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FE Review Materials Science and Structure of Matter
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  1. FE ReviewMaterials Science and Structure of Matter Jeffrey W. Fergus Materials Engineering Office: 281 Wilmore Phone: 844-3405 email: jwfergus@eng.auburn.edu

  2. Organization from 1996-7 Review Manual(same topics in 2004 review manual) • Crystallography • Materials Testing • Metallurgy

  3. Crystallography • Crystal structure • atoms/unit cell • packing factor • coordination number • Atomic bonding • Radioactive decay

  4. c b a b g a Bravais Lattice Crystal System Centering (x,y,z): Fractional coordinates - proportion of axis length, not absolute distanct P: Primitive: (x,y,z) I: Body-centered: (x,y,z); (x+½,y+½,z+½) C: Base-centered: (x,y,z); (x+½,y+½,z) F: Face-centered: (x,y,z); (x+½,y+½,z) (x+½,y,z+½); (x,y+½,z+½) Centering must apply to all atoms in unit cell.

  5. Bravais Lattices (14)

  6. Atoms Per Unit Cell • Corners - shared by eight unit cells (x 1/8) • (0,0,0)=(1,0,0)=(0,1,0)=(0,0,1)=(1,1,0)=(1,0,1)=(0,1,1)=(1,1,1) • Edges - shared by four unit cells (x 1/4) • (0,0,½)= (1,0,½)= (0,1,½)= (1,1,½) • Faces - shared by two unit cells (x 1/2) • (½,½,0)= (½,½,1)

  7. Common Metal Structures • Face-Centered Cubic (FCC) • 8 corners x 1/8 + 6 faces x 1/2 • 1 + 3 = 4 atoms/u.c. • Body-Centered Cubic (BCC) • 8 corners x 1/8 + 1 center • 1 + 1 = 2 atoms/u.c. • Hexagonal Close-Packed (HCP) • 8 corners x 1/8 + 1 middle • 1 + 1 = 2 atoms/u.c. • 12 hex. Corner x 1/6 +2 face x 1/2 + 3 middle = 6 atoms/u.c.

  8. a r a Packing Factor • Fraction of space occupied by atoms • For FCC • For BCC

  9. Density For nickel: - Atomic weight = 58.71 g/mole - Lattice parameter = 3.5239 Å=3.5239 x 10-8 cm - Avogrado’s No. = 6.02 x 1023 = 0.602 x 1024 = atoms/mole

  10. Close Packed (CN=12) Highest packing density for same sized spheres FCC and HCP structures

  11. Cube Center (CN=8) Same atoms: BCC Different atoms: CsCl

  12. Octahedral Site (CN=6) In FCC: - Center (½,½,½) - Edges (0,0,½),(0,½,0),(½,0,0) - 4 per unit cell - All filled - NaCl structure 8-sided shape

  13. Tetrahedral Site (CN=4) In FCC: - Divide cell into 8 boxes - center of small box - (¼,¼,¼),(¾,¼,¼),(¼,¾,¼),(¾,¾,¼) (¼,¼, ¾)(¾,¼, ¾),(¼,¾, ¾)(¾,¾, ¾) -8 per unit cell -All filled - CaF2 structure; half-filled - ZnS 4-sided shape

  14. Radius Ratio Rules Critical radius is size of atom which just fits in site Define minimum for bonding (i.e. atoms must touch to bond) CN 8 Critical Radius for CN 8 = 0.732 CN 6 Critical Radius for CN 6 = 0.414 CN 4 Critical Radius for CN 4 = 0.225 CN 3 planar

  15. Close Packed Plane C A A B A B HCP: ABABABABABABABAB FCC: ABCABCABCABCABC Same packing density (0.74) Same coordination (CN=12)

  16. Miller Indices (hkl) specific - No commas - No fractions - Negative indicated by bar overnumber Planes {hkl} family [hkl] specific Directions <hkl> family A family of planes includes all planes which are equivalent by symmetry - depends on crystal system. - For cubic: (110),(011) and (101) are all {110} - For tetragonal: (011) and (101) are {101} but (110) is not (ca)

  17. c b a Miller Indices - Directions -1 1/2 x 1/2 y -1 z -1/3 -1/3 (x 6) 1/2 1 x 1 y 1/4 z 1/2 (x 4) 1/4

  18. Miller Indices - Planes intercept reciprocal x 1/4 4 y  0 z -1/2 -2

  19. Miller Indices - Planes intercept reciprocal x 1/4 4 y -1/3 -3 z -1/2 -2

  20. Covalent sharing electrons strong directional Ionic trading of elecrons electrostatic attraction or ions strong non-directional Metallic metal ions in sea or electrons moderately strong non-directional Secondary Van der Waals H-bonding electrostatic attraction of electric dipole (local charge distribution weak Atomic Bonding

  21. Radioactive Decay • Loss of electrons/protons/neutrons • alpha - 2 protons / two neutrons (i.e He nucleus) • beta - electrons • gamma - energy • Exponential decay time time constant amount half life original amount

  22. Material Testing • Stress-strain relationships • engineering stress and strain • stress-strain curve • Testing methods • tensile test • endurance test • impact test

  23. Stress F F F A A A Tension: >0 Compression: <0

  24. Strain a q lo l h l lo

  25. Tensile Test Control length (l) Measure force (F) with load cell Reduced section used to limit portion of sample undergoing deformation

  26. Stress-Strain Curve Ultimate Tensile Strength Force decreases due to necking Yield Point Elastic Limit Proportionality Limit Stress Slope = E (Young’s Modulus) Strain Percent Elongation (total plastic deformation)

