US 3166416 A
Description (OCR text may contain errors)
United States Patent 3,166,416 PRGQESS PEGBUQENG DHPERSKN- HARDENED ALLQJS I David Kenneth Worn, Soiihnli, England, assignor to The International Nickel (Jornpauy, inc, New York, NY a corporation of Deiaware No Filed 2-5, 1961 ar. No. teases Claims priority, Great Britain, May 12, 196i),
The present invention relates to dispersion-hardened metals and/or alloys, and, more particularly, to the process for producing dispersion-hardened metals and/ or alloys'by powder metallurgical techniques.
It is known that the high-temperature strength of metals and/or alloys can be improved by incorporating a dispersion of very line particles of a non-metallic phase in a matrix of the metals and/or alloys. The function of the dispersed phase is to inhibit slip processes in the matrix. matrix should be hard and should have very low solubility in the metal and/or alloy matrix even at elevated temperatures. The resulting materials containing a metallic matrix having a non-metallic phase incorporated therein are commonly known as dispersion-strengthened and/ or dispersion-hardened metals and/ or alloys. These dispersion-hardened metals and/or alloys may be advantageously produced by powder metallurgical processes.
One method heretofore employed to produce such metals and/or alloys was to introduce another element into the matrix and to oxidize the introduced element in situ. More commonly, the method employed to produce dispersion-hardened metals and/or alloys was to disperse and incorporate particles of the non-metallic phase into the metallic matrix by intimately mixing them with a finely-divided powdered metal and/or alloy, compacting and sintering the resulting mixture, and working the sintered compact, e.g., by extrusion, forging, hot and/or cold Working, etc.
The properties of the products so processed depend, amongst other things, on the mean free path between discrete particles of the non-metallic phase. To obtain higher strength at room and elevated temperatures this distance should be as small as possible in order to obtain the maximum slip-inl1ibiting eiiect. Accordingly, when using powder metallurgical techniques, theparticle size of the metal and/or alloy components should also be as small as possible to insure maximum strength. I
It is also highly desirable that the metal and/ or alloy components be free from unintentional oxides other than, or course, those of the dispersed phase. a The reason for their exclusion is that the presence of such oxides may contaminate the dispersed phase and reduce its stability a televated temperatures.
The aforementioned requirements introduce problems in the production of certain types of dispersion-hardened alloys by powder metallurgical methods. In particular, these problems are especially troublesome in the production of dispersion-strengthened alloys wherein at least one of the elements forming the matrix of the alloy forms very stable oxides. For example, chromium is one such element that forms very stable oxides. Consequently, even though it has long been konwn that the resistance or" nickel to oxidation at elevated temperatures can be in- Thus, the non-metallic phase incorporated in the .mental chromium powder.
difficulties, the commercial production by powder met- 3,lti6, il6 Patented Jan. 19, 1965 creased by alloying it with chromium, the production of dispersion-strengthened nickel-chromium and/or nickelchromium base alloys present considerable diificulties. The chromium, for example, can not be mixed in the form of powder with nickel and the particles of the dispersed phase since, in practice, the chromium powder will have a diiiicultly reducible oxide layer which contaminates the dispersed phase and reduces the stability of the resulting alloy at high temperatures. The problem cannot be solved by simply employing a nickelchrornium alloy powder since, although such powders may be made by grinding and/or atomization, andare commercially available, their particle size is far too large, e.g., of the order or" about 50 microns, for them to be satisfactory. In addition, they invariably contain traces of chromium-rich oxides in much the same Way as ele- As a result of the foregoing allurgical methods of dispersion-strengthened alloys such as nickel-chromium alloys containing an element inthe matrix which forms very stable oxides has been greatly hampered.
Although attempts were made to overcome the foregoing difficulties and other disadvantages, none, as far as I am aware, was entirely successful when carried into practice commercially on an industrial scale.
It has now been discovered that dispersion-strengthened vide a novel powder metallurgical process for producing dispersion-hardened alloys substantially devoid of undesirable contamination such as that which reduces the stability of the alloys at high temperatures. T p I Another object of the invention is to provide sintered high-temperature alloys containing a hard, substantially insoluble, slip-inhibiting, non-metallic phase.
