United States Patent M 3,146,097 TIN BASE ALLOYS Lloyd R. Allen, Belmont, Mass, assignor to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts No Drawing. Filed Apr. 23, 1962, Ser. No. 189,273 Claims. (Cl. 75-175) This invention relates to tin base alloys and more particularly to allotropic tin alloys which will decrepitate to a fine powder at a predetermined time when subject to certain environmental conditions.

Recently it has been proposed to place into orbit around the earth one or more belts of thin microwave dipoles which will serve as an artificial scattering medium for radio signals in the centimeter band. It has been proposed that each dipole forming the belt of dipoles be a short length of metallic wire that will reflect radio waves.

One of the important problems connected with achieving orbital dipoles is that of putting objects or materials into relatively permanent orbits without prior assurance that there will be no harmful side elfects. Accordingly, the lifetime of the dipoles forming the orbital belts becomes a matter of importance. For example, at altitudes higher than 1000 kilometers atmospheric friction could not be depended upon to bring the dipoles down and in general solar radiation pressure produces variations in orbital eccentricity that are usually fairly small. It is known, however, that such radiation pressure variations can be quite large within a narrow range of altitude and orbital inclination where they become resonant with the period of precession of the major axis of the orbit. Within this narrow range the periodic lowering of perigee might well limit the lifetime to 1 or 2 years. Outside this range the belt lifetime might be 1 or 2 decades.

It has been determined that one possible way of limiting the lifetime of dipoles is to make the dipoles of white tin alloy which will undergo transformation to gray tin and disintegrate to powder under conditions in space. Depending on the size of the powder formed, solar radiation pressure may bring the powder to earth or solar light may sweep the powder into space.

The allotropic transformation of tin from white to gray tin has been the subject of much research in the hundred or so years since its existence has been known. The decrepitation to powder which results from [3 and a transformation and retransformation is due to a density change from 7.20 (pl-phase) to 5.75 (ct-phase). The rate of transformation of white tin (/S-phase) to gray tin (ocphase) is known to be a function of temperature. As White tin is lowered in temperature below the transformation temperature, which is approximately 13 C., the rate of transformation approaches a maximum rate which is close to about 35 to 45 C., at which point the rate is many times faster than at the transus temperature of approximately 13 C. As the temperature of the tin is further lowered the rate of transformation starts to decrease. The rate of decrease has not been investigated at temperatures below about 120 C.

The transformation of white to gray tin is nearly always preceded by an indefinite incubation period which may run into years during which no sign of the transformation can be detected. While the transformation has been studied in considerable detail the emphasis of such studies has ben directed towards means for preventing rather than 3,146,097 Patented Aug. 25, 1964 promoting the transformation. Accordingly, one of the main problems is that of providing allotropic tin alloys which promote the transformation of tin within a predetermined time.

Accordingly, a principal object of the present invention is to provide an allotropic tin alloy useful in producing dipoles which under orbital conditions will maintain electrical integrity and physical size for a predetermined time and which will decrepitate under orbital conditions in a predetermined time.

Another object of the invention is to provide tin base alloys which will undergo allotropic transformation and decrepitate to form a powder of high area-to-mass ratio.

Another object of the present invention is to provide tin alloys which, when subject to electron or neutron radiation, require no induction or nucleation period for initiation of transformation.

A further object of the invention is to provide tin alloys which are radiation sensitive and which will decrepitate to a fine powder within a predetermined time upon thermal cycling about the transus temperature.

A still further object of the present invention is to provide tin base alloys which when isothermally exposed to cold temperatures will transform from white to gray tin within a predetermined time.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the products possessing the features, properties and the relation of components and the processes involving the several steps and the relation and order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention reference should be had to the following detailed description.

The above objects are achieved and the problems indicated overcome by the tin alloys of this invention which contain as essential ingredients between 2 and 8 percent by weight of copper, about .05 to 4 percent by weight of germanium, the balance being tin.

