Seven Years of Uranium Alloy Development at Weldon Spring, 1959/1966. Page: 38 of 47
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made it clear that the beta to alpha transformation could be temporarily
suppressed if sufficient. alloy was present permitting isothermal transformation
at temperatures well down within the alpha phase region. It was also known that
in such circumstances, more than one mode of transformation could be -
experienced. In this sense, the beta to alpha transformation taking place in
the region perhaps above 900*F. would be by a nucleation and growth process,
whereas at low temperatures, the transformation would proceed by a shear
mechanism similar to the martensitic transformation in steel. It was,
therefore, of interest to learn the specific characteristics of the alloys that
had shown most promise in reactor testing to determine what degree of control
would be possible during beta heat treatment to provide a given transformation
product if reactor experience should indicate that it would be particularly
A quenching dilatometer was built3O'58>>2>64 to permit preheating in an inert
atmosphere at temperatures within either the beta or the gamma phase and then
very rapidly quenching into an agitated molten metal bath to undergo isothermal
transformation at an assortment of selected temperatures in the alpha phase.
TTT diagrams have been defined for a number of alloys ranging from a very
simple iron-silicon composition to the more complex Sicral F-2 type. The
ability to retain the beta phase during the quench was, of course, a sensitive
function of the amount of alloy present. The low alloys transformed so
promptly during the quench that it was not possible to cool past the upper nose
of the TTT diagram without undergoing transformation. Such alloys, therefore,
can only be provided with a structure created by the nucleation and growth type
of transformation, particularly because the cooling rates possible in an actual
core blank are much slower than those attained in the quenching dilatometer.
With more alloy present, the system-was more sluggish,.and the beta phase.could
actually be retained to quite low temperatures to undergo transformation by the
martensitic mode. In the case of the Sicral F-2 with high aluminum, an even
more complicated type of diagram was revealed since there is now evidence that
a third nose or C in the TTT diagram exists at an intermediate temperature.
The difference in transformation is not explicitly understood as yet, but it
possibly bears some resemblance of the iron-carbon system in which an acicular-
type transformation product (bainite) just below the nose differs from the
pearlitic structures formed above the noze.
In conjunction with this experimental work, a number of cooling curves were
carefully .measured for centrifugally cast core blanks to permit correlation of
the cooling behavior of an actual core under different quenching conditions
with the possible structure that would be expected on the basis of the TTT
diagram data. For the Sicral F-2 composition,. as an example, this comparison
with the experience gained in isothermal quenching to eliminate grain clusters
has suggested that the ideal transformation temperature is at or just below the
upper nose of the TTT curve.
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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/m1/38/: accessed March 18, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.