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### Lecture presentation

1. 1. CRYSTAL<br /> AND ITS <br />DEFECTS/ IMPERFECTION<br />
2. 2. In a crystal , the arrangement of atoms is in periodically repeating pattern<br />Periodic repeating units are called unit cells. <br />Unit cell = building block of crystals<br />Lattice = infinite, repeating arrangement of unit cells to make the crystal<br /> OR<br />An array of points such that every point has identical surroundings<br />
3. 3.
4. 4. Cubic Crystals<br />a = b= c =  =  = 90º<br />Simple Cubic (P)<br />Body Centred Cubic (I) – BCC<br />Face Centred Cubic (F) - FCC<br />PyriteCube<br />[1]<br />[1]<br />GarnetDodecahedron<br />Fluorite<br />Octahedron<br />[1]<br />
5. 5. Tetragonal Crystals<br /> a = b  c  =  =  = 90º<br />Simple Tetragonal<br />Body Centred Tetragonal<br />Zircon<br />[1]<br />[1]<br />[1]<br />[1] http://www.yourgemologist.com/crystalsystems.html<br />
6. 6. Orthorhombic Crystals<br /> a  b  c  =  =  = 90º<br />Simple Orthorhombic<br />Body Centred Orthorhombic<br />Face Centred Orthorhombic<br />End Centred Orthorhombic<br />[1]<br />Topaz<br />[1]<br />[1] http://www.yourgemologist.com/crystalsystems.html<br />
7. 7. Hexagonal Crystals<br /> a = b  c  =  = 90º  = 120º <br />Simple Hexagonal<br />Corundum<br />[1]<br />[1] http://www.yourgemologist.com/crystalsystems.html<br />
8. 8. 5. Rhombohedral Crystals<br /> a = b = c  =  =   90º<br /><ul><li>Rhombohedral (simple)</li></ul>[1]<br />[1]<br />Tourmaline<br />[1] http://www.yourgemologist.com/crystalsystems.html<br />
9. 9. Monoclinic Crystals<br /> a  b  c  =  = 90º  <br />Simple Monoclinic<br />End Centred (base centered) Monoclinic (A/C)<br />[1]<br />Kunzite<br />[1] http://www.yourgemologist.com/crystalsystems.html<br />
10. 10. 7. Triclinic Crystals<br /> a  b  c      <br /><ul><li>Simple Triclinic</li></ul>[1]<br />Amazonite<br />[1] http://www.yourgemologist.com/crystalsystems.html<br />
11. 11. RHOMBOHEDRAL<br />
12. 12. 4 atoms/unit cell<br />2 atoms/unit cell<br />1 atom/unit cell<br />(8 x 1/8 + 6 x 1/2 = 4)<br />(8 x 1/8 + 1 = 2)<br />(8 x 1/8 = 1)<br />11.4<br />
13. 13. Types of Crystals<br />Ionic Crystals<br /><ul><li>Lattice points occupied by cations and anions
14. 14. Held together by electrostatic attraction
15. 15. Hard, brittle, high melting point
16. 16. Poor conductor of heat and electricity</li></ul>CsCl<br />ZnS<br />CaF2<br />11.6<br />
17. 17. carbon<br />atoms<br />Types of Crystals<br />Covalent Crystals<br /><ul><li>Lattice points occupied by atoms
18. 18. Held together by covalent bonds
19. 19. Hard, high melting point
20. 20. Poor conductor of heat and electricity</li></ul>graphite<br />diamond<br />11.6<br />
21. 21. Types of Crystals<br />Molecular Crystals<br /><ul><li>Lattice points occupied by molecules
22. 22. Held together by intermolecular forces
23. 23. Soft, low melting point
24. 24. Poor conductor of heat and electricity</li></ul>11.6<br />
25. 25. nucleus &<br />inner shell e-<br />mobile “sea”<br />of e-<br />Types of Crystals<br />Metallic Crystals<br /><ul><li>Lattice points occupied by metal atoms
26. 26. Held together by metallic bonds
27. 27. Soft to hard, low to high melting point
28. 28. Good conductors of heat and electricity</li></ul>Cross Section of a Metallic Crystal<br />11.6<br />
29. 29. Types of Crystals<br />11.6<br />
30. 30.
31. 31.
32. 32.
33. 33.
34. 34.
35. 35.
36. 36.
37. 37.
38. 38.
39. 39.
40. 40.
41. 41.
42. 42.
43. 43. THANKS<br />
44. 44. Question : Why metals can be plastically deformed and why the plastic deformation properties could be changed to a very large degree by forging without changing the chemical properties ?<br />This phenomenon was explained by Taylor, Orowan and Olyani by using the concept of dislocations. <br />Dislocations are thought of as extra lattice planes inserted in the crystal but not extending through all of the crystal but ending in the dislocation line.<br />Motion of dislocations allows slip- Plastic Deformation –when interatomic bonds are fractured and reformed. Slip always occurs through dislocations motion.<br />
45. 45. Line Defects<br />Dislocations<br />
46. 46.
