CA1126055A - Abradable seal material and composition thereof - Google Patents

Abstract

ABRADABLE SEAL MATERIAL AND COMPOSITION THEREOF

ABSTRACT OF THE DISCLOSURE

An abradable seal material suitable for high tempera-ture application in turbomachinery comprising a sintered mat of (1) randomly disposed fine metal fibers, or (2) fine metal powders or (3) both fibers and powders. The metal fiber and powder are com-posed of an alloy consisting essentially of I, Al, Cr, II, or I, Al, Cr, III, wherein I is at least one member of the group Fe, Co, Ni, and Co plus Ni, II is a member of the group consisting of Y, Sc and Rare Earths, and III is at least one member of the group consisting of Si, Hf, Zr, Cb, and Ta. The exposed surfaces of the fibers and powder forming the seal are protected against oxidation at high temperatures by a coating of Al2O3 which is formed on the substrate. Said substrate has an Al content of at least 4% to re-place spalled Al2O3 and for "healing" any Al2O3 scale fractures.

Description

Il r~~ ID-3703 l ~ 5~

This application is a division of Canadian Ser. No. 226,606, filed, May 9, 1975.
BACKGROUND _Ei' TH~ INVENTION_ ¦1. Field of the Invention ¦ This invention relates to abradable materials useful in high operating temperature sealing members. The new and improved jmaterials described herein below are particularly useful as sealing surfaces between adjacent rotating and stationary elements of turbomachines, such as compressors and turbines, which operate at elevated temperatures.

2. Description of the Prior Art As is well known in the art, the efficiency of gas tur-i bine engines is dependent in part on the peripheral seal between the ends of the .rotor and stationary blades, and the adjacent en-gine structure. The use of abradable sealing materials is well known and a number of different methods have been proposed for employing such materials. See for example, U.S. Patent 3,413,136, wherein an abradable porous nickel sealing surface is formed on affected engine parts by a spray technique. U.S. Patent 3,519,282 shows an abradable, porous, metal fiber seal wherein the pores are filled with copper and/or nickel powder. The advantages of metal fiber seals are set forth therein in some detail. Porous metal seals are likewise employed for this purpose, as is shown for example by U.S. Patent 3,268,997 and 3,350,178. However, it has l ~1-'i 3~' 1 ¦¦ r~ ID-3703 ~ , l ~2~S

been found that due -to the high temperatures involved, oxidation of the seals employed here-tofore can occur, lessening the sealing capability and lowering efficiency.

SUMMARY OF THE INVENTION

A porous seal made of fine particles of an alloy having the composition I, Cr, Al, II; or I, Cr, Al, III, wherein I may be Co, Fe, Ni, and Co plus Ni, II may be Y, Sc, or Rare Earths, and III may be Si, Hf, Zr, Cb or Ta, and ~herein the seal is coated with an A1203 layer, is substantially protected against oxidation at high operating temperatures. The seal is abradable, and is fitted to the desired tolerance by placing it in its functional environment, e.g. a turboengine, and causing rotating elements of the equipment which contact the seal to rub away any projecting , portions.
The seal may be installed in unoxidized conditions and the protective A1203 coating is then formed during use. Alterna-tively, the seal may be first given an oxidizing treatment to form the A1203 coating as by exposure to a hot oxygen containing gas, l and then nstalled in the engine.
¦ Both fibers and powder, either alone or in combination, may be employed in making the seal. As employed herein, the term "parti 1GII is to be understood as covering botb the rine metal Il -2-'I
' s~

