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Lecture11-12 - Dislocations Materials Classes • Metals...

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1 1 Lecture 11-12 Mechanisms of plastic deformation and strengthening mechanisms 2 Dislocations & Materials Classes • 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 + + + + + + + + + + + + + + + + + + + + + + + + 3 Dislocation Motion Dislocations & plastic deformation 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. 4 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. 5 Slip System – Slip plane - plane allowing easiest slippage Wide interplanar spacings - highest planar densities – Slip direction - direction of movement - Highest linear densities FCC Slip occurs on {111} planes (close-packed) in <110> directions (close-packed) => total of 12 slip systems in FCC in BCC & HCP other slip systems occur Deformation Mechanisms Adapted from Fig. 7.6, Callister 7e. 6 Stress and Dislocation Motion • Crystals slip due to a resolved shear stress, τ R . • Applied tension can produce such a stress. slip plane normal, n s Resolved shear stress: τ R = F s / A s slip direction A S τ R τ R F S slip direction Relation between σ and τ R τ R = F S / A S F cos λ A /cos φ λ F F S φ n S A S A Applied tensile stress: = F / A σ slip direction F A F φ λ σ = τ cos cos R
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