US3893844A - Dispersion strengthened metals - Google Patents

Dispersion strengthened metals Download PDF

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US3893844A
US3893844A US384028A US38402873A US3893844A US 3893844 A US3893844 A US 3893844A US 384028 A US384028 A US 384028A US 38402873 A US38402873 A US 38402873A US 3893844 A US3893844 A US 3893844A
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alloy
metal
powder
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oxide
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Anil V Nadkarni
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SCM Metal Products Inc
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Assigned to SCM METAL PRODUCTS INC., WESTERN RESERVE BUILDING; 1468 WEST 9TH STREET; CLEVELAND, OHIO 44113 A CORP. OF DE. reassignment SCM METAL PRODUCTS INC., WESTERN RESERVE BUILDING; 1468 WEST 9TH STREET; CLEVELAND, OHIO 44113 A CORP. OF DE. ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: SCM CORPORATION, A NY. CORP.
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C32/00Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ

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  • ABSTRACT A dispersion-strengthened metal wherein the improvement comprises recrystallizing alloy powder prior to internal oxidation to increase the grain size of the alloy to at least about ASTM (E-1l2) Grain Size No. 6 to reduce the grain boundary area in the alloy pow der whereby the recrystallized alloy powder provides substantially improved structural properties in the dispersion-strengthened metal product.
  • Dispersion strengthening has been recognized in the past as a method for increasing strength and hardness of metals.
  • a solid solution alloy comprising a relatively noble matrix metal having relatively low heat or free energy of oxide formation and a solute metal having relatively high negative heat or free energy of oxide formation wherein the alloy is heated under oxidizing conditions to preferentially oxidize the solute metal.
  • This technique is known in the art as in situ internal oxidation of the solute metal to the solute metal oxide or more simply internal oxidation.”
  • Dispersion-strengthened metal products such as copper dispersions strengthened with aluminum oxide. have many commercial and industrial uses wherein high temperature strength properties and high electrical and/or thermal conductivities are desired or required in the finished product. Such commercial uses include frictional brake parts such as linings. facings. drums. and other machine parts for friction applications. Other commercial uses include electrical contact points. resistance welding electrodes. electrodes generally. electrical switches and switch gears. transistor assemblies. wires for solderless connections. wires for electrical motors. and many other uses requiring good electrical and thermal conductivities together with good strength and hardness at elevated temperatures.
  • Copending application Ser. No. 217.506 provides an improved alloy-oxidant mixture by providing for assimilation of the oxidant residue into the dispersion strengthened metal wherein the oxidant residue is dispersion strengthened during thermal coalescence by a hard. refractory metal oxide provided in the oxidant.
  • the oxidant residue formed during internal oxidation is not required to be removed from the dispersionstrengthened metal but rather is dispersion strengthened by the hard. refractory metal oxide during coalescence to form an integral part of the dispersionstrengthened metal stock.
  • a further objective and advantage of this invention is to provide reduced grain boundaries to substantially eliminate the concentration of solute metal oxide at the grain boundaries of the alloy during internal oxidation. This provides a dispersion-strengthened metal product having sustained resistance to preferential stress failure at the grain boundaries.
  • a still further objective is to provide a dispersionstrengthened metal product having a finer and more uniform distribution of dispersoid particles which result in improved elevated temperature properties.
  • a further object of this invention is to minimize formation of inner oxide film of solute metal oxide within the alloy during internal oxidation by increasing the grain size of the alloy prior to internal oxidation which advantageously permits higher oxidation temperatures as well as decreasing oxidation time.
  • a still further objective is to permit increased amounts of solute metal oxide which advantageously provide improved strength properties particularly at elevated temperatures.
  • a still further objective is to dispersion strengthen larger alloy particles which advantageously permits increased yield.
  • this invention provides an alloy-oxidant mixtureadapted to be internally oxidized wherein the alloy powder is recrystallized at elevated temperatures prior to the step of internal oxidation for a time sufficient to increase the grain size of the alloy powder to at least about ASTM (E-l l2) Grain Size No. 6.
  • FIG. I is a microphotograph magnified 500 times showing internally oxidized aluminum alloy powder without prior recrystallization.
  • FIG. 2 is a microphotograph magnified 500 times indicating internally oxidized aluminum alloy powder recrystallized prior to internal oxidation in accordance with this invention to obtain a grain size No. 6 as measured by ASTM Test E-l l2.
  • This invention pertains to dispersion-strengthened metals produced by internal oxidation of alloy powders wherein the improvement comprises recrystallizing the alloy powder to increase the grain size of the alloy prior to the internal oxidation step. Thereafter. the recrystallized powdered alloy is suitable for intimately intermix ing with oxidant. internally oxidized. and then co alesced and formed into dispersion-strengthened metal stock.
  • FIG. I indicates a copper alloy containing about 0.70% by weight of aluminum which was internally oxidized but without prior recrystallization. Shown on the outer periphery of the copper alloy I0 is a continuous internal oxide film 12 which has been found to inhibit internal oxidation of the aluminum.
  • FIG. 2 similarly indicates the same alloy composition. internally oxidized. but recrystallized prior to internal oxidation to achieve a grain size No. 6 as measured by ASTM Test E-l 12. No oxide film is formed on the outer periphery of the recrystallized alloy 14 as compared to nonrecrystallized alloy 10 shown in FIG. 1.
  • the preferred powder alloy comprises a relatively noble matrix metal having a negative free energy of oxide formation at C of up to 70 kilocalories per gram atom of oxygen. and a solute metal having a negative free energy of oxide formation exceeding that of the relatively noble matrix metal by at least about 60 kilocalories per gram atom of oxygen at 25C.
  • the relatively noble matrix metal and the solute metal are alloyed by conventional techniques such as melting them under inert or reducing conditions and thereafter comminuting the alloy by atomization or other conventional size-reduction techniques such as grinding or ball milling to form a particulate alloy having an average particle size of less than about 300 microns.
  • Suitable noble matrix metals include. for example. iron. cobalt.
  • solute metals include. for example. silicon. titanium. zirconium. aluminum. beryllium. thorium. chromium. magnesium. manganese.
