 32.80 39,00 PM 75 13.90 22.90 36.80 17.50 PM 76 11.20 19.90 33.80 11.00 SP 75 10.00 18.00 33.90 11.10

The creep rupture strengths of these alloys are higher than the creep rupture strengths of comparable intermetallic phases, for example they are higher than the creep rupture strengths of the binary alloys NiAl or NiAlCr.

Table 4a contains a comparison of the yield strength of 0.2% (in MPa) in the conventional high alloy alloy test given in the following tables for abbreviation as superalloy, binary NiAl alloys, and NiAl-Ta-Cr alloys by compression.

Table 4a

Temperature: Superalloy ^Ni 50 ^A1 50 NiAl-Ta-Cr alloy / PM 75 900 ° C 424 47 345/205 1000 ° C 135 26 186/126 1100 ° C 28 18 125/65

With respect to the limit of 0.2% yield strength, the advantage of the inventive alloy at temperatures above 1000 ° C has been demonstrated.

The following table 4b compares the static resistance of superalloys, binary NiAl alloys and new NiAl-Ta-Cr alloys to creep at ^--10-7 l / s MPa) and under pressure testing:

Table 4b

Temperature: Superalloy ^Ni 50 ^A1 50 NiAl-Ta-Cr alloy / PM 75 900 ° C 424 47 345/205 1000 ° C not identified 26 186/126 1100 ° C not identified 18 125/65

Compared to conventional high-alloy alloys, the NiAl-Ta-Cr alloy has the advantage that it has sufficient strength even at temperatures above 1050 ° C to 1150 ° C. With this alloy, there is no sudden decrease in strength, which would lead to lower strength values due to the release of the solidified phase.

Table 5 shows a comparison of the K _IC values of various ceramic materials known from industrial practice with those of NiAl-Ta-Cr alloys produced by powder metallurgy techniques.

Table 5

NiAl alloy NiAl-Ta- -Cr alloy NiAl-Ta- -Cr Pm NiAl-Ta- -Cr SP SiC To _IC / MPa m 4-5 * 4,5 8 8-11 4.6

* ReuB, Dissertation and RWTH Aachen

The toughness of the intermetallic NiAl base alloy is significantly better than the measured values for binary NiAl and SiC.

The alloy has a good oxidation resistance, which is of the order of 5.10 " ¹⁴ g ² cm ^-4 s and which is as equal or better than the oxidation resistance of the binary NiAl. Thus, unlike high-alloy alloys, no protective layers, for example of ceramic material, are required at high temperatures in the alloy according to the invention. This eliminates the connection problems between the ceramic and metal parts.

The alloy according to the invention also has sufficient thermal shock resistance. At 1350 ° C, 500 temperature cycles were achieved with the alloy of the invention without damaging the material.

Claims (12)

Intermetallic nickel-aluminum base alloy, characterized in that it comprises a predominantly binary NiAl phase and additionally also chromium and tantalum, the chromium and tantalum being present in the alloy in a total proportion of up to 12 at.% And optionally comprising at least one an element from the group consisting of iron, molybdenum, tungsten, niobium and hafnium with a proportion of individual elements up to 1 at.%, the sum of the proportions of added elements being not more than 3 at.%. Intermetallic alloy according to claim 1, characterized in that the proportion of the binary phase NiAl in the total alloy composition is in an atomic or molar amount of 70% to 95%, in particular 85% to 90%. Intermetallic alloy according to claim 1 or 2, characterized in that it contains 0.3 to 3.8 at.% Tantalum and 1.0 to 9.0 at.% Chromium. Intermetallic alloy according to claim 3, characterized in that it contains 0.3 to 0.9% tantalum and 1.0 to 3.0% chromium in atomic amounts. 5. An intermetallic alloy as claimed in claim 3 comprising 1.7 at.% To 3.0 at.% Tantalum and 6.0 at.% To 9.0 at.% Chromium. Intermetallic alloy according to claims 1 to 5, characterized in that it contains tantalum and chromium in a ratio of 1: 3 or less. Intermetallic alloy according to claims 1 to 5, characterized in that at least some of the NiAl grain boundaries are coarser Laves phase and within at least some nickel-aluminum grains there are particles of fine-grained Laves phase and α-chromium. Intermetallic alloy according to claim 7, characterized in that its structure contains, by volume, 5 to 11% of coarse Laves phase particles and 3 to 10% of fine-grained Laves phase and α-chromium particles in NiAl. 9. The intermetallic alloy of claim 8, wherein the structure contains about 11% of the Laves phase at the grain boundaries and about 10% of the binary NiAl precipitates in bulk. Intermetallic alloy according to claims 1 to 9, characterized in that it contains at least one element from the group comprising iron, molybdenum, tungsten, niobium and hafnium in an atomic amount of up to 1%, in total not more than 3%. Use of an intermetallic nickel-aluminum alloy containing predominantly the binary phase NiAl as well as tantalum and chromium, wherein the total proportion of tantalum and chromium in the alloy is at most 12% by weight, as a material for producing gas turbine structural elements. . Use of an alloy with a material composition according to any one of claims 1 to 10 as a material for producing articles of high strength (600 MPa at room temperature and 0.2% elongation), high heat strength (about 200 MPa at 800 ° C) and more than 90 MPa at 1000 ° C at 0.2% elongation), with good toughness (K _to at least 7 MPa / m), oxidation resistance (about 5.10 ¹⁴ g ² cm ⁴ s) and heat resistance rázům.