US3063834A - Magnesium alloys - Google Patents

Description

United States Patent M 3,063,834 MAGNESIUM ALLOYS Geolfrey Charles Edward Olds, Leicester, England, as-

signor to Associated Electrical Industries (Rugby) Lim-' ited, London, England, a British company No Drawing. Filed Oct. 20, 1958, Ser. No. 768,062 Claims priority, application Great Britain Oct. 25, 1957 2 Claims. (Cl. 75-468) This invention relates to magnesium alloys suitable, inter alia, for use as canning materials for the fuel element of nuclear reactors.

The important mechanical properties required in a canning material for uranium fuel in a thermal nuclear reactor are high creep ductility between 200 C. and 500 C.; small grain size and high stability of grain size up to 500 C.; freedom from excessive intercrystalline cavitation; high creep strength at low stresses at the highest operating temperatures and good weldability.

Alloys used for cans in natural uranium reactors must also possess very low neutron absorption cross-sections and for this reason magnesium is the most important metal to form the main constituent of any alloy used. Any alloying elements used must be such that the neutron absorption cross-section of the resulting alloy is adequately small. Magnesium alloys which have been considered up to now have been those in which the main alloying constituent is approximately 1% aluminium, magnesium alloys containing 0.5% zirconium, and those containing 0.5% zirconium with 0.5 %l% zinc.

The two alloys of magnesium with 0.5% zirconium, and magnesium with 0.5 zirconium and 0.5 %-1% zinc have many properties which make them suitable for reactor cans with the exception'that their creep strength at high temperatures of the order 500 C. is insufficiently high.

We have found that a small quantity of manganese alloyed in magnesium with zirconium or with zirconium and zinc produces an alloy with high creep resistance, yet still possessing the other desirable properties which are known to exist in binary magnesium-zirconium or ternary magnesium-zirconiumzinc alloys. These improved alloys a-re essentially single-phase alloys, which at high temperatures do not exhibit precipitation hardening which is usually associated with creep resistance.

An alloy according to the invention consists of zirconium, magnesium and manganese, and optionally zinc, the proportions, by weight, in the alloy lbeing 0.3-1.0% Zr, 0.050.5% Mn, 02.0% Zn and the remainder magnesium.

It is known that the useful liquid and solid solubility of Zr in Mg to form a single phase alloy is about 0.7%, above which percentage a zirconium precipitate is present in the final alloy. Also manganese has a limited liquid and solid solubility in magnesium, such that if the amount present in a binary alloy exceeds about 0.5% an undesirable precipitate is present in the alloy, which may lead to embrittlement. If Zr and Mn are both present in an Mg alloy (with or without the optional zinc content), each of the two elements limits the solubility of the other, i.e. a small Mn addition of a few tenths of 1% reduces the solubility of Zr to less than 0.7%. The presence of zinc in the quantities mentioned makes little difference to the aforementioned solubility and precipitation problems. The incorporation of Mn in the manufacture of Mg.Zr alloys (with or without zinc) therefore necessitates the reduction of the Zr content to less than 0.7%, both on grounds of obtaining the alloy elements in solution in the melt, and in producing a final alloy of single phase, which for the latter reason should also be free from serious em- 3,063,834 Patented Nov. 13, 1962 brittlement; TheZr andMn contents lof any Mg.Zr.Mn(Zn) alloys are, therefore, interdependent.

Furthermore, the preferred maximum Mn content for a canning material on grounds of neutron absorption economy is about 0.2%.

The preferred compositions according to this invention have been found to be within the following ranges: 0.3%-0.8% Zr, 0.05%0.3% Mn, remainder Mg.

In more detail, the following examples are given of preferred compositions which were found to be essentially single phase alloys:

Alloy (1) 0.5% Zr, 0.12% Alloy (2) 0.5% Zr, 0.12% Alloy (3) 0.3% Zr, 0.20% Mn, remainder Mg Alloy (4) 0.3% Zr, 0.20% Mn, 0.5% Zn, remainder Mg A comparison of the alloys given in Examples ('3) and (4), with the corresponding alloys given in Examples (1) and (2), illustrate the manner in which the proportion of Zirconium is reduced when the proportion of manganese is increased in order to allow for the mutual precipitation eflFects of these constituents.

The presence of zinc in the alloys improves the resistance to grain growth at high temperature, which if such growth is high leads to reduced ductility. The improve ment in creep resistance of the alloys containing manganese over their respective parent alloys of Mg-Zr(Zn) is shown by tensile test creep results as follows:

Mn, remainder Mg Mn, 0.5 Zn, remainder Mg Alloy Parent Mg. Zr. Zn.

5 999 oaeeulovmace The retention of adequately small grain size and resistance to serious grain growth at the upper temperature (500 C.), and the retention of adequate low temperature ductility (200 C.), in all the new alloys, both despite the reduced Zr content, is shown in the following tables. Also shown is the efiect of Zn in tending to promote even greater grain size stability.

Average Grain Diameter in Mierons after Annealing 500 hours at 500 0.

Alloy Percent Tensile Elongation at Fracture when Percent Tensile Elongation at Alloy Fracture when 3 4 Whatl elaim is: References Cited in the file of this patent l. An alloy consisting essentially, by Weight, of zirconi- UNITED STATES PATENTS um 0.5%, manganese 0.12%, and the remainder magnesi- 2,371,531 Mel Donald Mar. 13, 1945 2. An alloy consisting essentially of, by weight, zir- 5 2,788,272 Whltehead at 1957 conium 0.3%,manganese 0.2%, and the remainder mag- OTHER REFERENCES nesmm' The Influence of Zirconium Upon the solidification of Magnesium Alloys and Some of the Properties of Cast 10 Magnesium Alloys Containing Zirconium," by Franz Sauerwald. Zeitschrift fiir Metallkunde, vol. 40, 1949, pages 44 and 45.

Claims (1)

1. AN ALLOY CONSISTING ESSENTIALLY, BY WEIGHT, OF ZIRCONIUM 0.5%, MANGANESE 0.12%, AND THE REMAINDER MAGNESI-UM..