The invention also contemplates providing novel composite metal powders containing hard, non-metallic material dispersed therethrough, which powders are particularly suitable for powder metallurgy production of wrought dispersion-hardened alloys.
Other objects and advantages will become apparent from the following description:
Generally speaking, the present invention contemplates a unique powder metallurgical process for producing novel dispersion-hardened metals and/or alloys containing a matrix and a dispersed, slip-inhibiting, hard, nonmetallic phase that is substantially insoluble in the matrix even at temperatures up to about of the absolute melting point of the matrix by vapor depositing at least one of the matrix-forming substances in, into and/or on,
that is, infusing the agglomerates of the hard,'non-metal lic.
and a melting point of at least about 1500? C. and'being stable and substantially insoluble in the matrix at temperatures up to about 90% of the absolute melting point of the matrix are provided to form 'a particulate mass containing at least one initial matrix-forming substance and hard, non-metallic material. 'Thereafter, the'particulate mass is subjected to a vapor infusion treatment whereby at least one matrix-forming metal such as chromium, aluminum, etc., is vapor deposited on the particulate powder mass in amounts from about 1% to about 40% by Weight, e.g., 5% to 40% of the total matrix-formers; Silicon, in amounts up to 40%, by weight, of the total matrix-formers may also be vapor deposited as may up to about 1.0%, by weight, of at least one matrix-hardening metalloid, e.g., carbon. 7 g
It is essential thatprecautions are taken during the vapor infusion treatment to minimize the formation of a surface layer devoid of the non-metallic material thereby minimizing zones devoid of the slip-inhibiting phase in the final product. This may be achieved by mechanical agitation of the partculate mass'during vapor treatment and control of the vapor pressure of the element or ele- ,ments to be deposited, e.g., by the use of a partial pressure of inert gas. a
It is to be clearly appreciated that concentrations of impurity oxides such as oxides of iron, chromium, silicon, etc., must be less than about 0.05%. The infused mass is then consolidated, e.g., by extrusion, pressing, forming, hot and/ or cold working, etc., to form alloys containing hard particles dispersed in an alloy matrix.
In addition, the hard, nonmetallic particles are substantially homogeneously dispersed through the matrix and the mean free path between the discrete non-metallic insure maximum strength particles is at most about 10 microns and advantageously 7 less than about 3 microns.
The vapor deposition, accordingto the invention, is carried out by heating the, material 'to be infused to a temperature between about 800 C. to about 1200 C; in a substantially non-oxidizing atmosphere. 'Advantageously, the non-oxidizing atmosphere is a'vacuum, i.e., a pressure of about 2 microns of mercury or less, e.g., 0.5 micron of mercury. p
The substances comprising the initial matrix-forming substances are metallic components which can be metals and/ or alloys having a heat of oxidation relating to the lower oxide of no more than 70 kilocalories per gram atom of oxygen.(kcal./g.a. of oxygen) at 1 8 C. If, on the other hand, the heat of oxidation of the initial matrixforming substances is greater than 70 kcal./g.a. of oxygen, the oxides of the matrix-forming substances are very diificult to reduce under normal operating conditions. Thus, any alloys produced from such powders containing a substantially irreducible oxide are of little commercial value in that thenon-metallic phase will be contaminated.
Thus, elements such as chromium, aluminum, silicon, etc., cannot, as a practical matter, be employed as the initial matrix-forming powder because they form very stable oxides. Advantageously, the matrix powder is nickel, iron and/or alloys of these metals with each other because of their desirable high temperature characteristics. Additions of cobalt and molybdenum are suitably made in the form of hydrogen-reduced powders, and iron and nickel as their respective carbonyl metal powders. The hard, non-metallic constituent advantageously comprises a refractory metal oxide having a very high melting contemplation of this invention are cerium, thorium and/ or calcium sulfide. A silicide that is within the scope of this invention is molybdenum disilicide.
