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 Phases

Polymorphism is the property 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.

Figure 5 Cooling Curve for Unalloyed Uranium

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