US3114631A

US3114631A - Silver composition

Info

Publication number: US3114631A
Application number: US51671A
Authority: US
Inventors: George H Sistare; Charles D Coxe
Original assignee: Handy and Harman
Current assignee: Handy and Harman
Priority date: 1960-08-24
Filing date: 1960-08-24
Publication date: 1963-12-17
Anticipated expiration: 1980-12-17
Also published as: GB909349A

Description

Dec. 17, 1963 G. H. SISTARE ETAL SILVER COMPOSITION Filed Aug. 24, 1960 FIG. I

Plain silver-cadmium alloy 400 diameters magnification FIG. 2

Silver-cadmium alloy with additive 400 diameters magnification INVENTORS GEORGE H. SISTARE (LDICKEY COXE Quin. Eluowb Wnkow3M-n A inf-a,

ATTORNEYS 3,114,631 HIVER COMPOSITION George H. Sistare and Charles D. (Ioxe, Fairfield, Conm, assignors to Handy & Harman, New York, N.Y., a corporation of New York Filed Aug. 24, 1960, Ser. No. 51,671 6 Claims. (Cl. 75173) This invention relates to alloys for use in making electrical contacts for switches, relays, circuit breakers and the like. More particularly, it relates to silver-cadmium alloys which can be internally oxidized without adverse effect owing to the presence in the alloy of a small amount of silicon or aluminum or both. The invention contemplates providing the alloy in either unoxidized or internally oxidized form, and it further contemplates the provision of cold-Worked electrical contacts formed from the internally oxidized alloy and a method of making such contacts.

Cadmium oxide-silver compositions have long been recognized as superior materials for use in making electrical contacts of the type described. Powder metallurgy was originally relied upon for forming the desired combination of metal and metal oxide, but it is the usual practice now to cold-form the finished parts from silver-cadmium alloy wire and then internally oxidize them to provide a silver matrix in which cadmium oxide is dispersed. This method is by no means completely satisfactory, however, because the finished contacts are very often brittle and marred by longitudinal surface cracks as a result of the final oxidation step. Also, the process of internal oxidation must be carried out by the manufacturer of the contacts rather than by the supplier of the silver-cadmium wire.

The ultimate cause of the brittleness and longitudinal surface cracks characteristic of conventional internally oxidized electrical contacts is not known with complete certainty, but it appears that the platelet form in which the cadmium oxide appears in the silver matrix is a contributing factor. During oxidation, the cadmium oxide crystals grow rapidly at the interfaces of the relatively larger crystals of silver to form fiat platelets or flakes, and so produce dominant planes of cleavage in the structure at which later applied internal or external stresses tend to cause failure by cracking. The internal oxidation process itself sets up severe internal stresses (primarily hoop stresses) which produce the characteristic cracks along these planes of clevage. These stresses are due to the fact that as oxidation proceeds inwardly from the surface of the part to the core, it is accompanied by volumetric expansion; and as the interior of the alloy oxidizes and expands it subjects the previously oxidized outer layer to stresses which cause cracks to open at the surface along the planes defined by the platelets of cadmium oxide.

It is the primary purpose of the present invention to provide a silver-cadmium alloy for use in making electrical contacts which is not susceptible to such surface cracking when internally oxidized and which is even sufficiently ductile and workable after oxidation to be oxidized while still in the form of wire and later fabricated by cold-forming into finished contacts.

These objects are achieved according to the invention by a new electrical contact alloy of from 2 to 20 percent by weight of cadmium, from 4 to 18X l percent by weight of at least one element taken from the group consisting of aluminum and silicon, and the balance essentially all silver. The invention also provides the new alloy in the internally oxidized condition. It then includes the oxides of cadmium and of said element (aluminum or silicon) in a silver matrix, the cadmium oxide being characteristically in the form of substantially sphertates atent ice ical nodules substantially uniformly dispersed throughout the matrix, rather than platelets.

Electrical contacts are made in accordance with the invention by internally oxidizing an alloy of from 2 to 5 20 percent by weight of cadmium, from 4 to 18 10 percent by weight of at least one element taken from the group consisting of aluminum and silicon, and the balance essentially silver, and thereafter cold-forming the alloy into electrical contacts.

