Tham khảo tài liệu 'material science_ vol 1 of 2 - us doe (1993) episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Structure of Metals DOE-HDBK-1017 1-93 POLMORPHISM Alpha Phase The alpha a phase is stable at room temperature and has a crystal system characterized by 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 this condition when heated the phase expands in the X and Z directions and shrinks in the Y direction. Figure 6 shows what happens to the dimensions Ả angstrom one hundred-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 dimensional changes and gross distortions of the metal. Thus pure uranium is not used as a fuel but only in alloys or compounds. Beta Phase The beta P phase of uranium occurs at elevated temperatures. This phase has a tetragonal having four angles and four sides lattice structure and is quite complex. Rev. 0 Page 13 MS-01 POLYMORPHISM DOE-HDBK-1017 1-93 Structure of Metals Gamma Phase The gamma y 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 907 C causes a change from BCC alpha ferrite iron to the FCC gamma austenite form. 2. Zirconium is HCP alpha up to 863 C 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 carbon whereas alpha iron can dissolve only . 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
