US2567560A - Heat-treatment of copper-silver binary alloys - Google Patents

Description

Sept. 11, 1951 A. w. HODGE 2,567,560

HEAT TREATMENT OF COPPER-SILVER BINARY ALLOYS Filed May 6, 1948 2 Sheets-Sheet 1 FIGURE 1 Gu l 6 PER CENT SILVER FIGURE 3 TENSILE TENSILE STRENGTH IN lOOO RSI.

CONDUCTIWTY PER CENT IACS 5 6 7 PER CENT SILVER IN COPPER INVENTOR Allen W. Hodge ATTORNEY Sept. 11, 1951 A. w. HODGE I HEAT TREATMENT OF COPPER-SILVER BINARY ALLOYS 2 Sheets-Sheet 2 Filed May 6, 1948 FIGURE 2 D E A E R T m E H .EE 5%. o mv o 00 0 mm ne 8 mm 009 :BEEW 552E TREATED O 31] 6M 74.I 84.5 88.5 94.0 96.l 9Z4 98.4 99.0

REDUCTION IN AREA, PER CENT INVENTOR Allen W. Hodge evh 5 ATTORNEY Patented Sept. 11, 1951 HEAT-TREATMENT OF COPPER-SILVER BINARY ALLOYS Allen W. Hodge, Columbus-,Ohio, assignor, by mesneassignments, to Battelle. Development Corporation,..Columbus, Ohio, a,- corporation of Delaware Application'May. 6, 1948, Serial No. 25,509

2 Claims; (Cl. 14812.7)

This invention relates-to a method of-processing binary alloys of copper and silver andto the product thereby produced. The present 7 invention is particularly directed to the processing of the low-silver alloys wherein the silver does not exceed approximately 8 Binaryalloys of copper and silver in which copper is the'base' -metal have been known for H some time, but exist primarilyin the laboratory due 'to thefact that they arenot characterized bysufiiciently outstanding properties to offset the explicit expense of their production. These binary alloys, wherein the silver is present in amounts under 8%, do not compare favorably to the much'i cheaper metal copper. These alloys have a conductivity,'when' annealed, of about 90 of thatofpure copper; only'little'higher tensile strength and a lower'elongation: Consequently, very little work has been done with these alloys, particularly because preliminary processingiat tempts Were discouraged by the discovery that, while hardness increased after an age-hardening heat treatment, the tensile strength'wentdown.

The primary object of the present invention is to i provide a methodof. processing silvercopper alloys containing lessthan. about 8% of silver so as to produce aproduct having unusually high conductivity combined withiexcellent tensile strength.

Another object of this invention is to provide a method for aging and cold working a silvercopper binary alloy having a copper base wherein the. aging time is reduced to. less than onehalffhour.

Other objects and advantages of the present invention will become apparent from the following detailed description thereof when read in conjunction with: the accompanying drawings in which:

Figure l is agphase diagram of the copper-rich portion of the copper-silver system,

Figure 2 is a graph upon which curves are plotted to show the effect of aging and cold drawing upon the, properties of aparticular silver-copper wire, and

Figure 3 graphically illustrates the effect of variations of the silver content in the processed alloy.

It has been discovered, as described and claimed in the copending application, Serial No. 25,508, filed May 6, 1948, on Copper-Silver Alloys, now abandoned, that binary alloys of copper and silver can be aged and then coldworked to produce alloys having. unusual and useful properties. The present invention lies in the discovery that preliminary cold Working prior to aging'the alloy increases the hardening and strengthening effect of subsequent cold working o-f' the alloy over what-can be otherwise attained Withoutthe preliminary cold working treatment."

Furthermore; this preliminary cold Working'step lowers-the aging temperature required and great 1y diminishes the timenecessary for the aging treatment.

The process comprising the present invention isapplicable to alloys containing less than about 8% silver and which are composed of a single phase (alpha). These alloys fall within the area lying between lines I and 2 in Figure 1; Line I represents the limit of solid solubility at' any given temperature of silver in copper, whereas line 2 is the solidus line, that is, the line below which the alloy is entirely in the solid form;

singlealpha phase.

Thejun'ction of lines I and 2 in Figure l is designated point fl and is the-maximum limit of solid solubility of silver in copper. Line 4 indicates the temperature at which the eutectic of this system solidifies, that is, about 779 G.

Since pointfl indicates the maximum Solid solubility of silver in copper to be about 8%, silver in excess of this amount will producea two phase alloy, to which the present invention is not applicable. It is very difficult to obtain complete solid solubility of over-7% silver in copper, so'that it" is preferred to restrictthesilver con-- tent to 7 or slightly'lower. The lower limit for silver is about'3%, because very little benefit is derived from the present process using-alloys of lower silver content; The full effectoftheaging step is not found in alloys containing as little as 3% silver, so that the lower preferred limit is about 5% silver.