fibers and the finely divided metal powder which may be employed in fabricatincJ the seal.
The alloy composition is such that after the A1203 coating is Eormed the substrate contains at least 4% Al. Alloy compositions (given in weight percent) meeti.ng -the requirements are within the broad range of, by weight, ahout 15-25% Cr; about 5-20~ ~1; about .01-0~5% II and about 0.1-2.0~ III when I=Co or Ni, and about 0.1-1.0% III when I=Fe; and the remaindcr Co, Fe, Ni. The diameters oE the fiber and powder particles are within the broad ranges (in microns) of about 4 to 150 and about 4 to 100, respectively. The aspect ratios (L/D) of the fiber are broadly from 10 to about 4200, and the aspect ratio of the powder is around 1, but may be as great as 7 or 8.
Thus broadly, the invention contemplates a me-tal mat or compact resistant to oxidation at high temperatures, which comprises a sintered mass of fine metal par-ticles composed of a homogeneous alloy consisting essentially of the composition I, Cr, Al, II, or I, Cr, Al, III, wherein I is at least one member of the group consisting of Fe, Co, Nl, and mixtures of Co and Nl, II
20. is at least one member of the group consisting of Y, Sc and Rare Earths, and III is at least one member of the group consisting of Si, Hf, Zr, Cb and Ta. The components of the alloy have essentially the following weight percents: 10-27% Cr, 5-20% Al, .01-0.5% II; when I=Fe, then III=0.1-1.0%; and when I=Co, Ni or Co and Ni, then III=0.1-2.0; and I=the remainder, and the exposed surface of the particles are capable.of developing a protective coating of A1203 at least 0.5 micron in thickness over an under-lying substrate of at least 4% Al content.
In a further embodiment, this invention contemplates a metal particle resistant to oxidation at high temperatures com-posed of a homogeneous alloy consisting essentially oE the composition I, Cr, Al, II, or I, Cr, Al, III, wherein I is at , , , ~llZG~SS
least one member o~ the group consi-sting of Fe, Co, Ni, and mix-tures of Co and Ni; II is at least one member of the group consis-ting of Y, Sc, and Rare Ear-ths, and III is at least one member of the group consisting oE Si, ~IE, Zr, Cb, and Ta. The components of the alloy have essentially the Eollowing weigh~ percents:
10-27~ Cr, 5-20% Al, .01-0.5% II; when I=Co, Ni or Co and Ni, III=0.5-2.0% and when I=Fe, then III=0.1-1.0~; and I=the remain-der, and the exposed surEace of the particle being capable of developing a pxotective coating of A1203 at least 0.5 microns in thickness over an underlying particle substrate of at leas-t 4~ Al content.
In a still further embodiment, the invention also con-templates a machine workable homogeneous alloy of the composition I, Cr, ~1, II, or I, Cr, A1, III, wherein I is at least one member of the group consisting oE Fe, Co, Ni, and mixtures of Co and Ni;
II is at least one member of the group consisting of Y, Sc and Rare Earths, and III is at least one member of the group consist-ing of Si, Hf, Zr, Cb and Ta. The components of the alloy have essentially the following weight percents: 10-27~ Cr, 5-20% Al, ~01-0.5~ II; when I=Co, Ni or Co and Ni then III=0.5-2.0%; when I=Fe then III=0.1-1.0~; and the remainder=I.

BRIEF DESCRIPTION OF THE DRAWINGS
E'igure 1 is a fragmentary schematic cross-sectional view of a segment of a gas turbine engine employing the present invention;
Figure 2 is a greatly enlarged cross-sectional view of the metal fiber abradable seal embodiment of the invention Figure 3 is similar to Figure 2 except that the seal consists of fibers and powder;

Figure 4 is similar to Figure 2 but shows a metal powder seal, Figure 5 is a graph showing the relationship between the fiber diameter and the weight percent aluminum in the alloy necessary to provide a 0~5 micron A1203 coating on the fiber while retaining at least 4% Al in the fiber case.

3 SUMMARY OF BASIC OBJECTS

~¦ Among the objects of the invention are the following:

~ (a) To provide a bulk alloy, capable of being formed lC into particles by machining or spinning processes, having the com-positions I, Al, Cr, II, or I, Al, Cr, III; wherein I may be Co, Fe, or Ni, or both Co and Ni; and II may be Y, Sc, or Rare Earths, and III may be Si, Hf, Zr, Cb, or Ta, and wherein the composition by weight percent is:

Element ~ Preferred Range I remainder to 100% remainder to 100%
Al about 5-20 about 8-12 Cr about 10-27 about 15-17 when I=Ni or Co about 15-23 when I=Fe II about .01-0.5 abou-t .01-.25 when I=Ni or Co abou-t .01-0.1 when I=Fe .
20III about 0.1-2.0 when I=Co or Ni about 0.7-1.0 when I=Co or Ni about 0.1-1.0 when I=Fe about 0.4-0.6 w~hen I=Fe;

