DE1286870B - Intermetallic protective layer on workpieces made of niobium or niobium alloys and processes for their production - Google Patents

Description

1 2

The invention relates to an intermetallic protective formation or to other defect formations. In the opposite class high breaking strength, high rate of oxidation to molybdenum, which oxidizes to a high degree, resistance at temperatures between 538 and the oxide of niobium is non-volatile; it's from this 1371 ° C and high fatigue strength suitable for factory grounds, oxygen catches on the niobium Pieces of niobium or a niobium alloy, in particular to prevent 5 coating of the metal and in the event a niobium alloy, the 20 percent by weight of premature damage to the layer Tantalum, 15 weight percent tungsten, 5 weight-localized places these defects and oxygen percent Molybdenum and the remainder of niobium are made and limit an attack on the localized areas. Process for the production of this protective layer. Further advantages over niobium alloys

In gas turbine technology, the upper temperature io molybdenum alloys is that the niobium limit currently relatively ductile and at a low temperature due to the maximum turbine inlet alloys given. This inlet temperature is editable and that specific gravity in turn determined by the temperature, which is lower than that of niobium than that of molybdenum. Turbine blades - without suffering damage - niobium alloys are mainly used

can endure. In the past, the best high-grade alloys, if it is a question of oxy-temperature-resistant alloys, nickel and cobalt-degenerating atmospheres at temperatures up to about basic superalloys. Exposure to 1371 ° C from these just mentioned, and if they shovel components manufactured with these high alloys - such as turbine temperatures over a long period of time - can only be at most temperatures. Stress fracture-resistant, unbalances of 871 to 1038 ⁰ C can withstand a maximum. 20 layered niobium alloys withstand 538 ° C without

It has long been known that more resistant oxidations occur; at temperatures ability of metals to withstand high temperatures above 538 ° C, oxidation becomes a serious one largely on the respective melting points of the problem.

Metals is dependent and thus metals with high levels One has meanwhile resistant to oxidation Melting points mostly high temperatures to endure 25 intermetallic coatings found which capital. especially refractory metals (for example

The need for metallic materials, which niobium, molybdenum, tantalum and tungsten) at high Can withstand higher temperatures than those previously used to protect against oxidation. In general known materials such as nickel and are the more effective of these intermetallic layers Cobalt alloys or refractory metals such as silicides, aluminides and berylides of the base metal in particular chromium, niobium, molybdenum and tungsten. For layers for high melting points have the interest in metals with very high metals are both the layers and the layer melting points awakened. Until recently, carrier materials were thought to be important for the behavior of the Molybdenum is the best coated system for the purposes mentioned. Silicide layers on niobium and suitable, but at the high temperatures that occur, molybdenum can, for example, be very different doors oxidize molybdenum very strongly, mainly work; these behavioral differences are more like that This is because molybdenum oxide is to be ascribed to the coating at elevated temperatures. A doors is volatile. additional evidence of the importance of the shift

Ais basic material for alloys for high temperature carriers is that some types of coatings, which niobium seems to be suitable. One of its 4 °, for example, is effective on tantalum, on niobium technically most important physical properties is ineffective because they are too premature the high melting temperature of 2416 ° C and its deficiencies in localized places at high temperature ringer Neutron entrance cross-section. Niobium is des- door prone.

semi particularly suitable for use in construction For applying intermetallic layers on of airplanes, spaceships and nuclear reactors. 45 Niobium alloys can use several processes.

By nature, niobium is soft, ductile and detached, for example flame or plasma without any other deformable material. Although its spray, the slurry application technique, electrophoric melting temperature is about 2416 ⁰ C, niobium is used for table deposition, warming or the deposition temperatures of over 649 ⁰ C for construction from the gas phase. A method of deposition is too soft for the purpose. Niobium is also a very reactive 5 ° from the gas phase, which in some types of layers is metal, since it is used in large amounts of oxygen and expediently, the so-called probably also absorbs nitrogen if it is used in processes in which the atmosphere to be coated is exposed, which only small objects surrounded by a special mixture will contain amounts of these elements at relatively low levels, for example that at the temperatures to be coated. 55 Metal deposited on the workpiece (such as silicon, aluminum

The niobium processing technique has shown, such as minium, beryllium), an activator (usually a heavy one, an oxidation _{resistance and a halide salt such as NaCl, KF, NH 4} I, NH ₄ Cl and the like) resistance to high temperatures and a Inert filler material (ζ. Β. Al ₂ O ₃ , SiO ₂ , BeO, can only be achieved by alloying metals. Since it contains MgO and the like). The suitability of alloys based on 60 (made of steel or graphite, for example ceramic crucibles) of niobium in a suitable container depends largely on the temperature-resistant mixture then depends on a desired speed of these alloys; Representatives of the niobium alloys are heated to a protective atmosphere and need layers on this temperature as long as they are kept at temperatures until the desired layer is reached. which normally cause oxidations. Typical representatives of such coatings of this application process, layers, for example made from niobium-based niobium workpieces, are hard and niobium, of precise thickness. The main part of the niobium is brittle and has a tendency to crack at localized points - for example NbAl ₃ or NbSi ₂ and the like.