47. 47. Dislocations ( Line defects) :These defects produce lattice distortions centered about a line.<br />A dislocation is the edge of an extra inserted fractional plane of atoms. <br />Positive dislocation: extra fractional plane, Negative dislocation ( missing fractional plane)<br />Dislocations play a very important role in the deformation of crystals.<br />Slip : Plastic deformation when interatomic bonds are fractured and reformed . Slip always occurs through dislocations motion.<br />Slip plane : The plane in which a dislocation moves through a crystal.<br />
48. 48. When a shear stress is applied , the dislocation moves, one atomic row after another, until one part of the crystal is displaced relative to the other. The motion of the dislocation causes the crystal to be permanently deformed.<br />On either side of the dislocation, the crystal lattice is perfect but in the vicinity of the dislocation the lattice is severely distorted. For a positive edge dislocation , the presence of the extra half plane causes the atoms above the slip plane to be in compression while those below are in tension.<br />http://web.mit.edu/3.091/www/archives/Notes_6.pdf<br />
49. 49. Missing half plane A Defect<br />
50. 50. An extra half plane…<br />…or a missing half plane<br />
51. 51. What kind of defect is this?<br />A line defect?<br />Or a planar defect?<br />
52. 52. No extra plane!<br />Extra half plane<br />
53. 53. Missing plane<br />No missing plane!!!<br />
54. 54. An extra half plane…<br />EdgeDislocation<br />…or a missing half plane<br />
55. 55. If a plane ends abruptly inside a crystal we have a defect.<br />The whole of abruptly ending plane is not a defect<br />Only the edge of the plane can be considered as a defect<br />This is a line defect called an EDGE DISLOCATION<br />
56. 56. Callister FIGURE 4.3 The atom positions around an edge dislocation; extra half-plane of atoms shown in perspective. (Adapted from A. G. Guy, Essentials of Materials Science, McGraw-Hill Book Company, New York, 1976, p. 153.)<br />
57. 57. 1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />
58. 58. 1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />
59. 59. boundary<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />Burgers vector<br />Slip plane<br />b<br />slip<br /> no slip<br />= edge dislocation<br />
60. 60. t<br />b<br />Slip plane<br />slip<br />no slip<br />Dislocation: slip/no slip boundary<br />b: Burgers vectormagnitude and direction of the slip<br />dislocation<br />t: unit vector tangent to the dislocation line<br />
61. 61. Dislocation Line:A dislocation line is the boundary between slip and no slip regions of a crystal<br />Burgers vector:The magnitude and the direction of the slip is represented by a vector b called the Burgers vector,<br />Line vectorA unit vector t tangent to the dislocation line is called a tangent vector or the line vector.<br />
62. 62. boundary<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />1<br />2<br />3<br />4<br />5<br />6<br />7<br />8<br />9<br />Burgers vector<br />Slip plane<br />b<br />slip<br /> no slip<br />t<br />= edge dislocation<br />
63. 63. b t , b  t  Mixed dislocation<br />In general, there can be any angle between the Burgers vector b (magnitude and the direction of slip) and the line vector t (unit vector tangent to the dislocation line)<br />b  t  Edge dislocation<br />b  t  Screw dislocation<br />
64. 64. t<br />b<br />3<br />2<br />1<br />Screw Dislocation Line<br />b || t<br />
65. 65. If b || t<br />Then parallel planes  to the dislocation line lose their distinct identity and become one continuous spiral ramp<br />Hence the name SCREW DISLOCATION<br />
66. 66. Positive<br />Negative<br />Extra half plane above the slip plane<br />Extra half plane belowthe slip plane<br />Edge Dislocation<br />Left-handed spiral ramp<br />Right-handed spiral ramp<br />Screw Dislocation<br />b parallel to t<br />b antiparallel to t<br />
67. 67. 1<br />8<br />2<br />3<br />4<br />5<br />6<br />7<br />9<br />10<br />11<br />12<br />13<br />14<br />15<br />16<br />1<br />9<br />2<br />8<br />3<br />7<br />4<br />6<br />5<br />5<br />6<br />4<br />7<br />3<br />8<br />2<br />9<br />1<br />1<br />8<br />2<br />3<br />4<br />5<br />6<br />7<br />9<br />10<br />11<br />12<br />13<br />14<br />15<br />16<br />S<br />F<br />A closed Burgers Circuit in an ideal crystal<br />
68. 68. b<br />13<br />14<br />16<br />1<br />8<br />4<br />5<br />6<br />7<br />9<br />10<br />11<br />12<br />2<br />3<br />15<br />1<br />9<br />2<br />Map the same Burgers circuit on a real crystal<br />8<br />3<br />7<br />4<br />6<br />5<br />5<br />6<br />4<br />7<br />3<br />8<br />2<br />9<br />1<br />2<br />3<br />6<br />9<br />10<br />11<br />12<br />13<br />4<br />5<br />8<br />7<br />14<br />15<br />16<br />1<br />F<br />S<br /><br />RHFS convention<br />
69. 69. 57<br />Imperfections in Solids<br />Screw Dislocation<br />Screw Dislocation<br />b<br />Dislocation<br />line<br />(b)<br />Burgers vector b<br />(a)<br />Adapted from Fig. 5.9, Callister & Rethwisch 3e.<br />
70. 70. 58<br />Edge, Screw, and Mixed Dislocations<br />Mixed<br />Edge<br />Screw<br />Adapted from Fig. 5.10, Callister & Rethwisch 3e.<br />
71. 71.