~ 3703' ` - ~L2~35~ , I ~, I

(b) ~o provide a porous material comprising intermeshed~
fine metal fibers and/or powders of the a~oy of (a);

¦ (c) ~ro provide a new and improved seal cons;sting of the compacted and sintered porous material of (b);

, (d) To provide a seal as in (c) wherein the Al content of the fibers and/or powders is at least sufficient to form a pro-tective coating of .~1203 at least one half micron thick on the fiber or powder particle and still have at least a 4% Al content in the substra-te, (e) To provide a metal compact as in (d) wherein the fibers have a diameter of from about 4 to about 150 microns and preferably from about ~ to about 2~ microns, and most preferably ¦ from about 8 to about 12 microns, and the powder particles have a diameter of from about 4 to about 100 microns, preferably from about 8 to 40 microns and most preferably from abo~t 8 to about ¦25 microns;

(f) To provide a metal compact as in (e) wherein the aspect ra~io of the fibers is from about lO to about 4200, prefer-ably about 10 to about lO0 and most preferably around 50, and the 20 aspect ratio of the powder is about 1 to 8; .
.
(g) To provide a metal compact as in (f) wherein the Il ~, t~-3703 ~l~Z~;~5~ 1 ¦ Al content of the fiber is at least equal to that given in the equation % Al -( D -~ .04) 100 I wherein D is the diameter oE the fiber in microns;

(h) To provide a metal compact as in (f) wherein the Al eontent of the powder particle is at least equal to that given in the equation % Al =( D ~ 4) 100 wherein D is the diameter of the particle in microns;

(i) To provide a metal compact seal as in (g) and (h) 5 wherein the compacted and sintered fibers and/or powder particles are coated with a proteetive eoating of A1203 at least 0.5 micron in thickness;

(j) To provide a metal partiele compact wherein at least some of the particles have different a~oy compositions, falling within the broad ~efinition given in (a); and, (k) To provide metal particle compacts comprisiny NiCrAlY and NiCrAlSi alloys falling within the range set forth lin (a).
The above and further objects and features of the in-vention will become apparent from the following detailed descrip-tion and accompanying drawings.

26~S~
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jDESCRIP~ION OF PREFERRED EMBODIMENTS
~'''I . I
¦¦Referring to Figure 1, reference numeral 1 represents the outer casing structure of a turboengine which has structurally l,mounted thereon stationary turbine vanes 2, only one of which is ,!shown. Rotating blades 3 and 4 are secured to the rotor wheel (not;
shown) of the engine. Mounted between the blades 3 and 4 and fastened thereto is a sealing means 5, having an inner portion 6 and an outer portion 7. Abradable seals 8 and 9 are mounted on casing 1 adjacent to tips 10 and 11 of blades 3 and 4 and comprise 10 the outer seal. outer portion 7 of element 5 has projecting rings 12 integrally secured thereon, Thin, circular knife edges 13 are fastencd, in turn, on rings 12.
A seal land ring 14 is secured to theinner portion of vanes 2. Abradable seals 15 are mounted on the inner face of land 14 and comprise the inner seal. As an engine is brougllt to oper~
ating temperature and speed, knife edges 13 expand radially greater than the inner scal surfaces 15a to perrnit the ~nife edges 13 to rub into the seals 15, and thus provide efficient rotary 20 sealing action.
~ The abradable sealing material, described hereinafter in greater detail, consists of a compact of metal fibers, metal powders, or both, of an alloy of the composition I, Cr, Al, II, or ,11, ID-3703 ~Z6~SS