3 4

The cheaper layers for niobium (niobium aluminides, less ductile diffusion layers, such as

Silicides, berylides) have certain disadvantages, for example niobium aluminide (NbAl ₃ ), and even then it is not

wise rapid oxidation at low temperatures for eventual ones at low temperature, for example

(in the range of 704 ⁰ C) or at high temperatures at 704 ⁰ C, rapid oxidation destruction

(above 1093 ⁰ C). The most serious disadvantage 5 susceptible to being exposed to high temperatures beforehand,

of known layers for niobium, however, their, for example, 1204 ⁰ C, was exposed. The inventive

Tendency to break in local places. It has a very high protective layer

shown here that in particular two properties are thermal stability of their oxidation properties

help to keep these layers in local places and is also against contamination with

tend to break (destruction). These two intrinsically foreign ingredients or phases are insensitive,

stocks are: d. that is, such impurities reduce the protective effect

1. Only insignificantly impair the extraordinary brittleness of the layer

materials and invention also paws a process

“,., _A j ι ι · j · ι ζ ¹ ™ application of layers on niobium or a

^ Rmfhe expansion differences between _{niobium alloys} . _This method includes the following

Layer and carrier. preferred levels:

U.S. Patent Application Ser. No. 65 962 including i. Separation from the gas phase by a two-

tet a group of workable, ductile, in the case of staged application processes

mechanical stress break-proof niobium alloy _{2 galvanic} deposition (especially galvanic

rungs, which for use at high temperature * o _S i _e ration diffusion process) and a

Temperatures up to at least 1371 C are suitable. One . ,,. . _,. . ,,,.,

typical representative of this last group of alloy ³ · combined galvanization and deposition

is the compound Nb-20Ta-15W-5Mo, process from the gas phase,

where the numbers express percentages by weight. Even with compli-

With the present invention, a permanently shaped work piece, corners and edges becomes one

break-proof intermetallic protective layer created, almost uniform and continuous coating

which workpieces up to temperatures of 1371 ^{0 C.} Other features and advantages of the invention

made of niobium or a niobium alloy and which are explained in the following description,

despite the prevailing in niobium or a niobium alloy, according to the proposal according to the invention

the thermal expansion difference between the intermetallic protective layer made of cobalt aluminide

the protective layer and the niobium or a niobium (CoAl), which may still be in dissolved form

alloy results in a ductile layer that is not up to 30 atomic percent cobalt and up to 2 atomic percent

There is a tendency for errors to occur in local places. Aluminum in excess compared to the stoichiometric

In addition to protecting the workpiece made of niobium or the metric composition of CoAl in one a niobium alloy against oxidation at a high 35 contains such an amount that the total content of Temperatures this layer also has the cobalt in the layer between 48 and 80 atomic property, mechanical and thermal stresses percent. (The unit of measurement is atomic percent to be able to withstand, as it is defined for example in the case of nozzles by the assumption that in the case of the engines occur. The protective stoichiometric composition according to the invention is one of two layer is compatible with the base material of the 4 ° components existing connection the proportion of the Alloy so that the layer does not have a low-melting point component of 50 atomic percent.) According to the phases or phase mixtures, volatile compounds can be a further proposal according to the invention fertilize or form a thick, brittle coating; this protective layer essentially consists of a their mechanical properties are matched to the basic cobalt and niobium intermediate layer or adapted support. 45 be arranged, which is applied to the workpiece.

