US2789072A - Heat treated uranium alloy and method of preparing same - Google Patents

Description

D. w. WHITE, JR

3 She ets-Sheet 1 450 n g Y e 6% 400 E 9/0 2 o g o a m 96/ g 350 u 7 I00 xdm;

" 05 l 5 l0 50 I00 500 I000 rmz-muurss INVENTOR. DONALD W. WHlTE,Jr.

ATTORNEY Aprii 16, 1957 D. w. WHITE, JR

.HEAT TREATED URANIUM ALLOY AND METHOD OF PREPARING SAME Filed D80. 22, 1952 3 SheetS-Shqet 2 5 TIME-MINUTES INVENTOR.

DONALD w. WHlTE,Jr. BY

ATTORNEY HEAT TREATED URANIUM ALLOY AND METHOD OF PREPARING SAME Donald W. White, Jr., Burnt Hills, N. Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Application December 22, 1952, Serial No. 327,248

6 Claims. (Cl. 148-13) This invention deals with a process of dimensionally stabilizing uranium-base metals, and in particular uranium-chromium alloys subjected to nuclear irradiation, and with the products obtained by the process.

It is known that surface and dimensional stability of uranium materials subjected to nuclear irradiation are dependent on a fine and uniform grain size arbitrarily arranged without preferred orientation. A fine grain size has been obtained heretofore in uranium by working the metal at so-called alpha-phase temperatures, that is, at temperatures up to about 660 C. However, this uranium material is highly oriented and is dimensionally unstable when subjected to nuclear irradiation. When uranium is heated to temperatures at which it exists in the so-called beta-phase, that is, between 660 and 770 C., and is thereafter quenched to an alpha-phase temperature, a material having a relatively random grain orientation with improved dimensional stability is obtained. This process forms the subject matter of copending application Serial No. 235,115, filed by Raymond Ward and Alden B. Grenginger on July 3, 1951. However, the grains are relatively large and as a result some surface roughening of the uranium is encountered upon nuclear irradiation.

A further improvement has been obtained by alloying the uranium with a small amount of chromium metal and then beta-heating and alpha-quenching the alloy. This latter improvement forms the subject matter of copending application Serial No. 314,734, filed on October 14, 1952, by John R. Keeler and Raymond Ward.

It is an object of the present invention to obtain further improvements in the surface and dimensional stability of uranium-base materials when subjected to neutron irradiation as for example, when used in neutronic reactors such as power piles.

It is another object of this invention to provide a uranium metal which contains crystals of a very small uniform size arranged in a non-oriental distribution.

It is still another object of this invention to provide a uranium metal which retains its smooth surface even after exposure to neutron irradiation.

It is furthermore an object of this invention to provide a uranium metal which is comparatively hard.

It is also an object of this invention to provide a uranium alloy possessing the combination of excellent surface and dimensional stability, only slight dilution of the uranium and a relatively low neutron capture cross section.

It is finally also an object of this invention to provide a uranium-base metal that has a high degree of dimensional stability and a smooth surface and retains these properties after repeated thermocycling.

These and other objects are accomplished by alloying the uranium with from 0.1 to 10 atomic percent of chromium metal, heating the alloy obtained at least to betaphase temperatures, quenching the alloy to a selected temperature of the alpha-phase range, this selected temperature ranging in general from about 400 C. to 525 C.,' and holding it at this temperature until transformaatent I another test.

tion to alpha-phase crystals is complete. However, on alloys containing less than 0.6 atomic percent chromium, the minimum temperature to which the alloys must be quenched is higher than 400 C.

A number of uranium-chromium alloys were prepared for investigating the effect of holding temperatures in the alpha-phase or isothermal transformation on the physical and mechanical characteristics. Pure uranium metal and electrolytic chromium were used for this purpose in varying proportions; the metals were vacuummelted in zircon crucibles and cast into graphite molds. The alloys obtained, after annealing at 550 C., were swaged in the form of rods l inch in diameter. During the swaging process the alloys were annealed at 550 C. after every percent reduction in area. The rods were cut into specimens of equal length, annealed between 700 and 720 C. for several hours and then furnacecooled.

The specimens, after being coated with a protective film of silicone grease, were converted to beta-phase condition by immersion in a tin-lead bath of a temperature above 630 C., specifically of about 715 C., for 15 minutes. Thereafter the specimens were cooled down to alpha-phase temperature by immersion in another tin-lead bath of corresponding alpha-phase temperature.

The transformation of uranium from beta-phase or the gamma-phase to the alpha-phase condition is always accompanied by contraction. This phenomenon was utilized to ascertain the progress of this transformation; measurements of the contraction were carried out in regular intervals with a dilatometer, and the specimens were held in the alpha-temperature bath until the dilatometer measurements indicated that no further contraction was taking place.

Four diiterent alloys were prepared and used for the investigation which led to this invention, namely alloys containing 0.3, 0.6, 1.8 and 4 atomic percent of chromium, respectively. The temperature selected for isothermal transformation treatment ranged between and.

