US2453444A - Magnesium base lithium alloys - Google Patents

Description

Patented Nov. 9, 1948 MAGNESIUM BASE LITHIUM ALLOYS Alfred C. Loonam, New York, N. Y., assignor to Mathieson Chemical Corporation, a corporation of Virginia N Drawing.

Application June 28, 1945,

Serial No. 602,171

3 Claims. 1

This invention relates to magnesium-base alloys and has for its object the provision of an improved magnesium-base alloy containing lithium (hereinafter called magnesium-lithium alloys). Magnesium-lithium alloys have been studied a number of times by different investigators but, despite the fact that some of them have lower specific gravities than other magnesium alloys, they have failed to show any marked improvement in properties and therefore have not been successful.

I have found that sodium exercises a pronounced detrimental eifect on magnesiumlithium alloys and that its elimination or reduction to permissible low percentages in alloys characterized by having at least in part a bodycentered cubic (beta phase) crystal lattice structure results in alloys which are highly malleable, ductile and formable when cold, have less pronounced directional properties, and are possessed of good tensile strength and very satisfactory corrosion resistance. I have also found that certain other metals, notably zinc and aluminum, may be incorporated in the magnesium-lithium alloys with marked improvements, said improvements being augmented by the elimination or diminution of sodium. Cadmium and silver may also be similarly included in magnesium-lithium alloys.

My invention, accordingly, provides an improved magnesium-lithium alloy free of harmful amounts of sodium characterized by being partly or wholly comprised of a body-centered cubic (beta phase) crystal lattice structure. In a more complete embodiment, the invention provides a magnesium-lithium alloy composed of at least 70% of magnesium, from 3.6% to 12% of lithium, and a metal or metals having the capacity of improving the afore-mentioned physical properties, the allow being preferably completely free of sodium but in any event containing not over 0.1% of sodium.

In the absence of sodium, magnesium alloys of the close-packed hexagonal crystal structure (alpha crystals) are inferior from a standpoint of ductility to the magnesium-lithium alloys comprised partly or Wholly of a body-centered cubic crystal structure (beta crystals). The beta-containing alloys of magnesium have high ductility and the sodium-free magnesiumlithium alloys of the invention not only have high ductility and high tensile strengths but are characterized by exceptional elongation and reduction in area.

The addition of zinc, for example, to the 2 magnesium-lithium alloys, results in the formation of a beta phase in alloys containing as little as 3.6% of lithium. Zinc in amounts varying from 2% to 11% has a pronounced beneficial effect in the way of increased tensile strength, higher modulus of elasticity and better working properties. Such alloys are more fully described and claimed in my co-pending application Serial Number 616,162, filed September 14, 1945, now abandoned. The elimination of sodium from such alloys to amounts less than 0.1% gives exceptional improvements manifested by greater reduction in area and elongation under tension. An alloy containing 81.6% magnesium, 8.4% lithium, 10.0% zinc, and a trace of sodium had the following physical properties:

Tensile strength, p. s. i. 36,700 Tensile yield strength (dividers), p. s. i. 32,300 Elongation in 1 inch, per cent 20.8 Reduction in area, per cent 29.8

The variation in tensile properties and ductility with the sodium content of an alloy containing 87% magnesium. 9% lithium and 4% zinc is illustrated in the following table:

Elnngn' 1 Reduction Yield 'lensilc Eg 1 Area, Strength, Strength, inch E percent p. s. i. p. s. 1. 0. 035 35. 0 I 68. 0 24, 900 30, 900 O. 034 33. 0 66. 5 24, 600 31, 700 0. 023 34. 0 (i7. 3 24, 200 31, 400 O. 045 31. 5 62. 1 24, 700 '31, 500 O. 041 35. 0 59. 4 24, 600 31, 400 0. 028 34. 0 58. 2 24, 700 31, 800 O. 066 23. 5 34. 1 24, 200 31, O. 053 29. 5 46. 7 25, 400 31, 500 0. 096 10. (l 13. 7 24, 200 31,200 0. 128 10. 5 16. 3 23, 400 31, 000 O. 8. 5 12. 1 23, 900 30, 800

1 By spcetrographic analysis.

A beta phase is also formed in. magnesiumlithium alloys containing aluminum. My copending application Serial Number 616,163, filed September 14, 1945, now abandoned, describes more fully and claims magnesium-base lithium alloys containing from 1% to 8% of aluminum. Such aluminum alloys are also characterized by great tensile strength, high ductility, high modulus of elasticity and good cold forming properties. The freedom or substantial freedom of sodium from such alloys, i. e., the elimination of sodium to amounts below 0.1%, has pronounced beneficial effects on the alloy.

