WO1991019820A1

WO1991019820A1 - HIGH STRENGTH-HIGH CONDUCTIVITY Cu-Cr COMPOSITES PRODUCED BY SOLIDIFICATION/MECHANICAL REDUCTION

Info

Publication number: WO1991019820A1
Application number: PCT/US1991/003987
Authority: WO
Inventors: Edwin D. Gibson
Original assignee: Iowa State University Research Foundation, Inc.; Verhoeven, John, D.; Haupt, Gary, R.
Priority date: 1990-06-12
Filing date: 1991-06-06
Publication date: 1991-12-26

Abstract

A Cu-Cr melt is solidified to form a two phase, cast body (e.g., an ingot) and the body is mechanically reduced to form an 'in-situ' composite. The body in the solidified condition and/or the deformed condition is subjected to temperature and time at temperature conditions effective to precipitate dissolved Cr from the Cu matrix to improve the electrical conductivity of the composite.

Description

HIGH STRENGTH-HIGH CONDUCTIVITY Cu-Cr COMPOSITES PRODUCED BY SOLIDIFICATION/MECHANICAL REDUCTION

Contractual Origin Of The Invention

The United States Government has rights in this invention pursuant to Contract No. W-7 05-ENG-82 between the U.S. Department of Energy and Iowa State University, Ames, Iowa, which contract grants to the Iowa State University Research Foundation, Inc. the right to apply for this patent.

Field of the Invention

The present invention is a Continuation-In- Part of U.S. Serial No. 536,706 and relates to high strength, high conductivity Cu-Cr composites and to a method of making such composites.

Backgrond of the invention

Copper has many important uses in the form of wire, sheet, etc. , as a result of its desirable electrical and heat conducting properties. However, pure copper has relatively weak tensile strength. Prior art workers have sought to overcome this deficiency by forming what has been referred to as an "in-situ" composite of Cu and X where X is Nb, Ta, or other refractory metal. The composite so formed includes elongated strengthening filaments of the X constituent in a Cu matrix constituent.

"In-situ" Cu-X composites have been produced by a solidification/reduction process wherein a Cu-X melt is cast in a manner to form a two phase body (e.g., a two phase ingot having X dendrites dispersed in a Cu matrix) and the body is mechanically reduced (e.g., by swaging, rolling, drawing, etc.) to produce the composite. The composites produced in this way are quite ductile and can be mechanically reduced to very large strains without breakage. The mechanical reduction operation converts the X (refractory metal) dendrites into strengthening filaments which serve to reinforce and greatly increase the strength of the composite.

Prior art workers have been successful in making Cu-Nb, Cu-Ta and other Cu-X composites using the solidification/reduction process as a result of the quite low solubility of the refractory metal in Cu in the solid state. However, the present inventors have discovered that the solidification/ reduction process is not straightforwardly applicable to the production of "in-situ" Cu-Cr composites and, as heretofore practiced, does not provide a viable method for making high strength, high conductivity Cu-Cr composites. In particular, the present inventors have found that solidification/reduction processed "in-situ" Cu-Cr composites exhibit electrical conductivities on the order of only 50% of those exhibited by solidification/ reduction processed "in-situ" Cu-Nb and Cu-Ta composites having the same volume fraction Cu matrix.

Since Cu-Cr composites will enjoy a significant cost advantage over the corresponding Cu-X composites as a result of the lower cost of Cr compared to other refractory metals, there is a continuing desire to provide "in-situ" high strength, high conductivity Cu-Cr composites and a viable process for making such composites. The present invention has as an object to satisfy this desire.

r?Wrø TΥ ^«■>* ----- Invention

The present invention provides an improved process for making an "in-situ" high strength, high conductivity Cu-Cr composite using the solidification/ reduction process wherein a two phase, cast body (e.g., two phase ingot) is solidified and then mechanically deformed. The present invention relates to the discovery that the solidified, two phase body comprises a Cu matrix having not only the desired Cr dendrites dispersed therein but also having Cr dissolved in the matrix to a harmful level that significantly adversely affects the electrical conductivity of the composite. In particular, the present invention involves in the context of one embodiment of a solidification/reduction process for making an "in-situ" composite subjecting the two phase body to temperature and time at temperature conditions to precipitate sufficient dissolved Cr from the Cu matrix to substantially improve the electrical conductivity of the composite.

The present invention also involves in the context of another embodiment of a solidification/ reduction process for making an "in-situ" composite subjecting the two phase body in at least one of the solidified condition or the deformed condition to a heat treatment at a solutioning temperature above 900'C followed by appropriate cooling so that matrix ductility is increased to permit increased deformation of the ώody in the mechanical deformation stage of the process sequence, thereby allowing attainment of increased strength levels in the composite.

The present invention also involves in the context of still another embodiment of a solidification/reduction process for making an "in- situ" composite subjecting the solution heat treated body, before the deformation stage, to a low temperature heat treatment at a temperature lower than the solutioning temperature to precipitate dissolved Cr from the Cu matrix in a manner that increases the strength of the Cu matrix so as to enhance development of the elongated Cr filaments during the subsequent deformation stage.

The present invention also involves in the context of still another embodiment of a solidification reduction process subjecting the solution heat treated body, near the end of or after the deformation stage, to a low temperature heat treatment at a temperature lower than the solutioning temperature to age harden the Cu matrix in a mariner to complement the strength attributable to the Cr strengthening filaments so as to increase the overall strength of the composite.

In accordance with the method of the invention, an "in-situ" Cu-Cr composite is formed by solidifying a melt of Cu and Cr to form a two phase, cast body (e.g., a two phase ingot) comprising a Cu matrix and Cr dendrites (dendritic particles) randomly dispersed throughout the Cu matrix. The two phase body preferably comprises about 3 to about 40 volume % Cr dendrites, even more preferably about 5 to about 18 volume % Cr dendrites, and the balance the Cu matrix. The two phase body is then deformed (e.g. , mechanically reduced by swaging, drawing, etc.) to impart a filamentary shape to the Cr dendrites for composite strengthening purposes.