In any case, it is very important that the hard, nonmetallic constituent be substantially insoluble in the matrix and also be very stable at processing and service temperatures, i.e., the hard, non-metallic material must have a thermal dissociation equilibrium constant that is substantially zero or approaches zero at such temperatures. When the hard, non-metallic constituent does not possess such characteristics, any alloy containing such hard material lacks high temperature stability and is materially weakened. In addition, the hard, non-metallic constituent must have a particle size of about one micron and, advantageously, less than 0.5 micron, e.g., 0.1 .micron, to in dispersion-hardened alloys produced therefrom. i
The matrix-forming substances that are vapor-deposited can be any material that vaporizes under commercial operating conditions such as the elements chromium, aluminum, silicon (although silicon poses a rather special problem), etc.
deposited comprise metals and alloys that have a heat of formation of the most stable oxide at 18 C. of more than about 80 kcal./g.a. of oxygen.
Advantageous results are obtained in the production of particulate composite powders comprising matrix-forming substances and hard, non-metallic particles according to a this invention by mechanically mixing, e.g., by ball milling, rod milling, vibrating, etc., from about 0.1% to' about mixture of metallic powders and refractory metal oxides. V
This mixture is then, advantageously, infused with from about 20 parts to about 15 parts, by weight, of the vapor of at least one matrix-forming or alloying material, e.g., chromium, having a heat of oxidation at 18 C. of more than about 80 koal./g.a. of oxygen, byheating the alloying material to aternperature in the range or from about 400 C. to about 1200 C. in a vacuum, i.e., a pressure of less than 0.5 micron of mercury to produce a composite powder. The composite powder. is then consolidated by extrusion to form a dispersion-hardened and/ or strengthened alloy which contains from about 99.9 parts to about 80 parts by volume of nickel and chromium.
In carrying the invention into practice, it is advantageous to subject the initial powder mass to a reducing action before subjecting the mass to the vapors of matrixforming material in order to insure complete absence of any oxides of matrix-forming substances. This reducing operation is advantageously carried out in a hydrogen atmosphere although other reducing atmospheres may also be used. The operationis performed at a temperature of at least about 400 C. and not more than about 1000" C. This process lightly sinters the material. The
. sintered material may then be subjected to a pressing op point, e.g., alumina, thoria, beryllia, calcium oxide, or
an oxide of the rare earth group. However, carbides such as titanium and zirconium carbide and other substantially inert compounds such as sulfides and silicides may also be used. Among the sulfides that can be used within the eration prior to crushing before alloying by vapor deposi tion.
Metalloids, e.g., carbon, may be added to the particulate mass by subsequent treatment in any suitable atmosphere, e.g., the well-known carburizing atmospheres. The non metallic material may be dispersed in at least one matrixforming element byball milling, rod milling, vibrating,
7 If higher temperatures, higher than about 1000 C., are used in the initial reducing operation, the strength ofthe sintered material may be too great for it to be'sub- Advantageously, materials whichyield and/or release matrix-forming substances to be vapor sequently crushed into small lumps. In addition, the porosity of the sintered cake may be so low as to hinder the uniform diffusion of alloying vapor through it. After crushing it is advantageous to resubject the mixture to a reducing operation conducted at a temperature below about 1000 C., e.g., about 400 C. to about 1000 C., in an atmosphere such as hydrogen, to remove any traces of oxygen picked up in the crushing operation.
Alternatively, when dealing with relatively high concentrations of very fine non-metallic material, it is advantageous to compact the powder mass and pulverize this to granular form prior to the reducing heat treatment in hydrogen.
More than one matrix-forming constituent may be introduced by vapor treatment, and the treatments may be carried out either successively or simultaneously. Advantageously, successive treatment is utilized as it enables better control of the composition of the product and enables different temperatures to be used for each constituent if need be. In addition, chromium, aluminum, silicon, etc., may be added as vapor to nickel-base alloys, iron-base alloys, etc., containing hard, non-metallic particles to improve the resistance to oxidation.
Although it is advantageous to form the vapor of the other matrix-forming substance by vaporization of the pure element or any alloy containing said element, the vapor treatment may be carried out using a compound that will decompose on heating to form the element. Thus, the Vapor of chromium fluoride, for example, may be used in place of pure chromium vapor. Carbon may be introduced by any gas-carburizing process. Compounds which release oxygen on decomposition should, of course, be avoided.
