POLYMORPHISM

Material Science Volume 1 of 2
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POLYMORPHISM DOE-HDBK-1017/1-93 Structure of Metals POLYMORPHISM Metals are capable of existing in more than one form at a time. This chapter will discuss this property of metals. EO 1.7 DEFINE the term polymorphism. EO 1.8 IDENTIFY the ranges and names for the three polymorphism phases associated with uranium metal. EO 1.9 IDENTIFY the polymorphism phase that prevents pure uranium from being used as fuel. Polymorphism is the property Figure 5 Cooling Curve for Unalloyed Uranium or ability of a metal to exist in two or more crystalline forms depending upon temperature and composition. Most metals and metal alloys exhibit this property. Uranium is a good example of a metal that exhibits polymorphism. Uranium metal can exist in three different crystalline structures. Each structure exists at a specific phase, as illustrated in Figure 5. 1. The alpha phase, from room temperature to 663C 2. The beta phase, from 663C to 764C 3. The gamma phase, from 764C to its melting point of 1133C MS-01 Page 12 Rev. 0

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