In addition, the protection according to the invention has The process according to the invention for production Layer has sufficient diffusion stability (i.e. the protective layer can be deposited twice When the protective layer is applied, an initial phase emerges from the gas phase, with the metallic borrowed diffusion between the protective layer and the workpiece first with cobalt and then with Layer support on, but the resulting 50 aluminum is coated. The metallic work protective layer In this way, a stability piece is given a protective layer at working temperature compared to an oxidation and a further diffusion essentially of CoAl or of CoAl and one sion). The protective layer according to the invention consists of another layer, which consists essentially of also has a high associated cobalt and niobium-containing material at elevated temperatures Ductility. The inventive method leads to 55 is manufactured.

an adherent, uniform layer, which in another embodiment of the method

Properties already during the initial on- will suitably cobalt on the metallic

wear of the layer and deposited on the workpiece during use, for example by deposition

Working temperatures. dung from the gas phase, flame or plasma spraying,

The protective layer according to the invention grants at 60 mash application, hot rolling or the like. It arises

high temperatures (1204 ⁰ C) at a strength of this way a thin, approximately uniform

A cobalt layer on the workpiece surface for 0.05 mm or more for 100 hours. That with

Oxidation protection. In spite of this thin cobalt layer, this protective layer has a coated workpiece

a relatively large difference between the is then used as a cathode in a cobalt ion-containing

thermal expansion coefficient of the layer 65 brought galvanic bath in which the cobalt layer

and niobium or a niobium alloy is substantially given its desired strength. In further stages

higher resistance compared to thermal duration - this process is the one with the cobalt layer

Alternating stresses than the already known, provided workpiece as a cathode in an aluminum

Brought ion-containing galvanic bath in which forms an aluminum layer on the cobalt layer of the workpiece; following this coating with aluminum the workpiece is heated up to 1204 ° C, with CoAl as a surface layer the workpiece.

It is not possible to deposit cobalt on a niobium base metal in the usual galvanic way. However, it has been shown that cobalt can be deposited with good success if the Workpiece beforehand, for example by deposition from the gas phase, using an insert method a thin layer of cobalt has been applied.

When using an electroplating process to apply a cobalt layer to a base metal made of niobium or niobium alloys, it is also possible to pretreat the base metal by placing it as a cathode in an iron ion-containing galvanic bath in order to provide it there with a thin iron coating. The provided with the iron plating base metal should be over an extended period, for example 20 hours, degassed at elevated temperature, for example at 204 ⁰ C in a vacuum and then in a vacuum at a high temperature (about 1 hour at 704 ° C) of a diffusion be subjected. It has been shown that the thin iron coating on the base metal surface supports the adhesion of cobalt.

In yet another and preferred embodiment of the invention, the base metal is in a Electroplating bath containing cobalt ions brought after its surface was previously coated with a thin Cobalt or iron layer was provided, as already mentioned above. By introducing the base metal A cobalt deposit is obtained on the base metal in the electroplating bath containing cobalt ions. After the treatment in the electroplating bath, the base metal coated with cobalt is used placed in an alitating pack of powdery material; this material consists of one Aluminum donor and from a small amount of a volatile halogen-releasing substance as active additive and an inert filler. The base metal in this aluminizing pack is heated with volatilization of the halogen, whereby an outer surface layer is formed on the base metal forms, which for the most part consists of CoAl in the form of an alloy or an intermetallic Compound is composed. This latter procedure is particularly effective when one wants to achieve a continuous layer of CoAl on the base metal.

It has been found that the cobalt aluminide used to protect against oxidation of niobium or niobium alloys is superior to compounds such as NbSi ₂ , NbAl ₃ and their modifications.

The effectiveness of CoAl as a protective layer for

ίο Niobium or niobium alloys were determined in tests. A CoAl compound was made in solid form by making one of 68 percent by weight Cobalt and 32 weight percent aluminum using a tungsten electrode sheet melted under a partial pressure of the purest helium. The homogenized melt was cast into a rod which was 1.27 cm in diameter and 10.16 cm in length would have. This rod was then used as a model

ao samples for oxidation, thermal expansion and Diffusion tests processed.

The oxidation tests were carried out in air at temperatures of 538, 704, 871, 1038, 1204, 1371 ° C for 100 hours long carried out. During these tests, the

as sample samples at intervals of 25 hours approximately cooled to room temperature, weighed and visually checked. That showed at all temperatures CoAl excellent resistance to oxidation, as can be seen from Table 1:

Table 1

In contrast, aluminide compounds for coating niobium and its alloys formed by reaction with the base metal (that is, the MA1 ₃ type in which M represents the proportional amount of the base metal, for example NbAl ₃ or Nb-20Ta-15W-5Mo Al ₃ ) showed weakness Resistance to oxidation as shown in Table 2.

temperature Cumulative weight gain ⁰ C mg / ^cm2 / 100 hrs. 538 0.0 704 0.1 871 0.5 1038 0.5 1204 2.9 1371 4.7 *

Tabel

Coating material Test temperature
⁰ C Time
hours Cumulative
Weight gain
mg / cm ^a NbAl ₃ 538
704
871
1038
1204
704
1204 75
25th
60
100
100
25th
25th 48
213
30th
16
81
86
356 NbAl ₃ . NbAl ₃ NbAl ₃ NbAl ₃ (Nb-20Ta-15W-5Mo) Al ₃ ..
(Nb-20Ta-15W-5Mo) Al ₃ ..

