US3020154A - Aluminum alloy - Google Patents

Description

3,020,154 Patented Feb. 6, 1962 ALLOY Noble N. Ida, Towson, Md., assignor to Martin-Marietta Corporation, a corporation of Maryland No Drawing. Filed Apr. 24, 1953, Ser. No. 730,535 3 Claims. (Cl. 75-138) This invention relates to aluminum-base alloys especially suited for use in pressurized-water power reactors, and a method of preparing same.

Because extreme thermal and nuclear conditions obtain in the central region of a reactor, the selection of materials from which cores may be fabricated is limited. In research and critical reactors, which operate at low temperatures, core elements are generally constructed of aluminum in preference to stainless steel or zirconium. The application of aluminum metals as fuel element cladding reduces reprocessing costs appreciably below that for stainless steel or zirconium elements. Aluminum and aluminum-base alloys recommend themselves also because of their low thermal neutron absorption cross sections, relative stability towards radiation effects and excellent fabn'cabi-lity characteristics.

In view of these outstanding properties it would be highly desirable to utilize aluminum in pressurized-water power reactor cores. However, at the temperatures usually attained in such reactors, from about 500 F. upwards, almost all commercial aluminum alloys are severely corroded by water. Consequently, stainless steel and an. alloy of zirconium, Zircaloy 2, are the materials of choice. Both are considerably more expensive than alu minum. For example, an estimated saving of 80% in materials cost would be realized by cladding fuel elements with aluminum rather than zirconium. In comparison with stainless steel, aluminum has a much smaller absorption cross section for thermal neutrons, so that for a given power output less fissionable material is required in aluminum reactor cores than in stainless steel cores of similar design.

It is the purpose of this invention to provide aluminumbase alloys having excellent water-corrosion resistance and strength up to about 700 F. These alloys are especially useful in pressurized-water reactors as cladding for fuel elements, and are characterized by small thermal neutron absorption cross sections, excellent fabricability and low materials and reprocessing costs. It is expected that these alloys will be relatively stable towards radiation effects.

In accordance with the present invention an aluminum alloy having superior resistance to water corrosion up to about 700 F. as compared with existing commercial aluminum alloys is prepared from aluminum, nickel and titanium with or without additions of silicon and niobium.

The ranges by weight percent of the ingredients of the alloys are as follows:

Nickel 0.5 5.0

Titanium 0.5 3.5 Silicon 0.0- 1.0 Niobium 0.0 2.5 Aluminum Balance The general practise, heretofore, was to limit the percentages of titanium and nickel in aluminum to approximately 0.3% and 1.5%, respectively, since higher concentrations tend to cause segregation in and embrittlement of the resulting alloy. Accordingly, persons skilled in the art will readily appreciate the fact that unusually large amounts of nickel and titanium are used in the practise of this invention. Contrary to a priori predictions it has been determined that embrittlement and segregation can be largely inhibited it special measures are taken to insure uniform distribution of the precipitating phases and to suppress the formation of large grains during castmg.

A preferred melting practise followed in the preparation of these alloys employs standard apparatus. Alloying and pouring may be performed in air. After homogenation of the melt, it is cleared of dross, cast, partially cooled and rapid-quenched in water. In this manner grain growth is minimized and precipitation of intermetallics occurs uniformly throughout the cast ingot.

Although the accumulation of oxide dross during air melting of these alloys is not excessive, it can be reduced by the usual methods. For example, just prior to pouring, chlorine may be bubbled through the melt. Again, an empirically determined amount of AlCl or CaF about 2% to 10% by weight of the over-all melt, may be added.

A particular melting program found satisfactory will now be described. Predetermined quantities of aluminum, nickel and titanium, together with any additions of silicon or niobium or both, are melted in a graphite or zirconia crucible in a high-frequency induction-heated air melting furnace, and brought to a temperature of about 1450 F. This temperature is held for 2 to 5 hours so that melting and homogenation are complete. At the end of this period, dross is removed manually and the molten alloy is poured into a stainless steel mold. The mold may be painted with zirconia or other suitable refractory material to eliminate sticking of the ingot to the mold. The ingot, at a temperature of from 900 to 1100 F., is quenched in water. Grain growth and phase segregation are kept to a minimum by quenching at the higher temperatures. It has been observed, however, that neither phenomena present a problem if the ingot is not allowed to cool below about 900 F. before quenching. The cast structure is then homogenized at a temperature between about 500 C. and 600 C. for a minimum of six hours.