In a more specific and more preferred embodiment the alloy composition contains from about 3 to 7 percent by weight of copper, about .1 to 3 percent by weight of germanium, the balance being tin.

The alloys of the present invention can be prepared in accordance with conventional procedures and through recourse to known melting or casting techniques. Thus the individual metals can be melt cast together and the melt allowed to cool and solidify. The melting operation can be carried out, for example, in a resistance melting furnace, or by subjecting the charge to induction heating or gas heating in a skull or crucible type of container. Whatever the type of melting and casting employed, care should be exercised to protect the molten metal from contamination. This can be prevented, for example, by conducting operation under a vacuum or a non-reactive inert atmosphere such as argon, helium and the like or under a flux blanket.

The individual metals used to form the alloys and charged to a melting furnace can be in any desired form such as powder shot, wire, sponge and the like, and should be of the highest purity to insure production of satisfactorily pure alloy products.

In the present invention the alloys were prepared by introducing the desired weights of the individual metals into a crucible in a vacuum resistance melting furnace. The charge of metals to the crucible was then heated and cast under vacuum. The alloys prepared in this manner were then tested for transformation under isothermal and thermal cycling conditions by converting the cast alloy to foil or wire and then sealing a portion of the foil or wire in a glass tube. For exposure to isothermal temperatures the glass tube was immersed into a constant temperature bath of the desired temperature. In the thermal cycling the glass tube was alternately dipped into baths held at the desired temperatures. When a specimen alloy was subjected to an irradiation field strontium 90 was used as the source of radiation.

In accordance with the present invention it was determined that in order for a tin alloy to decrepitate to fine powder upon thermal cycling, it is necessary that the alloy be capable of retransforming from gray tin to a hard friable white state. Alloys which transform to hard white tin will upon subsequent cooling convert to gray tin and decrepitate to form small particles. In contrast alloys which retransform from gray tin to soft white tin will not decrepitate upon subsequent retransformation to gray tin.

It has further been determined that the alloys of the present invention are sensitive to electron and neutron radiation. For example, it has been discovered that when the alloys of the present invention are irradiated with amounts as small as 10 rads the incubation period is sufficiently reduced so as to be negligible. Thus the radiation sensitivity of the alloys of the present invention comprises one of the important features thereof. For example, when the present alloys are utilized as orbital dipoles, solar radiation will be sufficient to eliminate uncertain incubation times.

For a clearer understanding of the invention the following specific examples are given. These examples are only illustrative and are not to be considered as limiting the scope and underlying principles of the invention.

Example 1 In this example an alloy containing 7 percent by weight copper, /2 percent by weight germanium, the balance being tin, was prepared. The tin alloy was then converted to foil and a portion of the foil in the as rolled condition isothermally exposed to a temperature of 80 C. After 96 hours of isothermal exposure the tin alloy had transformed 100 percent from white tin to gray tin. Thus, when the alloy is in a state of mechanical strain such as that resulting from cold rolling, the alloy does not require an incubation or nucleation period to initiate transformation.

Exam plc 2 In this example an alloy containing 5% by weight copper, 3% by weight germanium, the balance being tin was prepared. The alloy was converted to foil. A portion of the alloy foil was inoculated with a few gray tin crystals and exposed to cold temperatures. When isothermally exposed to a temperature of-40 C. the tin alloy required 26 hours to transform from white tin to gray tin. When another portion of the tin foil was isothermally exposed to a temperature of 80 C., 100% conversion to gray tin was completed in 120 hours. The gray tin when exposed to a temperature of +60 C. for 1500 hours transformed to the hard white phase and decrepitated to powder upon subsequent exposure to 80" C. Another portion of the gray tin, when exposed to +70 C. for 100 hours, transformed to the hard white phase and decrepitated to powder upon subsequent exposure to -80 C.