In another example of this embodiment of the invention, a magnesium-lithium alloy was prepared containing 88.8% magnesium, 9.2% lithium and 2.0% aluminum with sodium eliminated to a trace. The alloy had the following physical properties:

Tensile strength, p. s. i. 32,700

Tensile yield strength (dividers), p. s. i. 27,000 Elongation, per cent 7.5 Reduction of area, per cent 51.6

An alloy having the composition 8 1.6% magnesium, 7.4% lithium, 4.0% aluminum and 1.0% zinc had the following physical properties:

The quaternary alloys of magnesium-lithium containing zinc and silver or cadmium, or silver and cadmium are also improved by the elimination or marked reduction of the sodium content. Unlike the magnesium-lithium-Zinc alloys which lose strength and gain ductility with increases in lithium, the alloys containing zinc and silver, or zinc and cadmium possess high strength and good ductility in the favorable ranges of lithium and zinc. The following are examples of such alloy: 81.9% magnesium, 10.1% lithium, 4% zinc and 4% silver; 78.3% magnesium, 9.7% lithium, 6% zinc and 6% cadmium; and 73% magnesium, 12% lithium, silver and 5% cadmium. All these quaternary alloys contain the important beta crystal lattice structure and are greatly improved'by the elimination of sodium to amounts below, say 0.1%. Such alloys are more fully de scribed and claimed in the co-pending application Alfred E. Hesse, Serial Number 616,160, filed September 14, 1945.

The alloys of the invention may be prepared in any suitable type of melting operation; for example, the magnesium may be melted in an iron crucible under a flux and the Zinc, aluminum, silver or cadmium, as the case may be, incorporated therein in any suitable manner. In order to avoid excessive oxidation of the lithium during the alloying process, it is necessary to use means for holding the lithium beneath the surface of the molten magnesium. An inverted perforated steel cup or phosphorizer may be used for this purpose. Temperatures of about 1300 to 1400 F. may be used for the operation.

The elimination of sodium may be accom plished by the use of sodium-free metals and a refining or treating flux that is either free of sodium or which lacks the capacity to add sodium to the alloy. From a practicable point of view, the use of sodium-free metals is all but impossible at this time.

An efficient and economical method of producing magnesium-lithium alloys which are adequately low in sodium involves treatment of the alloy with a special flux which is the subject of a co-pending application of Alfred H. Hesse, Serial Number 603,749, filed July 7, 1945. Briefly, thisfiux comprises a mixture of lithium chloride and a suitable fluoride, for example, lithium or potassium fluoride. An especially effective flux comprises about lithium chloride and about 25% lithium fluoride. The flux is of lighter weight than magnesium and the magnesium may be protected while bein melted. The alloying metals may be added beneath the flux and also protected from reactive gases and the like. The alloy may be agitated with the flux in accordance with any of the usual practices. The flux has a peculiar affinity for sodium and results in the elimination of sodium to amounts below 0.1%. One of the unusual characteristics of the flux is that it may contain initially small percentages of sodium salts and still have the property of removing sodium from the alloy.

The alloys undergoing treatment with the refining flux may also be treated by blowing the metal and flux with a gas, such as nitrogen or ammonia, to effect a reduction in the sodium.

The aforementioned treatments for the elimination or diminution of sodium. may also be practiced for the elimination or diminution of potassium.

I claim:

1. A magnesium-base alloy comprising at least 70% of magnesium, from 3.6% to 12% of lithium, and substantially less than 0.1% of sodium, said alloy having improved ductility because of the low sodium content.

2. A ma nesium-base alloy comprising at least 70% of ma nesium, from 3.6% to 12% of lithium, from 2% to 11% of zinc, and substantially less than 0.1% of sodium, said alloy having at least in part a beta structure and having improved ductility because of the low sodium'content.

3. A magnesium-base alloy comprising at least 70% of magnesium, from 3.6% to 12% of lithium, from 1% to 8% of aluminum, and substantially less than 0.1% of sodium, said alloy having at least in part a beta structure and having improved ductility because of the low sodium content.

ALFRED C. LOONAM.

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

UNITED STATES PATENTS Number Name Date 2,317,980 Dean May 4, 1943 2,376,868 Dean May 29, 1945 FOREIGN PATENTS Number Country Date 361,086 Germany Oct. 10, 1922 OTHER REFERENCES American Magnesium Corporation, Magnesium, First edition, pub. 1923 at Niagara Falls, N. Y. Pages 6 and 7.

Henry et al., The Lithium-lvlagnesium Equilibrium Diagram Trans. American Inst. Min. & Met. Engrs, 1934.. Vol. 111, pages 319 through 332.

Gruber et al., Zeitschrift fur Elektrochemie, 1934, vol. 40, pages and through 164.