Microscopic and analytical examinations of homogenized ingots prepared by the above method have established that precipitation of the various phases is uniform throughout, with grain size averaging about ASTM No. 4. In the ternary alloys, i.e., Al-Ni-Ti alloys, TiAl and NiAl structures have been identified.

ingots prepared as above are readily rolled into strip of. any desired thickness provided precautions are taken to prevent stringering and consequent embrittlement, which would cause edge cracking. I have found that hot rolling at about 450 C. gives excellent results. Prior to working, the ingot is preannealed for one hour at about 500-60 0 C. The annealed ingot is then given several reduction passes with intermediate half-hour anneals at 450 C. so that a working temperature of approximately 450 C. is maintained. Depending upon the size of the rollers, reductions of up to 60% per pass are possible. It is preferred to hot roll until a thickness of 0.250 inch is reached, and solution heat-treat the rolled strip at 620 F. for 2 hours. This soaking step is followed by a rapid quench in water. Further reduction can then be accomplished by cold rolling. In order to remove the effects of cold working, the final strip may be annealed at 800 F. for about one-half hour.

The compositions by Weight percent of several representative melts are as follows:

A series of corrosion tests on alloys of this invention and comparative studies of two aluminum alloys presently marketed were carried on simultaneously. I The two it is apparent that the subject alloys will have longer lifetimes in aqueous environments than the best aluminum alloys presently available for such use. In pressurizedwater reactors, for example, 10 mil fuel element claddings of either G or F can be expected to be serviceable for a period of at least one year. Cladding with 10 mils of alloy B extends this period considerably, such that, prescinding from a consideration of radiation effects, the cladding of alloy B will have a lifetime twenty times greater thanrthat of either commercial alloy.

The present alloys also exhibit a remarkable improvement in mechanical properties as compared to F and G.

alloys are designated herein as F and G, were specifically Temp Ultimate developed for high temperature water systems. Their Alloy g sg compositions are as follows:

200 25, 500 Alloy Composition by Weight Percent 500 11,700 600 6,385 000 9,000 F 6 1 Cu, 0.3 M11, 0.1 v, 0.15 Zr, balance A1. 600 7, 220 G 1 0 Ni, 0.5 Fe, 0.2 Si, balance 1. (S00 8, 385

Corrosion test specimens cut from a rolled sheet were prepared by conventional cleaning methods. The specimens were placed in an autoclave maintained at con stant pressure and temperature, and, at the end of each run, examined for surface appearance, flaking, penetration and weight loss. In no instance did the subject alloys exhibit any sign of a white corrosion product; rather a very thin and tightly adherent protective oxide layer having a dark or jet black appearance was formed. In contrast, white flaky corrosion products were present on the two commercial alloys. Other corrosion data are presented in Table I.

. TABLE I Typical water corrosion rates of the present alloys compared with F and G On the basis of the corrosion results listed in Table I Increases in high temperature tensile strength of 30% or more have been realized. Several specimens, fully annealed, were pulled at various temperatures, and the results are listed above.

I claim:

1. An aluminum alloy consisting by weight of about 0.5% to 2.0% nickel, 1.0% to 3.5% titanium and the balance being essentially aluminum.

2. An aluminum alloy consisting by weight of about 0.5% to 2.0% nickel, about 1.0% to 3.5% titanium, not more than about 0.5% silicon and the balance being essentially aluminum.

3. An aluminum alloy consisting by weight of about 0.5% to 2.0% nickel, about 1.0% to 3.5% titanium, silicon and niobium, each in amounts not more than about 0.5%, and the balance being essentially aluminum.

References Cited in the file of this patent V UNITED STATES PATENTS 2,263,823 Bonsack et al Nov. 25, 1941 2,463,022 Cooper et al Mar. 1, 1949 2,578,098 Southard Dec. 11, 1951 2,579,369 Dawe Dec. 18, 1951 2,781,261 Kamlet Feb. 12, 1957 2,871,176 Draley et al. Jan. 27, 1959

Claims (1)

1. AN ALUMINUM ALLOY CONSISTING BY WEIGHT OF ABOUT 0.5% TO 2.0% NICKEL, 1.0% TO 3.5% TITANIUM AND THE BALANCE BEING ESSENTIALLY ALUMINUM.