ENGR2000_dislocations - Introduction to Engineering...

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Introduction to Engineering Materials ENGR2000 Chapter 7: Dislocations and Strengthening Mechanisms Dr. Priya Thamburaj Goeser
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7.1 Introduction What is plastic deformation on a microscopic scale? Net movement of atoms in response to an applied stress Inter-atomic bonds are broken & reformed Motion of dislocations
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7.2 Basic Concepts Slip the process by which plastic deformation is produced by dislocation motion. Slip plane The crystallographic plane along which the dislocation line moves.
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An edge dislocation moves in response to an applied shear stress Applied stress is perpendicular to dislocation line Dislocation motion is parallel to applied stress Macroscopic plastic deformation results
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An edge dislocation moves in response to an applied shear stress dislocation motion
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An edge dislocation moves in response to an applied shear stress
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A screw dislocation moves in response to an applied shear stress Dislocation motion is perpendicular to applied stress Macroscopic plastic deformation results
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Dislocation Density Metals crystals 10 3 mm -2 Ceramic crystals 10 2 mm -2 -10 4 mm -2 Silicon single crystals 0.1 - 1 mm -2 e unit volum per length n dislocatio total density n dislocatio =
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7.3 Characteristics of Dislocations When metals are plastically deformed ~5 % of deformation energy is retained as internal energy (strain energy associated with dislocations) Remainder is dissipated as heat energy
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Lattice strains - due to an edge dislocation
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Two edge dislocations of the same sign and lying on the same slip plane compression tension
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Two edge dislocations of opposite sign and lying on the same slip plane
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Dislocation density during plastic deformation Increases significantly ~ 10 10 mm -2 Existing dislocations ‘multiply’
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Dislocation density during plastic deformation Grain boundaries, internal defects, surface defects (scratches, etc.) act as stress concentrations serve as dislocation formation sites
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7.4 Slip Systems Slip plane Preferred plane on which dislocation motion occurs. Plane with highest planar density. Slip direction Preferred directions on the slip plane along which dislocation motion occurs. Direction with highest linear density.
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ENGR2000_dislocations - Introduction to Engineering...

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