I, Cr, Al, III, wherein I is Fe, Ni, Co, or both Ni and Co, II is Y, Si, Sc or Rare Earths, and III is Y, Si, Hf, Zr, Cb, or Ta. In use under hiyh temperature oxidizing conditions, the compact com-prising intermeshed, compacted and sintered fibers and/or powders becomes coated wi-th a protective layer of A1203, which serves to reduce or prevent in large measure further oxidation.
The compact doés not contain any loose particles, and the terms "fibers" and "powder" when used in connection with the mat or compact stracture are to be understood as referring to the sintered and joined together fiber strands and powder particles as they exist in the structure.
; Tips 10 and 11, and knife edges 13 rub against the abradable material comprising seals 8, 9, and 15, during rotation of rotor blades 3 and 4. Any projectiong portions in the seal are rubbed away thereby establishing a minimum working clearance and a tight gas seal.
Figure 2 shows the modification having an interlaced structure of metallic fibers. The abradab~e seal in this modifica-tion consists of interlaced and randomly oriented fibers 16. In another embodiment of the invention, as shown in Figure 3, some of lthe voids between fibers are filled with a metal powder 17 of the same or similar alloy composition falling within the generic formu-~la, an the compact as a whole is sintered together SD that the g ' .

!1 ~ ID~3703 11~6~)55 I

~etallic fibers and the powder and bonded (fiber to fiber, powder ~article to powder particle, and fiber to powder particle) to form ¦la friable structure having the requisite physical strength to serve jlas a seal. In a further modification, shown in Figure 5, the com-¦ pact is made entirely of metal powder.
Some methods for making small diameter metal fibers ¦ usable in this invention are disclosed in U.S. Patents 3,394,213 3,505,039; 3,504,516; 3,277,564; 3,379,000, and 3,698,863, which are owned by assignee hereof. Methods for making metal fiber com-pacts or mats of the type described herein are shown in U.S. Patent~

3~127,668; 3,505,038; and 3,469,297, the latter two being owned bythe assignee hereof. Processes for making metal powders and metal powder compacts are well known to the art. See, for example, "Powder Metallurgy", Editor John Wulff, 1942, published by the , American Society for Metals; "Powder Metallurgy, Practice and Appllcation", by sands and Shakespeare, 1966, George Newnes Ltd., publisher, London; and U.S. patents 3,268,997; and 3,350,178.

ALLOY COMPOSITION

~¦ It ha9 been discovered that alloys having the composi-tion set forth below can be formed in bulk shapes and are machine-able and otherwise susceptible to metal working operations to form the metal particles which comprise the abradable seals of this ' -10-i!

, ~ ID-3703 1 ~1 Z6055 invention. Said alloys impart the requisite properties to the metal particles and compacts made therefrom and have the following compositions in weight percent:

TABLE I( ) ~lloy Ni(2) Fe Al cr Y(3) Si(4) A. NiCrAlY Broad Range Bal. - 5-20 10-27 .01.5 (or CoCrAlY) Preferred Range Bal. - 8.0~12 15-17 .01.25 B. NiCrAlSi Broad Range Bal. _ 5-20 10-27 _ .5-2( 10(or CoCrAlSi) Preferred Range Bal. - 8.0-12 15-17 - .7-1 C. FeCrAlY Broad Range - Bal. 5-20 10-27 .01.5 Preferred Range - Bal. 8.0-12 15-23 .01.1 D. FeCrAlSi Broad Range - Bal. 5-20 10-27 - .l-L~
Preferred Range - Bal. 8.0-12 15-23 - .4-.6 (1) Elements given in weight percent.
(2) Nickel may be substituted wholly or in part with cobalt.

(3) Yttrium may be substituted with scandium or the rare earth elements.
(4~ Silicon may be substituted with other known oxide stabilizers (Hf, Zr, Cb, Ta).

Allvys NiCrAlY and NiCrAlSi are preferred because of their lower melting points. Without being bound by any theory, it appears that the superior abrading properties result from the fact that the melting point of the seal alloy is lower than the conven-tional metals employed for the rotor blade tips and knife edges Il ~ 1I)-3703 - llZ61~S~

that are rubbed against the abradable seal material. Sufficient Al must be present in the alloy to provide a coating of A1203 result-ing Erom oxidation of the metal particle (fiber or powder) of at least 0.5 micron in thickness. Additionally, the substrate must retain an Al content of at least 4% in order to provide additional Al for "healing" any fractures in the A1203 scales or coating and for the replacement of any spalled A12O3.
While usually a single alloy will be used in making the fibers and powder, in order to meet specific requirements, mixtures of fibers and/or powders of different elemental or weight percent compositions falling within the generic alloy class and weight per-cent range may be employed in the compact. Where two or more alloys containing different elements are employed, under the high temperature conditions of use diffusion of elements can take place between particles of differing composition which are in contact.
As a result, highly complex alloy mixtures may be formed.
That the metal fibers employed in compacts of this invention could be made from the alloy compositions described here-in, having Al compositions above 5%, is quite surprising since the generally held opinion was that such alloys would be too brittle for the machining and other mechanical operations required to make the fibers.
It can be demonstrated mathematically -that the .