At some stage of the method of the invention, the two phase Cu-Cr body is subjected to temperature and time at temperature conditions e fective to precipitate sufficient dissolved Cr from the Cu matrix to improve the electrical conductivity of the composite. Preferably, sufficient Cr is precipitated to leave about 0.02 weight % or less Cr dissolved in solid solution in the Cu matrix. The two phase body may be subjected to the temperature and time at temperature (i.e., Cr precipitation conditions) in .conjunction with the solidification step or the deformation step or, alternately, as a separate heat treatment step before or after the deformation step. Moreover, the two phase body may be subjected to the Cr precipitation conditions at one or more times during the solidification/reduction process. For example, the two phase body may be heat treated in the solidified condition to precipitate Cr particulates, deformed to a selected extent, heat treated in the deformed condition to again precipitate Cr particulates and then further deformed to develop the desired composite strength.

In accordance with another embodiment of the invention, an "in-situ" Cu-Cr composite is formed in the manner described hereinabove by solidifying a melt of Cu and Cr to form a two phase body having Cr dendrites (dendrite particles) dispersed throughout the Cu matrix. The two phase body is deformed to impart a filamentary shape to the Cr dendrites for composite strengthening purposes. The two phase body is subjected in at least one of the solidified condition or the deformed condition to a heat treatment at a solutioning temperature above about 900^βC followed by appropriate cooling to improve matrix ductility and permit attainment of greater deformation levels during the deformation stage so as to increase composite strength levels achievable. The solution heat treated body is subjected to another heat treatment at a temperature lower than the solutioning temperature before or after the ~ deformation stage.

In a specific embodiment, the two phase body is subjected to the solution heat treatment in the solidified condition and the lower temperature heat treatment is carried out after the solution treated body is deformed. Alternately, the body is subjected to the lower temperature heat treatment immediately after the solution heat treatment and before the deformation step.

In another specific embodiment, the two phase body is subjected to the solution heat treatment after both the solidification and the deformation stages. The body is then subjected to the low temperature heat treatment after the solution heat treatment followed by one or more optional deformation steps.

In the above described embodiments, the low temperature heat treatment is conducted at a temperature of about 350^βC to about 500^βC or. alternately, at a higher temperature in excess of 500^βC, preferably from about 600°C to about 750°C.

The improved "in-situ" Cu-Cr composites of the present invention include a Cu matrix having elongated Cr strengthening filaments and having Cr in solid solution in the Cu matrix reduced to such a level (e.g., 0.02 weight % or less of the Cu matrix) that the composite exhibits electrical conductivity generally equivalent to the more expensive "in-situ" Cu-X (refractory metal) composites at a given ultimate strength level. In one embodiment of the invention, the Cu-Cr composite exhibits an electrical conductivity in the range of about 60% to about 75%

IλCS (International Annealed Copper Standard) with an UTS (Ultimate Tensile Strength) in the range of about 900 to about 1100 MPa. In another embodiment of the invention, Cu-Cr composite exhibits an electrical conductivity in the range of about 75% to 85% IACS with an UTS in the range of about 750 to about 900 MPa.

Description of the Drawings

Figure 1 is a flow sheet illustrating sequential method routes or steps for forming an "in-situ" Cu-Cr composite in accordance with the invention.

Figure 2 is a schematic elevational view of a cast ingot from which disk specimens and resistivity specimens are cut as shown.

Figure 3 is a graph of resistivity versus temperature for as-cast Cu-15 volume % Cr specimens with and without a Cr precipitation heat treatment.

Figure 4 is a graph of resistivity versus temperature for as-cast Cu-15 volume % Cr specimens (not heat treated) with and without mechanical deformation.

Figure 5 is a graph of resistivity versus temperature for as-cast/mechanically reduced Cu-15 volume % Cr specimens with and without a Cr precipitation heat treatment prior to deformation.

Figure 6A is a photomicrograph at 250X of a longitudinal section of a Cu-Cr composite prepared in ae.πmrήar.r_ts wi* h xamnle 2.

Figure 6B is a photomicrograph at 1000X of a longitudinal section of the Cu-Cr composite prepared in accordance with Example 2.

Figure 7 is another flow sheet illustrating sequential method routes or steps for forming an "in- situ" composite in accordance with another embodiment of the invention.

Figure 8 is a graph of strength versus conductivity for Cu-15 volume % Cr specimens of the invention that have been subjected to a high temperature solution heat treatment and various lower temperature age hardening heat treatments.

Figure 9 is a graph of strength versus conductivity for Cu-12.5 volume % Cr and Cu-15 volume % Cr specimens of the invention that have been subjected to heat treatments in accordance with different embodiments of the invention.

Figure 10 is a graph of strength versus conductivity for Cu-7 volume % Cr specimens of the invention that have been subjected to heat treatments in accordance with different embodiments of the invention.

Detailed Description of the Invention

Referring to Figure 1, the various routes or steps involved in practicing one embodiment of the method of the invention are illustrated. In this embodiment, a melt of Cu and Cr is initially prepared and solidified in a manner to form a two phase, cast body (e.g., a two phase ingot) comprising a Cu matrix and Cr dendrite particles dispersed randomly throughout the matrix. As will be discussed in more detail below, the Cu matrix also includes Cr dissolved in solid solution therein to a level that adversely affects the electrical conductivity of the composite.

The melt can be formed and cast in a variety of ways, such as conventional crucible melting of Cu particulates and Cr particulates in desired proportions and casting the melt into chill (cooled) molds (e.g., water cooled copper molds), uncooled molds (e.g., sand molds) or preheated molds. In practicing the invention, it is preferred to cast the Cu-Cr melt into chill molds to obtain a two phase, cast body (e.g., ingot) having fine, as-cast dendrites (dendritic particles) randomly dispersed throughout the Cu matrix and having a dendrite particle size in the range of about 2 to about 6 microns.