To produce a dispersion-hardened nickel-chromium alloy, for example, according to the invention, finelydivided nickel powders, advantageously fine carbonyl nickel powder, are mixed with particles of the dispersed, hard phase which advantageously are, for example, thoria having a particle size of less than about 0.5 micron, and advantageously less than about 0.1 micron, in amounts as hereinbefore set forth. r
The mixing is advantageously effected by ball milling the powders together. However, instead of mechanically mixing the metal powder the non-metallic hard particles the powder may be formed in the presence of the nonmetallic material, e.g., by reduction of the oxide of the metal. Thus, a mixture of calcium oxide and nickel for further'treatment according to the invention may be obtained by igniting a mixture of calcium and nickel formates and reducingthe nickel oxide in the resultingmixture to nickel by heating in hydrogen. In any event, the mixing is carried out before the treatment in chromium vapor as otherwise the nickel particles tend to agglomerate and thus increase the mean free path between the particles of the dispersed phase in the final product. The mixture is then heated in hydrogen. In order to remove traces of oxygen at a reasonable rate and lightly sinter the powder mixture this heating should take place at a temperature of at least about 400 C. and advantageously not more than about 1000 C.
The sintered cake is then crushed into small lumps, e.g., by grinding, jaw crushing, etc. They are then placed in a furnace which also contains chromium gnanules and again treated with hydrogen at a temperature in the'range of about 400 C. to about 1000 C. to remove any traces of oxygen picked up in the crushing operation. Subsequently, the furnace is evacuated and the temperature raised to about 1200 C. at which temperature chromium vapor is liberated from the chromium granules. During this step in the process, it is essential that oxygen is substantially entirely excluded from the furnace during this operation otherwise chromium-rich oxide may be formed.
The chromized aggregate is then removed from the furnace and consolidated advantageously by first compacting i-t, e.g., by hydrostatic pressure, and then extruding the compact in a metal sheath, for example, of nickel, at a temperature of about 1100 C. to about 1200 C. Advantageously, the compacted material may be subjected to a homogenizing treatment at about 1200 C. prior to extrusion to insure uniformity of the nickelchrom-ium matrix alloy.
For the purpose of giving those skilled in the art a better understanding of the invention the following illustrative example is given:
Example About parts by weight of carbonyl nickel powder having a particle size of about 2 microns were mechanically mixed with about 6 parts by weight of'thoria having a specific surface area of 15 square meters per gram (m /g.) and a corresponding mean particle size of about 0.05 micron by ball milling for 36. hours. In other words, the mixture of nickel and thoria contained, by weight, about 92.7% nickel and 7.3 thoria. In addition, the volumetric ratio of nickel powder tothoria was 14.421. The thoria used was produced by ignition of the oxalate, although thoria produced by the ignition of the nitrate or formate is also satisfactory. After ball milling, the mixture was removed and placed in a furnace having a dry hydrogen atmosphere and was heated to about 800 C. for about 15 hours. The mixture was then removed from the furnace and compacted under a pressure of 35 long tons per square inch (t.s.i.) and crushed to produce agglomerates of minus 20 to plus 30 British Standard mesh sieve size. The agglomerates were then mixed with an equal quantity by weight of crushed chromium granules of substantially greater size than the nickel-thoria granules. That is, the chromium granules had a mean particle size of greater than about plus 30 British Standard mesh. This mixture was then transferred to a cylindrical furnace which could be rotated and was heated to 1200 C. under a vacuum, i.e., a pressure of 0.5 micron of mercury. heated in the furnace at a temperature of about 400 C. for about 1 hour in the presence of substantially dry hydrogen to remove any traces of oxygen picked up during-the crushing. The furnace was then evacuated to a pressure of about 0.5 micron of mercury and argon was I admitted to result in a pressure of about 50 microns of mercury which was thereafter maintained. The temperature was raised to about l200 C. at which tempera- I ture the chromium vaporized. Heating was continued a for about 60 hours until 19 parts of chromium, by Weight, were infused in, into and/ or on the aggregate.
furnace. It was then consolidated at a pressure of about 25 t.s.i., heat treated for 16 hours at 1200 C., and extruded at a temperature of about 1100 C. An analysis of the extruded material was made and it was found that the metallic matrix contained, by weight, about nickel and about 20% chromium. It was also found that this-extruded compact was substantially devoid of nickel oxide and chromium oxide, i.e., less than about 0.01% of both. The amount of thoria present as the hard phase inthe alloy was found to be about 5% by volume of the total composition of the alloy. The mean free path between the discrete particles of thoria was found to be about 2 microns. The extruded compact was then stressrupture tested in air at a temperature of about 1150 C.
and a stress of about 1 t.s.i. and was found to have a Prior to this treatment the charge was I I The chromized aggregate was subsequently removed from the By the term particle size when appliedto the thoria is meant the size measured by examination of the finished alloy under the electron microscope.