72. 72. Glide of an Edge Dislocation<br /><br /><br />
73. 73. crss<br />Glide of an Edge Dislocation<br />crss is critical resolved shear stress on the slip plane in the direction of b.<br />crss<br />
74. 74. crss<br />Glide of an Edge Dislocation<br />crss is critical resolved shear stress on the slip plane in the direction of b.<br />crss<br />
75. 75. crss<br />Glide of an Edge Dislocation<br />crss is critical resolved shear stress on the slip plane in the direction of b.<br />crss<br />
76. 76. crss<br />Glide of an Edge Dislocation<br />crss is critical resolved shear stress on the slip plane in the direction of b.<br />crss<br />
77. 77. crss<br />Glide of an Edge Dislocation<br />A surface step of b is created if a dislocation sweeps over the entire slip plane<br />Surface step, not a dislocation<br />crss<br />
78. 78. Surface Defects<br />
79. 79. Surface Defects<br />External<br />Internal<br />Free surface<br />Grain boundary<br />Same phase<br />Stacking fault<br />Twin boundary<br />Interphase boundary<br />Different phases<br />
80. 80.
81. 81.
82. 82.
83. 83.
84. 84.
85. 85.
86. 86.
87. 87.
88. 88. Grain Boundary<br />Internal surface: grain boundary<br />Grain 2<br />Grain 1<br />A grain boundary is a boundary between two regions of identical crystal structure but different orientation<br />
89. 89. Optical Microscopy, Experiment 4<br />Photomicrograph an iron chromium alloy. 100X.<br />Callister, Fig. 4.12<br />
90. 90. Grain Boundary: low and high angle<br />One grain orientation can be obtained by rotation of another grain across the grain boundary about an axis through an angle <br />If the angle of rotation is high, it is called a high angle grain boundary<br />If the angle of rotation is low it is called a low angle grain boundary <br />
91. 91.
92. 92. Grain Boundary: tilt and twist<br />One grain orientation can be obtained by rotation of another grain across the grain boundary about an axis through an angle<br />If the axis of rotation lies in the boundary plane it is called tilt boundary- Edge dislocation<br />If the angle of rotation is perpendicular to the boundary plane it is called a twist boundary – Screw dislocation<br />
93. 93. B<br />b<br /><br />C<br /><br />B<br />C<br />2h<br /><br /><br />A<br />A<br />Edge dislocation model of a small angle tilt boundary<br />Tilt boundary<br />Grain 1<br />Grain 2<br />Or approximately<br />Eqn. 6.7<br />
94. 94. Stacking fault<br />CBACBACBA<br />ACBABACBA<br />Stacking fault<br />HCP<br />FCC<br />FCC<br />
95. 95.
96. 96.
97. 97. Influence of grain boundaries<br />Grain boundaries influence strength, ductility of metals and strain hardening.<br />Plastic deformation takes place through grain-boundary sliding. <br />Creep mechanism results from grain-boundary sliding.<br />At a low-melting-point, metals, strong metal can crack under very low stresses known as grain-boundary embrittlement.<br />
98. 98. Plastic Deformation of Polycrystalline Metals<br />During plastic deformation, mass continuity in grain boundaries is maintained. <br />The grains would become elongated in one direction and contract in the other.<br />Two types of anisotropy in metals:<br />Preferred orientation<br />Mechanical fibering<br />
99. 99. Recovery, Recrystallization,and Grain Growth<br />The temperature range and the time required depend on the material.<br />3 events take place during the heating:<br /><ul><li>Recovery
100. 100. number of mobile dislocations reduced
101. 101. Recrystallization </li></ul> - new grains form<br /><ul><li>Grain growth</li></ul> - grains grow bigger<br />
102. 102. Cold, Warm, and Hot Working<br />When plastic deformation above recrystallization temperature, it is called hot working, vice versa it is knowm as cold working.<br />