As an alternative to conventional crucible melting, the Cu-Cr melt may be formed by a consumable arc melting technique as described in Verhoeven et al. U.S. Patent 4,481,030. That process involves preparing a consumable electrode which is a mixture of Cu plus Cr consolidated by some suitable technique. The electrode is subjected to direct current arc aelting in an enclosed chamber containing an inert gas (e.g. , argon) at a gas pressure of about 650 mm Hg. The inert gas pressure should be sufficient to suppress boiling of liquid Cu at the liquidus temperature of the melt being produced. The invention also envisions use of other melting techniques such as consumable plasma melting, Vader melting process, vacuum induction melting and other conventional melting processes wherein the melt is formed and cast from a non-contaminating crucible. Although the invention is not limited to any particular melting and casting process, certain specific melting and casting processes are described in the Examples set forth below.

The two phase, as-cast body (ingot) generally comprises about 3 to about 40 volume % Cr dendritic particles and the balance the Cu matrix. Preferably, the two phase body comprises about 5 to about 18 volume % Cr dendritic particles. The original melt composition is selected to this end as those skilled in the art will appreciate.

As a result of the solubility of Cr in the Cu in the melt, the Cu matrix of the two phase, as-cast body will include Cr dissolved in solid solution therein in an amount typically of about 0.1 to about 0.6 weight % of the Cu matrix. This amount of dissolved Cr in the matrix is quite harmful to the electrical conductivity of the composite (i.e., it significantly decreases the electrical conductivity) . In accordance with the invention, the amount of dissolved Cr in the Cu matrix is reduced at some time during the process to about 0.02 weight % or less of the Cu matrix to substantially improve the electrical conductivity of the composite as will become apparent.

Referring to Figure 1, the two phase, cast body (ingot) is subjected to a mechanical deformation (reduction) operation to form an "in-situ Cu-Cr composite comprising a codeformed Cu matrix and elongated, ribbon-shaped Cr strengthening filaments dispersed in the matrix. As mentioned above, the Cr filaments increase the strength of the composite as compared to that of pure Cu. A large percentage reduction in area is used to form the "in-situ" Cu-Cr composite to a desired configuration, such as wire, rod, sheet and the like.

The mechanical reduction operation can be effected in one or more stages as will become apparent below. Typically, the reduction in area is described in terms of the parameter η that is equal to the natural logarithm of the ratio of the cross-sectional area of the body before reduction (A₀) to the cross-sectional area of the body after reduction (A_f) , i.e. ln A^A_f). In general, the InfA_g/A_f) is at least about 2-3, preferably about 6-8.

The mechanical reduction process can be carried out using known mechanical size reduction processes, such as swaging, rolling, forging, drawing and like processes (as well as combinations thereof) . These reduction processes can be carried out at room temperature or elevated temperature as will become apparent below. Although the invention is not limited to any particular mechanical reduction process, certain specific reduction processes are described in the Examples set forth below.

As mentioned above, in accordance with the invention, the dissolved Cr concentration of the Cu matrix is reduced at some time during the method of the invention. In particular, the two phase body (e.g., the two phase ingot) is subjected at one or more stages of the process to temperature and time at temperature conditions effective to precipitate sufficient dissolved Cr from the Cu matrix as to improve the electrical conductivity of the composite. Preferably, the concentration of dissolved Cr in the Cu matrix is reduced to about 0.02 weight % or less to this end.

The two phase body can be subjected to the temperature and time at temperature conditions to precipitate dissolved Cr (i.e., Cr precipitation conditions) in the solidified and/or the deformed condition of the body. For example, the two phase body can be subjected to the Cr precipitation conditions in the solidified condition of the body during the solidification step (i.e., during cool down) or in a separate heat treatment step following the solidification step and prior to the deformation step. The two phase body can be subjected to the Cr precipitation conditions in the deformed condition during the deformation step or in a separate heat treatment step following the deformation step.

In particular, in one embodiment of the invention, the two phase body is subjected to the Cr precipitation conditions during the solidification step by solidifying the Cu-Cr melt in a suitable mold and cooling the solidified melt at a sufficiently slow rate to precipitate dissolved Cr from the as-cast matrix. However, as a result of the relatively slow cooling rate from the casting temperature (e.g., l-5^βC/minute) , the Cr dendrites precipitated in the Cu matrix will exhibit a relatively larger dendrite particle size (e.g., 10 micron diameter) which is less desirable than the dendrite size produced by chill casting (e.g., 4 micron diameter) from a composite strength standpoint.

In another embodiment of the invention, the two phase body is subjected to the Cr precipitation conditions after the solidification step and before the deformation step by subjecting the as-cast body to a separate heat treatment step at suitable temperature and time at temperature conditions to precipitate dissolved Cr from the as-cast Cu matrix. Typically, the heat treatment is conducted at a temperature of at least about 350*C, preferably at least about iσø^Λ€-aπ more preferably from about 600*C to about 750"C for a time of at least about 60 minutes. The time at temperature will depend on the temperature employed

(e.g. , higher temperatures will require shorter times at temperature) as well as mass of the cast body. After holding at temperature, the heat treated body is cooled to room temperature at a slow cooling rate of 60'C/hour or less; for example, at a cooling rate of about 30^βC/hour to 60^βC/hour to complete the heat treatment step and provide sufficient precipitation of dissolved Cr from the Cu matrix.

In a preferred embodiment, the as-cast body is held at a temperature of about 750"C for 1 hour followed by furnace cooling to room temperature (for a time period of generally 24 hours) . If the body is cast in a chill mold, the cast body will initially be heated from room temperature to the temperature of 750"C. On the other hand, if the body is cast in a non-cooled (sand) or preheated mold, the mold can be stripped from the cast body after it has cooled to a temperature of about 750'C and the as-cast body is then subjected to the heat treatment step described above to precipitate the dissolved Cr from the matrix.