- Il ID-3703 ~ ~2~ 5 relationship between the ~1 content of the alloy (given as the ~ fraction of Al in metal) and the diameter in microns (D) of a fiber having a circular cross-section required to yield a 0.5 micron coating oE A1203 while retaining a 4% Al content in the core is:
percent Al in metal fiber =( D + .04) 100 While this gives the relationship between the minimum thickness of A1203 which is useful and the Al alloy content, it is . ¦to be understood that greater thicknesses are included within the iO scope of the invention, subject to the limitation that the sub-strate must contain at least 4% Al. Although this formula is based on fibers having a circular cross~section, it gives the minimum Al content required in the alloy for the diameter of any shape fiber.
The analogous relationship for the coating on a spheri-: cal metal powder particle is:
percent Al in metal spheres =( D 0 )100 .

¦¦ These relationships are shown graphically in Figure 5, ¦¦wherein it can be seen that the following relationships, for example, apply.

!! ~ ID-3703 Z~S~
1~ .

~iameter Min. % Al Required Min. % Al Required ¦in Microns for Fibers for Spheres ` 5 13.5 18.2 979 12.9 8.7 11.1 6,4 7.6 While this figure gives the relatiolls}lip for the mini-mum aluminum content required to provide a minimum thickness of 0.5 micron of A1203 while maintaining at least 4O/o aluminum in the substrate for oxide healing, it is to be understood that thinner coatings will be developed duriny the early oxidation ]ife and that graater thicknesses are included within the scope of the invention subject to the limitation that the substrate alloy must retain at least 4% aluminum after oxidation.

i PREPARATION OF COMPACT
Compacts formed solely from metal fibers may be made ¦ by a number of different processes. For example a thin web of ¦ metal fibers may be formed by an air layering process as taught in ¦~.S. Patent 3~505,038! or by the water slurry process shown in U.S.
¦IPatent 3,127,668. In order to increase the green strength of the ¦¦mat, cellulosic fibers, such as cotton linters, cellulose e~ters and ethers, rayon, etc., may be added to the mass of fibers being layered to form the raw mat.
The web of fibers so prepared is then compressed and `- !
!!

¦¦ J~ ID-3703 ~Z6~

s tered to form the compact. Its clenslty may he varied by varying ; the amount of compression applied to the web and/or by adding destructable materlal to the raw Eiber mat which is destroyed dur-ing the sintering step. As is well known to the art, particles of wood, plastic, or volatile compounds may be employed for this purpose.
Compacts of both metal fibers and powders can be made by first forming a web from metal fibers by any of the methods known to the art, then sifting the metal powder into the open spaces of the web, and finally compacting and sintering the mass of fibers and powders to bond the mass. In another modification, the fibers and powder can be mixed together in desired proportions, formed into a web by the methods of the prior art and then compact-~` ing and sintering. The powder content of the compact may vary from 10-50% and more preferably from 30-50%. The addition of powder to the raw mat reduces the porosity, and thus permits a desired density to be reached with less compaction. In sintering the metal fiber-powder web, not only are points of fiber interaction bonded l together, but the metal powder particles become bonded to the fibers and to each other.

Metal powder compacts may be prepared as shown general-ly in U.S. Patents 3,268,997 and 3,350,178~ Briefly, this involves mi~ing the powder with a liquid binder or cellulosic material for ~ -3703 ~L2~

reen st r eng th, ad ~ing vo la t i le ma ter ia ls to ms ke the c ompac t porous, compressing the mixture and sinterlng.

GEOMETI~Y OF PARTICLES

It has been further discovered that the qual~ty of results achieved in the rubbing step depends on the aspect ratio (ratio of length to diameter~ of the p~rticles making up the seal.
The broad range of aspect ratios of the metal fibers is from about 10 to about 4200 and more preferably 10 to 100, with the optimum ¦value being about 50. In the case oE the metal powders, the opti-Imum aspect ratio is 1, but may be as high as 7 or 8.