As the alloy cools down after the copper and silver havebeen intermingled inthe molten state, the copper-rich phase designated alpha in Figure l precipitates out first, leaving the alloy which solidifies last richer in silver. Conse-' quently; the as=cast alloy contains both alpha and'beta phase and must be solution annealed or solution heat treated to cause it to reform in the single'alpha phase. It is essential that the alloy be first put into a single phase by solution heat treatment in order to age it. The step of solution heat treatment is well known to metallurgists and need not be described here in detail since it forms only an incidental part of the presentinvention. It has been found, nevertheless, that solution heat treatment at about 745 C. continuedfor two hours and followed by a water quench produces highly satisfactory results, if repeated often enough. About four of these soaking periods have proven to be sufiicient-to put the alloy substantially or entirely into the Variations in time and temperaturefand' manner of quench may be made inthis procedurefwithout in anyway afiecting the present invention. .Itfis only critical that the increase gradually up to about 75% reduction in 1 area. Since the maximum benefit is derived from this step when reduction ranges from 75% to about 95%, this range is preferred.

The cold worked alloy is then aged by heating the alloy, usually in a molten salt bath or the like, until a marked drop in tensile strength is obtained. This phenomenon, as indicated above, is directly contrary to the usual age hardening treatment because normally other alloys that respond to the aging treatment increase in tensile strength upon aging. The aging effect is the combined product of time, temperature and composition, and the aging step itself, as it is practiced herein, is familiar to all metallurgists as being common practice in connection with a large number of alloys. In the present instance, it may be stated generally that temperatures between 350 C. and 500 C. produce excellent results with the binary copper-silver alloys to which this invention relates. The present aging treatment is remarkable, not only as respects the softening action above referred to, but as respects the short period of time required to effectively age this alloy. For example, an aging treatment of ten to fifteen minutes at about 400 C. produces very excellent results. This extremely short aging period is highly important from a commercial standpoint, because it greatly increases the rate of production.

After aging, the alloy is again cold worked until a tensile strength and hardness has been obtained suflicient for the purpose to which the alloy is to be put. The exact extent of this second cold working step will depend almost entirely upon the end use of the alloy, but normally sufficient cold working is employed to at least give the alloy a tensile strength greater than that which it had prior to the aging treatment. Actual practice has shown that maximum tensile strengths are obtained when the alloy is cold worked to about 97% reduction in area after the aging treatment. For example, an alloy of 6% to 7% silver, balance copper, aged 10 minutes to minutes at 400 C. and cold worked before and after aging in this preferred manner will have a better combination of tensile strength and electrical conductivity than any other known alloy. Substantially the same result will be obtained with an aging time of 5 minutes at 425 C.

A graphic illustration of the effect of the treatment defined in the present application upon an alloy containing 6.5% silver and the balance copper is clearly shown in Fig. 2. The tensile strength is plotted against the per cent reduction in area on a logarithmic scale, and the alloy was tested in the form of a wire. The annealed alloy, having a tensile strength of 45,000 p. s. i., increases in tensile strength as it is cold worked up to the point of 74.1% reduction in area, at which point the alloy was aged. The aged alloy, represented by the dotted line, dropped in tensile strength rapidly from about 89,000 p. s. i. to 76,000 p. s. i. upon aging. However, it required less than additional reduction of area to raise the tensile strength of the heat-treated or aged alloy above that of the unheat-treated alloy represented by the solid line on Figure 2. Upon further cold working of the heat-treated or aged alloy, it is clearly apparent that this alloy was much more responsive to the cold working treatment than the alloy which was not cold worked prior to aging. At 97% additional reduction in area after aging the alloy attained a tensile strength of 165,000 p. s. i. and exceeded the tensile strength of the unaged alloy by 40,000 p. s.i.

Not only does the present process produce an alloy having greatly increased tensile strength, but the conductivity, as indicated in Figure 2, is also increased somewhat by the aging treatments. As above pointed out, the amount of increase in conductivity is dependent upon the length of the aging treatment. In Figure 2, the aging treatment was 12 minutes at 400 C. in a nitrate salt bath and Was much too short for any substantial increase in conductivity to be obtained. This treatment is intended to give maximum strength with an electrical conductivity of at least 70% of that of pure copper.

In order to better enable those skilled in the art to more readily practice the present invention, the following examples are set forth:

Example 1 Aging 1 Tensile Conductiv- Time, fi gg Strength, ity percent Minutes p. s. i. IACS Example 2 An alloy of 5% silver and copper was treated in identical manner to that of Example 1. The wire produced had the following properties:

Aging Tensile Conductiv- Time, g g gs Strength, ity, percent Minutes p. s. i. IAOS Example 3 An alloy of 6% silver and 94% copper was treated identically to the foregoing examples and had the following properties:

Oonduc Aging Diameter, Tensile tivity, Time, Inches Strength, Per Cent Minutes p. s. i. IAGS Example 4 An alloy of 7% silver, 93% copper was treated as the foregoing examples down to the aging step. This wire was then aged at various times and 5 temperatures to obtain the maximum strength possible at each temperature after a reduction of area of 97% following aging. These times were found to be:

