Chapter 7 - Micrographofsodiumchloride showingcleavagesteps...

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http://www.ann.jussieu.fr/~elrhabi/Curriculum_fichiers/numerical.html Micrograph of sodium chloride showing cleavage steps 1
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2 Very pure metals yield at low stress: y 20 – 40 MPa Remember that for metals, E 100 – 200 GPa  y 2 10 ‐4 E How much stress does it take to shear atomic planes? You have to break and reform many bonds simultaneously. This requires high levels of stress: y th 0.05 E Pure metals should be strong. .. … so why are they so soft? We need a mechanism to explain deformation. (click to play)
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4 The concept of a line defect was introduced in chapter 4. Edge dislocation Moves in response to shear stress applied perpendicular to its line ሺparallel to stressሻ Motion similar to that of a caterpillar Screw dislocation Direction of movement is perpendicular to the stress direction Dislocation motion distorts the crystal. How does this work? What’s the mechanism? We call this slip or glide Dislocation ሺclick to playሻ
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5 The concept of a dislocation was postulated in the 1930’s. Early evidence was indirect. Etch pits Slip trace on polished surface
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6 In the 1950s electron microscopy allowed us to “ see ”dislocations . Dislocation tangles in strained stainless steel
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7 Two‐dimensional Crystal Dislocation starts here Dislocation glide path Downward Shearingforce (click to play)
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9 All crystals contain dislocations. They’re like growth defects. Let’s define dislocation density ሺ ሻ, as the total length of a dislocation line / unit volume. semiconductor grade Si wafers  10 5 / m 2 typical cast metal  10 10 / m 2 heavily deformed metals  10 15 / m 2 Dislocations create local distortion. .. … and these distortions store energy Lattice strains: slight displacement of atoms usually imposed by crystalline defects Imposed on atoms neighbouring a dislocation line Screw dislocation lattice strains are purely shear
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10 Regions of tension and compression interact with each other. Dislocations attract and repel each other. ‐dislocations with opposite sign and same slip plane will attract each other ሺ dislocation annihilation will occur
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11 Remember how we defined Burger’s vector in chapter 5 edge dislocation screw dislocation
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12 Slip occurs on specific crystallographic planes and directions
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This note was uploaded on 10/14/2011 for the course ENGINEER CHEM ENG 3 taught by Professor Ghosh during the Spring '11 term at McMaster University.

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Chapter 7 - Micrographofsodiumchloride showingcleavagesteps...

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