  • the alloy composition comprises about 0.01% to about 2 weight percent of solute metal with the balance being relatively noble matrix metal and. if desired, minor amounts of conventional additives to improve abrasion resistance. hardness. conductivity. and other selected properties.
  • the comminuted alloy powder is first recrystallized at elevated annealing temperatures of the alloy to increase the grain size of the alloy powder to achieve at least about ASTM Grain Size No. 6 measured by ASTM Test No. El 12.
  • the process of recrystallization ordinarily consists of annealing the alloy to produce a new grain structure. Re crystallization diagrams for most metals and alloys are published. as indicated in Practical Metallurgy. by Sachs and Van Horn. particularly Chapter 5. 3rd printing. 1943. and incorporated herein by reference.
  • An nealing temperatures depend on the alloy to be dispersion strengthened and such temperatures are high enough to efficiently cause recrystallization but at temperatures substantially lower than the melting point of the alloy.
  • the alloy powder is internally oxidized by conventional methods such as disclosed in prior art processes disclosed hereinbefore and identi lied as patents issued to Schreiner. McDonald. and Grant. and the same are included herein by reference.
  • the Schreiner patent. US. Pat. No. 1488.1 83 provides a suitable method of internal oxidation of an alloy by controlling partial pressure of oxygen produced by dissociation of metal oxide within a two-compartment chamber.
  • the McDonald patent. US. Pat. No. 3.552.954 provides internal oxidation of a copper alloy within an oxygen atmosphere to saturate the copper with oxygen and thereafter reducing with hydrogen.
  • the Grant patent. US. Pat. No. 3.179.515 suggests internal oxidation by first oxidizing a copper alloy in air to form a surface layer of Cu O followed by continued heating to diffuse oxygen into the copper matrix followed by hydrogen reduction.
  • a particularly preferred method of internally oxidizing is disclosed in said copending application. Ser. No. 217.506. and provides for an intimate admixture of alloy powder with oxidant.
  • the disclosed oxidant comprises a pulverant. in situ heat-reducible metal oxide having a negative free energy of formation ranging up to kilocalories per gram atom of oxygen at 25C in intimate interspersion with discrete particles of hard. refractory metal oxide. the negative free energy of formation of said hard. refractory metal oxide exceeding the negative free energy of formation of said heat' reducible metal oxide by at least 60 kilocalories per gram atom of oxygen at 25C.
  • Suitable heat-reducible metal oxides include. for example, oxides of iron. cobalt. nickel. copper.
  • Suitable hard. refractory metal oxides include. for example. oxides of silicon. titanium. zirconium. aluminum. beryllium. thorium. chromium. magnesium. manganese.
  • the matrix metal must be relatively noble with respect to the solute metal so that the solute metal will be preferentially oxidized. This is achieved by selecting the solute metal such that its negative free energy of oxide formation at 25C is at least 60 kilocalories per gram atom of oxygen greater than the negative free energy of formation of the oxide of the matrix metal at 25C. Generally. such solute metals have a negative free energy of oxide formation per gram atom of oxygen of over kilocalories and preferably over 120 kilocalories. Similarly.
  • the metal moiety of the heat-reducible metal oxide in the oxidant preferably is the same metal as matrix metal present in the alloy to be internally oxidized. although the heatreducible metal oxide moiety can be different.
  • the hard. refractory metal oxide in the oxidant preferably is the same as the solute metal oxide formed in the alloy during internal oxidation of the alloy. although the refractory metal oxide in the oxidant can be different from the solute metal oxide in the internally oxidized alloy. as more particularly set forth in said copending application. Ser. No. 217.506.
  • the oxidant for internally oxidizing the powdered alloy by the preferred method is a mixture of an in situ heat-reducible metal oxide and a hard. refractory metal oxide.
  • a suitable oxidant include. for example. decomposing an oxide'forming salt of a refractory metal on particles of heat-reducible metal oxide in the micron or sub micron range. or coprecipitation of oxide-forming compounds from their respective salt solutions. or physical blending of the desired oxide components.
  • At least about 0.1 weight parts of oxidant are combined with weight parts of powder alloy and desirably between about 0.1 to 20 weight parts of oxidant.
  • Preferably. about 0.1 to 10 weight parts of oxidant are combined with about I00 weight parts of powder alloy.
  • the exact proportions of the oxidant relative to the alloy depends on the solute metal of the alloy to be oxidized, and the oxygen content of the oxidant.
  • the amount of such oxidant to be added may be determined by the stoichiometric amount of oxygen required to completely oxidize the solute metal. in this regard.
  • the heat-reducible metai oxide is added in sufficient amounts to completely oxidize the solute metal in the alloy, whereas the amount of hard, refractory metal oxide depends upon the amount of heat-reducible metal oxide.
  • the residue of heat-reducible metal oxide present after internal oxidation is dispersion strengthened by coalescence by the hard, refractory metal oxide. Sufficient oxidant is utilized to completely oxidize the solute metal in the alloy. however. if excessive oxidant is utilized. the resulting internally oxidized metal powder may then be reduced with hydrogen at temperatures of about l500F for time sufficient to reduce residual oxygen.
  • Electrolytic tough-pitch grade copper rods are melted in an inert refractory crucible in an inductionheating furnace under reducing conditions at about 2300F.
  • Metallic aluminum shavings are introduced into the molten copper in the proportion of 0.33% by weight of the resulting molten metallic mass.
  • the molten solution of aluminum in copper is then superheated to 2400F. atomized through an atomizing aperture in a jet of nitrogen (alternatively other inert gases or water or steam can be used as the atomizing fluid) to yield an atomized copper-aluminum alloy powder which substantially all passes a lOO-mesh.
  • nitrogen alternatively other inert gases or water or steam can be used as the atomizing fluid
  • U.S. Sieve indicating that the average particle size is less than about I49 microns.
  • the atomized and screened alloy powder is annealed at a temperature of about l600F for about an hour in an argon atmosphere to recrystallize and yield a grain size in the recrystallized alloy powder of at least about ASTM Grain Size 6 according to ASTM Test E-l12. Preferably. the grains are as large as possible to minimize grain boundary area in the powder.
  • the alloy powder is then ready for use in combination with the oxidant.