Aging Aging Maximum Conduc- Temper- Time, Tensile tivity,

ature, Maximum Strength, Per Cent O. Strength p. s. i. IACS Minutes 375 20 162, 000 70. 400 7. 167, 000 (18. 0 425 3. 0 169, 000 69. 0

In general, as indicated above, the time of heat treatment becomes shorter and more critical as the temperature increases, and the maximum strength is higher with higher temperatures. Also, it has been found that the ductility of the wire is poor at the point of maximum strength, but that nearly as high a strength is obtained With higher electrical conductivity and better ductility is reached after longer aging periods, such as 5 minutes at 425 C., 12 minutes at 400 C., 30 minutes at 375 C. (at 375 C. aging temperature, the ductility is definitely poorer than at higher temperatures).

Example 5 An alloy of 6% silver, 0.1% magnesium, balance copper, was solution heat treated and quenched from 750 C. in the form of wire of 0.1%-inch diameter. This wire was then cold drawn 82% reduction of area and aged. Following the aging treatment, the wire was again drawn to 97% reduction of area. This wire had the properties noted below:

Aging Conduc Temp tit? start ature, Minuts S i Per Cent 0. A08

Example 6 An alloy of 6.5% silver, 0.02% magnesium, balance copper was solution heat treated and quenched from 745 C. in the form of inch diameter rod. This was then drawn to 94% reduction of area, aged 12 minutes at 400 C., and then drawn to 97% reduction of area. The wire (.011 inch diameter) had the following properties:

Tensile Conduc- Strength tivity sile strength of the unaged alloy is also shown for comparison purposes. Optimum properties appear to be reached at about 7% silver, although the tensile strength begins to level off at about 6% silver. Alloys containing 4% silver have lower tensile strengths, but have somewhat higher conductivity than alloys containing greater amounts of silver. Even the lowest tensile strength for 4% silver alloys (138,000 p. s. i.) is an improvement of more than 200% over pure copper. This increase in tensile strength is of supreme importance in producing light weight communication cable.

It is apparent from the above-detailed description that the present invention relates to the processing of copper-silver alloys containing from V 3 to 8% silver, which comprises cold working the alloy prior to aging as well as after the aging step. By practicing the present invention in its preferred embodiment, a product may be produced having a tensile strength in excess of 160,000 p. s. i. and a conductivity greater than 70% of that of copper. The combination of these two properties provides a product so unusual that the relatively high cost of the binary silver-copper alloys is offset thereby.

What is claimed is:

1. A method of processing binary alloys of copper and silver which contain at least 3% and not more than 8% silver, which comprises solution heat treating and quenching the alloy, whereby substantially all of the alloy is in the copper-rich single phase, cold working the resulting alloy until a cross-sectional reduction in area of at least 75% and not more than is obtained, aging the cold-worked alloy at temperatures ranging from 350 C. to 500 C. for a period of at least five minutes and less than thirty minutes, and cold working the aged alloy until an additional reduction in cross-sectional area of approximately 97% is produced.

2. A binary alloy of copper and silver which contains at least 3 per cent and not more than 8 per cent silver, in the treated state, produced by the process comprising solution heat-treating and quenching the alloy, whereby substantially all of the alloy is in the copper-rich single phase, coldworking the resulting alloy until a cross-sectional reduction in area of at least 75% and not more than 95% is obtained, aging the coldworked alloy at temperatures ranging from 350 C. to 500 C. for a period of at least five minutes and less than thirty minutes, and cold-working the aged alloy until an additional reduction in cross-sectional area of approximately 97% is produced.

ALLEN W. HODGE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,281,691 Hensel et a1 May 5, 1942 FOREIGN PATENTS Number Country Date 521,927 Great Britain June 14, 1940 OTHER REFERENCES Transactions, A. I. M. E., vol. 99, 1932, pages 112-113.

Age Hardening of Metals, by American Society for Metals, pp. 143 and 321. 1940.

Claims (1)

1. A METHOD OF PROCESSING BINARY ALLOYS OF COPPER AND SILVER WHICH CONTAIN AT LEAST 3% AND NOT MORE THAN 8% SILVER, WHICH COMPRISES SOLUTION HEAT TREATING AND QUENCHING THE ALLOY, WHEREBY SUBSTANTIALLY ALL OF THE ALLOY IS IN THE COPPER-RICH SINGLE PHASE, COLD WORKING THE RESULTING ALLOY UNTIL A CROSS-SECTIONAL REDUCTION IN AREA OF AT LEAST 75% AND NOT MORE THAN 95% IS OBTAINED, AGING THE COLD-WORKED ALLOY AT TEMPERATURES RANGINGNG FROM 350* C. TO 500* C. FOR A PERIOD OF AT LEAST FIVE MINUTES AND LESS THAN THIRTY MINUTES, AND COLD WORKING THE AGED ALLOY UNTIL AN ADDITIONAL REDUCTION IN CROSS-SECTIONAL AREA OF APPROXIMATELY 97% IS PRODUCED.

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