The invention is described below with reference to the accompanying drawings, in which FIG. 1 shows the microstructure of an internally oxi dized alloy of silver and cadmium; and

FIG. 2 shows the microstructure of an internally oxidized silver-cadmium alloy containing silicon or alumi num in the percentages contemplated by the invention.

It has been discovered that by the addition of small amounts of silicon or aluminum or both to a silver-cadmium alloy as described, the resulting cadmium oxide crystals form in substantially spherical nodules, not platelets or flakes, which are substantially uniformly dispersed throughout the matrix. As a result the ductility of the oxidized alloy is considerably improved. The distinct nodular form of the cadmium oxide crystals which typifies the new alloy appears to result from nucleation of the crystallizing cadmium oxide by previously formed evenly dispersed small crystals of silicon or aluminum oxide. This even dispersion and prior formation of the silicon or aluminum oxide crystals results from the fact that low concentrations of both silicon and aluminum are in solid solution in the silver-cadmium alloy before oxidation, and their oxides have significantly higher heats of formation that cadmium oxide. Hence, the silicon or aluminum oxidizes first as the alloy is subjected to oxidation, and provides uniformly dispersed crystal nuclei about which the cadmium oxide crystals form. It is clear, therefore, that the solid solubility in a silver-cadmium alloy and the higher heat of formation of their oxides than cadmium oxide are properties of both silicon and aluminum which contribute to the remarkable success of the new alloy.

The advantages of the new alloy are shown clearly in the table below, wherein a conventional internally oxidized silver-cadmium alloy containing 9% cadmium is compared to similar alloys which are modified by the addition of aluminum and silicon in accordance with the invention. FIG. 1 and 2 of the accompanying drawing are referred to in the table to show the microstructure of the old and new alloys respectively.

Weight Ultimate Elou- Freedom Con- Additive Percent Strength, gation, From tact Crystal 10- 1,000 Percent Surface Heads Structure p.s.i. Cracks 0 31-35 21-25 Poor Poor Fig. 1. 5. 5 32-36 16-20 Good Goo Fig. 2. 11 32-36 16-20 do do Fig. 2. 5.5 B l-38 16-20 o do Fig.2. D 11 34-38 16-20 do do Fig. 2. Aluminum 22 30-34 3-7 Poor Poor Fig. 2. Silicon 22 31-35 9-13 do do. Fig.2.

the fine nodular form of the cadmium oxide, provides considerably more ductility and workability than in the alloy of FIG. 1. As mentioned previously, this uniformity in the FIG. 2 alloy is believed to be caused by nucleation of the cadmium oxide by crystals of aluminum or silicon oxide, about which the cadmium oxide crystallizes in intricately dispersed spheroidal form. Because of their higher heats of formation, the oxides of aluminum and silicon are formed closer behind the advancing oxidation front, during oxidation of the alloy, than does cadmium oxide; and because the silicon or aluminum content is in solid solution, their oxides are formed very uniformly dispersed throughout the alloy.

It is to be noted that aluminum and silicon do not achieve this advantageous structural modification of the silver-cadmium alloy on account of their deoxidizing properties. Indeed, many deoxidizers, such as lithium and phosphorous (which are among these most commonly used), not only do not improve the alloy structure but actually reduce its ductility and workability.

Following is an example of the preparation of an electrical contact in accordance with the invention: An alloy of 91 percent by weight of silver and 9 percent by weight of cadmium was melted by conventional means and heated to about 2000 F. Silicon metal, in the form of a silicon-silver master alloy, was added to the silver-cadmium melt in an amount equivalent to 12x10 percent by weight. The melt was then cast, rolled into rods, and drawn into wire of 0.120 inch in diameter. This wire was internally oxidized by heating at 1550 F. in air for 72 hours. After such oxidation, the wire was sufficiently ductile to be cold drawn to .090 inch in diameter. It was then annealed at 1000 F. and was cold drawn to .088 inch in diameter. Finally, it was cold-headed into the rivet form typical of an electrical contact. The finished contacts were hard in the headed portion and free of cracks.