In still another embodiment of the invention, the two phase body is subjected to the Cr precipitation conditions during the deformation step by deforming the as-cast body at a suitable elevated temperature. For example, the as-cast body may be warm or hot extruded, rolled, forged, drawn, swaged or otherwise deformed under conditions of temperature and time at temperature to precipitate the dissolved Cr from the Cu matrix to the extent desired. Generally, deformation at a body temperature of at least about 350^βC is employed to this end. Warm deformation of the body is typically conducted at about 400^βC to about 500"C. Hot deformation of the body is typically carried out at about 500"C to 800*C. After warm or hot deformation, it is desirable to slowly cool the deformed body to room temperature to insure complete precipitation of dissolved Cr from the Cu matrix. If the body is cast in a non-cooled or preheated mold, the mold can be stripped from the body when its temperature reaches a suitable temperature for deformation and the body can be immediately deformed with or without supplemental heating thereof.

As will be explained below, the warm and/or hot working (deformation) of the as-cast body may be used in combination with a subsequent cold (room temperature) working step to develop the desired strength level for the composite. Various cold (room temperature) deformation processes may be employed and include cold extrusion, rolling, drawing as well as other cold working processes. In still another embodiment of the invention, the two phase body is subjected to the Cr precipitation conditions after the deformation step by subjecting the deformed body to a separate heat treatment step under suitable temperature and time at temperature conditions to precipitate the dissolved Cr from the deformed Cu matrix. The temperature/time at temperature/slow cooling will correspond to those set forth above for the heat treatment step conducted after the solidification step and before the deformation step. However, the Cr precipitation conditions are selected in dependence upon the degree of previous deformation (i.e., Cr filament size) so as to avoid substantial Cr filament coarsening that could reduce the composite strength for a given deformation (reduction) . As will become apparent below, heat treatment of the body after deformation results in enhanced precipitation of the dissolved Cr from the matrix and resultant enhanced electrical conductivity of the composite.

In practicing the method of the invention, the Cr precipitation treatment may be conducted in different sequences/combinations with the solidification/reduction steps and may be conducted more than once during the process. For example, the as-cast body may be heat treated after solidification in a separate heat treatment step under Cr precipitation conditions, deformed (cold, warm or hot deformed) to certain extent, heat treated again under Cr precipitation conditions and then deformed (cold, warm or hot deformed) to achieve a desired composite strength. Alternately, the as-cast body could be initially deformed, heat treated (under Cr precipitation conditions) after deformation, deformed again, heat treated again and then finally deformed to provide a desired composite strength. Various processing step sequences/combinations are illustrated in the Examples set forth below. The invention, however, is not limited to any particular sequence/ combination of steps.

As a result of the aforementioned Cr precipitation treatments, the Cr dissolved in solid solution in the Cu matrix is reduced to substantially improve the electrical conductivity of the composite. The as-cast Cr dendrites and/or deformed Cr filaments present in the Cu matrix are believed to facilitate or foster precipitation of Cr from the Cu matrix at the as-cast Cr dendrites/deformed Cr filament interfaces during the Cr precipitation treatments described hereinabove.

The following Examples are offered to illustrate features of the invention in further detail without limiting the scope thereof.

EXAMPLE 1

An ingot of Cu-15 volume % Cr was prepared by melting Cu particulates and Cr particulates in suitable proportions in a A1₂0₃ crucible and then pouring the Cu-Cr melt into a chill mold (e.g., a cylindrical copper mold) . The melt solidified in about one minute as a cylindrical ingot having a diameter of about 2 inches and a length of about 8 inches. The ingot is represented schematically in Figure 2.

Disks D-l and D-2 were cut from the ingot near the bottom and the top of the ingot. The top disk was then severed to produce specimen strips S-1A and S-2U as shown in Fig. 2. Strip S-1A was heat treated at 750 'C for 24 hours then slow cooled (furnace cooled for about 24 hours) to room temperature. The strip S-2U was not annealed and instead was left in the as-cast condition. The resistivity versus temperature for these specimens is shown in Figure 3. The variation of resistivity with temperature was developed by progressively heating the specimen to increasing temperatures (heating rate=l'C/min) , measuring the resistivity at close intervals during heating using a standard four probe DC technique (e.g., see J. Applied Phys. ___H (1989) p. 1293) and, after heating to 810*C, slowly cooling (e.g., furnace cooling) to room temperature. Resistivity is also measured at close intervals as the specimen cools. The arrow (Figs. 3-4) pointing up indicates resistivity measurements made on the heating cycle, and the arrow pointing down indicates measurements made on the cooling cycle.

As is apparent, the 750'C/slow cool treatment produced a considerable reduction in the resistivity of the as-cast material as a result of the precipitation of Cr from solid solution in the as-cast copper matrix. Included on the lower right of Figure 3 is the electrical conductivity of the two specimens measured at room temperature before and after the resistivity test. The electrical conductivity is obtained by dividing 1.72 by the resistivity having units of microhm-cm and multiplying by 100. The value for pure Cu is approximately 101% IACS' (International Annealed Copper Standard) .

The room temperature conductivity of the unannealed as-cast specimen S-2U increased from 47.4% to 75.7% as a result of the heating to 810"C/slow cooling to room temperature of the resistivity test. The specimen S-1A annealed at 750'C prior to the resistivity test did not change its room temperature conductivity significantly as a result of heating to 810'C and cooling to room temperature during the test. This behavior indicates that the initial 750°C/slow cool Cr precipitation treatment already caused the Cr to precipitate from the Cu and improve the conductivity of the composite.

This Example is offered to illustrate the effectiveness of the 750'C/slow cool Cr precipitation treatment in increasing electrical conductivity of the composite. In order to isolate the effect of heat treatment on the electrical conductivity, the specimens were not mechanically reduced. The effect of mechanical reduction is illustrated below. In accordance with the invention, the as-cast, two phase body is subjected to the Cr precipitation conditions and mechanical reduction operation during the process to impart both high strength and high conductivity to the composite.