  • Part C Preparation of the Internally Oxidizable Alloy Powder-oxidant Mixture
  • the alloy powder of Part A is thoroughly mixed with the oxidant powder of Part B in the proportion of 2.12 parts of oxidant to I00 parts of alloy powder.
  • the mixing is accomplished in a ball-mill. although a conventional V-cone blending device can alternatively be used.
  • Part D Internal Oxidation of the Alloy Powder
  • the alloy powder-oxidant mixture of Part C is then charged to an internal oxidation vessel which is then sealed.
  • the oxidation vessel is copper or copper-lined steel to avoid contamination of the alloy powder-oxide mixture during oxidation.
  • the alloy powder-oxidant mixture is then brought to a temperature of about l750F and maintained at this temperature for about 30 minutes to effectuate internal oxidation of the alloy powder.
  • the internal oxidation can be carried out on a continuous basis using a continuous belt furnace maintained under an inert atmosphere.
  • substantially all of the aluminum in the alloy powder has been oxidized to M 0 and substantially all of the cuprous oxide in the oxidant has been reduced to metallic copper.
  • the particles of internally oxidized alloy comprise 99.37% by weight of copper plus minor amounts of impurities and 0.63% by weight of M 0
  • the oxidant residue comprises 99.3771 copper particles and 0.63% A1 0 particles.
  • the overall internally oxidized metal powder composition comprises 98.2 I'/( internally oxidized alloy powder and 1.79% oxidant residue.
  • Part E Reduction of the Internally Oxidized Metal Powder
  • the internally oxidized metal powder of Part D is then placed in a reducing atmosphere of hydrogen at a temperature of about l500F for one hour to reduce any residual copper oxide.
  • Part F Thermal Coalescence or Consolidation of the internally Oxidized Metal Powder
  • the internally oxidized and reduced metal powder of Part E is then changed under an inert argon atmosphere to a thin-walled copper can having a diameter of about 7 inches and equipped with a feed tube.
  • the can and its contents are heated to about l700F and the feed tube sealed.
  • the feed tube is attached to a vacuum pump; and the can is evacuated while the temperature of the can is brought to l700F to remove any occluded gas from the powder. After evacuation at a pressure of l X l0 mm of Hg for minutes at 1700F. the feed tube is sealed and disconnected from the vacuum pump.
  • the sealed can is then placed in a ram-type extrusion press and is extruded to form extrudate in the shape of cylindrical bar stock having a diameter of about L25 inches.
  • the bar stock comprises about 99.37% copper having dispersed throughout 0.637: (or about I 5% by volume) of M 0 particles and has a density of about 99.3% of the theoretical density.
  • the bar stock has an electrical conductivity of 88% lACS". a tensile strength of about 72.000 psi, an elongation of 19% using ASTM Test E-8 (for a test specimen 0. lo inch diameter and 0.65 inch gage length) and a Rockwell hardness of about 75 units on the 8 scale. All property measurements reported in the example are conducted at room ized powder are each blended with stoichiometric amounts of oxidant consisting of an intimate mixture of submicron Cu O and N and internally oxidized, as indicated in Example l.
  • cold i S f ii are h if i iglz ff extrusion or cold drawing to form workpieces having H t en moedwre a I l f treatment at [500 F for an hour in argon.
  • the Vicker s particular tensile strengths according to conventional Cold working techniques (DPH) Diamond Pyramid Hardnesses are measured in insumce when the bar Stock is reduced to 50% kilograms per mm at a 15 gram load and the results in cross-sectional area by coldswaging.
  • the test results show su- Wllh the exception that [ha gram growth p perior resistance to softening upon heating of the diseluded. Both powd and are d, P persion-strengthened workpiece of this invention. ished. and etched.
  • the non-recrystallized (no grain growth) powder (b) shows a continuous internal oxide EXAMPLE 2 film after internal oxidation. whereas the recrystallized (grain growth) powder (a) does not indicate an oxide A copper alloy similar to Example I and containing m vi k Diamond pyramid Harm-3SS (DPH) weight Percent of aluminum nllrOgellFmm'lZed 5g taken at l5 g.
  • the atomized powder is subjected to recrystallim 1 :1 ⁇ e g mm outs e or t e zation treatment at l800F for one hour under argon Oxlde I atmosphere prior to internal oxidation.
  • recrystallim 1 :1 ⁇ e g mm outs e or t e zation treatment at l800F for one hour under argon Oxlde I atmosphere prior to internal oxidation.
  • Both the nontreated atomized powder and the recrystallized atomwell hardness. electrical conductivity and ultimate tensile strength are determined and are set forth in Table 2.
  • a nickel alloy containing 0.45% aluminum by weight is nitrogen atomized to produce an alloy powder.
  • the alloy powder is divided into two fractions. namely, one fraction recrystallized in a grain growth step. and the other fraction did not undergo grain growth.
  • the atomized nickel alloy powder is subjected to recrystallization treatment at about l800F for one hour under argon atmosphere to achieve a grain size No. 6. Both the recrystallized fraction and the non-treated fraction of the nickel alloy powder is then mixed with L89 weight parts of pulverant oxidant comprising l.87 parts of nickel dioxide and 0.02 parts of aluminum oxide per I00 weight parts of powder alloy.
  • the nickel alloy and oxidant mixtures are then internally oxidized as indicated in Example I at l750F in argon for about 3 hours. Both fractions are then reduced with hydrogen at I500F for about one hour to remove any excess oxygen.
  • the reduced powder mixtures are then cold compacted and hot forged in the manner set forth in Example 2 and tested.
  • the recrystallized alloy fraction undergoing grain growth to achieve a grain size number of at least 6 indicates substantially improved Vickers hardness. Rockwell hardness. electrical conductivity. and tensile strength when compared to the other nonrecrystallized alloy fraction.
  • EXAMPLE 6 A silver alloy containing 99.04% silver and 0.48% aluminum is nitrogen atomized in a manner similar to Example I to produce an alloy powder. The alloy powder is then separated into two fractions wherein one fraction is recrystallized with a grain growth step whereas the second fraction is not treated to undergo grain growth. In the grain growth fraction. the atomized alloy powder is subjected to recrystallization treatment at about l500F for about one hour under argon atmosphere to achieve a grain size of No. 6. Both the recrystallization and the non-treated fractions are combined with 6.35 parts of pulverant oxidant comprising 6.24 parts of silver oxide and 0.1 1 parts of aluminum oxide.
