Chapter 3_Part_3

Chapter 3_Part_3 - ¾ 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: ¾ Cd, Mg, Zn, Ti have this crystal structure 14 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids ¾ Unit cell has two lattice parameters a and c. Ideal ratio c/a = 1.633 ¾ The coordination number, CN = 12 (same as in FCC) ¾ 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 ¾ Atomic packing factor, APF = 0.74 (same as in FCC) ¾ All atoms are equivalent Hexagonal Close-Packed Crystal Structure (II) a c 15 MSE 2090: Introduction to Materials Science Chapter 3, Structure of solids ¾ Both FCC and HCP crystal structures have atomic packing factors of 0.74 (maximum possible value) ¾ Both FCC and HCP crystal structures may be generated by the stacking of close-packed planes ¾ 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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This note was uploaded on 02/14/2012 for the course MSE 2090 taught by Professor Leoindzhiglei during the Fall '10 term at UVA.

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Chapter 3_Part_3 - ¾ Cd Mg Zn Ti have this crystal...

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