EXAMPLE 2

A strip S-3U cut from the bottom disk D-2 of Figure 2 was mechanically reduced (by room temperature swaging and drawing) in the as-cast condition (i.e., without heat treatment) to an η of 3.3. The resistivity versus temperature of this specimen S-3U and of specimen S-2U is set forth in Fig. 4. Included in Fig. 4 is the change in the room temperature conductivity for these specimens measured before and after the resistivity test. It is apparent that the mechanical working of specimen S-3U has caused the conductivity after the 810'C anneal (during the resistivity test) to increase to 82.6% versus

75.7% for the specimen S-2U. These results show that the mechanical working is apparently causing the amount of Cr which is precipitating from the Cu matrix to increase. The microstructure of the Cu-Cr composite so produced is shown in Fig. 6A-6B. EXAMPLE 3

Referring to Fig. 2, the as-cast ingot was cut into sections H-l and H-2. The ingot section H-l was given the 750"C anneal described above, but the ingot section H-2 was not. Heat treatment of ingot section H-l constitutes step (2) of this exemplary process after ingot solidification which comprises step (l) .

These sections H-l,H-2 were mechanically reduced (by room temperature rod rolling and swaging) to rod having a diameter of 0.3 inch and then by room temperature drawing to wire having a diameter of 0.098 inches. This corresponds to an η of 6. This mechanical reduction constitutes step (3) of this exemplary process.

The resistivity and ultimate tensile strength of both ingot sections H-l and H-2 were measured. Fig. 5 compares the resistivity versus temperature curves for specimens from the two sections H-l,H-2. The resistivity treatment (heat to 810*C/slow cool to room temperature) is seen to produce a dramatic reduction in the room temperature resistivity of the specimen from ingot section H-2 after it is drawn to an η of 6. This reduction in resistivity indicates that room temperature mechanical reduction by such a large amount does not cause the dissolved Cr to precipitate from solid solution.

Hence, the Cr precipitation treatment is necessary to achieve the lower resistivity and corresponding higher conductivity of 83.4% IACS versus only 49.6% IACS for the specimen from ingot section H-2.

After mechanical reduction (before resistivity testing) , the specimen formed from ingot section H-l (annealed) exhibited a conductivity of 83.4% IACS and an ultimate tensile strength (UTS) of 567 MPa versus a conductivity of 49.6% IACS and UTS of 589 MPa for the specimen formed from the unannealed ingot section H-2. These results illustrate the importance of the annealing heat treatment in producing optimum values of both a high strength and high conductivity.

EXAMPLE 4

The same processing steps are used as set forth in example 3 except the heat treatment (Cr precipitation treatment) is conducted after the cast ingot has been reduced to a diameter of 0.3 inches. This processing sequence will produce the same or better electrical conductivity for the composite with no loss in composite strength.

EXAMPLE 5

The same processing steps (1) and (3) are used as set forth in example 3 except that, in step (1) , the solidified ingot is prepared by casting the melt into a non-cooled mold (such as a sand mold) and the cooling rate is slow enough (e.g., l-5*C/minute) to allow precipitation of the Cr on cool down of the solidified melt from the solidification temperature.

Heat treatment step 2 of example 3 is eliminated since the Cr precipitation conditions are provided as the solidified melt cooled to room temperature. However, the strength of the composite will be lower at the same deformation (value of η) because of the larger as-cast Cr dendrite particle size. EXAMPLE 6

The same processing steps are used as set forth in example 3 except the cast ingot is prepared by casting the melt into a non-cooled mold (such as a sand mold). After the ingot has cooled to 750"C, the mold is stripped from the ingot and steps 2 and 3 of example 3 are carried out.

EXAMPLE 7

The same processing steps are used as set forth in example 3 except the cast ingot is prepared by casting the melt into non-cooled molds (such as a sand mold) . After the ingot has cooled to around 750*C, the mold is stripped from the ingot and the ingot is hot deformed (by extrusion) at about 500"C to 750*C to an i) value of around 2 to 3. Deformation step (3) of example 3 is then carried out.

E AMPLE 8

The same processing steps are used as set forth in example 3 except heat treatment step (2) is eliminated and the cast ingot is hot deformed (by extrusion) in the temperature range of about 500"C to 800*C to an η of about 3. The hot deformed ingot is then cold deformed (by swaging and/or drawing) to higher η values of about 8.

EXAMPLE 9

The same processing steps are used as set forth in example 3 except that the deformation step (3) is carried out in the temperature range of about.

350'C to 500'C.

Referring to Figure 7, various routes or steps involved in practicing another embodiment of the invention are illustrated. In these embodiments, a melt of Cu and Cr is prepared and solidified in the manner described hereinabove with respect to the _____ embodiment of Figure 1. The two phase, cast body (ingot) produced is subjected to a mechanical deformation (reduction) operation also in a manner described hereinabove for the embodiment of Figure 1 to form an "in-situ" Cu-Cr composite comprising the aforementioned codeformed Cu matrix and elongated, ribbon-shaped Cr strengthening filaments dispersed in the Cu matrix. This embodiment differs from the embodiment of Figure 1 in that the two phase body in at least one of the solidified condition or the deformed condition is subjected to a high temperature solution heat treatment at a solutioning temperature above about 900'C followed by cooling (e.g., a water quench) . After the solution heat treatment at some time during the subsequent process sequence, the body is also subjected to a low temperature heat treatment which may comprise the Cr precipitation treatment described hereinabove (e.g., from about 600*C to about 750"C) and/or a lower temperature heat treatment (e.g., from about 350*C to about 500*C) to be described below in more detail.