According to the invention, the resistance is 65 The CoAl sample, which for 100 hours at

had been oxidized by CoAl against destruction during cyclic oxy-1204 ° C, became another

dation far better than the normal niobium oxidation test. She was in an oven

Alloy Trialuminides. from 1204 ° C to room temperature without damage

cooled down and then kept at a temperature of 704 ^° C. for 100 hours without any significant damage occurring, which proved the excellent properties of the material according to the invention. Even if trialuminide coatings for niobium and its alloys are made such that they have good oxidation ^{resistance in the range of 538 and 1371 °} C. due to favorable modifications, then these can be ^{destroyed quickly at low temperatures (704 °} C.) if they have been previously were oxidized at 1204 ^{0 C.} A diffusion dressing comprising metallurgically bonded to a representative CoAl niobium alloy, Nb-20Ta-15W-5Mo, was 100 hours maintained at 1204 ⁰ C for. The diffusion dressing treated in this way showed a very slight decrease in the good properties of the CoÄl layer. A metallographic examination of the bandage treated in this way still showed a uniform, 0.025 mm thick diffusion zone. Hardness tests showed that the total width of the diffusion zone was 0.038 mm and that an additional, outwardly invisible dissolution of the components occurred. The diffusion stability between CoAl as a coating and Nb-20Ta-15 W-5Mo as a representative for alloy base metals was therefore more than sufficient for the new and good result according to the invention. The thermal expansion of CoAl was measured in a strain gauge, at temperatures between room temperature and 1371 ⁰ C (s. Table 3).

Table 3

material

Mean linear thermal

Expansion coefficient

between 20 ⁰ C and the respectively

specified temperatures

cm / cm / ⁰ C / 10 ⁶

-SOO 1000 1500 2000 2500 CoAl 11.5
7.2
6.6 12.8
7.5
7th 13.8
7.9
7.4 15.1
8.3
7.7 17.1
8.4
8.1 Nb Nb-20Ta-15W-5Mo ...

According to the invention, the ductility at high temperatures of CoAl is so great that it unexpectedly becomes compensates for the large difference in thermal expansion between CoAl and the niobium base metals. Two different base metals were used for the samples:

1. unalloyed niobium in the form of a 0.63 cm rod and

2. an alloy of Nb-20Ta-15W-5Mo, which hereinafter referred to only as "the alloy".

The alloy was in the form of a rod 0.32 to 0.63 cm in diameter.

The CoAl layers can be used on all suitable niobium alloys and not only on those described here be applied. The CoAl layers can be applied to the niobium base metals not only according to the special methods described here are applied, but also by other methods, for example Flame or plasma spray processes, slurry application processes, hot rolling processes and the like.

According to the invention, CoAl layers can be applied to the base metals in a two-stage application process be applied, in which the first stage is the embedding of a chemically cleaned and ground workpiece made of base metal into a das The pack containing the following mixture is:

15 percent by weight cobalt powder,
3 percent by weight NH ₄ C1 powder,

82 percent by weight Al ₂ O ₃ powder.

The desired results can also be achieved with packings which contain about 10 to 40 percent by weight, preferably 15 to 30 percent by weight cobalt, about 2 to 12 percent by weight, preferably 3 to 9 percent by weight NH ₄ Cl and an inert filler material, for example Al ₂ O ₃ powder contain. The packs located in closed steel containers or graphite shells can be subjected to ^{various thermal treatments at temperatures between 760 and 1204 ° C. for about 4 to 8 hours in an argon atmosphere, as required.}

ao During this thermal treatment, the cobalt reacts with the base metal, for example with the formation of the compounds M ₃ Co and M ₂ Co ₃ or with the formation of other cobalt-rich layers on the base metal surface; M represents roughly proportional

a £ tional components of the base metal, e.g. B. Nb ₃ or Nb ₂ . Cobalt-rich layers on niobium contain, for example, the compounds NB ₃ Co and Nb ₂ Co ₃ , while similar compounds on "the alloy"

(Nb-20Ta-15W-5 Mo) ₃ Co

and

(Nb-20Ta-15W-5Mo) ₂ Co ₃

are. The thicknesses of the cobalt-rich layers vary from 0.00127 to 0.152 mm. They each depend on the specific coating conditions. For the subsequent treatment, thicknesses of about 0.0127 to 0.0508 mm are desirable. According to the invention, the second stage consists in embedding the aforementioned cobalt-coated niobium or alloy base metal in aluminizing packings, which contain ¹ I ₂ to 15 percent by weight, advantageously about 1 to 5 percent by weight, but preferably about 1 percent by weight of aluminum powder, about 0.05 to 3 percent by weight Percent by weight, but preferably about 1 percent by weight of a powdery halogen salt and the remainder an inert filler material, for example Al ₂ O ₃ . A mixture which consists of about 1 percent by weight of aluminum, about 1 percent by weight of halogen salt and of inert, high-melting material has proven to be extremely favorable. The aluminizing step can also easily be carried out using aluminum and inert filler material without a halogen salt.

