Chapter 7 Slides

Chapter 7 Slides - Chapter 7 Lectures begin on R Mar. 6,...

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Chapter 7 Chapter 7 Lectures begin on R Mar. 6, 2008
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2 Chapter 7 Chapter 7 Dislocations and Dislocations and Strengthening Mechanisms Strengthening Mechanisms
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3 Dislocations ( Dislocations ( Dn Dn ) in different classes of materials ) in different classes of materials • Covalent Ceramics (Si, diamond): Motion hard. -directional (angular) bonding • Ionic Ceramics (NaCl): Motion hard. -need to avoid ++ and - - neighbors. ++++ + + + --- - - - - • Metals: D n motion easier. -non-directional bonding -close-packed directions for slip. electron cloud ion cores + + + + + + + + + + + + +++++ + + + + + + + Bond deformations are smaller in metals and displacement vector b is smaller – therefore slip is easier
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4 Dislocation Motion Dislocation Motion Dislocations & plastic deformation Cubic & hexagonal metals - plastic deformation by plastic shear or slip in which one plane of atoms slides over an adjacent plane by passage of a dislocation If dislocations don't move, plastic deformation doesn't occur! It is the MOBILE dislocation density that matters
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5 Dislocation Motion Dislocation Motion Dislocation moves along slip plane in slip direction perpendicular to dislocation line Slip direction same direction as (parallel to) Burgers vector Dislocation motion requires shear stress Edge dislocation Screw dislocation
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6 Strain fields occur at dislocations Strain fields occur at dislocations The edge component has both compressive (extra half plane) and tensile (below plane) Screw has pure shear Mixed is mixed These strain fields produce forces that can repel or attract See at right an attractive interaction that leads to dislocation annihilation – this occurs during annealing More generally plastic flow leads to a little annihilation and much multiplication – The energy of plastic flow is partly stored (~10%) in creating these defects and mostly dissipated in irreversible work (heat) Strain field around edge dislocation Annihilation process during annealing
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7 – Slip plane - plane allowing easiest slippage • Wide interplanar spacings - highest planar densities – Slip direction - direction of movement - Highest linear densities – FCC Slip occurs on any of the 4 {111} (close-packed) planes in any of the 3 <110> (close-packed) directions => total of 12 slip systems in FCC – in BCC – closest packed planes are {110}, cpd <111>, slip may also occur on other almost as close packed planes {211} and {321} – HCP slips primarily on the basal A and B planes {0001} along the cpd <1120> but pyramidal plane ({1010} and {1011}) slip is also possible Slip systems Slip systems the combination of slip plane and slip the combination of slip plane and slip direction direction Note the use of {hkl} to mean ALL planes of the type (hkl) and similarly <uvw> for ALL directions of the type [uvw]
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8 Shear Stress on the Slip Plane Shear Stress on the Slip Plane • Slip is caused by the shear stress, τ R , resolved on the slip plane • Applied tension can produce the needed shear stress.
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This note was uploaded on 04/20/2008 for the course EML 3234 taught by Professor Hellstrom during the Spring '08 term at FSU.

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Chapter 7 Slides - Chapter 7 Lectures begin on R Mar. 6,...

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