  • Both fractions are then internally oxidized at 1200F in argon for about I hour in the manner indicated in Example Each fraction is reduced. compacted. and hot forged as indicated in Example 2.
  • the recrystallized fraction exhibits substantially improved Vickers hardness. Rockwell hardness. electrical conductivity. and tensile strength when compared to the non-treated fraction.
  • An improved powdered alloy suitable for dispersion strengthening by internal oxidation comprising:
  • an alloy comprising a relatively noble matrix metal having a negative free energy of oxide formation at 25C of up to kilocalories per gram atom of oxygen and a solute metal having a negative free energy of oxide formation exceeding the free energy of oxide formation of said noble matrix metal by at least about 60 kilocalories per gram atom of oxygen at 25C;
  • said powdered alloy being recrystallized and having 21 Grain Size of at least Number 6 as measured by ASTM E-l l2.

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Abstract

A dispersion-strengthened metal wherein the improvement comprises recrystallizing alloy powder prior to internal oxidation to increase the grain size of the alloy to at least about ASTM (E-112) Grain Size No. 6 to reduce the grain boundary area in the alloy powder whereby the recrystallized alloy powder provides substantially improved structural properties in the dispersion-strengthened metal product.

Description

United States Patent 191 Nadkarni DISPERSION STRENGTHENED METALS [75] Inventor: Anil V. Nadkarni, Baltimore, Md.
[73] Assignee: SCM Corporation, New York, NY.
[22] Filed: July 30, 1973 [2l] Appl. No.: 384,028
Related U.S. Application Data [63] Continuation-impart of Ser. No. 217,506, Jan. 13,
1972, Pat. No. 3,779,7l4.
[52] U.S. Cl 75/0.5 R; 75/0.5 B; 75/0.5 BA; 75/0.5 BB; 7510.5 BC; 75/206 [51] Int. Cl B22f 9/00 [58] Field of Search 75/0.5 B, 0.5 BA, 0.5 R, 75/0.5 BB, 0.5 BC, 206
[56] References Cited UNITED STATES PATENTS 2,539,298 l/l95l Doty et al 75/O.5 BC
[451 July s, 1975 3,45l,803 6/1969 Cerulli 75/0.5 BC 3,488,183 1/1970 Schreiner et al.... IS/0.5 B 3,492,113 1/1970 Shafer et a]. 75/0.5 BC 3,505,059 4/1970 Cerulli 75/0.5 BC 3,552,954 l/l97l McDonald 7.5/0.5 BC
Primary Examiner-W. Stallard Attorney, Agent, or Firm-Thomas M. Schmitz [57] ABSTRACT A dispersion-strengthened metal wherein the improvement comprises recrystallizing alloy powder prior to internal oxidation to increase the grain size of the alloy to at least about ASTM (E-1l2) Grain Size No. 6 to reduce the grain boundary area in the alloy pow der whereby the recrystallized alloy powder provides substantially improved structural properties in the dispersion-strengthened metal product.
1 Claim, 2 Drawing Figures 1 DISPERSION STRENGTHENED METALS This is a continuation-in-part of copending application Ser. No. 2l7.5()6 filed on Jan. I3. 1972. now U.S. Pat. No. 3.779.714 and said application is incorporated herein by reference.
BACKGROUND OF THE INVENTION Dispersion strengthening has been recognized in the past as a method for increasing strength and hardness of metals. A solid solution alloy comprising a relatively noble matrix metal having relatively low heat or free energy of oxide formation and a solute metal having relatively high negative heat or free energy of oxide formation wherein the alloy is heated under oxidizing conditions to preferentially oxidize the solute metal. This technique is known in the art as in situ internal oxidation of the solute metal to the solute metal oxide or more simply internal oxidation."
Dispersion-strengthened metal products. such as copper dispersions strengthened with aluminum oxide. have many commercial and industrial uses wherein high temperature strength properties and high electrical and/or thermal conductivities are desired or required in the finished product. Such commercial uses include frictional brake parts such as linings. facings. drums. and other machine parts for friction applications. Other commercial uses include electrical contact points. resistance welding electrodes. electrodes generally. electrical switches and switch gears. transistor assemblies. wires for solderless connections. wires for electrical motors. and many other uses requiring good electrical and thermal conductivities together with good strength and hardness at elevated temperatures.
Several prior art processes for internal oxidation have been suggested. such as disclosed in the Schreiner patent. U.S. Pat No. 3.488.185; the McDonald patent. U.S. Pat. No. 3.552.954; and the Grant patent. U.S. Pat. No. 3.179.5l5. The prior art processes require delicate control over the partial pressure of oxygen during oxidation. or require removal of an oxidant residue which otherwise would form defects in the dispersionstrengthened metal.
Copending application Ser. No. 217.506 provides an improved alloy-oxidant mixture by providing for assimilation of the oxidant residue into the dispersion strengthened metal wherein the oxidant residue is dispersion strengthened during thermal coalescence by a hard. refractory metal oxide provided in the oxidant. The oxidant residue formed during internal oxidation is not required to be removed from the dispersionstrengthened metal but rather is dispersion strengthened by the hard. refractory metal oxide during coalescence to form an integral part of the dispersionstrengthened metal stock.
It has been found that dispersion-strengthened metals produced by internal oxidation have substantially improved properties if the alloy powder is recrystallized prior to internal oxidation in order to increase the grain size of the alloy powder to at least ASTM Grain Size No. 6 as measured by ASTM Test No. E-l l2.
Accordingly. it is a primary objective of this invention to increase the grain size and reduce the grain boundary area in the alloy powder prior to the step of internal oxidation in processes for dispersion strengthening of metals.
A further objective and advantage of this invention is to provide reduced grain boundaries to substantially eliminate the concentration of solute metal oxide at the grain boundaries of the alloy during internal oxidation. This provides a dispersion-strengthened metal product having sustained resistance to preferential stress failure at the grain boundaries.
