Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966. Page: 22 of 47
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more confirmed, the high-temperature annealed providing a coarse grain size
after a following beta treatment.
The next natural question was that of how the particle size of the precipitates
exerted its influence on the alpha grain size resulting from a later beta heat
treatment. Soaking studies in the beta phase were conducted to determine the
effect on the alloy precipitates; and it was learned that they tended to
dissolve, the required time being related to the alloy level. With 300 Fe and
200 Si, a period of at least six hours was needed to dissolve the precipitates
completely. Interestingly enough, however, even with complete solution and a
prolonged beta phase soak (carried in some instances to 82 hours at 1350 F.),
the alpha grain size resulting. from a quench from the beta after such prolonged
treatments was still fine grained if the precipitates had been unagglomerated
prior to being heated to the beta temperature. A different behavior was
observed, however, on multiple beta treatments. In such a procedure, the final
alpha grain size tended to coarsen as the number of quenches increased, leading
to an ultimate stable grain size regardless of the total number of cycles into
the beta phase. This final grain size was appreciably coarser than that
existing after the initial beta quench.
On the basis of these early experiments, a certain number of tentative
conclusions was drawn: (1) It seemed apparent that the size and quantity of
precipitates as determined by the condition of time and temperature experienced
in the alpha phase did, in turn, control the number of beta grains later
nucleated in undergoing transformation from the alpha to beta phase; (2) The
beta grain size once established did.not appear to undergo grain growth to any
appreciable extent regardless of the-time at temperature in the beta phase; (3)
Since with an adequate beta soak, the original array of precipitates would be
completely dissolved, the final grain size on quenching from the beta phase was
therefore presumed to be determined by the beta grain size itself and not by any
function fulfilled by the original precipitate system other than the establish-
ment of that same beta grain size (There. are other aspects of beta-to-alpha
transformation not considered here that will be discussed in a later section
having to do with the mode of transformation as determined by cooling rate.
This preliminary deduction, however, appeared to satisfy the need of the moment
and, in fact, still appears to hold true.); (4) The coarsening behavior observed
after multiple beta heat.treatment might be. interpreted as meaning that in
conditions. leading sto complete dissolution of the precipitate and a minimum
opportunity for reprecipitation in the-alpha range before a subsequent beta
treatment, a portion of the effect of alloy in the uranium could be considered
to be suppressed, and a..coarser grain size could be expected.
The role of the precipitate as a reversible phenomenon was rather readily
demonstrated by a number of experimental heat-treatments.S,15 In these experi-
ments, it was determined. that uranium treated with a standard iron-silicon
additive could be altered in grain- size successively as desired by selecting an
appropriate annealing temperature in the alpha range as previously quoted. The
evidence showed that a fine grain size could be always achieved by annealing any
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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/m1/22/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.