These packings, located in graphite shells, are then subjected to a thermal treatment in an argon atmosphere at a temperature between about 871 and 1204 ° C for a period of about ¹ _1/2 to 4 hours in order to achieve the desired thickness of the CoAl layer (layer thicknesses between 0.00254 and 0.127 mm were reached). It has been found that one must perform the alitization process at temperatures of about 1093 ⁰ C to achieve the desired CoAl layers. At temperatures of 1093 ⁰ C and below the CoAl structure tends to absorb a large amount of aluminum and thereby provide compounds which contain more aluminum than COAJ

809 702/1316

9 10

and a local defect occurred at temperatures of about 1177 ° C in liquid 704 ° C, the could not Phase. Such low-melting compounds have poor properties of trialuminide layers fertilize disrupt the desired properties of the materials observed from niobium or niobium alloys Layer. will.

The good behavior of the CoAI layers also affects 5 If niobium is aluminized to form 0.05 to 0.127 mm of the connection and the structure of the layers, thick NbAl ₃ layers, it is no longer just because of the method of use capable of more than four oxidation cycles (20 hours) when the layers are made. 1204 ° C without damage in localized areas

As for the compound, chemical ones have to endure. Since NbAl _3, a compound with low analyzes, has shown that after the application process there is solubility for its components and therefore a CoAl layer has up to 20 percent by weight less ductility than CoAl, it cannot contain impurities in dissolved form; able to withstand repeated thermal cycling stresses and the main impurities are iron and tungsten, although it is a more favorable thermije depending on the type of process used. These see the difference in expansion coefficients between impurities do not lead to any significant deterioration and niobium exhibits as CoAl. Changes in the physical structure and do not adversely affect the good behavior of these layers. "

For a better understanding of the invention are in the sample bars with 0.32 cm in diameter and 1.27 cm

some examples are given below. ao length and pattern strips with an area of

2.4 cm ^a and a thickness of 0.159 cm made of a Nb-20Ta-15W-5Mo alloy were

Example 1 Reversal of the staged application process with CoAl be

layers. The cobalt coating treatment was

To 1.27 cm long, have a diameter of 0.63 cm as in a steel container made which contained the folaufweisenden pattern rods unalloyed niobium constricting mixture: were prepared by the above-described two-stage ₁₅ weight percent cobalt powder,

Application process CoAl layers applied. The _{6 percent by weight N} H ₄ C1 powder,

The cobalt was applied using a _{? 9} weight percent Al ₂ O ₃ powder.

Feed mixture through a graphite container 30

guided. The treatment lasted 4 hours at one temperature

The mixture had the following composition: temperature of 1093 ⁰ C. During this Kobaltbeschich-

15 percent by weight cobalt powder, treatment treatment, the packs rotated with a

3 percent by weight of NH ₄ C1 powder, revolutions per minute. The resulting cobalt-rich

82 percent by weight Al ₂ O ₃ powder. 35 layers were 0.0508 mm thick. Those loaded with Koba t

layered sample pieces were then an aluminum

The treatment lasted 8 hours at 1093 ° C. The niumbeschichtungsbehandlung for 1 hour at the resulting cobalt-rich layers had undergone in a graphite shell a 1204 ^0C. The thickness of 0.0762 mm. The graphite shell coated with cobalt contained a mixture consisting of: Sample rods were then for 2 hours at 1204 ⁰ C 40 ₅ weight percent aluminum powder,

in a graphite shell of an aluminum coating _{95 weight} ; _percent Al ₂ O ₃ -PuIVeT.

subjected to treatment. The graphite cup included

a mixture of the following composition: During the aluminum application treatment ro-

5 percent by weight aluminum powder, ^tied ^ ^Pack ™ S ™ t one revolution per minute. the