A still further objective is to provide a dispersionstrengthened metal product having a finer and more uniform distribution of dispersoid particles which result in improved elevated temperature properties.
A further object of this invention is to minimize formation of inner oxide film of solute metal oxide within the alloy during internal oxidation by increasing the grain size of the alloy prior to internal oxidation which advantageously permits higher oxidation temperatures as well as decreasing oxidation time.
A still further objective is to permit increased amounts of solute metal oxide which advantageously provide improved strength properties particularly at elevated temperatures.
A still further objective is to dispersion strengthen larger alloy particles which advantageously permits increased yield.
These and other advantages will become more apparent from the detailed description of the invention.
SUMMARY OF THE INVENTION Briefly. this invention provides an alloy-oxidant mixtureadapted to be internally oxidized wherein the alloy powder is recrystallized at elevated temperatures prior to the step of internal oxidation for a time sufficient to increase the grain size of the alloy powder to at least about ASTM (E-l l2) Grain Size No. 6.
In the drawings:
FIG. I is a microphotograph magnified 500 times showing internally oxidized aluminum alloy powder without prior recrystallization; and
FIG. 2 is a microphotograph magnified 500 times indicating internally oxidized aluminum alloy powder recrystallized prior to internal oxidation in accordance with this invention to obtain a grain size No. 6 as measured by ASTM Test E-l l2.
DETAILED DESCRIPTION OF THE INVENTION This invention pertains to dispersion-strengthened metals produced by internal oxidation of alloy powders wherein the improvement comprises recrystallizing the alloy powder to increase the grain size of the alloy prior to the internal oxidation step. Thereafter. the recrystallized powdered alloy is suitable for intimately intermix ing with oxidant. internally oxidized. and then co alesced and formed into dispersion-strengthened metal stock.
Referring first to the drawings. FIG. I indicates a copper alloy containing about 0.70% by weight of aluminum which was internally oxidized but without prior recrystallization. Shown on the outer periphery of the copper alloy I0 is a continuous internal oxide film 12 which has been found to inhibit internal oxidation of the aluminum. FIG. 2 similarly indicates the same alloy composition. internally oxidized. but recrystallized prior to internal oxidation to achieve a grain size No. 6 as measured by ASTM Test E-l 12. No oxide film is formed on the outer periphery of the recrystallized alloy 14 as compared to nonrecrystallized alloy 10 shown in FIG. 1.
Referring now to the powdered alloy. the preferred powder alloy comprises a relatively noble matrix metal having a negative free energy of oxide formation at C of up to 70 kilocalories per gram atom of oxygen. and a solute metal having a negative free energy of oxide formation exceeding that of the relatively noble matrix metal by at least about 60 kilocalories per gram atom of oxygen at 25C. The relatively noble matrix metal and the solute metal are alloyed by conventional techniques such as melting them under inert or reducing conditions and thereafter comminuting the alloy by atomization or other conventional size-reduction techniques such as grinding or ball milling to form a particulate alloy having an average particle size of less than about 300 microns. Suitable noble matrix metals include. for example. iron. cobalt. nickel. copper, cadmium. thallium. germanium. tin. lead, antimony. bismuth. molybdenum. tungsten. rhenium. indium. silver. gold. ruthenium. palladium. osmium. platinum. and rhodium. Suitable solute metals include. for example. silicon. titanium. zirconium. aluminum. beryllium. thorium. chromium. magnesium. manganese. The alloy composition comprises about 0.01% to about 2 weight percent of solute metal with the balance being relatively noble matrix metal and. if desired, minor amounts of conventional additives to improve abrasion resistance. hardness. conductivity. and other selected properties.
In accordance with this invention. the comminuted alloy powder is first recrystallized at elevated annealing temperatures of the alloy to increase the grain size of the alloy powder to achieve at least about ASTM Grain Size No. 6 measured by ASTM Test No. El 12. The process of recrystallization ordinarily consists of annealing the alloy to produce a new grain structure. Re crystallization diagrams for most metals and alloys are published. as indicated in Practical Metallurgy. by Sachs and Van Horn. particularly Chapter 5. 3rd printing. 1943. and incorporated herein by reference. An nealing temperatures depend on the alloy to be dispersion strengthened and such temperatures are high enough to efficiently cause recrystallization but at temperatures substantially lower than the melting point of the alloy. For a predominantly copper alloy with minor amounts of aluminum. for example. desirable recrystallization takes place after heating for at least an hour at a temperature of about 1600F in an inert atmosphere such as argon. Much lower recrystallization tempera tures may be utilized. such as 600F to 700F. but lower temperatures necessitate increased heat times. Increasing the grain size of the alloy powder prior to internal oxidation has been found to effectively minimize the tendency for solute metal oxide to concentrate at the powder grain boundaries during internal oxidation which undesirably may cause early failure under stress at the grain boundaries in the final dispersionstrengthened metal product.
After recrystallization. the alloy powder is internally oxidized by conventional methods such as disclosed in prior art processes disclosed hereinbefore and identi lied as patents issued to Schreiner. McDonald. and Grant. and the same are included herein by reference. For example. the Schreiner patent. US. Pat. No. 1488.1 83 provides a suitable method of internal oxidation of an alloy by controlling partial pressure of oxygen produced by dissociation of metal oxide within a two-compartment chamber. The McDonald patent. US. Pat. No. 3.552.954. provides internal oxidation of a copper alloy within an oxygen atmosphere to saturate the copper with oxygen and thereafter reducing with hydrogen. The Grant patent. US. Pat. No. 3.179.515. suggests internal oxidation by first oxidizing a copper alloy in air to form a surface layer of Cu O followed by continued heating to diffuse oxygen into the copper matrix followed by hydrogen reduction.
A particularly preferred method of internally oxidizing is disclosed in said copending application. Ser. No. 217.506. and provides for an intimate admixture of alloy powder with oxidant. The disclosed oxidant comprises a pulverant. in situ heat-reducible metal oxide having a negative free energy of formation ranging up to kilocalories per gram atom of oxygen at 25C in intimate interspersion with discrete particles of hard. refractory metal oxide. the negative free energy of formation of said hard. refractory metal oxide exceeding the negative free energy of formation of said heat' reducible metal oxide by at least 60 kilocalories per gram atom of oxygen at 25C. Suitable heat-reducible metal oxides include. for example, oxides of iron. cobalt. nickel. copper. cadmium. thallium. germanium. tin. lead. antimony. bismuth. molybdenum. tungsten. rhenium. indium. silver. gold. ruthenium. palladium. osmium. platinum. and rhodium. Suitable hard. refractory metal oxides include. for example. oxides of silicon. titanium. zirconium. aluminum. beryllium. thorium. chromium. magnesium. manganese.
