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1、Why Study Structures of Metals and Ceramics?,properties of some materials,crystal structures,directly related by,For example:,Magnesium(Mg) Beryllium(Be),Having same crystal structure,more brittle than,Gold(Au) Silver(Ag),Having same crystal structure,Crystalline and Noncrystalline,Different propert
2、y,having the same composition but different structure,3.1 INTRODUCTION,Chapter 2 was concerned atomic bonding.,the electron structure of the individual atoms.,In this chapter,next level of the structure of materials the arrangements of atoms in the solid state,Introducing the concepts,crystal and no
3、ncrystal,For crystal structure,unit cell,crystallographic directions and planes,metals and ceramics,Single crystals and polycrystalline,非晶体,晶体,晶胞,单晶体,多晶体,3.2 Fundamental Concepts,Crystalline. The state of a solid material characterized by a periodic and repeating three-dimensional array of atoms, io
4、ns, or molecules.,long-range order,the atoms will position themselves in a repetitive three-dimensional pattern, in which each atom is bonded to its nearest-neighbor atoms.,All metals, many ceramic materials, and certain polymers form crystalline structures,Materials,crystalline,Noncrystalline or am
5、orphous,No long-range atomic order,长程有序,The description of crystalline,atomic hard sphere model,Unit Cells 晶胞,3.3 Unit Cells,hard sphere lattice unit cells,晶格,atomic hard sphere model :atoms (or ions) are thought of as being solid spheres having well-defined diameters.,lattice. The regular geometric
6、al arrangement of points in crystal space.(means a three-dimensional array of points coinciding with atom positions),unit cells:The basic structural unit of a crystal structure. It is generally defined in terms of atom (or ion) positions within a parallelepiped or prisms volume.,the lattice paramete
7、rs:The combination of unit cell edge lengths and interaxial angles that defines the unit cell geometry. a, b, c, and , , .,硬球模型,晶格,晶胞,晶格常数,平行六面体,棱柱,Crystal structure. For crystalline materials, the manner in which atoms or ions are arrayed in space. It is defined in terms of the unit cell geometry a
8、nd the atom positions within the unit cell.,A unit cell is chosen to represent the symmetry of the crystal structure, the unit cell is the basic structural unit or building block of the crystal structure,对称性,3.4 Metallic Crystal Structures,no restrictions as to the number and position of nearest-nei
9、ghbor atoms,dense atomic packings,Three relatively simple crystal structures,face-centered cubic, body-centered cubic, and hexagonal close-packed,1.The Face-centered Cubic Crystal Structure,A crystal structure found in some of the common elemental metals. Within the cubic unit cell, atoms are locate
10、d at all corner and face-centered positions.,面心立方晶格,无限制,原子密堆,FIGURE 3.1 For the face -centered cubic crystal structure: (a) a hard sphere unit cell representation, (b) a reduced-sphere unit cell, and (c) an aggregate of many atoms.,a=b=c and = = = 90o atoms located at each of the corners and the cen
11、ters of all the cube faces.,the face-centered cubic (FCC),1).the cube edge length a and the atomic radius R are related through,2).The number of atom in unit cell (n):,原子半径,晶胞 原子数,3). Coordination Number (CN):The number of atomic or ionic nearest neighbors. For “FCC” crystal structure: CN = 12,配位数,4
12、) Atomic Packing Factor (APF): The fraction of the volume of a unit cell that is occupied by hard sphere atoms or ions.,紧密系数(致密度),2.The Body-centered Cubic Crystal Structure,body-centered cubic (BCC),a=b=c and = = = 90o atoms are located at corner and cell center positions.,FIGURE 3.2 For the body-c
13、entered cubic crystal structure, (a) a hard sphere unit cell representation, (b) a reduced-sphere unit cell, and (c) an aggregate of many atoms.,体心立方晶格,1) the cube edge length a and the atomic radius R are related through,2) The number of atom in unit cell (n):,3) CN = 8,4),3.The Hexagonal Close-pac
14、ked Crystal Structure,hexagonal close-packed (HCP):,The top and bottom faces of the unit cell consist of six atoms that form regular hexagons and surround a single atom in the center. Another plane that provides three additional atoms to the unit cell is situated between the top and bottom planes.,a
15、=bc c/a=1.633 and a = b = 90o g =120o,FIGURE 3.3 For the hexagonal close-packed crystal structure, (a) a reduced-sphere unit cell and (b) an aggregate of many atoms.,密排六方晶格,2).The number of atom in unit cell (n):,3) CN = 12,4),1).the cube edge length a and the atomic radius R are related through,a,c
16、,铬,镉,钴,3.5 Density Computationsmetals,n: number of atoms associated with each unit cell A: atomic weight Vc: volume of the unit cell NA: Avogadros number (6.0231023 atoms/mol),EXAMPLE:,Copper,R = 0.128 nm (1.28 ),FCC,ACu = 63.5 g/mol,The literature value for the density of copper is 8.94 g/cm3,3.6 Ceramic Crystal Structures,Ceramics least two elements or more, crystal structures more complex,atomic bonding from purely ionic to totally covalent, sometimes, two