1 percent by weight NaCl powder, "the resulting layer was a uniform CoAl-

94 percent by weight Al ₂ O ₃ powder. Layer of O, O5O8mm thick which a small one

Amount of a dispersed MA1 ₃ phase contained. Between

The resulting layers were 0.06 mm thick. The CoAl layer and the alloy base CoAl layers with a small amount of a dispersed metal contained a 0.0254 mm thick layer of yeasted NbAl ₃ phase. Between these CoAl layers 50, an incompletely reacted cobalt-rich material, and the carrier metal of niobium, one could thin a coated sample for 100 hours

Layers incompletely reacted cobalt-rich for a long time at 1204 ° C. in the one described in Example 1 Observe the layer material. Way oxidized. At no time were the test

One of the coated sample bars was observed to have defects in the layer for 100 hours. The whole is oxidized long at 1204 ° C. During the oxidation, the specific gravity increased during the oxy-dation. The sample rods from the oxidation loss was 4.2 mg / cm ² .

taken and cooled down to room temperature niobium alloys, on the other hand, can if they are below

(after 1.5, 3.0, 4.5, 20, 25, 50, 75 and 100 hours formation of a 0.0762 to 0.152 mm thick MAl ₃ total time), weighed and checked visually. None of the layers are aluminized, generally no more than the time of this test, deficiencies in layers 60 six or seven oxidation cycles (50 to 75 hours) were observed. Withstand the cumulative increase in specific gravity without causing damage to localized weight spots during the oxidation process. The high ductility of CoAl makes 4.3 mg / cm ² . Following this 100-hour oxy- this against repeated thermal cyclical dation at 1204 ⁰ C, the sample rods in the furnace were more resistant to voltage stresses than ^{cooled from 1204 0} C to room temperature and then 65 typical MA1 ₃ compounds, although the latter at 704 ⁰ Oxidized again for 28 hours (five heating and thermal expansion with the Le cooling stages). Although the sharp alloys from which they are formed, better fit together at the corner of the sample stick during treatment.

Example 3

1.27 cm long sample rods 0.32 cm in diameter and 0.159 cm thick sample strips of Ni> -2QTa-15 W-5 Mo alloy, measuring 2.4 cm ^{2 in} area, were added using the two-stage insertion process CoAl coated. The cobalt was applied in a steel container containing a mixture of the following composition:

15 percent by weight cobalt powder,
3 percent by weight NH ₄ Cl-PuIvCr,
82 percent by weight Al ₂ O ₃ powder.

The treatment lasted 8 hours and was carried out at 1093 ° C. During the treatment, the packs rotated 180 ° every 30 minutes. The resulting cobalt-rich layers were 0.0762 mm thick. The cobalt-coated sample strips were then subjected to an aluminum coating treatment; it lasted 1 stand at 1204 ⁰ C and was carried out in a graphite bowl, which contained a mixture of the following composition:

5 percent by weight aluminum powder,

1 percent by weight NaQ powder,

94 percent by weight Al ₂ O ₃ powder.

During the aluminum coating treatment, the package rotated 180 degrees every 30 minutes. The resulting layers were rough and irregular, but mainly consisted of an approximately 0.1 mm thick CoAl layer, which was located on a thin layer between the CoAl layer and the base metal and consisting of incompletely reacted, cobalt-rich layer material . The amount of MA1 ₃ phase in the CoAl layer was here significantly greater than in one of Examples 1 and 2; in some areas the continuity of the CoAl layer was _{disturbed by this MA1 3} phase.

A coated sample rod was ^{oxidized for 100 hours at 1204 °} C. in approximately the same way as in Example 1. At no time during the test could defects in the coating be observed. However, the total increase in specific gravity during this experiment was 12.3 mg / cm ² or about three times the values measured on the sample bars of Examples 1 and 2. Although the

Coating of this example - it was heavily _{contaminated by an MA1 3} phase - formed a protection for the alloy base metal for 100 hours at 1204 ^° C., its oxidation behavior was significantly worse than that of the relatively less contaminated layers of Examples 1 and 2.

Nevertheless, this MAl ₉ -contaminated layer on Grand des CoAl and its good ductility was able to withstand repeated thermally cyclical stresses.

to withstand stresses, which proves the superiority of these layers over the layers produced on the basis of MA1 _3.

Example 4

Sample bars and sample strips made of an Nb-2ÖTa-1 5 W-5 Mo alloy were used the two-stage application process with CoAl

»Ο coated. The cobalt overlay treatment was like carried out in example 2. The treatment lasted 5 hours at 982 ° C. A 0.0254 mm was created strong cobalt-rich layer. It was found to be useful after the cobalt application treatment

»To undertake 5 annealing in an argon atmosphere for 4 hours at 1204 ^° C. Such samples coated with cobalt - with and without the above-mentioned annealing treatment - were then subjected to an aluminum coating treatment in a pack. The package contained the following powder mixture:

1 percent by weight aluminum,
1 percent by weight NaCl,
98 percent by weight AI ₂ O ₃ .

