Structure of MetalsDOE-HDBK-1017/1-93POLMORPHISMThe alpha (a) phase is stable at room temperature and has a crystal system characterizedby three unequal axes at right angles.In the alpha phase, the properties of the lattice are different in the X, Y, and Z axes.This is because of the regular recurring state of the atoms is different. Because of thiscondition, when heated the phase expands in the X and Z directions and shrinks in theY direction. Figure 6 shows what happens to the dimensions (Å = angstrom, onehundred-millionth of a centimeter) of a unit cell of alpha uranium upon being heated.As shown, heating and cooling of alpha phase uranium can lead to drastic dimensionalchanges and gross distortions of the metal. Thus, pure uranium is not used as a fuel,but only in alloys or compounds. Figure 6 Change in Alpha Uranium Upon Heating From 0 to 300CThe beta (b) phase of uranium occurs at elevated temperatures. This phase has atetragonal (having four angles and four sides) lattice structure and is quite complex.Rev. 0Page 13MS-01
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