In another example of the invention, an alloy of 91 percent by Weight of silver and 9 percent by weight of cadmium was prepared and heated to 200 P. Then 8 x" percent by weight of aluminum was added to the melt and the metal was cast, rolled, and drawn into wire 0.120 inch in diameter. After internal oxidation by heating at 1550 F. in air for 72 hours, the wire was cold drawn to .090 inch in diameter, annealed at 1000 F and cold finished to .088 inch in diameter. The wire was then cold-headed to form rivets which were hard in the headed portion and free from surface cracks.

It is apparent from these examples that silicon and aluminum are both highly effective for making the new contact alloys. In certain instances, however, silicon may be preferred because it does not produce as much dross during melting as does aluminum. Moreover, silicon is preferred for production because its presence in casting shops where silver alloys are being melted is unobjectionable, whereas aluminum is sometimes sought to be excluded from such shops because it is an objectionable contaminant in other silver alloys being produced.

Any of the nominal compositions of silver-cadmium oxide which have been commercially produced heretofor may be prepared in accordance with the invention. Such compositions generally contain from 2 to percent by weight of cadmium and the balance essentially silver. The most commonly used materials, however, are these nominally containing 10 to 15 percent by weight of cadmium. Impurities normally'present in silver-cadmium alloys, or silver-cadmium oxide compositions, may be present without harmful effect in the alloys of this invention.

The silicon and aluminum additives which modify the alloy structure in accordance with the invention may be used singly or together. When used jointly their total amount preferably is in the same range as for either one alone when used singly, that is at least 4 10 percent by weight but not more than 18x10 percent by weight of the alloy. In most instances, the preferred range for the silicon or aluminum is from 5 to 12x10 percent by weight.

Less than about *4 10 percent of silicon or aluminum or their combined total is not suficient for the purposes of this invention, and more than about 18x10- percent is harmful to the properties of the alloy as shown in the last two items of the table in column 2.

We claim:

1. An internally oxidized alloy produced by the internal oxidation of a solid solution alloy consisting essentially of cadmium in an amount of from 2% to 20%, by weight, from 4 to l8 10 percent, by Weight, of at least one element from the group consisting of aluminum and silicon and the balance silver, said cadmium being substantially in the form of spherical oxide nodules substantially uniformly dispersed throughout the silver matrix.

2. An electrical contact element composed of an alloy according to claim 1 in the plastically-deformed cold worked condition.

3. An internally oxidized alloy according to claim 1 in which the cadmium is present in an amount of from about 10 to 15 percent, by weight, and the element of said group is aluminum and is present in the alloy in amount of from 5 to 12 l0 percent by weight.

4. An electrical contact element composed of an alloy according to claim 3 in the plastically-deformed coldworked condition.

5. An internally oxidized alloy according to claim 1 in which cadmium is present in the alloy in amount of from 10 to 15 percent, by weight, and the element from said group is silicon and is present in the alloy in an amount from 5 to 12 1O- percent, by weight.

6. An electrical contact element composed of the internally oxidized alloy of claim 5 in the plasticallydeformed cold-worked condition.

References Cited in the file of this patent UNITED STATES PATENTS 2,258,492 Hensel et al Oct. 7, 1941 2,396,101 Hensel et a1 Mar. 5, 1946 2,796,346 Stumbock June 18, 1957 2,932,595 Pfiurnm Apr. 12, 1960 FOREIGN PATENTS 611,813 Great Britain Nov. 4, 1948 542,630 Canada June 25, 1957

Claims

1. AN INTERNALLY OXIDIZED ALLOY PRODUCED BY THE INTERNAL OXIDATION OF A SOLID SOLUTION ALLOY CONSISTING ESSENTIALLY OF CADMIUM IN AN AMOUNT OF FROM 2% TO 20%, BY WEIGHT, FROM 4 TO 18X10-3 PERCENT, BY WEIGHT, OF AT LEAST ONE ELEMENT FROM THE GROUP CONSISTING OF ALUMINUM AND SILICON AND THE BALANCE SILVER, SAID CADMIUM BEING SUBSTANTIALLY IN THE FORM OF SPHERICAL OXIDE NODULES SUBSTANTIALLY UNIFORMLY DISPERSED THROUGHOUT THE SILVER MATRIX.