The packing located in a graphite dish rotated during the two-hour ^{aluminum coating treatment, which took place at 1204 °} C., at one revolution per minute. CoAl coatings were created with a thickness of about 0.0254 mm.

Between the CoA! and the base metal remaining cobalt-rich layers (not CoAl) with a thickness of about 0.0254 mm.

Coated sample bars were heated for 100 hours at temperatures of 704 and 1204 ° C Fluctuations between the test temperatures and the room temperature (see Example 1) oxidized. the Oxidation test results are shown in Table 4.

Tabel

Pretreatment

Shift life in hours

Oxidation test results
704 ⁰ C

cumulative increase

of the specific

Weight *

mg / cm ^% shift life
in hours

1204 ⁰ C

cumulative increase

of the specific

Weight *

mg / cm ²

Annealed at 1093 ⁰ C
Not annealed ..

100
100

* After 100 hours at the respective temperature.

During the tests with these samples, no defects in the coating could be found, which in turn speaks for the good protective effect of CoAl layers on niobium alloys. A slow cooling had no measurable effect on the layer.

0.66 100 3.50

0.46 100 5.58

Example 5

2.54 cm long sample rods made of Nb-20 Ta-15 W-5 Mo alloy with a diameter of 0.475 cm were coated with CoAl layers in the following way:

An iron layer 0.254 mm thick was electrodeposited onto the sample pieces. The galvanic deposition took place in an iron formate bath with a pH of 4.2 at 62 ° C. and 3.2 amps / dm ² and lasted 5 minutes. The samples were then degassed at 204 ° C in a vacuum for 20 hours; Following the degassing, there was ^{the possibility of diffusion at 704 °} C. in a vacuum for 1 hour. Then one opened and one could see an oxidation of the base metal.

A metallographic test showed that there was practically no CoAl left even in the uninjured areas. Calculations showed that 0.0127 mm CoAl were consumed by the combined oxidation and diffusion processes with the parent metal at 1204 ⁰ C within 20 hours (about 0.0127 mm would be for a 20-hour protection when plating performed in an ordinary io 1,204 ⁰ C just enough). The samples of this cobalt sulfate and chloride bath at 3.8 amps / dm ² and example which first coated electrolytically at 46 ° C for 15 minutes to form a 0.0127 mm cobalt and aluminum and then a heavy cobalt precipitate. The second deposition was subjected to heat treatment, behaved electrolytically using a comparison similar to the steamer plate used in Examples 1 and 2 for 20 minutes in a molten sample. .

A bath of 80 percent by weight AlCl ₃ and 20 Ge The coating according to the invention of basic percent by weight NaCl at 163 ° C. and 4.3 amps / dm ^{2 of} metals from niobium or a niobium alloy is now sufficient. The aluminum contract treatment was then and advantageous. CoAl as an intermetallic compound by galvanic application in a galvanic has a number of previously disregarded ether hydride aluminum baths for 15 minutes long with Zim- 20 good properties, which it carried out at an excellent temperature and 3.2 Amp./dm ² ; The protective layer material for niobium or niobium alloy does not make a stoehiometric amount of alumi- nium. ;

nium, which then reacted with cobalt under BiI- In particular, the good oxidation dung of CoAl. The samples were resistant to CoAl in the important temperature then under ¹ J ₅ atmospheres of very pure argon in the range up to about 1371 ° C, in which the hitherto

Encapsulated quartz. Finally, they were subjected to the following heat treatment to form a CoAl layer subjected:

1st heating in I ¹ I ₂ hours to 649 ° C; this temperature was held for 4 hours;

2. Heating to 677 ° C in 20 minutes; the temperature was held for 2 hours;

3. Heating to 927 ° C in 40 minutes; the temperature was 15 ¹ Z held _{for 2} hours;

4. Heating to 982 ° C in 30 minutes; the temperature was held for 1 hour;

5. Heating to 1093 ° C in 30 minutes; the temperature was held for 2 hours;

6. Heating to 1149 ^° C. in 20 minutes; the temperature was held for 2 hours;

7. Heating to 1204 ° C in 30 minutes; the temperature was held for 3 hours;

8. Cooling of the quartz capsule to room temperature by means of air, breaking the capsule and removing it of the sample pieces.

After the heat treatment, a small amount of powder was left on the sample pieces and on the Detected inside of the quartz tube. The resulting CoAl layers were about 0.0127 mm thick and covered with a thin, 0.0025 mm thick, unidentified layer.

