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LOGAM

LOGAM. STRUKTUR LOGAM. SUSUNAN ATOM-ATOM DALAM KRISTAL LOGAM. Susunan 2 dimensi antara lain : Bujur sangkar heksagonal. DALAM 2 DIMENSI SUSUNAN BUJUR SANGKAR : Setiap atom logam bersingungan dengan 4 atom sejenis Bukan susunan rapat SUSUNAN HEKSAGONAL

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LOGAM

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  1. LOGAM STRUKTUR LOGAM

  2. SUSUNAN ATOM-ATOM DALAM KRISTAL LOGAM • Susunan 2 dimensiantara lain : • Bujursangkar • heksagonal

  3. DALAM 2 DIMENSI • SUSUNAN BUJUR SANGKAR : • Setiap atom logambersingungandengan 4 atom sejenis • Bukansusunanrapat • SUSUNAN HEKSAGONAL • Setiap atom logambersinggungandengan 6 atom sejenis • susunanrapat (close packing atau closest packed) • lebihrapatdibandingkanbujursangkar • Padastrukturdiatasterdapat tempat2 lowong yang disebutinterstitial site (tempatselilitan)

  4.  Metallic Crystal Structures • How can we stack metal atoms to minimize empty space? 2-dimensions vs. Now stack these 2-D layers to make 3-D structures

  5. Atom-atom logamdalamsusunanhcp 2 dimensimembentuksuatulapisan • Lapisanakanmenempatiselilitan • Padasusunanrapat 3 dimensiada 2 macam: • Hcp = hexagonal close packed • Ccp = cubic close packed

  6. HCP • Menunjukkanhcpdari lap A dan B • B menempatiselilitan A • Setiap atom logambersinggungandengan 6 atom darilapisan yang samadan 3 atom dari lap atasdan 3 atom dari lap bawah • Bilangankoord = 12

  7. This means that a total of 12(1/6) + 2(1/2) + 3 = 6 atoms are inside the unit cell

  8. TempatSelilitan HCP • Tempatselilitan Tetrahedral (T) • Terbentuk 3 atom padalapisan A dan 1 atom pada lap B • TempatselilitanOktahedral • Terbentukdari 3 atom pada lap A dan 3 atom pada lap B

  9. Tempatselilitanpada HCP

  10. Dari susunanhcpdapatdibuatselsatuanheksagonaldengan c = 1,633 a • Bila c ‡ 1,633 a selsatuannyatetapheksagonaltapibukansusunanrapat

  11. SUSUNAN RAPAT KUBUS • Gambardibawahmerupakanpembentukansusunanrapatkubusdari lap A, B, C • Tempatselilitandari lap A diisioleh atom-atom pada lap B • Tempatselilitanpada lap B diisioleh atom-atom lap C • Tempatselilitanpada lap C diisioleh atom-atom lap A • Dst ……ABCABCBC…. • susunanberulanginidisebutsusunanrapatkubus • Bilkoordinasi 12 • Terdapat 2 macamtempatselilitan Tetrahedral danoktahedral

  12. Dari susunanrapatkubusdapatdibuatselsatuan kubusberpusatmuka (face centered cubik = fcc) Atom-atom sejenismenempatipojok-pojokdanpusatmukakubus Selsatuanterdiridari 2 atom pada lap A 6 atom pada lap B dan 6 atom pada lap C

  13. Susunanlogamkubus yang bukansusunanrapat • Bcc • Sc = pc

  14. Simple Cubic Structure (SC) • Rare due to low packing density (only Po has this structure) • Close-packed directions are cube edges. • Coordination # = 6 (# nearest neighbors) Click once on image to start animation (Courtesy P.M. Anderson)

  15. Atomic Packing Factor (APF):SC volume atoms atom 4 a 3 unit cell p (0.5a) 3 R=0.5a volume close-packed directions unit cell contains 8 x 1/8 = 1 atom/unit cell Adapted from Fig. 3.24, Callister & Rethwisch 8e. Volume of atoms in unit cell* APF = Volume of unit cell *assume hard spheres • APF for a simple cubic structure = 0.52 1 APF = 3 a

  16. Body Centered Cubic Structure (BCC) • Atoms touch each other along cube diagonals. --Note: All atoms are identical; the center atom is shaded differently only for ease of viewing. ex: Cr, W, Fe (), Tantalum, Molybdenum • Coordination # = 8 Adapted from Fig. 3.2, Callister & Rethwisch 8e. Click once on image to start animation (Courtesy P.M. Anderson) 2 atoms/unit cell: 1 center + 8 corners x 1/8

  17. Atomic Packing Factor: BCC a 3 a 2 Close-packed directions: R 3 a length = 4R = a atoms volume 4 3 p ( 3 a/4 ) 2 unit cell atom 3 APF = volume 3 a unit cell • APF for a body-centered cubic structure = 0.68 a Adapted from Fig. 3.2(a), Callister & Rethwisch 8e.

  18. Face Centered Cubic Structure (FCC) • Atoms touch each other along face diagonals. --Note: All atoms are identical; the face-centered atoms are shaded differently only for ease of viewing. ex: Al, Cu, Au, Pb, Ni, Pt, Ag • Coordination # = 12 Adapted from Fig. 3.1, Callister & Rethwisch 8e. Click once on image to start animation 4 atoms/unit cell: 6 face x 1/2 + 8 corners x 1/8 (Courtesy P.M. Anderson)

  19. Atomic Packing Factor: FCC 2 a Unit cell contains: 6 x1/2 + 8 x1/8 = 4 atoms/unit cell a atoms volume 4 3 p ( 2 a/4 ) 4 unit cell atom 3 APF = volume 3 a unit cell • APF for a face-centered cubic structure = 0.74 maximum achievable APF Close-packed directions: 2 a length = 4R = Adapted from Fig. 3.1(a), Callister & Rethwisch 8e.

  20. B B B B C C C A A A B B B B B B B A sites C C C C C C B B sites sites B B B B C sites FCC Stacking Sequence • ABCABC... Stacking Sequence • 2D Projection A • FCC Unit Cell B C

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