Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966. Page: 25 of 47
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and analysis were not particularly successful with the precipitates encountered
in the uranium system because of their extreme fineness.- It was, therefore,
extremely fortunate that an electron microprobe unit became available at the
Weldon Spring laboratory in time for use in the phase diagram work. This
instrument, after important modification by the staff,6,17,-18,19,2s became
capable of determining the composition of precipitates as small as micronss in
diameter, including the identification and analysis of substitutional impurities
in the compounds normally to within + 100 ppm but, on occasion to within
+ 15 ppm (i.e., to + .0015 w/o of the precipitate composition). It can be
appreciated that such precision in analysis of a phase which, in itself,
represents only a minute fraction of the total sample, provided an unparalleled
advantage in pursuing the nature of the microconstituents in these systems.
Binary Alloy Systems
Experimental alloys in the U-Al, U-Si, and U-Fe systems were prepared by
thermite reduction of UF4 with magnesium in four-kilogram, laboratory-scale
bombs. Alloys of aluminum ranged from < 10 ppm to a maximum of 4000, silicon
alloys extended from 16 ppm to 5000, and iron compositions varied from 18 ppm
to 5000. Details of the constitutional diagrams were explored, employing
microprobe analyses, differential thermal analyses, and optical
metallography.,2122,23,53,54,55,58,59 The samples used in optical metallo-
graphic and electron probe analyses were equilibrated for 75 hours in vacuum
(50 hours in the case of the silicon alloy) at the.selected temperatures in the
three phases under study and then, breaking the vacuum with helium, immediately
Solid solubility limits for aluminum, silicon, and iron were determined in each
allotropic phase except that, in the case of aluminum, a precise determination
in the alpha phase was not possible because of the existence of a very fine and
uniformly dispersed precipitate (believed to be U-Al2) which prevented the
location of areas in the matrix sufficiently free of precipitate to permit a
realistic electron probe analysis of the matrix itself.
Each system showed an increasing solubility limit for the additive element in
question in each phase with a marked step-wise advance in solubility in general
in undergoing transformation from alpha to beta and from beta to gamma phases.
The U-Si system was found to be unusual in one respect in that transformation
from beta to gamma reduced the solubility limit for silicon to almost one-half
of the level in the beta phase just below the transformation temperature. This
arises because of a peritectoid reaction in which gamma 'uranium plus U3Si react
to form beta uranium and vice versa.
In the more general case, the higher solubility limit in the higher temperature
phases is, of course, most significant in considering the need for precipitation
of compounds for the purpose of grain size control. Iron and'silicon within the
limits which have been used are completely soluble in the beta phase; but the
behavior of aluminum is very much a function of beta phase temperature. At a
4* ** r t * *. .. . .. . 4
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Fellows, J. A. Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966., report, January 1, 1966; Weldon Spring, Missouri. (https://digital.library.unt.edu/ark:/67531/metadc1033773/m1/25/: accessed March 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.