The beta () phase of uranium occurs at elevated temperatures. This phase has a tetragonal (having four angles and four sides) lattice structure and is quite complex.

Gamma Phase

The gamma () 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.

Additional Examples

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%.

Summary

The important information in this chapter is summarized below.

Polymorphism Summary

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.

ALLOYS

Most of the materials used in structural engineering or component fabrication are metals. Alloying is a common practice because metallic bonds allow joining of different types of metals.

EO 1.10 DEFINE the term alloy.

EO 1.11 DESCRIBE an alloy as to the three possible microstructures and the two general characteristics as compared to pure metals.

EO 1.12 IDENTIFY the two desirable properties of type 304 stainless steel.

Al

An alloy is a mixture of two or more materials, at least one of which is a metal. Alloys can have a microstructure consisting of solid solutions, where secondary atoms are introduced as substitutionals or interstitials (discussed further in the next chapter and Module 5, Plant Materials) in a crystal lattice. An alloy might also be a crystal with a metallic compound at each lattice point. In addition, alloys may be composed of secondary crystals imbedded in a primary polycrystalline matrix. This type of alloy is called a composite (although the term "composite" does not necessarily imply that the component materials are metals). Module 2, Properties of Metals, discusses how different elements change the physical properties of a metal.