  27. 0.2% Offset Yield Strength 0.2% offset yield strength Stress Strain 0.2% strain

  28. True/Engineering Stress/Strain Stress Strain Engineering (initial dimensions) True (instantaneous dimensions) Using and

  29. True/Engineering Stress/Strain True True stress does not decrease Engineering Decrease in engineering stress due to decreased load required in the reduces cross-sectional area of the neck. Stress Strain

  30. Strain Hardening Plastic deformation require larger load after defomation. Sample dimensions are decreased, so stress is even higher Stress Onset of plastic deformation after reloading Strain

  31. Bending Test Four-point Three-point F F/2 F/2 w h L L/2 By summing moment in cantilever beam Tension at bottom, compression at top

  32. Hardness • Resistance to plastic deformation • Related to yield strength • Most common indentation test • make indentation • measure size or depth of indentation • macro- and micro- tests • Scales: Rockwell, Brinell, Vickers, Knoop

  33. Impact Toughness: combination of strength and ductility - energy for fracture Charpy V-notch hi hf Fracture energy = mghi -mghf

  34. Ductile-Brittle Failure • Ductile • Plastic deformation • cup-cone / fibrous fracture surface • Brittle • little or no plastic deformation • cleaved fracture surface Ductile-Brittle Transition Tempeature (DBTT) Fracture Energy Temperature

  35. Creep / Stress Relaxation • Load below yield strength - elastic deformation only • Over long time plastic deformation occurs • Requires diffusion, so usually a high-temperature process • Activation energy, Q (or EA)

  36. Creep /Stress Relaxation Creep Stress Relaxation F F time time fixed strain F F fixed load Permanent deformation

  37. Fatigue Repeated application of load - number of cycles, rather than time important. smax 0 Fatigue Limit (ferrous metals) smin Stress save Ds smax Number of Cycles to Failure 0 smin

  38. Metallurgy • Corrosion • Diffusion • Phase Diagrams • lever rule • Fe-Fe3C diagram • Gibb’s phase rule • Thermal processing

  39. Corrosion Resistance • Thermodynamics vs. Kinetics • Thermodynamics - stable phases • Kinetic - rate to form stable phases • Active vs. Passive • Active: reaction products ions or gas - non protective • Passive: reaction products - protective layer • Corrosion resistance • Inert (noble): gold, platinum • Passivation: aluminum oxide (alumina) on aluminum, chromia on stainless steel

  40. Electrode Potential • Tendency of metal to give up electron • Oxidation (anode) • M = M2+ + 2e- (loss electrons) • Reduction (cathode) • M2+ + 2e- = M (gain electrons) • LEO (loss electrons oxidation) goes GER (gain electrons reduction)

  41. Corrosion Reactions • Oxidation - metal (anode) • M = M2+ + 2e- • Reduction - in solution (cathode) • 2H+ + 2e- = H2 • 2H+ + ½O2 + 2e- = H2O • H2O + ½O2 + 2e- = 2OH- • Overall Reactions • M + 2H+ =M2+ + H2 • M + 2H+ + ½O2 = M2+ + H2O • M + H2O + ½O2 = M2+ + 2OH- = M(OH)2

  42. Galvanic Corrosion / Protection • At joint between dissimiliar metals • reaction rate of active metal increases • reaction of less active metal decreases • Galvanic corrosion • high corrosion rate at galvanic couple • presence of Cu increase the local corrosion rate of Fe • Galvanic protection • Galvanized steel • presence of Zn decreases the local corrosion rate of Fe • Galvanic protection • Mg or Zn connected to Fe decrease corrosion rate

  43. Waterline Corrosion • Oxygen concentration in water leads to variation in local corrosion rates Higher corrosion rate near oxygen access Rust just below water surface Rings of rust left from water drops

  44. Diffusion • Atoms moving within solid state • Required defects (e.g. vacancies) • Diffusion thermally activated • Diffusion constant follows Ahrrenius relationship Activation Energy Boltzman’s constant Gas constant Temperature

  45. Steady-State Diffusion • Fick’s first law (1-D) • J = flux (amount/area/time) • For steady state DC Dx

  46. Phase Equilibria • Gibb’s Phase Rule • P + F = C + 2 (Police Force = Cops + 2) • P = number of phases • F = degrees of freedom • C = number of components (undivided units) • 2: Temperature and Pressure • One-component system • F = 1 + 2 - P = 3 - P • Two-component system • F = 2 + 2 - P = 4 - P • Two-component system at constant pressure • F = 2 + 1 - P = 3 - P “2” becomes “1” at constant pressure

  47. Pressure-Temperature Diagram One component: H2O If formation of H2 and O2 were considered there would be two components (H and O) Two-phase line: Change T (P) require specific change in P (T) (F=1) Single-phase area: can change T and P indepenently (F=2) water Pressure ice Three-phase point: One occurs at specific T and P (triple point) (F=0) watervapor Temperature

  48. Phase Diagrams Two-component @ constant pressure Three-phase - horizontal line Peritectic L +solid (d) solid (g) d L d + L d + g Eutectic L  2 solids (g + b) b + L g + L g Temperature g + b Eutectoid solid (g) 2 solids (a + b) a + g a + b a b (pure B, neglibible solubility of A) A B Composition (%B)

  49. Lever Law • Phase diagram give compositions of phases • two-phase boundaries in 2-phase mixture • Mass balance generate lever law Solid Comp. (XS) Alloy Comp. (Xalloy) Opposite arm over total length Liquid Comp. (XL) Right arm for solid L Temperature Left arm for liquid S A Composition (%B) B

  50. 256°C 12.8 wt% Sn 70 wt% Pb -30 wt% Sn At 183.1°C First solid