A coated sample was subjected to an oxidation test for 20 hours at 1204 ° C. After a total of 1.5, 3.0, 4.5 and 20 hours it was cooled to room temperature, weighed and checked visually. The weight gain during the first cycle (1.5 hours) was high because of the oxidation of the thin, unidentified surface layer just mentioned; however, during the next two cycles (3 hours) the weight gain was only mg / cm ² . This corresponded to a parabolic Oxydationsverlauf [about 0.3 (mg / cm ^{2) 2} / hr.] In contrast to 65 [0.2 (mg / cm ^{a) 2} / hr.] In Example 1 and 2. After hours at a Temperature of 1204 ⁰ C occurred at several points of the sample destruction layer materials used - such as niobium, aluminides, silicides and berylides - poorly proven and also the very good diffusion stability of CoAl when it is used for coating niobium or niobium alloys, and their excellent ductility at elevated temperatures.

CoAl layers also have the advantage over layers of the MAl ₃ type that their resistance to oxidation at low temperatures on niobium alloy base metals is not impaired by previous exposure to high temperatures. CoAl layers can also be produced using a wide variety of processes. They can also be contaminated with foreign constituents or phases without their protective function diminishing as a result.

Claims (16)

Patent claims: 1. Intermetallic protective layer of high breaking strength, high resistance to oxidation at temperatures between 538 and 1371 ° C and high fatigue strength for workpieces made of niobium or a niobium alloy, in particular a niobium alloy, which consists of 20 percent by weight of tantalum, 15 percent by weight tungsten, 5 percent by weight molybdenum and the remainder niobium consists thereby characterized in that the intermetallic protective layer consists of cobalt aluminide (CoAl), optionally in dissolved form up to 30 atomic percent cobalt and up to 2 atomic percent Aluminum in excess over the stoichiometric composition of CoAl in one contains such an amount that the total cobalt content in the layer is between 48 and 80 atomic percent lies. 2. Intermetallic protective layer according to claim 1, characterized in that it consists of 68 percent by weight Cobalt and 32 weight percent aluminum. 3. Intermetallic protective layer according to claim 1 or 2, characterized in that it is on one is arranged consisting essentially of cobalt and niobium intermediate layer. 4. A method for producing a protective layer according to any one of claims 1 to 3, characterized in that that the workpiece is coated first with cobalt, then with aluminum and for training the cobalt aluminide layer is subjected to a heat treatment. 5. The method according to claim 4, characterized in that the cobalt layer on galvanic Ways is applied. 6. The method according to claim 4 or 5, characterized in that the workpiece in front of galvanic coating with cobalt coated with a thin diffusion layer of iron will. 7. The method according to claim 6, characterized in that the iron layer on galvanic Ways is applied. 8. The method according to claim 6, characterized in that the workpiece before the galvanic Coating with cobalt is provided with a thin diffusion layer of cobalt. 9. The method according to any one of claims 4 to 8, characterized in that the workpiece for Application of the cobalt layer in a powder mixture from a cobalt donor, preferably metallic Cobalt, a small amount of volatile halogen-releasing substance and an inert filler packed and heated until the halogen evaporates. 10. The method according to claim 9, characterized in that heating to a temperature between 760 and 1204 ⁰ C is carried out in a non-oxidizing atmosphere. 11. The method according to claim 9 and in particular claim 10, characterized in that a powder mixture of 10 to 40, preferably 15 to 30 percent by weight cobalt, 2 to 12, preferably 3 to 9 percent by weight inorganic halide and inert filler used as the remainder will. 12. The method according to any one of claims 4 to 11, characterized in that the coating of the cobalt-coated workpiece is made with aluminum by electroplating. 13. The method according to any one of claims 4 to 12, characterized in that heating to a temperature of about 1200 ⁰ C is carried out after the aluminum coating. 14. The method according to any one of claims 4 to 13, characterized in that the coated with cobalt Workpiece for applying the aluminum in a powder mixture from an aluminum dispenser, preferably metallic aluminum, donating a small amount of volatile halogen The substance and an inert filler are packed and heated until the halogen has evaporated will. 15. The method according to claim 14, characterized in that the workpiece is ^{heated for 1 to 4 hours over 1094 0} C in a non-oxidizing atmosphere. 16. The method according to claim 14 and in particular 15, characterized in that a powder mixture from 0.5 to 15, preferably about 1 to 5 percent by weight of aluminum, at least 0.05 percent by weight, preferably 1 percent by weight inorganic halide and an inert, high melting point material is used as the remainder. 809702/1316