The two phase body is subjected to the solution heat treatment at a temperature and time at temperature to provide the Cu matrix with a near equilibrium amount of dissolved Cr; for example, about 0.8 weight % for the Cu-Cr materials described above. In the solution treated condition, a substantial portion (e.g., 90%) of the Cr dendrites remain in the Cu matrix as a result of the limited solubility of the Cr in the matrix (e.g., about 0.8 weight %) for subsequent deformation processing to fine, elongated, ribbon-shaped strengthening filaments. The body is cooled from the solutioning temperature to ambient temperature sufficiently fast to retain the dissolved Cr in solution in the Cu matrix. A water quench is typically employed to this end. This treatment improves the ductility of the matrix and thereby increases the amount of deformation possible in the mechanical reduction operation so as to increase the strength level attainable in the "in-situ" composite.

As mentioned, the solution treatment can be carried out on the two phase body in the solidified condition or the deformed condition. For example, the two phase body can be subjected to the solution heat treatment in the solidified condition by heating the cast body before any mechanical deformation operation is performed thereon. Alternately, the solution heat treatment can be applied to the two phase body after it is deformed partially in the mechanical reduction operation, such as intermediate successive mechanical reduction steps. Use of the solution heat treatment at some time in the process sequence improves matrix ductility and increases the amount of deformation impartible to the body in the mechanical reduction operation. The two phase body is typically solution heat treated at a solutioning temperature high enough to dissolve an appropriate amount of Cr into the Cu matrix. Preferably, the body is heated at a solutioning temperature above 900*C. For example, for the Cu-Cr materials described hereinabove, the solution heat treatment involves heating from about 1000 to about 1050'C, such as at 1010'C, for about 45 to about 120 minutes. A water quench from the solutioning temperature is used to retain the dissolved Cr in solution in the Cu matrix, although other sufficiently rapid cooling techniques may be used.

Following the solution heat treatment and before or after mechanical deformation operations, the solution heat treated body is subjected to a low temperature heat treatment at a temperature lower than the solutioning temperature to precipitate dissolved Cr from the Cu matrix. The low temperature heat treatment can be conducted before the deformation operation to increase the flow strength of the Cu matrix relative to the retained Cr dendrites to enhance development of elongated, ribbon-shaped Cr filaments during subsequent deformation of the body. For example, strengthening of the Cu matrix forces the retained Cr dendrite particles dispersed in the matrix to deform more readily and thereby develop the desired texture (110) sooner and more uniformly during the deformation operation.

If conducted after the mechanical reduction operation, the low temperature treatment is carried out under controlled temperature and time at temperature conditions to achieve the desired precipitation of dissolved Cr from the Cu matrix while avoiding coarsening of the fine, elongated, ribbon- shaped Cr filaments to the extent that the composite strength attributable to the filaments is adversely affected. The low temperature heat treatment preferably is conducted at a temperature from about 350*C to about 500*C, such as about 450 ' C, for a time of about 30 to about 360 minutes in air or other atmosphere regardless of whether it is conducted before or after the mechanical deformation operation.

The low temperature heat treatment may optionally comprise the Cr precipitation treatment described hereinabove conducted at a slightly higher temperature (e.g., in excess of 500 * C and preferably from about 600'C to about 750^*C). However, the time at this higher temperature should be controlled to minimize coarsening of the fine, elongated, strengthening Cr filaments, if present in the Cu matrix (i.e., if the low temperature treatment is conducted after the mechanical reduction operation) . This Cr precipitation heat treatment should be followed by a further deformation step in most cases in order to increase the strength of the composite attributable to the Cr filaments.

Exemplary process sequences of this embodiment of the method of the invention are illustrated herebelow for purposes of illustration and not limitation. In the sequences listed herebelow, the following designations are used:

1. Solution Treat (Soln Tr) : Heat the sample to a solutioning temperature and hold long enough to dissolve Cr into the Cu matrix, and quench to room temperature.

2. Low Temperature Precipitation Treatment (LTPT) : Heat the sample to a low temperature, on the order of 350 to 500'C, hold a specific time, and cool to room temperature at any rate. 3. Age Harden (Age Hard.): Solution treat, quench and age. This treatment differs from 1 and 2 above only in that the steps of the treatment are conducted without any deformation processing between them.

4. High Temperature Precipitation Treatment (HTPT) : Heat the sample to a high temperature, on the order of 650 to 750*C, hold a specific time, and cool to room temperature very slowly.

Sequences Utilizing Soln Tr and not Age Hard. Solidify* Soln Tr*** Def. Proc.** LEFT*** Def. Prof. -» * Produ Solidify-* Soln Tr* Def. Rroc.*IiEPT * * Produ Solidify* Def. Prcc.*Soln Tr*Def. Proc.** IIEPT * - Produ Solidify* Def. Proc* Soln Tr* Def. Proc* LTPT* Def. Proc.* Produ

Sequences Utilizing Age Hard, and not Soln Tr Solidify* Age Hard.* Def Proc. -» __> ». produ Solidify* Age Hard.* Def. Proc.* TPT -» -, _• produ Solidify* Def. Proc.* Age Hard. _• _• -, produ Solidify* Def. Proc.* Age Hard.* Def. Proc. * * * Produc

Sequences Involving Other Combinations of the Four Thermal Treatments Solidify** HIS-T** Def. Proc.* Soln Tr* Def. Proc.* MPT * Produc Solidify* HUT* Def. Proc.* Age Hard.* Def. Proc. * Produc Solidify* HTFIV Def. Proc.* Age Hard.* Def. Proc* LTPT * Produc Solidify* Age Hard.** Def. Proc.* Soln Tr* Def. Proc.* LTPT * Produc

Sequences of First Embodiment Solidify¹* HUT* Def. Proc. -» _» __, produc Solidify* Def. Proc.* HTPT -» -» _• Produc Solidify* Def. Proc.* HTPT* Def. Proc. * * * Produc The following Examples are offered to illustrate features of the embodiments of the invention in further detail without limiting the scope thereof.

