Chapter 3_Part_3

Chapter 3_Part_3 - Cd, Mg, Zn, Ti have this crystal...

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11 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids Body-Centered Cubic (BCC) Crystal Structure (I) Atom at each corner and at center of cubic unit cell Cr, α -Fe, Mo have this crystal structure
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12 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids ¾ The hard spheres touch one another along cube diagonal the cube edge length, a= 4R/ 3 ¾ The coordination number, CN = 8 ¾ Number of atoms per unit cell, n = 2 Center atom (1) shared by no other cells: 1 x 1 = 1 8 corner atoms shared by eight cells: 8 x 1/8 = 1 ¾ Atomic packing factor, APF = 0.68 ¾ Corner and center atoms are equivalent Body-Centered Cubic Crystal Structure (II) a
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13 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids Hexagonal Close-Packed Crystal Structure (I) ¾ HCP is one more common structure of metallic crystals ¾ Six atoms form regular hexagon, surrounding one atom in center. Another plane is situated halfway up unit cell (c-axis), with 3 additional atoms situated at interstices of hexagonal (close-packed) planes
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Unformatted text preview: Cd, Mg, Zn, Ti have this crystal structure 14 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids Unit cell has two lattice parameters a and c. Ideal ratio c/a = 1.633 The coordination number, CN = 12 (same as in FCC) Number of atoms per unit cell, n = 6 . 3 mid-plane atoms shared by no other cells: 3 x 1 = 3 12 hexagonal corner atoms shared by 6 cells: 12 x 1/6 = 2 2 top/bottom plane center atoms shared by 2 cells: 2 x 1/2 = 1 Atomic packing factor, APF = 0.74 (same as in FCC) All atoms are equivalent Hexagonal Close-Packed Crystal Structure (II) a c 15 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids Both FCC and HCP crystal structures have atomic packing factors of 0.74 (maximum possible value) Both FCC and HCP crystal structures may be generated by the stacking of close-packed planes The difference between the two structures is in the stacking sequence Close-packed Structures (FCC and HCP) HCP: ABABAB. .. FCC: ABCABCABC...
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Chapter 3_Part_3 - Cd, Mg, Zn, Ti have this crystal...

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