¦ The fibers may range in diameter from about 4 microns to~about 150 microns, preferably about 5 to about 25 microns, with about 8 to about 12 microns being most preferred. In mixed fiber compacts, it has been found preferable to have the fiber sizes . widely separated, such as for example, a mixture comprising fibers ranging in size of about 100 to 150 microns mixed with fibers of ¦about 8 to 12 microns. The metal powder may range in individual particle diameter from about 4 to about 100 microns, preferably about 5 to about 75 mlcrons, and most preferably about 5 to about 25 microns. The term "diameter" as employed in the specification and claims is intended to include not only the diameters of cylin-drical fibers and spherical particles of powder, but also, in the case of non-cylindrical fibers and non-spherical powders, an equi-valent theoretical diameter. In this case of a cylinder, the ratio '- ' 'I
, ~ LD-3703 - ~2~

of volume (V) to surface area (A) is D/~, where D is the diameter, and the cases of a sphere is D/6. The -theoretical diameter of a fiber is determined by measuring its geometrical configuration, determining its surface area and volume, and substituti.ng in the equation A 4. Similarly, in the case of a powder particle, the ratio of the volume to area is multiplied by 6 to give the theoret-ical di.ameter.
While the formulae for the minimum A1203 coating, pre-viously set forth, are based on particles having circular cross-sections, they give the minimum Al content for any shape particleshaving the same theoretlcal diameters as the circular cross-section particles.
DENSITY OF COMPACT

The density of the compact is an important property . which has a considerable effect on the utility of the seal. For ., purposes of this specification, "density" is defined as the weight . of a unit volume of the compact or mat divided by the weight of th~
same volume of a solid metal of the same material used to make the . particles. Thus, a compact or mat having a 20% density has 20%
of the weight of an equal volume of the solid metal. "Porosity" ic 100 minus the density; a compact having a 20% density has an 80%
porosity.
The density of the compact can vary from about 10 to about 50%, preferably from about 14 to about 30%. Optimum density :~ varies with the intended use. For use in an inner air seal of a . -17-.

.

1l ~ ID-3703 l:~Z~5~

~gas turbine where the abradable material is rubbed with a knife edge, as with elements 13 and 15 in Figure 1, the compact density should preferably be within the range of 14 to 20%. However, with a rotary blade tip seal, as shown in e].ements 8, 9, 10, 11 in Figure 1, the optimum density is about 21%. In general, for higher tip speeds and higher gas velocities in the turbine, higher densi-ties are preferred for added erosion resistance.
When employing powders either in combination with fibers or alone, densities of about 14% to about 70% may be readily obtained. Densities of about 30 to about 7~/O may be employed when the compact consists of metal powders with a density range of 30-40~/O being preferred.
Methods for varying compact densities have been dis-cussed above in connection with methods for preparing the compact.
.. .
Example In making the seals, the metal particles, prepared by any of the methods o the prior art, are formed into a mat of the required shape and the desired density. As set forth herein above, the compact may consist of fibers and/or metal powder. After form-ing, the mat is sintered to give it the desired strength, subjectedto high temperature oxidation conditions to form an A1~03 coating, and is then mounted in the turboengine where it is rubbed to the proper seal clearance. Laboratory tests have shown abradable seals il ID-3703 S~ I

oE this invention to be satisfactory in resisting oxidation in air up to a temperature oE 1515F for 10,000 hours.
The Eollowing example is directed to the making of a NiCrAlY seal:

NiCrAlY fibers having a surface area equivalent to a 5-6 micron circular fiber and an average aspect ratio of about 60 was formed into a web from a fiber slurry. This fiber had the com-position of 15.7% Cr, 9.5% Al, 0.21% Yttrium, balance Nickel. The apparent density of the formed web was 13.7%. It was sintered in vacuum for 6 hours at 2150F and compacted in a rolling mlll to 22%
density. As a result of change in surface area of the fibers during sintering, the finished product had a surface area equiva-lent to an 11-13 micron circular fiber.
, As a result of some volatization in sintering, the chemical composition of the finished product was 13.8-15.0% Cr, 8.7-9.4% Al, 0.18% Y, balance Ni. Strips were cut from the finish-ed NiCrAlY fiber metal sheet, then were roll formed and brazed into a test stator ring and subjected to a temperature of about 1650F for 500 hours thereby forming at least a 0.5 micron A1203 ¦ coating on the exposed surfaces of the fiber metal.