In any particular combination of the matrix metal and the solute metal in the alloy to be internally oxidized by the preferred method. the matrix metal must be relatively noble with respect to the solute metal so that the solute metal will be preferentially oxidized. This is achieved by selecting the solute metal such that its negative free energy of oxide formation at 25C is at least 60 kilocalories per gram atom of oxygen greater than the negative free energy of formation of the oxide of the matrix metal at 25C. Generally. such solute metals have a negative free energy of oxide formation per gram atom of oxygen of over kilocalories and preferably over 120 kilocalories. Similarly. the metal moiety of the heat-reducible metal oxide in the oxidant preferably is the same metal as matrix metal present in the alloy to be internally oxidized. although the heatreducible metal oxide moiety can be different. Similarly. the hard. refractory metal oxide in the oxidant preferably is the same as the solute metal oxide formed in the alloy during internal oxidation of the alloy. although the refractory metal oxide in the oxidant can be different from the solute metal oxide in the internally oxidized alloy. as more particularly set forth in said copending application. Ser. No. 217.506.
As indicated. the oxidant for internally oxidizing the powdered alloy by the preferred method is a mixture of an in situ heat-reducible metal oxide and a hard. refractory metal oxide. Several methods of forming a suitable oxidant are described in said copending application Ser. No. 217.506 and include. for example. decomposing an oxide'forming salt of a refractory metal on particles of heat-reducible metal oxide in the micron or sub micron range. or coprecipitation of oxide-forming compounds from their respective salt solutions. or physical blending of the desired oxide components.
In preparing the alloy-oxidant mixture in accordance with the preferred method. at least about 0.1 weight parts of oxidant are combined with weight parts of powder alloy and desirably between about 0.1 to 20 weight parts of oxidant. Preferably. about 0.1 to 10 weight parts of oxidant are combined with about I00 weight parts of powder alloy. The exact proportions of the oxidant relative to the alloy depends on the solute metal of the alloy to be oxidized, and the oxygen content of the oxidant. The amount of such oxidant to be added may be determined by the stoichiometric amount of oxygen required to completely oxidize the solute metal. in this regard. the heat-reducible metai oxide is added in sufficient amounts to completely oxidize the solute metal in the alloy, whereas the amount of hard, refractory metal oxide depends upon the amount of heat-reducible metal oxide. The residue of heat-reducible metal oxide present after internal oxidation is dispersion strengthened by coalescence by the hard, refractory metal oxide. Sufficient oxidant is utilized to completely oxidize the solute metal in the alloy. however. if excessive oxidant is utilized. the resulting internally oxidized metal powder may then be reduced with hydrogen at temperatures of about l500F for time sufficient to reduce residual oxygen.
The following illustrative examples are included to further explain the invention and are not intended to be limiting All parts are by weight and all temperatures are in degrees Fahrenheit. unless otherwise stated.
EXAMPLE 1 Part A Preparation of the Alloy Powder Electrolytic tough-pitch grade copper rods are melted in an inert refractory crucible in an inductionheating furnace under reducing conditions at about 2300F. Metallic aluminum shavings are introduced into the molten copper in the proportion of 0.33% by weight of the resulting molten metallic mass.
The molten solution of aluminum in copper is then superheated to 2400F. atomized through an atomizing aperture in a jet of nitrogen (alternatively other inert gases or water or steam can be used as the atomizing fluid) to yield an atomized copper-aluminum alloy powder which substantially all passes a lOO-mesh. U.S. Sieve indicating that the average particle size is less than about I49 microns.
The atomized and screened alloy powder is annealed at a temperature of about l600F for about an hour in an argon atmosphere to recrystallize and yield a grain size in the recrystallized alloy powder of at least about ASTM Grain Size 6 according to ASTM Test E-l12. Preferably. the grains are as large as possible to minimize grain boundary area in the powder. The alloy powder is then ready for use in combination with the oxidant.
Part B Preparation of the Oxidant One hundred parts of commercially available cuprous oxide (Cu O) with an average particle size of about 1 to 2 microns are mixed with 4.l parts of an 7 aqueous solution of A1(NO,-;);, 9H O to form a slurry of cuprous oxide in aluminum nitrate solution. The solution of aluminum nitrate is slurried with cuprous oxide particles. and the stirring is continued with mild heating at 200F until the water has evaporated and the mixture is almost dry. The mixture is then heated at a temperature of about SOO F for hour to decompose the aluminum nitrate into aluminum oxide. The resulting agglomerate is then ground to form fine oxidant powder which passes a 325-mesh sieve. The resulting oxidant powder comprises 77.4371 Cu- O. 22.01% CuO, and 0.56% M 0 by weight.
Part C Preparation of the Internally Oxidizable Alloy Powder-oxidant Mixture The alloy powder of Part A is thoroughly mixed with the oxidant powder of Part B in the proportion of 2.12 parts of oxidant to I00 parts of alloy powder. The mixing is accomplished in a ball-mill. although a conventional V-cone blending device can alternatively be used.
Part D Internal Oxidation of the Alloy Powder The alloy powder-oxidant mixture of Part C is then charged to an internal oxidation vessel which is then sealed. The oxidation vessel is copper or copper-lined steel to avoid contamination of the alloy powder-oxide mixture during oxidation.
The alloy powder-oxidant mixture is then brought to a temperature of about l750F and maintained at this temperature for about 30 minutes to effectuate internal oxidation of the alloy powder. Alternatively, the internal oxidation can be carried out on a continuous basis using a continuous belt furnace maintained under an inert atmosphere.