EXAMPLE 11

An ingot of Cu-15% Cr was chill cast in a one inch diameter mold and was HTPT processed at 750^βC/3 hr. and cooled to room temperature in a 48 hr. linear ramp. It was swaged to a diameter of 0.25 inches and then solution treated at 1010*C and water quenched and then drawn to a diameter of 0.028 inches. The strength/conductivity of the wire in this solution treated and deformation processed condition is shown on the upper left of Fig. 8. The wire was then given various LTPT processes as shown in the upper 4 points (open boxes) on Fig. 8. It is seen that the 12 hr/420'C LTPT produced the maximum advantage over the known materials, such as the Ni modified Cu-Be 17510 alloy and the other low Cr/Zr/Mg copper alloys, shown in Fig. 8. At a %IACS of 75, the strength of this sample shows a 60% improvement over conventional materials, with a strength improvement from around 630 MPa to 1010 MPa. Alternately, at a strength level of 1010 MPa, the conductivity was improved over conventional materials from around 54 to 75 %IACS, an improvement of 39%.

The effectiveness of the solution treatment in improving the maximum amount of deformation was also demonstrated in this example. A piece of the original 1 inch casting after the initial HTPT processing was swaged and drawn without the solution treatment. It was found that this material began to breakup in the drawing dies at a diameter of 0.080 inches. With the solution treatment, however, the material was drawn to 0.025 inches before breaking was encountered.

This example is a demonstration of sequence number 9, Solidify_* HTPT* Def. Proc* Soln Tr* Def. Proc* LTPT.

SXAEPLE 12

A one inch ingot of Cu-15% Cr was chill cast, swaged to a diameter of 0.25 inches and was given the HTPT process. It was then drawn to a diameter of 0.100 inches. The strength and conductivity (solid triangle) are shown on Fig. 9. This is an example of a sequence covered in the first embodiment of the invention, sequence 15. It is seen in Fig. 9 that although a significant increase has been obtained relative to the conventional materials, the properties are not quite as good as the material of Example 11.

EXAM LE 13

A one inch ingot of Cu-12.5% Cr was chill cast and swaged to a diameter of 0.25 inch and was given the HTPT process. It was then drawn to a diameter of 0.064 inch. The strength and conductivity (solid box) are shown on Fig. 9. This is also an example of sequence 15 but with the composition changed to 12.5% Cr.

EXAMPLE 14

λ one inch ingot of Cu-12.5% Cr was chill cast and given the HTPT process in the as-cast condition. It was then swaged to a diameter of 0.25 inch followed by drawing to 0.064 inch. The strength/ conductivity (solid diamond) are shown on Fig. 9. This is an example of sequence 13.

EXAMPLE 15

An ingot of Cu-15% Cr was chill cast in a one inch diameter mold and then given the HTPT process. It was swaged to a diameter of 0.25 inch and the age hardened by heating to 1010*C, quenching and aging at 450*C for 5 h. In this age hardened condition it was possible to draw the material to a diameter of 0.040 inch. Again, this is an improvement in the maximum allowable deformation over the as-cast ingot given a HTPT and shows an advantage for achieving higher strength alloys. The wire was tested at a diameter of 0.048 inch and the strength/ conductivity (upper open circle) is shown on Fig. 9 as Example 14 (Seq. 10) . In this condition the sample corresponds to sequence 10.

Another sample was not given a LTPT process of 418*C/12 hr., and the strength/conductivity are shown on Fig. 9 as the point (lower open circle) labeled Example 14 (Seq. 11) . It is an example of sequence 11. EXAMPLE 16

An ingot of Cu-7% Cr was chill cast in a one inch diameter mold and was swaged to a diameter of 0.800 inches, solution treated at 1010'C and water quenched. It was then drawn to a diameter of 0.064 inch and given a LTPT at 414*C/12 hrs. The wire was drawn to a final diameter 0.0179 inch and tested for strength and conductivity. The results (EX 16) are _ shown in Fig. 10.

EXAMPLE 17

An ingot of Cu-7% Cr was chill cast in a one inch diameter mold. It was swaged to a diameter of 0.800 inch, solution treated at 1010*C and water quenched. It was then drawn to a wire of a diameter of 0.025 inch and given a LTPT at 414'C/12 hrs. The wire was drawn to a final diameter 0.0142 inch and tested for strength and conductivity. The results (EX 17) are shown in Fig. 10.

EXAMPLE 18

An ingot of Cu-7% was chill cast in a one inch diameter mold. It was swaged to a diameter of 0.800 inch, solution treated at 1010*C and water quenched. It was then drawn to a wire of a diameter of 0.064 inch and given the HTPT. The wire was drawn to a final diameter of 0.0142 inch and tested for strength and conductivity (EX 18) shown in Fig. 10.

EXAMPLE 19

An ingot of Cu-7% Cr was chill cast in a one inch diameter mold. It was swaged to a diameter of 0.800 inch and age hardened. It was then drawn to a diameter of 0.064 and given the HTPT. The wire was drawn to a final diameter of 0.0142 inch and tested, for strength and conductivity (EX 19) shown in Fig. 10.

Generally, Cu-Cr composites of the present invention include a Cu matrix having elongated Cr strengthening filaments and having Cr in solid solution in the Cu matrix reduced to such a level (e.g., 0.02 weight % or less of the Cu matrix) that the composite exhibits electrical conductivity generally equivalent to the more expensive "in-situ" Cu-X (refractory metal) composites at a given ultimate strength level. In one embodiment of the invention, the Cu-Cr composite exhibits an electrical conductivity in the range of about 60% to about 75% IACS (International Annealed Copper Standard) with an UTS (Ultimate Tensile Strength) in the range of about 900 to about 1100 MPa. In another embodiment of the invention, the Cu-Cr composite exhibits an electrical conductivity in the range of 75% to about 85% IACS with an UTS in the range of about 750 to about 900 MPa. Electrical conductivity and UTS are inversely related for the Cu-Cr composites as is apparent from the Examples set forth above.