' It is to be understood that the rare earth elementswmay be added to nickel-base oxidation-resistant alloys to improve their hightemperature corrosion resistance. For example, the addition. of cerium up to about 0.1%, by Weight, maybe made. These additions may advantageously be made to nickel-base dispersion-hardened alloys described herein by infusion during or subsequent to the principal infusion treatment.
The present invention is applicable to the formation of a vast variety of alloys, having a greatrvariety of uses 7 such as turbine blades, etc. particularly applicable for introducingtalloying constituents such as chromium, silicon, aluminum, magnesium, etc., which readily form oxides which may contaminate the dispersed phase to nickel-base alloys, cobalt-base alloys, iron-base alloys, molybdenum-base alloys, etc.
weight, about 6 parts of thoria having a mean particle size of about 0.05 micron and about 75 parts, by weight, of carbonyl nickel powder having a mean particle size of less than about 2 microns, ball milling said nickel powder and thoria to form a substantially homogeneous mixture, heating said mixture to about 400 C. in a hydrogen atmosphere to remove'traces of oxygen, pressing said mixture to form a compacted-mixture, crushing the compacted mixture, reheating the crushed mixture to about 400 C. in the presence of hydrogen'to remove The present invention is' traces of oxygen, infusing said mixture by vapor-deposition in an essentially oxygen-free atmosphere with about 19 parts, by weight, of chromium to form chromized aggregates and extruding said aggregates to produce a substantially homogeneous, dispersion-hardened alloy containing not more than 0.05%, by Weight, of metal oxides other than thoria.
V 2. A process for producing a dispersion-hardened nickel-chromium alloy which comprises providing from about 0.1% to about 20% by volume of the total composition of the alloy of thoria havinga particle size of up to about 0.1 micron and from about 80 parts to about '62 parts, by weight, of nickel powder having a particle mixture, heating said mixture to a temperature between about 400 C. and about 1000 C. in a hydrogen atmosphere to remove traces of oxygen, crushing the mixture, reheating the crushed mixture to a' -temperature between about 400 C. and about 1000 C. in the presence of hydrogen to remove traces of oxygen, infusing said mixture by vapor-deposition in an essentially oxygen-free atmosphere with from about 20 parts to about 15 parts,
by weight, of chromium to form a chromium-infused mixture containing 100 parts, by weight, of nickel, chromium and thoria and extruding said infused mixtureinto a substantially homogeneous dispersion-hardened alloy containing not more than 0.05%, by weight, of metal oxides other than thoria.
3. A process for producing a dispersion-hardened nickel-chromium alloy which comprises providing from about 0.1% .to about 20%, by volume of the total composition of the alloy, of thoria having a particle size of up to about 0.5 micron and from about 80 parts to about 62 parts, by weight, of nickel powder having a particle size of up to' about 2 microns, mechanically mixing said nickel size of less than about 2 microns, ball milling said nickel powder and thoria to form a substantially homogeneous powder andthoria to form a substantially homogeneous mixture, heating said mixture to a temperature between about'400 Cpand about 1000 C. in a hydrogen atmosphere' to remove traces of oxygen, infusing said mixture by vapor-deposition in an essentially oXygen-free -atmosphere with from about 20 parts'to about 15 parts of chromium, by weight, to form' a chromium-infused mix-t ture containing 100 parts, by weight, of nickel, chromium and thoria and consolidating said infused mixture into a substantially homogeneous structure having a thoria phase dispersed therethrough and containing not more than 0.05%, by weight, of metal oxides other than thoria.
