US3528806A - Method for producing binary aluminium-niobium alloys - Google Patents

Description

United States Patent 3,528,806 METHOD FOR PRODUCING BINARY ALUMINIUM-NIOBIUM ALLOYS Giorgio Beghi, Gavirate, and Giovanni Piatti, Varese,

Italy, assignors to European Atomic Energy Community (Euratom), Brussels, Belgium No Drawing. Filed July 10, 1967, Ser. No. 652,038 Claims priority, application Italy, July 25, 1966, 17,089/66 Int. Cl. C22c 1/02, 21/00 US. Cl. 75138 4 Claims ABSTRACT OF THE DISCLOSURE A process for obtaining binary aluminum-niobium alloys having improved dimensional stability under high heat conditions, rendering them useful for especially critical applications. The process involves producing such an alloy including an aluminum matrix containing a dispersion of aluminum-niobium particles in certain specified quantity relationships by melting the constituents of the alloy at a temperature above 1800 C. and then casting the molten material under conditions of high speed cooling upon a body of high heat conductivity such as a thick copper body and thereafter compacting the alloys so obtained at a temperature of at least 580 C. In cases where a liquid is present the temperature is restricted to not more than 750 C.

Alloys with an aluminium matrix attract great interest in the nuclear field, particularly (but not exclusively) as the sheaths of fuel elements, mainly because of the small neutron-capture cross-section of this element. The compatibility with organic liquids (mixtures of terphenyls) up to temperatures of about 450 C. is also good. The conditions that these alloys must satisfy to be employed usefully in reactors are essentially as follows:

(a) Good mechanical properties up to a temperature of 450-500 C., so as to be able to withstand all the influences to which the sheaths are subjected over very long periods (several years), and

(b) Good manufacturing and processing characteristics (extrusion, turning, Welding, etc.) so that the preparation of the fuel elements or other structures will not be difiicult.

The only aluminium alloy so far considered in the nuclear field for the above purposes is SAP, made of sintered Al and A1 0 in different types whose A1 0 contents vary from 4% to 14% by weight. The mechanical strength of these alloys under heat is good, but there is the disadvantage of an elongation that decreases with the temperature to very low values, about 0.5% uniform elongation at 450 C. This makes it necessary to take great precautions when designing reactors to prevent the material from being deformed. Heat-resistant aluminium alloys, which have better deformation capacities than SAP, cannot be considered because of their very low mechanical strength at temperatures above 350 C.

A more advantageous solution of the problem would therefore be the provision of a material having a mechanical strength under heat similar to that of SAP together with a good deformation capacity, and also good heat stability ensuring good behaviour for a long time.

The solution proposed by the invention consists essentially in a binary aluminium-niobium alloy composed of a matrix of aluminium containing a fine, uniform dispersion of particles of Nb A1 measuring about 1 micron and having a Nb percentage of up to 20% by weight in relation to the total of the matrix and Nb A1 particles, preferably between 5% and 12% "ice The invention also provides a process for obtaining the above-mentioned alloy, according to which Nb A1 and A1 are melted completely at a temperature above 1800 (above the melting point of Nb A1 and this is followed by casting under conditions of high cooling speed and compaction under heat (at least 580-600, or in the presence of liquid metal, but no higher than 750 C.) and extrusion if desired.

The dispersion of Nb A1 in Al is very heat-stable at temperatures between about 400 C. and 500 C. for very long times of several thousand hours. The dispersion according to the invention may be produced by the following successive stages:

(1) The production at a high temperature of an Al-Nb alloy containing up to 20% of Nb,

(2) Rapid solidification from a high temperature (e.g. casting of ingots a few millimetres thick in very thick chill moulds of copper or other metal of high heat conductivity, spraying, splat cooling), the very high cooling speed producing a structure having particles of Nb, A1 of about 1 micron and even less, and

(3) The extrusion of ingots prepared with the pieces obtained as in (2).

Three examples, which do not limit the scope of the invention, will now be described.

EXAMPLE 1 A master alloy, consisting mainly of the compound Nb A1 was prepared by completely melting equal parts by weight of aluminium and niobium in a helium atmosphere in an arc furnace with a tungsten electrode. A high frequency generator was used to melt the master alloy, and aluminium was added in a quantity such that an alloy containing 10% by weight of niobium was obtained. The alloy was melted in a cylindrical graphite crucible under argon flux at a temperature higher than 1800 C. When melting was complete, the alloy was poured into a cool cylindrical copper mould, which had a diameter of 200 mm. and a height of 300 mm., and in which a recess 3 mm. thick and mm. long was formed. Because of the large mass of copper, very rapidly cooled sheets of alloy having a homogeneous structure were obtained. Discs and pieces were cut from the resulting castings and used, after suitable cleaning, to form an ingot having a diameter of 54 mm. This ingot, wrapped in thin aluminium foil, was preheated for 3 hours to 580 C. and then extruded in the form of a bar having a diameter of 9 mm. Tension specimens obtained from this bar had the following mechanical properties:

Temperature of test: 450 C. Uniform elongation: 2.5%. A 5 elongation: 60%

Under similar conditions, specimens, having the same dimensions, of SAP (4% of A1 0 and 7% of A1 0 yielded the following values:

Uniform elongation: 0.5% A 5 elongation: 9% and 5.5% respectively.

EXAMPLE 2 The master alloy in Example 1 was used. The alloy was melted in a horizontal graphite crucible heated by a high-frequency generator in an atmosphere with a slight excess pressure of argon. Al and the Nb A1 were put in a crucible in proportions such as to form alloys containing 10% of niobium; the material was melted at a temperature higher than 1800 C. Then, with manipulation from outside, the molten metal was poured onto vertically superimposed inclined copper discs with a slope such that the molten metal ran from one to the other, becoming subdivided. The copper structure was kept cold by internal water circulation. Solidified pellets and particles were obtained with a high cooling speed. The rest of the treatment was as in Example 1.

EXAMPLE 3 This was similar to Example 1, except that the alloy 5 was simply poured, after being melted in a graphite crucible, onto a copper plate 20 mm. thick. The plate was horizontal but it may, if preferred, be slightly inclined.

We claim:

1. A process for obtaining a binary aluminum-niobium alloy containing Nb A1 particles dispersed in an aluminum matrix and having a niobium content up to 20% by weight comprising the step of completely melting the constituents of the alloy at a temperature above 1800 C., casting the molten material under conditions of high cooling speed and compacting the obtained alloy at a temperature of at least 580 C. and when liquid is present, at no more than 750 C.

2. The process as claimed in claim 1 wherein the high speed cooling is effected by depositing the said molten material upon a metal body of high heat conductivity characteristics, such as copper moulds, or copper plates.

3. The process as claimed in claim 2 wherein the high speed cooling is eifected by delivery of the molten metal as a spray onto the said body.

4. The process as claimed in claim 2 wherein said cooling upon said body is effected by the technique known as splat cooling.

References Cited UNITED STATES PATENTS 3,231,344 1/1966 Beaver et a1. 75--l38 3,297,415 1/1967 Allen 75-138 3,360,350 12/1967 Sama 75138 RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 75.5