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ALLOYS |
POLYMORPHISM
DOE-HDBK-1017/1-93
Structure of Metals
The gamma (g) phase of uranium is formed at temperatures above those required for
beta phase stability. In the gamma phase, the lattice structure is BCC and expands
equally in all directions when heated.
Two additional examples of polymorphism are listed below.
1.
Heating iron to 907°C causes a change from BCC (alpha, ferrite) iron
to the FCC (gamma, austenite) form.
2.
Zirconium is HCP (alpha) up to 863°C, where it transforms to the BCC
(beta, zirconium) form.
The properties of one polymorphic form of the same metal will differ from those of another
polymorphic form. For example, gamma iron can dissolve up to 1.7% carbon, whereas alpha
iron can dissolve only 0.03%.
The important information in this chapter is summarized below.
Polymorphism is the property or ability of a metal to exist in two or more
crystalline forms depending upon temperature and composition.
Metal can exist in three phases or crystalline structures.
Uranium metal phases are:
Alpha - Room temperature to 663°C
Beta - 663°C to 764°C
Gamma - 764°C to 1133°C
Alpha phase prevents pure uranium from being used as fuel because of
expansion properties.
MS-01
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