630 C. The periods of time required for the begining and completion of phase transformation at the various temperatures were determined. The findings were plotted as diagrams; they are shown in the accompanying drawings, Figures 1-4. In these diagrams, the temperatures used for the isothermal transformation are plotted on the ordinate, and the periods of time necessary for the beginning of alpha-phase formation (indicated by crosses) and those necessary for complete transformation (indicated by circles) are entered on the logarithmic-scale abscissa.

As previously mentioned, the specimens were directly quenched from the beta-phase to the alpha-phase temperature selected for isothermal transformation; however, for the alloy containing 0.3 atomic percent of chromium and the isothermal temperature of 100 C., uenching in ice water and reheating in boiling water were found necessary, because otherwise some transformation would take place in the transition through intermediate temperature ranges and thus give misleading results. Two control experiments indicated that this procedure did not bring about erroneous findings. These control experiments were carried out with an alloy containing 0.6 atomic percent of chromium, one set at 100 and one at 200 C.; in each instance, quenching was carried out to the temperature of isothermal treatment, directly in one test an d with ice water followed by reheating in In both instances identical results were obtained when quenching was carried out directly and when it was effected via the intermediate step of cooling in ice water. V I

The curves shown in the drawings are single-C- shaped for the alloys having chromium contents of 1.8

and 4 atomic percent and double-C-shaped for those having 0.3 and 0.6 atomic percent of chromium. Considering the upper Cs of the double-6 curves, it will be obvious that the maximum rate, of transformation in all four alloys is at about 570 C., while the maximum rate of transformation in the two lower-C sections is located between 250 and 300 C. This lower maximum rate is faster than that of the upper Cs; however, it was found that the grain size, when the isothermal treatment was carried out within the range of the lower US, was very irregular, while a uniform and finer grain size was obtained when treatment was carried out in the lower portion of the upper Cs. Since the dimensional stability is closely related to, and dependent upon, the grain size, treatment in the lower portion of the upper range is required.

Actually, while the curves in Figures 3 and 4- are single-c shaped and those in Figures 1 and 2 are double-C-shaped, the single Cs in fact correspond to the upper Us in the double -shaped curves insofar as the practice of the present invention is concerned. In other words, in those cases where only a single C is shown, it is believed there actually exists a lower 0 curve at lower temperatures than those employed in the particular experiments.

In the following Table I the grain diameters of a uranium alloy containing 0.6 atomic percent of chromium are given as they were obtained by isothermal transformation treatment at various temperatures shown.

Table I Temperature of isothermal Position on diagram, transformation, O. Fig. 2

Upper C do....

This table indicates that the smallest grain sizes are obtained when the temperature of isothermal transformation is in the upper 0 up to slightly above 515 C. and that larger grains are obtained at higher temperatures as well as at all temperatures in the region of the lower 0. Optimum transformation temperature range for the 0.6 atomic percent-alloy is from about 400 C., i. e., the transition temperature between the upper and lower 0, and a maximum temperature of about 525 C. The upper limit of 525 C. holds for all of the alloys, but the lower limit varies with the chromium content and broadly may be defined as the transition temperature between the two Cs. However, as long times-at-temperature are required to obtain a complete transformation of alloys having a high chromium content even when operating well above the transition temperatures, for practical purposes the isothermal transformation treatment will usually be carried out at temperatures of at least 400 C.

i The chromium content itself was found also to have a slight bearing on the grain size. For instance, after isothermal treatment at 515 C., a relationship between chromium content and grain size was determined as shown in Table II. These data show that the grain size slightly decreases with increasing chromium content.

1.8 at. percent Cr 0.0 35 0.03

-In order to determine whether the above-described improvements are actually due to the isothermal treat- 4 at. percent Cr ment, the uranium-chromium alloys were beta-heated as described above, but then, instead of holding them at the alpha-temperature until complete transformation, they were merely quenched to room temperature. It was found that the optimum properties achieved by the isothermal treatment of this invention were not obtained by beta-heating and quenching alone. Moreover, the alloys having higher chromium contents (above 1.8 atomic percent) showed a tendency to crack when thermally cycled.

The hardness of the isothermally treated alloys was also found to be improved by the isothermal treatment, the lowest temperatures of the upper C's (and of the single Cs) yielding the highest hardness values. The hardness was also found to increase with increasing chromium content.

A convenient laboratory means for predicting dimensional stability of a uranium sample under irradiation is a thermal cycling test, in which the sample is repeatedly heated and cooled. As a rule a uranium material which exhibits dimensional or surface instability during this test will be similarly unstable under irradiation. Therefore, in order to determine the dimensional stability of some of the alloys treated by the process of this invention, isothermally treated specimens were sealed in Pyrex tubes in which a partial pressure of helium prevailed, and the specimens were then thermally cycled. The results are compiled in Table III.