At the end of the 30-minute internal oxidation period. substantially all of the aluminum in the alloy powder has been oxidized to M 0 and substantially all of the cuprous oxide in the oxidant has been reduced to metallic copper. The particles of internally oxidized alloy comprise 99.37% by weight of copper plus minor amounts of impurities and 0.63% by weight of M 0 The oxidant residue comprises 99.3771 copper particles and 0.63% A1 0 particles. The overall internally oxidized metal powder composition comprises 98.2 I'/( internally oxidized alloy powder and 1.79% oxidant residue.
Part E Reduction of the Internally Oxidized Metal Powder The internally oxidized metal powder of Part D is then placed in a reducing atmosphere of hydrogen at a temperature of about l500F for one hour to reduce any residual copper oxide.
Part F Thermal Coalescence or Consolidation of the internally Oxidized Metal Powder The internally oxidized and reduced metal powder of Part E is then changed under an inert argon atmosphere to a thin-walled copper can having a diameter of about 7 inches and equipped with a feed tube. The can and its contents are heated to about l700F and the feed tube sealed. Alternatively instead of using the inert gas atmosphere. the feed tube is attached to a vacuum pump; and the can is evacuated while the temperature of the can is brought to l700F to remove any occluded gas from the powder. After evacuation at a pressure of l X l0 mm of Hg for minutes at 1700F. the feed tube is sealed and disconnected from the vacuum pump.
The sealed can is then placed in a ram-type extrusion press and is extruded to form extrudate in the shape of cylindrical bar stock having a diameter of about L25 inches. This corresponds to an extrusion ratio of about 3l:l (i.e., the ratio of the cross-sectional area of the can to the ratio of the cross-sectional area of the extrudate).
The bar stock comprises about 99.37% copper having dispersed throughout 0.637: (or about I 5% by volume) of M 0 particles and has a density of about 99.3% of the theoretical density. The bar stock has an electrical conductivity of 88% lACS". a tensile strength of about 72.000 psi, an elongation of 19% using ASTM Test E-8 (for a test specimen 0. lo inch diameter and 0.65 inch gage length) and a Rockwell hardness of about 75 units on the 8 scale. All property measurements reported in the example are conducted at room ized powder are each blended with stoichiometric amounts of oxidant consisting of an intimate mixture of submicron Cu O and N and internally oxidized, as indicated in Example l. at 1750F in argon for about temperature. The bar stock is substantially uniform and 5 minutes. Both the non-treated and the recrystallized does not P055655 Compositional l mixtures are then reduced at l500F for one hour in mally result when the spent oxidant is present in the hydrogen to remove any excess Oxygen. The reduced dlspel'slon'slrengtheged Workplecepowder mixtures are then cold compacted in a rectan- *lntcrnational Annealed oppcr Standard A copper wire I mctcr long weighing 1 gram. having a resistance 01015328 ohms. at 20C has X0 glfldr Cm X 1 cm i a Compacting pressure 01:40 a conductivity of 100% lACS (Kirk-Othmcr: Encyclopedia ol'Chcmi- (SI [0 yield a compact with l cm square cross section, Tiglgrolgg iagcycond Edition. Volume Vl. lnterscicncc Publishers. Th compact i h f d to 993% d i d h The bar stock is suitable for use as is. or it can be cold Cold forged to give about r educuon in area vick' worked by swaging, forging. rolling. wire drawing. cold i S f ii are h if i iglz ff extrusion or cold drawing to form workpieces having H t en moedwre a I l f treatment at [500 F for an hour in argon. The Vicker s particular tensile strengths according to conventional Cold working techniques (DPH) Diamond Pyramid Hardnesses are measured in insumce when the bar Stock is reduced to 50% kilograms per mm at a 15 gram load and the results in cross-sectional area by coldswaging. the tensile thereof are mdlcmed Table 1 belowstrength is 80.000 psi, the elongation is 1371, and Rock- 2 Tflbl 1 Shows that higher mhel'em hardnesses well B hardness is 84 units and conductivity is 86% obtained when the atomized alloy powder is recrystallACS. lized rior to internal oxidation concurrent with an imp This swaged material with a Rockwell B hardness of proved resistance to softening upon further heating to 84 units and prepared by the procedure of Example I l d temperatures.
TABLE 1 Vickers Hardness Grain Size Vicker's Hardness DPH (annealed l hr. at Mesh (average DPH (as forged) 1S00F) kglmm at Sample No. Fraction Processing diameter) kg/mm at 15g. load 15 g. load 2(a) -80+325 with grain 0.045 mm l49 kg/mm I39 kg/mm growth 2(b) 80+325 without 0.00397 mm 122 kg/mm 103 kglmm grain growth 2(c) --325 with grain 0.045 mm 153 ltg/mm l53 ltg/mm growth 2(d) 325 without 0.00397 mm [50 kg/mm l32 kg/mm grain growth is annealed along with a commercial copper-chromium EXAMPLE 3 F 1 I t alloy (0.9/r Cr) at various temperatures for one hour 40 An alloy of Copper huvmg 0.70% aluminum ls in argon. Improved hardness values are obtained by anpared in a manner set forth in Emm e l an A) one nealing for one hour at the various temperatures rangf p ing from 100F to 1500F. In another experiment. these Sample 0 duo) pc'wdfir procebbed m a same two materials are annealed together at l000F in nersctlorth i Example 1 mcludmg the step of recrys' argon Samples are removed from the annealing tallization to increase the grain size prior to internal oxnace at various time intervals. cooled to room temperaf A seconfl Sample (b) Processed slmllarly but ture. and tested for hardness. The test results show su- Wllh the exception that [ha gram growth p perior resistance to softening upon heating of the diseluded. Both powd and are d, P persion-strengthened workpiece of this invention. ished. and etched. The non-recrystallized (no grain growth) powder (b) shows a continuous internal oxide EXAMPLE 2 film after internal oxidation. whereas the recrystallized (grain growth) powder (a) does not indicate an oxide A copper alloy similar to Example I and containing m vi k Diamond pyramid Harm-3SS (DPH) weight Percent of aluminum nllrOgellFmm'lZed 5g taken at l5 g. load on the internally oxidized powders l P' 9) powder- The alloy p'owder dmfied (a) and (b) after internal oxidation in a manner set g a?- f 2 fl s g 9a; forth in Example 1 indicate the following hardnesses: and a -3... mes raction. ac ract on is rea e wi Recrystallized powder (a): 89 kg/mm2 a grain growth step and compared with a fraction that u I Non-recrystallized powder (b): 94kg/mminside the did not undergo grain growth. ln the grain growth trac- & I] n 161 k 2 d t h tion. the atomized powder is subjected to recrystallim 1 :1} e g mm outs e or t e zation treatment at l800F for one hour under argon Oxlde I atmosphere prior to internal oxidation. Prior to recrysh Powders and are t n Canned in a 1.25 tallization. the average grain diameter is 0.00397 millimch diameter and 2 Inch long copper containers and meters. whereas after recrystallization the average extruded at l700F into 0.25 inch diameter rods. Rockgrain diameter is 0.045 millimeters. Both the nontreated atomized powder and the recrystallized atomwell hardness. electrical conductivity and ultimate tensile strength are determined and are set forth in Table 2.