From the above description it is apparent that the resent invention includes the formation of metal compacts suitable I .

.

il r-~ ID-3703 for high temperature seals Erom Eine metal parti.cles which are !~apable o:E forming a thi.n protective coating oE A1203 over their ¦~surfaces, to the alloy composition of said particles, and to the .relationship between the particle dlameter and the Al content of ~the alloy.

Il '. l . -20-

Claims (10)

WHAT IS CLAIMED IS:

1. A metal particle resistant to oxidation at high tem-peratures composed of a homogeneous alloy consisting essentially of the composition I, Cr, Al, II, or I, Cr, Al, III, wherein I is at least one member of the group consisting of Fe, Co, Ni, and mix-tures of Co and Ni, II is at least one member of the group consis-tinq of Y, Sc, and Rare Earths, and III is at least one member of the group consisting of Si, Hf, Zr, Cb, and Ta, the components of the alloy having essentially the following weight percents: 10-27%
Cr, 5-20% Al, .01-0.5% II; when I=Co, Ni or Co and Ni, III=0.5-2.0%
and when I=Fe, then III=0.1-1.0%; and I=the remainder, the exposed surface of the particle being capable of developing a protective coating of Al2O3 at least 0.5 microns in thickness over an under-lying particle substrate of at least 4% Al content.

2. The article of claim 1 wherein the particle is a fiber having a diameter of from about 4 to about 150 microns, an aspect ratio of from about 10 to about 4200, and the Al content of the alloy is equal to at least that given in the equation:
D being the particle diameter in microns.

3. The article of claim 1 wherein said particle is a metal powder particle having a diameter of from about 4 to about 100 microns, an aspect ratio of about 1 to 8, and the Al content of the alloy is equal to:

wherein D is the particle diameter in microns.

4. The fiber particle of Claim 1, Claim 2 or Claim 3 wherein the alloy is Ni, Cr, Al, Y and the components have the following weight percents: 15-17% Cr, 8.0-12% Al, .01-0.25% Y and the balance Ni.

5. The fiber particle of Claim 1, Claim 2 or Claim 3 wherein the alloy is Fe, Cr, Al, Y and the components have the following weight percents: 15-23% Cr, 8.0-12% Al, .01-0.1% Y and the balance Fe.

6. The fiber particle of Claim 1, Claim 2 or Claim 3 wherein the alloy is Fe, Cr, Al, Si and the components have the following weight percents: 15-23% Cr, 8.0-12% Al, 0.4-0.6% Si and the balance Fe.

7. A machine workable homogeneous alloy of the composition I, Cr, Al, II, or I, Cr, Al, III, wherein I is at least one member of the group consisting of Fe, Co, Ni, and mixtures of Co and Ni; II is at least one member of the group consisting of Y, Sc and Rare Earths, and III is at least one member of the group consisting of Si, Hf, Zr, Cb and Ta, and wherein the components of the alloy have essentially the following weight percents: 10-27% Cr, 5-20% Al, .01-0.5% II;
when I=Co, Ni or Co and Ni then III=0.5-2.0%; when I-Fe then III=0.1-1.0%; the remainder-I, and the exposed surface of said homogeneous alloy being capable of developing a protective coating of Al2O3 at least 0.5 microns in thickness over an under-lying particle substrate of at least 4% Al content.

8. The alloy of Claim 7 having essentially the composition Ni, Cr, Al, Y of the following weight percents:
15-17% Cr, 8.0-12% Al, .01-0.25% Y and the balance Ni.

9. The alloy of Claim 7 having essentially the composition Fe, Cr, Al, Y and the components have the following weight percents: 15-23% Cr, 8.0-12% Al, .01-1.0% Y
and the balance Fe.

10. The alloy of Claim 7 having essentially the composition Fe, Cr, Al, Si and the components have the following weight percents: 15-23% Cr, 8.0-12% Al, 0.4-0.6% Si and the balance Fe.