4. A process for producing adispersion-hardened nickel-chromium alloy which compries providing from about 0.1% to about 20%, by volume of the total composition of the alloy, of'thoria having a particle size of' up to about'0.5 micron and from about 90 to about 46 parts, by Weight, of nickel powder having a particle size of up to about 5 microns, mechanically mixing said nickel powder and thoria to form a substantially homogeneous mixture, heating said mixture in hydrogen at a temper-1 ature between about 400 C. and 1000 C. to remove traces of oxygen, infusing saidmixture by vapor-deposition in an essentially oxygen-free atmosphere with from 7.
about 7 .7 parts to about 40 parts, by weight, of chromium to form a chromium-infused mixture 7 containing 100 parts,.by weight, of nickel, chromium and thoria and con solidating said infused mixture into a substantiallyhomogeneous structure having athoria phase dispersed theretbrough andcontaining not more than 0.05%, by weight, of metal oxides other than thoria.
5; A process for producing a dispersion-hardened alloy containing a matrix and a hard phase dispersed throughout the matrix which comprisesproviding from about 0.1% to about 20%, byrvolur ne of the total composition of the alloy, of at least one refractory metal oxide having a particle size of up to about 1 micron, a melting point of at least about 15 00 C. and being stable at temperatures up to about 90% of the absolute melting point of the matrix and from about 90 to about 46 parts, by weight, of at least 7 one initial matrix-forming metal having a heat of oxidation at 18 C. of less than about 70 kilocalories per gram atom of oxygen and having a particle size of up to'about 5 microns, mechanically mixing said refractory metal oxide and said matrix-forming metal to form a substanr' tially homogeneous mixture, heating said mixture in hydrogen at a temperature'between about 400? C. and
- 10001C. to remove traces of oxygen, infusing said mixture by vapor-deposition in an essentially oxygen-free atmosphere with from about 7.7 parts to about 40 parts,
by weight, of at least one matrix-forming metal having a heat of oxidation at, 18 C. of greater than about kilocalories per gram atom of oxygen to form an infused mixture containing 100 parts, by weight, of matrix-form ing metals and refractory metal oxide and consolidating said infused mixture into a substantiallyhomogeneous,
structure having a refractory metal oxide phase dispersed therethrough. I e
6. A process'for producing a dispersion-hardened alloy containing a hard phase dispersed throughout the matrix which comprises providing from about 0.1% to about 20%, by volume of the total composition of the alloy, of hard, non-metallic material having a particle size of upto about 1 micron, a melting point ofat least about 1500 C. and being stable at temperatures up to about of the absolute melting point of the matrix and fromabout 90 to about 46 parts, by Weight, of at least one initial matrix-forming metal selected from the group consisting of nickel, iron, cobalt, copper, molybdenum and tungsten to form a particulate mass, heating said mass in hydrogento remove traces'of oxygen, infusing the resulting mass by vapor-deposition in an essentially oxygenfree atmosphere with from about 7.7 parts to about 40 parts, by weight, of at least one matrix-forming metal material having a heat of oxidation at 18 C. of greater than about 80 kilocalories per gram of oxygen and up to about 1 part, by Weight, of matrix-forming metalloid material to form an infused mass containing 100 parts, by weight, of matrix-forming materials and hard, non-metallie material and consolidating said infused mass into a substantially homogeneous structure having a hard nonmetallic phase dispersed therethrough.
References Cited in the file of this patent UNITED STATES PATENTS 10 a Sindeband et a1. Oct. 27, Washken Sept. 20, Alexander et a1 Feb. 21, Healy Nov. 14, Alexander et a1. Jan. 30, Funkhouser Mar. 6;
UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,166,416 January 19, 1965 David Kenneth Worn It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column 1, 1ine'52, for "or" read of line 55, for "a televated" read at elevated column 2, line 4, for "can not" read cannot column 3, line 25, for "partculate" read particulate column 9, line 1, after "gram" insert atom Signed and sealed this 6th day of July 1965.
ERNEST W. SWIDER EDWARD J. BRENNER Attcsting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,166,416 January 19, 1965 David Kenneth Worn It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.
Column 1, line 52, for "or" read of line 55, for "a televated" read at elevated column 2, line 4, for "can not" read cannot column 3, line 25, for "partculate" read particulate column 9, line 1, after "gram" insert H atom Signed and sealed this 6th day of July 1965.
ERNEST W. SWIDER EDWARD J. BRENNER Attesting Officer 7 Commissioner of Patents