Example 3 In this example an alloy containing 3 percent by weight of copper, 3 percent by weight of germanium, the balance being tin, was prepared. The alloy in the form of foil 4. was first irradiated with 1 megarad of 1 mev. electron at 0 C. The alloy was then isothermally exposed to temperatures of 6 C., 40" C., and C. The tin alloy foils were completely converted to gray tin in 96 hours at 60 C., 48 hours at 40 C., and 48 hours at 80 C. The gray tin foil when heated to a temperature of +40 C. for 100 hours transformed to hard white tin. When subsequently exposed to 80 C., the hard white tin converted to gray tin and decrepitated to powder.

Example 4 In this example a 2 mil (.002 inch) diameter wire was made from an alloy similar to that of Example 3. The alloy wire was irradiated with 1 megrad of 1 mev. electron and isothermally exposed to a temperature of 80 C. to convert the alloy wire to gray tin. The alloy wire was then exposed to a temperature of 60 C. for about 36 hours whereupon the wire retransformed from gray tin to hard brittle white tin. Upon subsequent exposure to a temperature of 80 C. for about 24 hours the wire transformed to gray tin and decrepitated to a mixture of particles varying in size from 1 mm. to powder. When the wire was recycled to 60 C. for 24 hours then to 80 C. for about 16 hours no pieces of the wire were larger than .1 mm. in any dimension. Continued cycling increased the number of particles of 10 microns or less in diameter until after 4 cycles the wire had completely decrepitated to particles of 10 microns or below.

Example 5 In this example a tin alloy contining 5 percent by weight of copper, 1 percent by weight of germanium, the balance being tin, was formed and converted into a 2 mil wire. A small section of the wire (approximately 3 mm.) was then transformed to the gray tin phase by irradiation and isothermal exposure at -80 C. Exposure of the gray tin phase to a temperature of 60 C. for 2200 hours caused reversion to the hard brittle white phase which decrepitated to powder upon its reversion to the gray phase at -80" C. Three cycles between +60 C. and 80 C. completely converted the wire to a powder having average particle size of less than about l020 microns.

Example 6 In this example a tin alloy containing 5 percent by weight of copper, 2 percent by weight of germanium, the balance being tin, was formed and converted into a 2 mil wire. Similar to Example 5 a small section of the wire was then transformed to the gray tin phase by irradiation and isothermal exposure at 80 C. Exposure of the gray tin phase to a temperature of 60 C. for 1000 hours caused reversion to the hard brittle white phase which decrepitated to powder upon its reconversion to the gray phase at 80 C. Five cycles between +60 C. and 40 C. completely converted the wire to a powder having an average particle size less than about 10 microns.

Since certain changes may be made in the above products and process without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An allotropic tin base alloy consisting essentially of about 2 to 8 percent by weight of copper, about .05 to 4 percnt by weight of germanium, the balance being essentially tin.

2. An allotropic tin base alloy consisting essentially of about 3 to 7 percent by weight of copper, about .1 to 3 percent by weight of germanium, the balance being essentially tin.

3. An allotropic alloy consisting essentially of about 5 percent by weight of copper about 1 percent by weight of germanium, the balance being essentially tin.

4. An allotropic alloy consisting essentially of about 5 percent by weight of copper, about 2 percent by weight germanium, the balance being essentially tin.

5. An allotropic alloy consisting essentially of about 5 pecent by weight of copper, abouut 3 percent by weight 6 OTHER REFERENCES Hansen: Constitution of Binary Alloys, McGraw-Hill Book Company, Inc., New York, 1958, pp. 633-636.

Jaffee et al.: Technology of Germanium, Transactions germanium the balance being essentially 5 American Electrochemical Society, vol. 89 (1946), page 287. References Cited m the file of thls patent Guerter et al.: The Systems Tin-Germanium and Tin- UNITED STATES PATENTS Beryllium, Technical Publications of the International Tin 1,869,378 Konigsberg Au 2, 1932 Research and Development Council, Series A, Number 2,745,046 Lark-Horovitz et a1. May 8, 1956 1 1 p