While the invention has been described in terms of specific embodiments thereof, it is not intended to be limited thereto but rather only to the extent set forth in the following claims.

Claims

CLAIMS :

1. A method of making a filamentary strengthened Cu-Cr composite, comprising the steps of:

(a) solidifying a melt of Cu and Cr to form a two phase body comprising a Cu matrix and Cr dendrites dispersed throughout the Cu matrix, said matrix having Cr dissolved therein, _ _ . _ _ _.._. . _

(b) deforming the two phase body to impart a filamentary shape to the Cr dendrites, and

(c) subjecting the two phase body in at least one of the solidified condition or the deformed condition to temperature and time at temperature conditions effective to precipitate the dissolved Cr from the Cu matrix to improve the electrical conductivity thereof.

2. The method of claim 1 wherein said body comprises about 3 volume % to about 40 volume % Cr dendrites and the balance the Cu matrix.

3. The method of claim 2 wherein said body comprises about 5 volume % to about 18 volume % Cr dendrites and the balance Cu matrix.

4. The method of claim 1 wherein the two phase body is subjected to said temperature and time at temperature conditions in the solidified condition by solidifying the melt in step (a) and cooling the solidified melt at a sufficiently slow rate to precipitate dissolved Cr from the as-cast Cu matrix.

5. The method of claim 1 wherein the two phase body is subjected to said temperature and time at temperature conditions in the solidified condition by heating the cast body after step (a) and before step (b) to a temperature of at least about 400*C and _ then cooling to room temperature.

6. The method of claim 5 wherein the two phase body is heated to a temperature from about 600"C to about 750*C.

7. The method of claim 1 wherein the two phase body is subjected to said temperature and time at temperature conditions in the deformed condition by deforming the body in step (b) at a temperature of at least about 350"C and then cooling to room temperature.

8. The method of claim 7 wherein the body is deformed at a temperature of about 400^βC to about 500^βC.

9. The method of claim 7 wherein the body is deformed at a temperature of about 500*C to about

800'C.

10. The method of claim 7 wherein the body is deformed to an η of about 2 to about 3 where η is equal to the natural logarithm of the ratio of the original body cross-sectional area to the final body cross-sectional area.

11. The method of claim 7 including the further step, after step (b) , of deforming the body at room temperature.

12. The method of claim 1 wherein the two phase body is subjected to said temperature and time at temperature conditions in the deformed condition by heating the deformed body after step (b) to a temperature of at least about 350*C and then cooling to room temperature.

13. The method of claim 12 wherein the body is heated to a temperature of about 600*C to about 750*C.

14. The method of claim 12 including the further step, after heating the body, of deforming the body at room temperature.

15. A method of making a filamentary strengthened Cu-Cr composite having high strength and conductivity, comprising the steps of:

a) solidifying a melt of Cu and Cr to form a two phase body comprising a Cu matrix and Cr dendrites dispersed throughout the Cu matrix, said matrix having Cr dissolved therein,

b) deforming the two phase body to impart a filamentary shape to the Cr dendrites so as to form elongated Cr strengthening filaments dispersed in the matrix, c) subjecting the two phase body in at least one of the solidified condition or the deformed condition to a heat treatment at a solutioning temperature above about 900^βC to improve ductility of the matrix, and

d) subjecting the solution heat treated body before or after deformation to another heat treatment at a temperature lower than said solutioning temperature so as to precipitate Cr dissolved in the matrix at the solutioning temperature from the Cu matrix.

16. The method of claim 15 wherein the two phase body is subjected to the solution heat treatment in the solidified condition by heating above about 900*C after step a) and before step b) .

17. The method of claim 16 wherein the solution heat treated body is subjected to said another heat treatment after it is deformed in step b) and under temperature and time at temperature conditions to avoid deleterious coarsening of the Cr filaments.

18. The method of claim 17 wherein the solution heat treated body is subjected to said another heat treatment before deformation thereof.

19. The method of claim 18 wherein the heat treated body is deformed after said another heat treatment.

20. The method of claim 15 wherein the two phase body is subjected to the solution heat treatment after step a) and step b) .

21. The method of claim 20 wherein the solution heat treated body is subjected to said another heat treatment after the solution heat treatment.

22. The method of claim 21 including the further step after said another heat treatment of further deforming the heat treated body.

23. The method of claims 15, 17, 18, and 21 wherein said another heat treatment is conducted at a temperature of about 350'C to about 500"C.

24. The method of claims 15, 17, 18, and 21 wherein said another heat treatment is conducted at a temperature in excess of 500^βC and is followed by a subsequent deformation step of said body.

5

25. The method of claim 24 wherein said another heat treatment is conducted at a temperature of about 600 *C to about 750*C.

_10_ __. 26. The method of claim 15 including the further step of subjecting the two phase body in the solidified condition to said another heat treatment before conducting steps b) , c) and d) .

15 27. An "in-situ" Cu-Cr composite formed by solidification of a Cu-Cr melt and mechanical reduction of the solidified melt, said composite comprising a Cu matrix having elongated Cr strengthening filaments dispersed in the matrix, said

20 Cu matrix having Cr precipitated from solid solution in said matrix to an extent as to improve the electrical conductivity of the composite.

28. The composite of claim 27 comprising about 3 to about 40 volume % Cr filaments and the balance the Cu matrix.

29. The composite of claim 27 comprising about 5 to about 18 volume % Cr filaments and the balance the Cu matrix.

30. The composite of claim 27 wherein the Cr is precipitated from the Cu matrix to provide 0.02 weight % or less of Cr dissolved in solid solution in said Cu matrix.

31. The composite of claim 27 exhibiting an electrical conductivity in the range of about 60% to about 75% IACS with an ultimate tensile strength in the range of about 900 to about 1100 MPa.

32. The composite of claim 27 exhibiting an electrical conductivity in the range of about 75% to about 85% IACS with an ultimate tensile strength in the range of about 750 to about 900 MPa.