Table III Temp. of Dimensional Sample Iso- Change, Percent Number Material thermal Surface Treatment, C. Diarn. Length percent CL... 449 +0. 13 +0. 06 A1 percent Cr.... 449 +0. 13 +0.10 A1 percent On... 489 +0.10 +0. 02 A2 percent CL... 489 +0. 08 0. 08 A2 percent Gr-.. 513 +0.05 0 A3 percent Cr.... 513 +0. 10 0 A3 percent Or 547 +0. 03 +0. 02 A4 1 .percent Or 547 +0.05 02 A4 percent Or 581 +0. 03 0. 02 A5 1 percent CL.-. 581 0. ()8 +0.02 A5 percent CL... 614 0. 28 0. 34 C percent 01".... 614. -0. 35 0. 40 C percent Cr.. +0. 48 +0.20 B4 percent CL.-. 265 +0. 83 0. 07 B3 percent CI.... 377 Specimen oxidized in test percent Cr.... 3% +0. 13 +0.20 B1 percent Or 452 +0. 28 +0. 60 B1 percent Cr.... 515 +0.08 +0. 91 B2 percent Gr.... 574 +0.20 +0. 49 B7 percent Cr.. 624 +0.58 +1. (34 C4 percent 01".... 100 +1.05 +0.20 G3 percent Gr.... 265 +0. 94 +0. 24 C2 percent Cr... 376 Specimen oxidized in percent CL... 452 +0. 70 +0. 38 C1 percent Cr.... 480 +0. 48 +0. 49 B6 percent Cr-... 515 0. 13 +0. 91 B3 percent Gr.... 574 +0. 40 +0. 47 B5 percent Or.... 624 +0.88 +1. 66 05 While 500 cycles between 100 and 500 C. were used for samples No. l-l2, samples No. 13-28 were subjected to 554 cycles between 100 and 525 C.

It will be noted that the dimensional changes of the samples prepared in accordance with the present invention are extremely low as compared with known uranium products. For example, when subjected to the thermal cycling test employed, a fine-grain uranium obtained by working the metal at so-called alpha-phase temperatures, that is, at temperatures up to about 660 C., will usually exhibit a change in length of about 15 to 20 percent while a uranium sample which has been heated to a beta-phase temperature of from 660 to 770 C. and then quenched to an alpha-phase temperature will change in length about l-2 percent and will also exhibit some surface roughening or blistering.

Table III, in addition to the dimensional changes, lists the surface conditions of the isothermally treated specimens in the last column, A indicating a very smooth, virtually unchanged surface, B a smooth surface, and C a rough surface comparable to that of the best unalloyed uranium. Minor variations within the A, B and C groups are shown by added numbers, 1 being the smoothest, 2 the next smoothest surface, and so on.

Table HI shows that with temperatures between 400 and 525 C., the finest grain size and the greatest dimensional plus surface stability were obtained.

The foregoing data also show that such uranium alloy-s can be used as structural members in devices, other than reactors, where the devices are subjected to such fluctuations of temperature and where unalloyed uranium would be unsuitable due to a greater change in dimensions and surfaces.

As has been mentioned in the beginning, the process of this invention is applicable to alloys containing from 1.0 to 10 atomic percent of chromium; it is especially well suitable for alloys containing from 0.3 to 4 atomic percent of chromium.

It will be understood that this invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.

What is claimed is:

1. A process of producing small grain size uranium of great dimensional and surface stability, comprising incorporating from 0.1 to 10 atomic percent of chromium into the uranium, heating the alloy obtained to an elevated temperature above the alpha-to-beta transformation temperature, rapidly cooling the alloy to a temperature of between about 525 and 400 C., and holding the alloy at this latter temperature until transformation to alpha-phase crystals is complete.

2. The process of claim 1 wherein the alloy contains from 0.3 to 4 atomic percent of chromium.

3. The process of claim 2 wherein the elevated temperature is between 660 and 770 C. and is maintained for 15 minutes.

4. The process of claim 3 wherein the elevated temperature is about 715 C.

5. A composition of matter obtained by the process of claim 1.

6. A composition of matter obtained by the process of claim 2.

References Cited in the file of this patent UNITED STATES PATENTS Morris July 31, 1956 OTHER REFERENCES Metallic Uranium, 14 pages, declassified September 23,

(Copy in Patent Office Library.)

Claims (1)

1. A PROCESS OF PRODUCING SMALL GRAIN SIZE URANIUM OF GREAT DIMENSIONAL AND SURFACE STABILITY, COMPRISING INCORPORATING FROM 0.1 TO 10 ATOMIC PERCENT OF CHROMIUM INTO THE URANIUM, HEATING THE ALLOY OBTAINED TO AN ELEVATED TEMPERATURE ABOVE THE ALPHA-TO-BETA TRANSFORMATION TEMPERATURE, RAPIDLY COOLING THE ALLOY TO A TEMPERTURE OF BETWEEN ABOUT 525 AND 400*C., AND HOLDING THE ALLOY AT THIS LATTER TEMPERATURE UNTIL TRANSFORMATION TO ALPHA-PHASE CRYSTALS IS COMPLETE.

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