Additional Examples

Alpha Phase

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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 907C causes a change from BCC (alpha, ferrite) iron to the FCC (gamma, austenite) form. 2. Zirconium is HCP (alpha) up to 863C, 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 663C Beta - 663C to 764C Gamma - 764C to 1133C Alpha phase prevents pure uranium from being used as fuel because of expansion properties. MS-01 Page 14 Rev. 0

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