TABLE 2 Electrical Sample Rockwell Conductivity Tensile No. Processing Hardness IAC S Strength 3( a) with grain 90 79% 85.000 psi growth 3th) without 83 75% 80,000 psi grain growth EXAMPLE 4 Two fractions of -l mesh copper alloy powder containing 0.70% aluminum are surface oxidized at 450C for about V2 hour to pick up sufficient oxygen for complete oxidation of the 0.70% Al in the alloy powder. One fraction (a) is previously recrystallized for increasing the grain size in the manner set forth in Example 2. but the other fraction (b) is not subjected to a grain growth step. Both fractions (a) and (b) are internally oxidized at l750F for hour and then reduced to remove any excess oxygen. Each fraction is individually canned within a L inch diameter and 2 inch long copper container. preheated. and extruded at about I700F into 0.25 inch diameter rods. Rockwell hardness. electrical conductivity. and ultimate tensile strength on these rods are shown in Table 3.
In accordance with the procedures set forth in Example l. a nickel alloy containing 0.45% aluminum by weight is nitrogen atomized to produce an alloy powder. The alloy powder is divided into two fractions. namely, one fraction recrystallized in a grain growth step. and the other fraction did not undergo grain growth. In the grain growth fraction. the atomized nickel alloy powder is subjected to recrystallization treatment at about l800F for one hour under argon atmosphere to achieve a grain size No. 6. Both the recrystallized fraction and the non-treated fraction of the nickel alloy powder is then mixed with L89 weight parts of pulverant oxidant comprising l.87 parts of nickel dioxide and 0.02 parts of aluminum oxide per I00 weight parts of powder alloy. The nickel alloy and oxidant mixtures are then internally oxidized as indicated in Example I at l750F in argon for about 3 hours. Both fractions are then reduced with hydrogen at I500F for about one hour to remove any excess oxygen. The reduced powder mixtures are then cold compacted and hot forged in the manner set forth in Example 2 and tested. The recrystallized alloy fraction undergoing grain growth to achieve a grain size number of at least 6 indicates substantially improved Vickers hardness. Rockwell hardness. electrical conductivity. and tensile strength when compared to the other nonrecrystallized alloy fraction.
EXAMPLE 6 A silver alloy containing 99.04% silver and 0.48% aluminum is nitrogen atomized in a manner similar to Example I to produce an alloy powder. The alloy powder is then separated into two fractions wherein one fraction is recrystallized with a grain growth step whereas the second fraction is not treated to undergo grain growth. In the grain growth fraction. the atomized alloy powder is subjected to recrystallization treatment at about l500F for about one hour under argon atmosphere to achieve a grain size of No. 6. Both the recrystallization and the non-treated fractions are combined with 6.35 parts of pulverant oxidant comprising 6.24 parts of silver oxide and 0.1 1 parts of aluminum oxide. Both fractions are then internally oxidized at 1200F in argon for about I hour in the manner indicated in Example Each fraction is reduced. compacted. and hot forged as indicated in Example 2. The recrystallized fraction exhibits substantially improved Vickers hardness. Rockwell hardness. electrical conductivity. and tensile strength when compared to the non-treated fraction.
The foregoing examples indicate that recrystallizing and increasing the grain size of the alloy powder prior to internal oxidation substantially improves the physical properties of dispersion strengthened metal products. The examples are not intended to be limiting to the scope of this invention as defined in the following claims.
I claim:
1. An improved powdered alloy suitable for dispersion strengthening by internal oxidation. comprising:
an alloy comprising a relatively noble matrix metal having a negative free energy of oxide formation at 25C of up to kilocalories per gram atom of oxygen and a solute metal having a negative free energy of oxide formation exceeding the free energy of oxide formation of said noble matrix metal by at least about 60 kilocalories per gram atom of oxygen at 25C; and
said powdered alloy being recrystallized and having 21 Grain Size of at least Number 6 as measured by ASTM E-l l2.

Claims (1)

1. AN IMPROVED POWDERD ALLOY SUITABLE FOR DISPERSION STRENGTHENING BY INTERNAL OXIDATION, COMPRISING: AN ALLOY COMPRISING A RELATIVELY NOBLE MATRIX METAL HAVING A NEGATIVE FREE ENERGY OF OXIDE FORMATION AT 25*C OF UP TO 70 KILOCALORIES PER GRAM ATOM OF OXYGEN AND A SOLUTE METAL HAVING A NEGATIVE FREE ENERGY OF OXIDE FORMATION EXCEEDING THE FREE ENERGY OF OXIDE FORMATION NOBLE MATRIX METAL BY AT LEAST ABOUT 60 KILOCALORIES PER GRAM ATOM OF OXYGEN AT 25*C, AND SAID POWDERED ALLOY BEING RECRYSTALLIZED AND HAVING A GRAIN SIZE OF AT LEAST BUMBER 6 AS MEASURED BY ASTME - 112.
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US4077816A (en) * 1973-07-30 1978-03-07 Scm Corporation Dispersion-strengthened metals
US4288024A (en) * 1977-10-25 1981-09-08 The Nippert Company Method for making a bimetal resistance welding electrode
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US6225591B1 (en) 1997-11-20 2001-05-01 The Nippert Company Resistance welding electrode and process for making

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