EGN3365-7

EGN3365-7 - Chapter 7: Dislocations & Strengthening...

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Chapter 7 - 1 ISSUES TO ADDRESS. .. • Why are dislocations observed primarily in metals and alloys? • How are strength and dislocation motion related? • How do we increase strength? • How can heating change strength and other properties? Chapter 7: Dislocations & Strengthening Mechanisms
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Chapter 7 - 2 Recall dislocations Edge dislocation Screw dislocation Plastic deformation corresponds to motion of large number of dislocations!
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Chapter 7 - 3 • Covalent Ceramics (Si, diamond): Motion hard. -directional (angular) bonding • Ionic Ceramics (NaCl): Motion hard. -need to avoid ++ and - - neighbors. + + + + + + + + + + + - - - - - - - - - - • Metals: Disl. motion easier. -non-directional bonding -close-packed directions for slip. electron cloud ion cores + + + + + + + + + + + + + + + + + + + + + + + +
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Chapter 7 - 4 Dislocation Motion • Cubic & hexagonal metals - plastic deformation by plastic shear or slip where one plane of atoms slides over adjacent plane by defect motion (dislocations). • If dislocations don't move, deformation doesn't occur! Adapted from Fig. 7.1, Callister 7e.
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Chapter 7 - 5 http://www.learner.org/jnorth/tm/monarch/LarvaLocomotion.html From Callister 6e resource CD. Slip: plastic deformation by dislocation motion. Slip plane: crystallographic plane on which dislocation line traverses.
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Chapter 7 - 6 Dislocation Motion • Dislocation moves along slip plane in slip direction perpendicular to dislocation line • Slip direction same direction as Burgers vector Edge dislocation Screw dislocation Adapted from Fig. 7.2, Callister 7e.
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Chapter 7 - 7 Lattice strain around dislocations Edge dislocation : Screw dislocation : Pure shear strain.
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Chapter 7 - 8 Stress fields from dislocations with no applied stresses. Like ones Repel Opposites attract Two halves make a whole. No strain from missing half row. Obstacles, e.g. GB, twins, particles (precipitates). Larger back stress w/ many. Dislocation pile-ups: traffic jam Interacting Edge Dislocations Interacting Edge Dislocations D. Johnson
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Chapter 7 - 9 Different slip planes and less than 45 0 , like ones still repel. Different slip planes and greater than 45 0 , like ones attract. Final configuration that is stable (w/o applied stress). Low-angle grain boundary (Collection of edges). Interacting Edges Interacting Edges Interacting Edges Interacting Edges D. Johnson
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Chapter 7 - 10 Less than 45 0 , opposites attract. Greater than 45 0 , opposites repel. Final stable configuration that minimizes local stress. Array of opposing edges. Interacting Oppositely Oriented Edges Interacting Oppositely Oriented Edges Interacting Oppositely Oriented Edges Interacting Oppositely Oriented Edges D. Johnson
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Slip System – Slip plane - plane allowing easiest slippage • Wide interplanar spacings - highest planar densities – Slip direction - direction of movement -
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EGN3365-7 - Chapter 7: Dislocations & Strengthening...

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